FMDS0788-Flammable Storage Tank

October 2011Page 1 of 44

FLAMMABLE LIQUID STORAGE TANKS

Table of ContentsPage

1.0 SCOPE .................................................................................................................................................... 31.1 Changes ........................................................................................................................................... 3

2.0 LOSS PREVENTION RECOMMENDATIONS ........................................................................................ 32.1 Construction and Location ............................................................................................................... 3

2.1.1 General .................................................................................................................................... 32.1.2. Aboveground tanks ............................................................................................................... 62.1.3 Buried Tanks ........................................................................................................................... 92.1.4 Indoor Tanks ......................................................................................................................... 102.1.5 Intermediate Bulk Containers (IBC) ..................................................................................... 112.1.6 Protection against Flooding ................................................................................................. 122.1.7 Earthquake ........................................................................................................................... 13

2.2 Occupancy ...................................................................................................................................... 132.2.1 General ................................................................................................................................. 132.2.2 Normal and Emergency Venting .......................................................................................... 142.2.3 Asphalt Tanks ....................................................................................................................... 23

2.3 Protection ....................................................................................................................................... 242.3.1 Indoor Tanks ......................................................................................................................... 242.3.2 Outdoor Tanks ...................................................................................................................... 242.3.3 Water Supply ........................................................................................................................ 25

2.4 Operation and Maintenance ........................................................................................................... 262.4.1 Repair, Reconditioning, and Abandonment ...................................................................... 27

2.5 Ignition Source Control .................................................................................................................... 273.0 SUPPORT FOR RECOMMENDATIONS .............................................................................................. 28

3.1 Background information ................................................................................................................. 283.1.1 Hazards ................................................................................................................................ 283.1.2 Types of Tanks .................................................................................................................... 293.1.3 Indoor Tanks ......................................................................................................................... 313.1.4 Tank Spacing and Containment ............................................................................................ 323.1.5 Manifolded Vents ................................................................................................................. 323.1.6 Asphalt Tanks ....................................................................................................................... 343.1.7 Fire Protection ..................................................................................................................... 35

3.2 Loss History .................................................................................................................................... 353.2.1 Storage Tanks ...................................................................................................................... 353.2.2 Manifolded Vents .................................................................................................................. 36

4.0 REFERENCES ...................................................................................................................................... 364.1 FM Global ........................................................................................................................................ 364.2 NFPA ................................................................................................................................................ 374.3 Others ............................................................................................................................................. 37

APPENDIX A GLOSSARY OF TERMS ....................................................................................................... 38APPENDIX B DOCUMENT REVISION HISTORY ....................................................................................... 40APPENDIX C HYDROCARBON FIRE DURATION .................................................................................... 41APPENDIX D HAZARDS ............................................................................................................................ 43

List of FiguresFig. 1. Horizontal aboveground tank .............................................................................................................. 5Fig. 2. Buried tank with flame arrester ........................................................................................................ 10

FM GlobalProperty Loss Prevention Data Sheets 7-88

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Fig. 3. Enclosed indoor tank ........................................................................................................................ 12Fig. 4. Cone roof vertical tank ..................................................................................................................... 13Fig. 5. Safe gauging methods ..................................................................................................................... 15Fig. 6. Manifolded tanks .............................................................................................................................. 22Fig. 7. Required pipe sizing if detonation arrester is smaller than nearby piping ...................................... 22Fig. 8. Improper piping around detonation arrester ..................................................................................... 22Fig. 9. Open top double deck ...................................................................................................................... 30Fig. 10. Open top pontoon .......................................................................................................................... 30Fig. 11. Pan-type covered tanks .................................................................................................................. 31Fig. 12. Detonation arrester ......................................................................................................................... 32Fig. 13. Storage tank with flame arrester .................................................................................................... 33Fig. 14. End-of-line flame arrester ............................................................................................................... 33Fig. 15. End-of-line flame arrester with pipe-away flange ........................................................................... 34Fig. 16. Backflash interrupter ....................................................................................................................... 34Fig. 17. Typical conservation vent ................................................................................................................ 38

List of TablesTable 1. Support (Saddle) Width for Horizontal Steel Flammable Liquid Tanks ............................................ 4Table 2. Spacing for Flammable Liquid Storage Tanks and Loading/Unloading Stations ............................ 6Table 3. Spacing for Flammable Liquid Tank Containment Dikes ................................................................ 7Table 4. Indoor Tank Quantity Limits ........................................................................................................... 10Table 5. Size of Opening for Normal Venting ............................................................................................... 16Table 6. Required Thermal (Normal) Venting Capacity 1 ............................................................................. 17Table 7. Typical Vent Line Size for Buried Tanks ......................................................................................... 18Table 8. Capacities for Emergency Relief of Excessive Internal Pressure forAboveground Tanks Operating at 1 psig (7 kPa) or less1 ............................................................................ 19Table 9. Values for L (M)1/2 ........................................................................................................................... 20Table 10. Environmental Factors for Emergency Venting Capacity (use only one factor) .......................... 21Table 11. Sprinkler Density for Storage Tank Rooms, gpm/ft2(mm/min) ...................................................... 24Table 12. Hose Stream Demand for TANKS Storing Flammable Liquids 1 ................................................. 26Table 13. Estimated Water Demand for Fixed Foam Protection for a full Surface Fire. ............................. 26Table 14. Safety Distances for Hot Work, Open Flames, Maintenance, Repair or Modification ................. 28Table 15. Losses over US$100,000 by Occupancy Class ........................................................................... 35Table 16. Losses over US$100,000 by Engineering Peril ............................................................................ 36Table 17a. Relationship Between Fuel Volume, Pool Size, and Fire Duration (English) ............................. 41Table 17b. Relationship Between Fuel Volume, Pool Size, and Fire Duration (metric) ............................... 42Table 18a. Flow Rate, Pool Diameter, Heat Release Rate, and Flame Height for a

Flowing Kerosene Fire (English) ................................................................................................. 42Table 18b. Flow Rate, Pool Diameter, Heat Release Rate, and Flame Height for a

Flowing Kerosene Fire (Metric) ................................................................................................... 42

7-88 Flammable Liquid Storage TanksPage 2 FM Global Property Loss Prevention Data Sheets

©2007 Factory Mutual Insurance Company. All rights reserved.

1.0 SCOPE

In this data sheet, the term ‘‘flammable liquid’’ is used synonymously for all three classes of liquids. Where rec-ommendations vary because of flash point, that is indicated.

The recommendations in this data sheet apply to chemically stable or unstable flammable liquids when storedin atmospheric pressure (operating at less than 1 psig [0.07 barg]) or low pressure (operating over 1 psig[0.07 barg] and less than 15 psig [1 barg]) tanks.

This data sheet applies to storage in horizontal and vertical tanks usually constructed of metal and locatedaboveground, underground, or inside buildings. Storage in floating roof tanks is not addressed. For guid-ance on floating roof tanks, refer to NFPA 30, Flammable Liquid Storage in Portable Containers, or equiva-lent national or international standard.

This data sheet provides requirements for intermediate bulk containers (IBCs), when used to supply liquidsto a process. Storage of IBC is covered by DS 7-29, Flammable Liquid Storage in Portable Containers.

This data sheet does not cover all aspects of pumping operations as represented by load and unload racks,pump pads at tank farms, or fuel pumping and transfer systems in buildings. Spacing criteria for some ofthese peripheral operations are provided in Table 2. For other aspects, refer to DS 7-32, Flammable LiquidOperations.

The recommendations for drainage, fire protection, separation, or diking do not apply to day tanks or other pro-cess tanks. Locate and protect those tanks in accordance with the appropriate FM Global data sheet, suchas Data Sheet 7-14, Fire and Explosion Protection for Flammable Liquid, Flammable Gas, and LiquefiedFlammable Gas Processing Equipment and Supporting Structures; Data Sheet 7-30, Solvent ExtractionPlants; Data Sheet 7-32, Flammable Liquid Operations, or Data Sheet 7-43/17-2, Loss Prevention in Chemi-cal Plants.

This data sheet does not apply to flammable liquids or gases stored in pressure vessels above 15 psig (103kPa). See Data Sheet 7-55, Liquefied Petroleum Gases, for such storages.

This data sheet addresses methods to prevent flame propagation throughout low-pressure flammable liq-uid storage tanks that are manifolded together to reduce atmospheric emissions where the presence of anignitable vapor-air mixture in normal operation is likely.

This data sheet does not address preventing flame propagation in fuel gas piping systems (see Data Sheet6-10, Process Furnaces) in systems handling acetylene (see Data Sheet 7-51, Acetylene) or in fume col-lection systems for process equipment (see Data Sheet 7-78, Industrial Exhaust Systems).

1.1 Changes

October 2011. The reference in Table 10, note 1 was corrected from 2.2.2-5 to 2.1.2-5.

2.0 LOSS PREVENTION RECOMMENDATIONS

2.1 Construction and Location

2.1.1 General

1. Construct atmospheric tanks (operating at less than 1 psig [0.07 barg]) in accordance with the followingrecognized engineering standards or suitable national or international equivalents:

a) API (American Petroleum Institute) Standard 650, Welded Steel Tanks for Oil Storage

b) UL (Underwriters Laboratories) 142, Standard for Steel Aboveground Tanks for Flammable and Com-bustible Liquids

c) UL 2080, Fire Resistant Tanks for Flammable and Combustible Liquids

d) UL 2085, Protected Aboveground Tanks for Flammable and Combustible Liquids

e) UL 2244, Standard for Aboveground Flammable Liquid Tank Systems

f) UL 58, Standard for Steel Underground Tanks for Flammable and Combustible Liquids

Flammable Liquid Storage Tanks 7-88FM Global Property Loss Prevention Data Sheets Page 3


2. Construct low-pressure tanks (operating at more than 1 psig [0.07 barg] but less than 15 psig [1 barg])in accordance with the following recognized engineering standards or suitable national or international equiva-lents:

a) API Standard 620, Design and Construction of Large, Welded, Low-Pressure Storage Tanks

b) Code for Unfired Pressure Vessels, Section VIII, Division 1 of the ASME Boiler and Pressure VesselCode

c) EN BS 14015, Specification for Design and Manufacture of Site Built, Vertical, Cylindrical, Flat-Bottomed, Aboveground, Welded, Steel Tanks for the Storage OF Liquids at Ambient Temperatures andAbove

d) EN BS 12285, Part 1, Workshop Fabricated Steel Tanks — Horizontal Cylindrical Single and DoubleSkin Tanks for Underground Storage of Flammable and Non-Flammable Water Polluting Liquids

e) EN BS 12285, Part 2, Workshop Fabricated Steel Tanks — Horizontal Cylindrical Single and DoubleSkin Tanks for Aboveground Storage of Flammable and Non-Flammable Water Polluting Liquids

3. Design supports for horizontal cylindrical tanks to minimize settlement or lateral movement that could resultin overstress or rupture of the tank or associated pipe and fittings.

a) Provide supports of fire-resistive construction (e.g., saddles of reinforced concrete as shown in Fig. 1),with at least one-third of the circumference of the tank bearing on the supports. Protect reinforcing steelin concrete saddles with at least 2 in. (50 mm) of concrete.

b) Design saddles in accordance with the following table.

Table 1. Support (Saddle) Width for Horizontal Steel Flammable Liquid Tanks

Capacity, gal (m3) ≤ 550 (2.1)> 550 ≤ 1100

(2.1 – 4.2)> 1100 ≤ 9,000

(4.2 – 34)> 1100 ≤ 35,000

(4.2 – 133)

> 35,000≤ 50,000

(133 – 189)Tank diameter,

in (mm)48 (1220) 64 (1625) 76 (1930) 144 (3660) 144 (3660)

Saddle width,in (mm)

4.5 (115) 6 (150) 6 (150) 9 (230) 10 (255)

c) Provide fireproofing of tank supports that are structural steel with a material having a fire resistanceof 2 hrs (concrete in accordance with DS 1-21 or an FM Approved coating rated for process structure ortank protection) or protected with automatic water spray nozzles in accordance with DS 4-1N.

d) Provide bracing to prevent movement in locations subject to earthquakes.

e) In an area subject to flooding, anchor tanks to prevent either full or empty tanks from floating duringa rise in water level up to the maximum flood stage. Details are given in Section 2.2.6, Protection againstFlooding.

4. Pressure vessels and low-pressure tanks may be used as atmospheric storage tanks. Where unstable liq-uids are stored, see 2.2.2.2 – 7.

5. Fixed tanks of combustible construction (usually glass fiber-reinforced plastic) may be used:

a) for underground installation.

b) where required by the properties of the liquid stored.

c) for liquids with flash point greater than 200°F (93°C) stored outdoors where not exposed to the leak-age of liquids with lower flash point.

d) for liquids with flash point greater than 200°F (93°C) (or any flash point if required as in b) stored indoorswith suitable automatic sprinkler protection and containment and installed in accordance with 2.1.4, below.

6. When glass fiber-reinforced plastic (FRP) tanks are used:

a) Construct the tank in accordance with the following recognized engineering standards or suitablenational or international equivalents:



1. UL 1316, Standard for Glass Fiber Reinforced Plastic Underground Storage Tanks for PetroleumProducts, Alcohols, and Alcohol-Gasoline Mixtures

2. API Specification 12P, Fiberglass Reinforced Plastic Tanks

3. ASTM D3299-Standard Specification for Filament Wound Glass Fiber Reinforced Thermoset ResinChemical Resistant Tanks.

4. ASTM D4097-Standard Specification for Contact Molded Glass Fiber Reinforced Thermoset ResinChemical Resistant Tanks.

5. EN BS 13121 GRP Tanks and Vessels for Use Aboveground.Part 1: Raw materials – specification and acceptance conditionsPart 2: Composite materials – chemical resistancePart 3: Design and WorkmanshipPart 4: Delivery, installation and maintenance

b) Install aboveground tanks on a concrete pad in the vertical position only.

c) Store only chemically stable liquids, compatible with the reinforced plastic.

d) Provide separate dikes for all reinforced plastic tanks over 2,500 gal (9.5 m3).

e) Provide spacing for all reinforced plastic tanks in accordance with Table 2.

f) On tanks containing liquids with flash point less than 100°F (38°C), install conductive metal fill and dis-charge lines, supported internally and extending to within 3 in. (76 mm) of the tank bottom, and providea static ground to dissipate charges that can accumulate during filling operations.

g) Where tanks are located indoors, provide automatic sprinkler protection designed in accordance withsection 2.3.1, below. Automatic sprinkler protection may be omitted in low-value buildings that haveadequate space separation from important buildings and structures.

h) Install buried tanks in strict conformance to the manufacturer’s recommendations.

7. Construct tanks that have special features, such as corrosion resistance, with strength equivalent to thatof steel tanks.

Fig. 1. Horizontal aboveground tank



8. Concrete tanks without liners may be used for the storage of liquids with flash points higher than 100°F(38°C) and specific gravities greater than 0.8.

9. Concrete tanks with combustible or noncombustible liners may be used for the storage of liquids with flashpoints lower than 100°F (38°C) when designed in accordance with recognized engineering standards.Choose the type and thickness of the liner depending on the properties of the liquid to be stored.

10. Provide rectangular steel tanks with internal braces to withstand hydrostatic loads in accordance with rec-ognized engineering standards.

11. Where combustible plastic insulation is used on storage tanks, install a proper fire protective coatingover the insulation or install FM Approved Class 1 insulation. See Data Sheet 1-57, Plastic in Construction, for additional guidance.

12. Prior to being placed in service, test all tanks in accordance with the standard under which they were con-structed; generally, by filling the tanks with water and observing them for leakage. (PRIOR 2.1.1.5)

2.1.2. Aboveground tanks

1. Locate aboveground tanks on ground sloping away from main facility buildings and plant utility installa-tions. On hilly terrain, provide drainage or dikes to bypass buildings or installations at lower levels.

2. Locate tanks with respect to buildings and other tanks in accordance with Table 2.

Table 2. Spacing for Flammable Liquid Storage Tanks and Loading/Unloading Stations

Liquid Flash Point (1)(2)

≤ 140°F (60°C) > 140°F (60°C)Stable liquids, tank to bldgs of non combustible or betterconstruction (See Appendix A) or open process structures (3)

1 D (min 75 ft, 23 m) 0.5 D (min 50 ft, 15 m)

Stable liquids, tank to buildings of combustible construction(See Appendix A)

2 D (min 125 ft, 38 m) 1 D (min 75 ft, 23 m)

Stable liquids in listed UL 2080, 2085 and 2244 containers See 2.1.2 – 6Unstable liquids, tank to bldgs of any construction 2 D (min 125 ft, 38 m) 1 D (min 75 ft, 23 m)Stable liquids, tank to tank 0.5 D (min 3 ft, 0.9 m) 0.5 D (min 3 ft, 0.9 m)Unstable liquids, tank to tank 1 D (min 5 ft, 1.5 m) 1 D (min 5 ft, 1.5 m)Tank truck and railcar loading/unloading to tank, (4) 75 ft (23 m) 50 (15 m)Tanks (single or multiple) to LPG storage minimum 100 ft (30 m) or 1 D

Notes1 Where tanks are equipped with internal heating systems and store liquids subject to boil over, froth over, or slop over, evaluate as if con-taining liquids with flash points = 140°F (60°C), regardless of their flashpoint.2 D refers to the diameter of the largest flammable liquid tank.3 Open process structure refers to areas of one or multiple levels used to manufacture chemicals. Intermediate tanks considered part ofthe process are excluded from this spacing requirement.4 For separation between loading/unloading facilities and buildings, see DS 7-32.

3. Provide containment for tanks containing flammable liquids with flash points below 200°F (93°C) by remoteimpounding, dikes around the tanks, or secondary containment. (Environmental or other government regu-lations may require containment for smaller tanks.)

4. Construct dikes used to provide containment around the tanks according to the following criteria:

a) Size dikes to hold 100% of the contents of the largest tank within the diked area. The volume occu-pied by this tank below the top of the dike may be considered part of the dike capacity unless the liquidstored is subject to boil over. The volumes of all other tanks below the top of the dike must be deductedwhen calculating dike capacity.

b) Construct dike walls of earth, steel, concrete, or solid masonry, designed to be liquid-tight and to with-stand a full hydrostatic head by release of tank contents.

c) Provide earthen walls 3 ft (1 m) or more in height with a flat section at the top not less than 2 ft (0.6m) wide with the wall slope consistent with the angle of repose of the material of which the wall is con-structed.



d) Control vegetation, desirable protection against erosion, so as not to impede fire fighters or add tothe fire hazard.

e) Limit the height of dikes, regardless of construction, to 6 ft (2 m) to minimize the chances of pocket-ing flammable vapors and to facilitate fire fighting.

f) Provide drainage to remove water from within diked areas at a minimum uniform slope of 1% awayfrom tanks toward a sump, a drain box, or other means of disposal located at a safe distance from thetank.

g) Design drains to prevent liquids from entering natural water courses, public sewers, or drains. Trapdrain lines and provide valves on the lines, outside the dike, so they are accessible under fire condi-tions. Protect the traps from freezing.

h) Limit dikes to contain an aggregate capacity of 5,000,000 gal (18,900 m3), except were individual tankcapacity exceeds 5,000,000 gal (18,900 m3) in which case, ensure the dike contains only one tank.

i) Subdivide any dike containing two or more tanks by intermediate dikes or channels as follows:

1. Stable liquids in weak seam roof tanks

a. Subdivision for each tank over 420,000 gal (1,590 m3)

b. Subdivision for each group of tanks with total capacity up to 630,000 gal (2,390 m3), none individu-ally > 420,000 gal (1,590 m3)

2. Stable liquids in horizontal tanks or vertical cone roof tanks

a. Subdivision for each tank over 100,000 gal (380 m3)

b. Subdivision for each group of tanks with total capacity up to 150,000 gal (570 m3), none individu-ally >100,000 gal (380 m3)

3. Unstable liquids in any type of tank need individual subdivision.

4. Unstable liquids in any type of tank protected by water spray in accordance with Data Sheet 4-1Ncan follow the subdivision requirements in “2” above.

j) Build intermediate dikes at least 18 in high.

k) Provide separation between a tank and the dike wall of at least one-half the tank diameter.

l) Provide separation between buildings and dike wall in accordance with Table 3.

m) Where tanks are arranged in more than two adjacent rows or in an irregular pattern, provide greaterspacing between tanks, additional dikes, or roadways so all tanks are accessible for firefighting.

Table 3. Spacing for Flammable Liquid Tank Containment Dikes

Liquid Flash Point (1)(2)

≤ 140°F (60°C) > 140°F (60°C)Stable liquids, dike wall to buildings of noncombustible orbetter construction (See Appendix A) or open processstructures (3)

1 D (min 75 ft; 23 m;max. 200 ft, 61 m)

0.5 D (min 50 ft; 15 m;max. 200 ft, 61 m)

Stable liquids, dike wall to buildings of combustible construction(See Appendix A)

2 D (min 125 ft, 38 m;300 ft, 91 m)

1 D (min 75 ft, 23 m; 300ft, 91 m)

Unstable liquids, dike wall to buildings any construction 2 D (min 125 ft, 38 m;300 ft, 91 m)

1 D (min 75 ft, 23 m; 300ft, 91 m)

Notes1 Where dikes contain tanks equipped with internal heating systems and store liquids subject to boil over, froth over, or slop over, protectas liquids with flash points = 140°F (60°C) regardless of their flashpoint.2 D usually refers to the longest dike dimension, length, width, or diameter (if circular).However, where a non-circular dike is present, basethe spacing to the exposure on the exposing dimension, i.e., the side that most directly faces the exposed structure, vessel or other dike,not necessarily the longest dimension.3 Open process structure refers to areas of one or multiple levels used to manufacture chemicals. Intermediate tanks considered part ofthe process are excluded from this spacing requirement.

5. Design remote impounding used for containment in accordance with the following criteria:



a) Provide drainage within diked areas at a minimum uniform slope of 1% away from tanks toward theimpounding basin.

b) Route drainage between the tanks and the impounding basin so that if the liquid in the system is ignited,it will not seriously expose tanks or important buildings (DS 7-83 can provide valuable guidance on thedesign of the drainage system).

c) Provide the impounding basin with a minimum capacity equal to twice the largest tank that could drainto it.

d) Equip the impounding basin with means to drain off accumulations of water from precipitation.

e) Separate the impounding basin from important buildings and facilities according to the size of the basinand the exposure potential to the building, as follows:

1. From buildings of ordinary or combustible construction (or from buildings containing hazardous mate-rials) having extensive window areas or associated combustible yard storage, spacing distance = 1.8× basin diameter or diagonal.

2. From buildings of fire resistive or noncombustible construction not having extensive window areas,hazardous materials, or associated combustible yard storage, spacing distance = 0.6 × basin diam-eter or diagonal.

3. From flammable liquid storage tanks, spacing distance = 0.3 × basin diameter or diagonal.

f) Provide each diked and/or subdivided area with drainage systems leading to the impounding basin.Hydraulically design the drainage system from each diked or subdivided area as follows:

1. Provide drainage capacity from each subdivision in a dike capable of carrying off liquid at a ratenot less than that which could be released assuming a break in a bottom connection from the largestfull tank within the subdivision, or the maximum tank fill rate, whichever is greater.

2. Use drainage system piping that is a minimum of 24 in. (60 cm) diameter.

3. Route piping under subdivisions and dikes to culverts or open channels leading to the impoundingbasin.

4. Design culverts or open channels with the capacity to carry off liquid from all the drainage connec-tions within the diked area having the largest single tank, with the connections flowing at their maxi-mum capacity.

5. Design the culverts or channels with additional capacity to carry off the maximum expected dis-charge of water from fire fighting operations.

6. Locate open channels a minimum of 50 ft (15 m) from important buildings and facilities.

7. Provide roads with culverts over the channels at intervals to permit access to the tanks for mainte-nance or emergencies.

8. Pave channels with asphalt or concrete, or line them with smooth stone, metal, or compacted clayto prevent growth of vegetation that could restrict liquid flow.

9. Provide a minimum of 1% slope for channels and culverts. Steeper slopes are advisable to reduceculvert or channel size.

6. Secondary containment tanks (double skinned) listed as meeting the requirements of UL 2080, 2085, 2244,and EN BS 12285, Part 2:

a) are limited to a capacity of 12,000 gal (45 m3) (Locate tanks exceeding 12,000 gal (45 m3) in accor-dance with Table 2 and meet all of the remaining criteria in b through i).

b) are limited to the storage of stable flammable liquids.

c) are spaced a minimum of 5 ft (1.5 m) from building walls or openings.

d) are spaced a minimum of 3 ft (1 m) from adjacent tanks of the same type.

e) are protected against vehicle impact by suitable barriers except where the tank is specifically listedand marked as having passed vehicle impact testing.



f) are provided with a means to prevent siphon flow from the tank.

g) are provided with a means, accessible to the delivery operator, for determining the level of liquid inthe tank.

h) are provided with a means to prevent overfilling by sounding an alarm when the liquid level in the tankreaches 90 percent of capacity and by automatically stopping delivery of liquid to the tank when the liq-uid level in the tank reaches 95 percent of capacity, without restricting or interfering with the proper func-tioning of the normal vent or the emergency vent.

i) do not need additional spill containment by way of impounding basins or drainage.

2.1.3 Buried Tanks

1. Locate buried tanks at least 5 ft (1.5 m) from building foundations and 2 ft (0.6 m) from other tanks andpipelines. Where a choice of location is offered, choose the one farthest removed from below-grade openareas such as pits and basements under important buildings.

2. Anchor the tanks where groundwater conditions are bad or where flooding is possible (Fig. 2). Detailsare given in Section 2.1.6, Protection Against Flooding, below.

3. Cover buried tanks with 2 ft (0.6 m) of earth, except under concrete paving at least 4 in. (100 mm) thick,where 1 ft (0.3 m) of earth is sufficient. In either case, provide an additional 1 ft (0.3 m) of cover at tank loca-tions over which heavy vehicles pass. Reinforce paving over the tank and extend at least 1 ft (0.3 m) beyondthe tank perimeter in all directions to transmit the superimposed load to foundations beside the tank.

4 The equivalent of a location below ground may be obtained with a substantial portion of a tank above grade.Earth is placed over the tank to form a 1 to 2 ft (0.3 to 0.6 m) cover at the angle of repose of the fill used.A concrete retaining wall or lock-sheet steel piling may be placed around the tank and filled with earth toreduce space requirements.

5. Protect tanks against corrosion as follows:

a) Provide at least 6 in. (150 mm) of well compacted clean gravel or sand around the tank.

b) Locate the tanks above the groundwater level.

c) Provide a protective coating on steel tanks. The base coat, usually applied by the manufacturer, actsas a primer. The outer coating, applied in the field, needs to be compatible with the base coat.

d) Patch-paint portions of the protective coating damaged when the tank is installed.

e) Cathodic protection is an acceptable alternate to protective coatings.

6. Provide openings for normal venting in accordance with Section 2.2.2.1.2. Venting for fire exposure isunnecessary.



2.1.4 Indoor Tanks

1. Limit the quantity of flammable liquids in indoor tanks in accordance with Table 4 below.

Table 4. Indoor Tank Quantity Limits

Liquid Flash Point Location of Tank(s)Maximum Indoor Storage, One or More Tanks

gal m3

≤ 200°F (93°C) Upper floor 2,000 7.5Grade level 2,000 7.5Basement Not permitted Not permitted

>200°F (93°C)3 Upper floor 5,000 20Grade level 2 50,000 1 190 1

Basement 5,000 20

Notes1 Not over 25,000 gal (95 m3) in one tank2 Limit FRP tanks to 5,000 gal (20 m3) and to liquids with flash points greater than 200°F (93°C) in accordance with section 2.1.1 – 5.3 Tanks containing liquids heated to within 25°F (14°C) of their flash point are evaluated in the ≤ 200°F (93°C) category.

2. Arrange tanks meeting the quantity limits of Table 4 as follows:

a) When located at grade level, provide a cut-off room for the purpose of containing the liquid storagetank(s). Provide concrete or masonry construction with a minimum one-hour fire-rating for the cut-off room,including similarly rated doors for any openings into the main building. Important structural steel needsto be similarly protected for one-hour fire resistance. Design the covering specifically for a hydrocarbon fireexposure. Locate the room along an outside wall with openings accessible to firefighters.

b) When located above or below grade level, provide a room separated from other occupancies by a wallof at least 3-hour fire-rated concrete or masonry construction and a 3-hour fire-rated resistive coveringon any exposed steel. Design the covering specifically for a hydrocarbon fire exposure.

c) Repair spalled areas of fire-resistive coatings on structural framing if the spalled area exceeds morethan 4 in2 (25 cm2).

Fig. 2. Buried tank with flame arrester



d) Provide hard-piped fill lines terminating outside the building.

e) Provide overflow protection using a high-liquid-level device that automatically shuts down filling opera-tions on detection of a high liquid level in the tank using an automatic safety shut-off valve. Locate thesafety shut-off valve as close to the tanker truck connection as possible.

f) Arrange the discharge line to exit the top of the tank. For tanks with bottom discharge lines, provide a fus-ible link-operated safety shut-off valve (SSOV) at the tank outlet.

g) Provide fire detection that automatically interrupts fill or discharge operations using automatic safetyshut-off valves. Approved water flow alarms, heat, smoke, or flame detection are acceptable means ofdetecting a fire.

h) Provide detection for a liquid spill that automatically interrupts fill or discharge operations using auto-matic safety shut-off valves where the tank room is in a non-industrial occupancy, e.g., retail, office, edu-cation, or residential.

i) Provide properly sized normal and emergency relief vents hard-piped to a safe location outside the build-ing. See Section 2.2.2 below.

j) Provide drainage designed to dispose of the discharge from all sprinklers in the room, as well as spilledliquids, in accordance with Data Sheet 7-83, Drainage Systems for Flammable Liquids.

k) Where drainage to a safe location is impractical, provide containment sufficient to hold the contentsof all spilled liquid plus a minimum of a 2 in. (5 cm) freeboard and an FM Approved special protection sys-tem. Containment can be provided with curbs, dikes, and existing walls. Ensure floors and walls are liquid-tight for the height of the required containment. Spilled liquid can include release from the storage tanksor from an uncontrolled release during filling operations.

l) Support tanks either directly on the floor or in accordance with Section 2.1.1 – 3.

3. An alternative arrangement to “2” above is to locate the tank in a liquid tight, concrete or brick-walled enclo-sure with the space between the tank and the wall filled with sand to a depth of 1 ft (0.3 m) above the tankas in Figure 3. In this case, there are no quantity limitations.

a) Provide hard-piped fill lines terminating outside the building.

b) Provide overflow protection using a high-liquid-level device that automatically shuts down filling opera-tions on detection of a high liquid level in the tank using an automatic safety shut-off valve. Locate thesafety shut-off valve as close to the tanker truck connection as possible.

c) Provide properly sized normal and emergency relief vents hard piped to a safe location outside the build-ing. See Section 2.2.2.1.2 below.

4. Arrange pumps located inside buildings as follows:

a) Install positive displacement pumps.

b) Arrange the pump to take suction from the top of the tank. Elevate the pumps to the same elevationas the top of the tank or provide an anti-siphon device. Locate the anti-siphon device as close to the tankoutlet as possible. (Some volatile liquids may require special pumping arrangements.)

c) Where a pump takes suction from the bottom of a tank, or when the pump is a centrifugal type, pro-vide a safety shut-off valve at the tank, interlocked to shut down the pump in the event of a leak or fire.

d) Provide a pressure-relief valve down stream of any positive displacement pump and pipe it back to thetank.

e) Implement all other requirements for flammable and combustible liquid transfer systems as requiredin DS 7-32 under “transfer by pumping”.

2.1.5 Intermediate Bulk Containers (IBC)

1. Store metal, composite, or plastic IBCs inside or outside in accordance with the requirements of DS 7-29,Flammable Liquid Storage in Portable Containers.

2. Treat IBCs used to supply flammable liquid to any type of process as indoor tanks and implement the rec-ommendations in Section 2.1.4.



2.1.6 Protection against Flooding

1. Locate tanks (aboveground or buried) so that at least 30 percent of their allowable storage capacity extendsabove the 100-year flood level, or secure the tank by one of the following methods:

a) Anchor the tank to resist movement.

b) Attach the tank to a foundation of steel and/or concrete having sufficient weight to provide adequateload for the tank when submerged by flood water to the 100-year flood level (Fig. 2). If the tank can bewater-loaded, the anchoring load can be calculated assuming a full tank; otherwise assume an empty tank.

c) Adequately secure the tank from floating by other means.

d) Fill the tank (buried), or to at least 70% capacity (above-ground), with water from a dependable sup-ply (if that is not impractical or hazardous due to the contents of the tank).

2. Construct any tank that is assumed to be submerged empty to safely resist external pressures.

3. Extend tank vents or other openings that are not liquid-tight above the 100-year flood level.

4. Provide tight closures at tank openings to prevent displacement of the tank contents by flood waters.

5. Where water filling is required to prevent tank floating, develop an emergency plan that includes a watersupply for water loading that is independent of public water or power supplies to allow for filling the tanksto increase their weight.

6. Prior to water loading, close all valves not used in connection with the filling operation.

7. Facility fire protection water may be used for filling if:

a) other water supplies are lost.

b) normal plant operations are terminated and a fire watch is started.

c) the loading operation is done through hoses and is constantly attended.

d) no fire pump, divisional, or sprinkler control valves have to be shut

Fig. 3. Enclosed indoor tank



e) the water supplies are not drawn down to seriously affect the required duration of sprinkler or hosestream discharge.

f) the loading operation is immediately shut down in the event of a fire emergency.

8. When filling is complete, close and lock all valves on connecting pipe.

2.1.7 Earthquake

1. In FM Seismic Zones 150 and under, provide restraint and appropriate flexibility in piping connectionsand associated tanks, pipe headers and piping systems per the requirements in DS 1-11, Fire Following Earth-quakes.

2. Where tanks are located indoors, arrange all liquid transfer operations to be shut down during a seismicevent using seismic shutoff valves.

2.2 Occupancy

2.2.1 General

1. Make pipe connections to tanks above the normal liquid level.

2. Extend fill, return, and similar pipes below the lowest level of liquid in the tank or within about 6 in. (150mm) of the tank bottom,(Fig. 1).

3. Where bottom connections are present:

a) provide steel shutoff valves bolted or welded to the first flange connection on the tank.

b) keep valves closed except when liquid is being transferred. (Fig. 4).

c) for tanks exceeding 10,000 gal (28 m3), provide valves that are manually controllable from a remote loca-tion.

d) provide a liquid-tight closure, such as a valve, plug, or blind, or a combination of these, on connec-tions below the liquid level through which liquid does not normally flow.

4. Where pumps are provided, implement the requirements for flammable and combustible liquid transfer sys-tems as required in DS 7-32 under “transfer by pumping‘‘.

Fig. 4. Cone roof vertical tank



5. Do not permit piping in dikes to pass through a dike wall to an area containing other tanks which couldallow a spill or fire to spread to adjacent tanks.

6. Provide manway openings with a bolted, gasketed cover that is kept closed except when the tank is openedfor examination or maintenance (Fig. 4).

7. Provide level-gauging or measuring devices for all tanks.

8. Where manual gauging connections are present:

a) Where liquids with flash point below 100°F (38°C) are present, use a method that will not expose thevapor space to outside atmosphere.

b) Avoid gauging equipment that will release large quantities of liquid if the equipment is damaged mechani-cally or by an exposure fire.

c) Where a rod and gauging well is provided, extend a pipe down into the tank below the level of the suc-tion intake (Fig. 5[a] ) to provide a liquid seal at the bottom of the well that prevents vapors above themain body of liquid from escaping during gauging.

d) Install FM Approved devices for safe gauging (level measurement) of tanks .

9. Provide high-level alarms that sound at an attended location.

10. Arrange heating equipment for tanks as follows:

a) Provide heat only in the vicinity of the suction intake for tanks storing liquids with flash point below200°F (93°C).

b) Provide only enough heat to ensure free flow of the liquid.

c) Arrange suction pipe or outlet pipe connections to ensure that heating coils will always be sub-merged.

d) For metal tanks, use steam, hot water or FM Approved electric heaters. For reinforced plastic tanks,use only steam or hot water.

e) Steam heating coils are commonly used on tanks containing No. 5 and No. 6 fuel oil and similar liq-uids to reduce their viscosity for pumping. In one acceptable arrangement, a horizontal open-ended shellor box contains the steam coils, and suction is taken from inside the shell. Another arrangement con-sists of a vertical spiral steam coil located around a top-connected suction pipe; this is acceptable if thefill opening is trapped or the fill pipe is extended below the level of the suction intake (Fig.1).

f) Provide a steam pressure-relief valve close to the tank, set at about 5 psi (35 kPa) over normal work-ing pressure, if steam is supplied through a reducing valve.

g) Provide FM Approved low-liquid- level and high-temperature interlocks to shut off the heating system.

11. Closely monitor all fill operations either by operator standing by or remote reading level gauges at an occu-pied location.

2.2.2 Normal and Emergency Venting

1. Provide normal venting to permit the intake and discharge of air during emptying and filling operationsand to permit expansion and contraction of vapor due to temperature changes.

2. Provide emergency venting to prevent the tank becoming overpressurized by fire exposure.

3. Where a mixture of several liquids is stored in the same tank, use the most volatile for the design basis.

4. Normal and emergency venting can be provided by one opening with a minimum capacity equivalent tothe emergency vent requirement.

5. Provide normally closed venting devices (conservation vents) on tanks storing liquids with flash pointsless than 73°F (23°C) and boiling points less than 100°F (38°C).

6. Provide normally closed venting devices (conservation vents) or an FM Approved flash arrester on tanksstoring liquids with flash points above or equal to 73°F (23°C) and below 100°F (38°C), and with boiling pointsabove 100°F (38°C) or liquids that can be heated to their flash points under normal operating conditions.



7. Prevent condensation in flame arresters on tanks containing liquids that solidify during cold weather by pro-viding a heating arrangement such as a steam coil at the arrester.

8. Where polymerization of a material may occur at the arrester, provide a dual arrester equipped with a three-way valve so one arrester is always in service.

9. Where vent pipes are necessary to conduct vapors to a safe location, install them as follows:

a) Terminate vents close enough above the tank to avoid imposing a dangerous liquid head on the tankif liquid overflows through the vent.

b) Extend vent pipe connections from indoor tanks to outside the building.

c) Terminate vents at a location free of potential ignition sources and away from openings through whichvapors can leak back into the building or locations where combustible construction would be exposedby a fire burning at the end of the vent.

Fig. 5. Safe gauging methods



d) Arrange horizontal runs of pipe to drain back to the tank.

e) Arrange the outlet and drains of vents on tanks operating at pressures in excess of 2.5 psig (17 kPa)so they do not direct vapor discharge back onto the tank.

f) Terminate open vents either with a weather protective hood or a U-bend to keep out rain and providecoarse screens to prevent foreign matter from obstructing the pipe.

g) Do not permit manifolding of tank vents for vapor recovery or air pollution control except in accor-dance with Manifolded Vents in Section 2.2.2.3.

2.2.2.1 Normal venting

2.2.2.1.1 Aboveground Tanks

1. For tanks with less than 50,000 gal (189 m3) capacity, the vent opening to meet normal venting require-ments can be in accordance with Table 5 but at least as large as the largest of the fill or withdrawal con-nection.

Table 5. Size of Opening for Normal Venting

Tank Capacity, gals (m3) Minimum diameter, nominal pipe size, in. (mm)Less than 2,500 (9.5) 1 ¼ (30)

2,500 – 3,000 (9.5 – 11) 1 1⁄2 (40)3,001 – 10,000 (11 – 38) 2 (50)

10,001 – 20,000 (38 – 76) 2 1⁄2 (65)20,001 – 35,000 (76 – 132) 3 (75)

35,001 – 50, 000 (132 – 189) 4 (100)

2. For tanks with a capacity exceeding 50,000 gal (189 m3) provide venting as follows:

a) Provide inbreathing (vacuum) capacity of 1 ft3/hr free air for each 7.5 gal/hr of the maximum empty-ing rate (1 m3/hr inbreathing capacity for each 1 m3/hr emptying rate) plus the thermal venting capacitygiven in Table 6.

b) For tanks storing liquid with a flash point ≤ 100°F (38°C), provide outbreathing (pressure) capacity of1 ft3/hr free air for each 3.5 gal/hr of the maximum tank filling rate (1 m3/hr free air for each 0.47 m3/hr ofthe maximum tank filling rate) plus the thermal venting capacity given in Table 6.

c) For tanks storing liquids with a flash point >100°F (38°C), provide outbreathing (pressure) capacity of1 ft3/hr free air for each 7.0 gal/hr of the maximum tank filling rate (1 m3/hr free air for each 0.94 m3/hrof the maximum tank filling rate) plus the thermal venting capacity given in Table 6.



Table 6. Required Thermal (Normal) Venting Capacity 1

Tank Capacity VacuumPressure

Liquid Flash Pointgal 42-gal

barrelsm3 All Stocks ≤ 100°F (38°C) >100°F (38°C)

ft3/hr m3/hr ft3/hr m3/hr ft3/hr m3/hr42,000 1,000 160 1,000 28 1,000 28 600 1784,000 2,000 320 2,000 57 2,000 57 1,200 34126,000 3,000 480 3,000 85 3,000 85 1,800 51168,000 4,000 640 4,000 113 4,000 113 2,400 68210,000 5,000 800 5,000 142 5,000 142 3,000 85420,000 10,000 1,600 10,000 280 10,000 280 6,000 170630,000 15,000 2,400 15,000 420 15,000 420 9,000 255840,000 20,000 3,200 20,000 570 20,000 570 12,000 340

1,050,000 25,000 4,000 24,000 680 24,000 680 15,000 4201,260,000 30,000 4,800 28,000 790 28,000 790 17,000 4801,470,000 35,000 5,600 31,000 880 31,000 880 19,000 5401,680,000 40,000 6,400 34,000 960 34,000 960 21,000 5901,890,000 45,000 7,200 37,000 1,050 37,000 1,050 23,000 6502,100,000 50,000 8,000 40,000 1,130 40,000 1,130 24,000 6802,520,000 60,000 9,600 44,000 1,250 44,000 1,250 27,000 7602,940,000 70,000 11,200 48,000 1,360 48,000 1,360 29,000 8203,360,000 80,000 12,800 52,000 1,470 52,000 1,470 31,000 8803,780,000 90,000 14,400 56,000 1,590 56,000 1,590 34,000 9604,200,000 100,000 16,000 60,000 1,700 60,000 1,700 36,000 1,0205,049,000 120,000 19,200 68,000 1,930 68,000 1,930 41,000 1,1605,880,000 140,000 22,400 75,000 2,120 75,000 2,120 45,000 1,2706,720,000 160,000 25,600 82,000 2,320 82,000 2,320 50,000 1,4207,560,000 180,000 28,800 90,000 2,550 90,000 2,550 54,000 1,530

1. Based on API Standard 2000, Venting Atmospheric and Low Pressure Storage Tanks, 5th Edition, 1998.(These requirements are also in NFPA 30)

2.2.2.1.2 Buried Tanks

1. Provide vent pipes sized in accordance with Table 7 for the maximum flow in or out of the tank, but notless than 1.25 in. (30 mm) nominal inside diameter to prevent blowback of vapor or liquid at the fill open-ing while the tank is being filled.

2. Extend vents a minimum of 12 ft (3.7 m) aboveground level for liquids with flash points below or equalto 100°F (38°C), and a minimum of 6 ft (1.8 m) aboveground level for liquids with flash points above 100°F(38°C).

3. Arrange vent pipes without traps or pockets so liquid condensate can drain back to the tank.

4. Arrange vent pipes to discharge upward or horizontally away from adjacent walls.

5. Locate vent outlets so vapors will not be trapped by eaves or other obstructions and at least 5 ft (1.5 m)from building openings and 15 ft (4.5 m) from powered air-intake devices.



Table 7. Typical Vent Line Size for Buried Tanks

Maximum In/Out Flow Vent Pipe Lengthgpm m3/hr 50 ft 15 m 100 ft 30 m 200 ft 60 m

100 20 1-1⁄4 in 30 mm 1-1⁄4 in 30 mm 1-1⁄4 in 30 mm200 45 1-1⁄4 in 30 mm 1-1⁄4 in 30 mm 1-1⁄4 in 30 mm300 70 1-1⁄4 in 30 mm 1-1⁄4 in 30 mm 1-1⁄2 in 40 mm400 90 1-1⁄4 in 30 mm 1-1⁄2 in 40 mm 2 in 50 mm500 115 1-1⁄2 in 40 mm 1-1⁄2 in 40 mm 2 in 50 mm600 135 1-1⁄2 in 40 mm 2 in 50 mm 2 in 50 mm700 160 2 in 50 mm 2 in 50 mm 2 in 50 mm800 180 2 in 50 mm 2 in 50 mm 3 in 75 mm900 205 2 in 50 mm 2 in 50 mm 3 in 75 mm1000 225 2 in 50 mm 2 in 50 mm 3 in 75 mm

2.2.2.2 Emergency Venting

1. Provide aboveground storage tanks containing stable liquids with emergency relief venting in the form ofconstruction or a device to relieve excessive internal pressure that develops from fire exposure.

a) Relieving construction can be in the form of a floating roof or weak seam roof.

b) A relieving device can be in the form of a floating manhole arranged for relieving, an open pipe, or a pres-sure relief valve suitable for the service. (UL 142, July 2002, section 8.10 – 12, provides design criteriafor floating manways.)

c) Emergency relief venting can be provided by the same device used for normal venting, provided it hasadequate capacity and pressure rating.

d) Stamp each commercial venting device, regardless of type, with its start-to-open pressure, the pres-sure at which it reaches its full-open position, and the flow capacity of the device at that pressure. Expressall flow capacities in either cubic feet per hour of air at 60°F and 14.7 psia or cubic meters per hour ofair at 15°C and 100 kPa absolute.

e) Emergency venting is not required for FRP tanks as the tank will fail at around 200°F (93°C)

f) Emergency venting is not required for tanks over 12,000 gal (45 m3) capacity containing liquids withflash points above 200°F (93°C) that are not exposed to spills from liquids with flash point less than or equalto 200°F (93°C ). Note: Normal in-and out-breathing is still required.

2. Where stable liquids are stored in tanks operating at 1 psig (7 kPa) or less, provide relief capacity/sizeof the relieving device or construction in accordance with Table 8 below. (Note: Tanks with weak seam roofconstruction have adequate emergency venting but would need normal venting for in-breathing and out-breathing)



Table 8. Capacities for Emergency Relief of Excessive Internal Pressure forAboveground Tanks Operating at 1 psig (7 kPa) or less1

Wetted area of tank 2 Vent Capacity 3 Minimum opening, NPS 4

ft2 m2 ft3 free air perhour (ft3/hr)

m3 free air perhour (m3/hr)

in mm

20 1.9 21,100 597 2 5030 2.8 31,600 894 2 5040 3.7 42,100 1,191 3 7550 4.6 52,700 1,491 3 7560 5.6 63,200 1,789 3 7570 6.5 73,700 2,086 4 10080 7.4 84,200 2,383 4 10090 8.4 94,800 2,683 4 100

100 9.3 105,000 2,970 4 100120 11.2 126,000 3,570 5 125140 13.0 147,000 4,160 5 125160 14.9 168,000 4,750 5 125180 16.7 190,000 5,380 5 125200 18.6 211,000 5,970 6 150250 23.2 239,000 6,760 6 150300 27.9 265,000 7,500 6 150350 32.5 288,000 8,150 8 200400 37.2 312,000 8,830 8 200500 46.4 354,000 10,020 8 200600 55.7 392,000 11,090 8 200700 65.0 428,000 12,110 8 200800 74.3 462,000 13,070 8 200900 83.6 493,000 13,950 8 200

1,000 92.9 524,000 14,830 10 2501,200 112 557,000 15,760 10 2501,400 130 587,000 16,610 10 2501,600 149 614,000 17,380 10 2501,800 167 639,000 18,080 10 2502,000 186 662,000 18,730 10 2502,400 223 704,000 19,920 10 250

2,800 and over5 260 and over5 742,000 21,000 10 2501. This table is based on hexane. For other materials, Table 9 can be used for vent capacity adjustments.2. The wetted area of the tank is defined as 55% of the total exposed area of a sphere or spheroid, 75% of the total exposed area of a hori-zontal tank, and the first 30 ft (10 m) above grade of the exposed shell area of a vertical tank. Include the bottom surface area of verti-cal tanks mounted on supports, above grade.3. Based on atmospheric pressure of 14.7 psia and 60°F (100 kPa abs. and 15°C)4. Based on open vent pipes of the noted diameter not more than 12 in. (0.3 m) long with a tank venting pressure of not more than 2.5psig (17 kPa).5 For tanks operating at pressures less than 1 psig (7 kPa) and having wetted areas exceeding 2800 ft2 (260 m2), complete fire involve-ment is unlikely and overheating will probably cause loss of metal strength in the vapor space before the development of a maximum vapor-evolution rate. For such tanks, the maximum listed relief capacity is adequate.For tanks operating at more than 1 psig (7 kPa) and having wetted areas exceeding 2800 ft2 (260 m2), the venting requirements are pro-vided in Section 2.2.2.2 — 3.

3. For tanks operating at pressures greater than 1 psig (7 kPa) and having exposed wetted areas greaterthan 2800 ft2 (260 m2), calculate the emergency venting capacity by one of the following formulae:

V = 1107 A0.82 (English)V = 220 A0.82 (metric)

Where V = hexane vent requirement, ft3/hr or m3/hr (at standard conditions)A = exposed wetted surface, ft2 or m2



4. Where the stored liquid is other than hexane, adjust the emergency venting capacity as follows:

V’ = V 1337 / L (M)1/2

V’ = V 3110 / L (M)1/2

Where: V = hexane vent requirement from Table 8, ft3/hr or m3/hrV’ = stored liquid vent requirement, ft3/hr or m3/hr

L = latent heat of vaporization of stored liquid, Btu/lb or kJ/kgM = molecular weight of stored liquid, no units

Table 9 lists L (M)1/2 for a number of common liquids. Data on other liquids can be found in most hand-books.

Note: the vent capacity determined from Table 8 is conservative compared to the other liquids listed in Table9; that is, if the capacity of the existing vents is adequate for hexane, it will be adequate for most other liq-uids.

Table 9. Values for L (M)1/2

Chemical L (M)1/2 Chemical L (M)1/2

English (1) Metric (1) English (1) Metric (1)

Acetic Acid 1350 3140 Ethyl acetate 1477 3436Acetic Anhydride 1792 4168 Ethyl alcohol 2500 5815

Acetone 1708 3973 Ethyl chloride 1340 3117Acetonitrile 2000 4652 Ethylene

dichloride1363 3170

n-Amyl alcohol 2025 4710 Ethyl ether 1310 3047iso-Amyl alcohol 1990 4629 Furan 1362 3168

Aniline 1795 4012 Furfural 1962 4564Benzene 1493 3473 Gasoline 1370–1470 3187–3419

n-Butyl acetate 1432 3331 n-Heptane 1383 3217n-Butyl alcohol 2185 5082 n-Hexane 1337 3110

iso-Butyl alcohol 2135 4966 Methyl alcohol 2680 6234Carbon disulfide 1310 3047 Methyl ethyl

ketone1623 3775

Chlorobenzene 1422 3308 n-Octane 1412 3284Cyclohexane 1414 3289 n-Pentane 1300 3024Cyclohexanol 1953 4543 n-Propyl acetate 1468 3415

Cyclohexanone 1625 3780 n-Propyl alcohol 2295 5338o-Dichlorobenzene 1455 3384 iso-Propyl alcohol 2225 5175

cis-Dichloroethylene

1350 3140 Tetrahydrofuran 1428 3322

Diethylamine 1403 3263 Toluene 1500 3489Dimethyl

acetamide1997 4645 o-Xylene 1538 3577

Dimethyl amine 1676 3898(1) L (heat of vaporization) in Btu/lb or kJ/kg

5. The venting capacity as determined by 2, 3 and 4 above, can be reduced for the effect of drainage, sprin-klers, insulation and low heat of combustion liquids (alcohols) using the Environmental Factors presentedin Table 10.

6. The total emergency venting capacity can be provided with specific construction or devices alone or in com-bination with the opening(s) provided for normal venting.



Table 10. Environmental Factors for Emergency Venting Capacity (use only one factor)

Environmental Factor (F) 1 Basic 4 For low heat of combustion liquids 4, 5

Drainage 1 0.5 0.25Water spray or sprinklers 2 & drainage 0.3 0.15

Water spray or sprinklers only 0.3 0.15Insulated 3 0.3 0.15

Water spray & insulated 0.15 0.15None of the above 1.0 0.5

1 Adequate drainage to remote impoundment in accordance with 2.2.2 – 5 above2 FM Approved water-spray installations in accordance with DS 4-1N or automatic sprinklers in accordance with Section 2.4.1 below.3 FM Approved coating rated for process structure or tank protection4 Use either basic credit or low heat of combustion credit, not both5 Capacity reduction permitted for liquids whose heat of combustion and rate of burning are equal to or less than those of ethyl alcohol (etha-nol)

7. Where unstable liquids are stored, provide tank-venting capacity that accounts for the effects of heat orgas produced by polymerization, decomposition, or self reactivity and the possibility of a two-phase relief. Fol-low the design guidance for reactive systems in DS 7-49, Emergency Venting of Vessels.

2.2.2.3 Manifolded Vents

1. Do not manifold vent collection systems of tanks containing incompatible materials.

2. Do not manifold vent pipes from tanks containing liquids with flash points below or equal to 100°F (38°C)with tanks containing liquids with flash points above 100°F (38°C).

3. Protect low-pressure storage tanks interconnected with fume recovery or collection systems against explo-sion propagation if they normally contain ignitable mixtures AND ignition sources could be (spontaneous heat-ing) or are normally present (continuous flames as in flares, fume incinerators, etc.) by one of the followingmethods:

a) Oxidant concentration reduction (e.g., inerting or purging). This method is limited to operations with-out open manway activities, such as sampling, liquid or solids addition, etc. (NOTE: Do not use inertingin tanks with monomers containing inhibitors that require oxygen to maintain activity. Examples: hydro-quinone and methyl ether of hydroquinone.) See Data Sheet 7-59, Inerting and Purging of Tanks, Pro-cess Vessels, and Equipment.

b) Combustible concentration reduction (e.g., ventilation). See Data Sheet 7-78, Industrial Exhaust Sys-tems.

c) Explosion isolation (detonation arresters).

4. Where an explosion isolation system is needed, provide Approved detonation arresters as follows (Fig. 6 ):

a) At each tank, in the piping connecting it to the vapor recovery system.

b) At the end of the manifold immediately upstream of the feed nozzle for any vapor processing equip-ment; for example, incinerators and scrubbers.

Note: Detonation arresters may not be appropriate in systems where powders are handled or added on aregular basis. The arrester could become plugged and fail to handle normal in-and-out breathing.

5. Provide detonation arresters with temperature sensors on each side, and as close as possible to the faceof the arresting element. Arrange the sensor to automatically close valves or initiate other actions that willeliminate the possibility of a stabilized flame burning on the arrester element. Do not locate the sensor in athermowell that will delay its response. If the sensor is to be a metal-sheathed thermocouple, it must be ofsmall diameter, e.g., 1⁄4 in. (6 mm), and must be inserted bare through a suitable packing gland.

6. Within 120 pipe diameters of the detonation arrester, install piping of equal or smaller diameter than thedetonation arrester.

(Fig.s 7 and 8 are showing pipe sizing around dda to meet this criteria)



7. Where conditions of operation will significantly exceed approximately atmospheric pressure and tempera-ture, specifically test detonation arresters under the actual operating conditions. Detonation arresters arecapable of successfully stopping detonation fronts only in systems initially at approximately atmospheric pres-sure and temperature.

8. Install detonation arresters where easily accessible for maintenance and inspection.

9. Install vapor-collection system piping in accordance with ASME B31.3, Chemical Plant and PetroleumRefinery Piping, or international equivalent, designed for a maximum allowable working pressure of 150 psig(10 barg).

10. Provide the flow capacity in common portions of manifolded vapor collection piping for the maximumflow of all vents connected to that portion of the system.

Fig. 6. Manifolded tanks

Fig. 7. Required pipe sizing if detonation arrester is smaller than nearby piping

Fig. 8. Improper piping around detonation arrester



11. Consider insulation and/or heat tracing of the system and arrester in cold climates where freezing or con-densation of the vapor is possible. The heat tracing must be kept below the accepted operating range ofthe arrester.

2.2.2.4 Indoor Tanks

1. Provide continuous low-level mechanical ventilation as specified in DS 7-32, Section 2.1.3.1, Ventilation.

2. Provide inert gas blanketing where tanks store liquid with a flash point less than 100°F (38°C).

2.2.3 Asphalt Tanks

Asphalt storage tanks have been a frequent source of fire or explosion events. In addition to the other cri-teria applying to outdoor tanks, the following represent good operating practice.

1. Ensure tank roofs are watertight.

2. Inspect tanks vents and the underside of the roof for accumulation of condensed material on a regularbasis and keep records of the inspection results.

3. Use tanks with weak seam roof (pressure relieving) construction per API 650 or similar.

4. Provide tanks with only one breather vent to minimize introduction of air into the vapor space.

5. Keep roof gauging and manway hatches closed to prevent unintended entry of air into the vapor space.

6. Use gauging hatches rather than manways when checking liquid level to minimize air entry into the tankvapor space.

7. Do not use pressure-vacuum (conservation) vents as condensed materials could prevent operation ofthe vent.

a) Where inerting of the vapor space is used, conservation vents will be needed.

b) Inject the inert gas below the vents to keep them free of accumulations.

c) Inspect the vents on a regular basis and keep records of the inspection results

8. Maintain tank liquid levels above any internal heating coils that could cause localized overheating, crack-ing of the liquids generating light ends and creating condensed deposits on the roof. Provide a reliablemethod to monitor tank liquid level.

9. Route supply piping for heating systems below the lowest liquid surface level or insulate the pipe with a non-permeable material.

10. Monitor the tank temperature with sensors located where it will be representative of bulk liquid tempera-ture. Keep sensors away from tank walls, near submerged heating coils, or and below normal operating lev-els.

11. Maintain tank temperatures at safe levels with the following considerations:

a) Keep temperatures at least 25°F below the flash point (out of the flammable range).

b) Keep temperatures out of the range of 212°F–265°F (100°C- 130°C) to avoid water condensation.

c) Temperatures above 350°F (177°C) encourage asphalt condensation on the roof surface. Deposit canoxidize, generate heat and possibly autoignite above 375°F (190°C).

d) Provide inert gas blanketing (oxygen concentration of 3% to 5%) for tanks operating at 350°F to 450°F(177°C-232°C) to prevent oxidation of deposits.

e) Don’t store materials at temperatures above 450°F (232°C) which can promote cracking and produc-tion of light hydrocarbons and increase the likelihood of operation in the flammable range.

12. Don’t allow entry of piping or any fixtures to or through the tank roof which would hinder deployment ofthe weak seam roof in an explosion.

13. Inspect internal tank heating coils for cracks, corrosion, and other damage whenever the tank is out of ser-vice and keep records of the inspection results.



14. Take precautions to safely oxidize pyrophoric deposits before taking the tank out of service (see APIRP 2016 for details).

15. Follow a written procedure for returning long-idled tanks to service that addresses at least the follow-ing:

a) Water accumulations that could boil on heating.

b) Residual product that may heat irregularly with localized overheating until the entire contents havereached a uniform temperature.

c) Lighter products that might have been previously in the tank and addition of hot material that could rap-idly vaporize material and exceed vent capacity or cause the vapor space to enter the flammable range.

16. Develop an emergency response plan to address fire, explosion, and unexpected liquid release that iden-tifies the hazards, site layout, protection equipment, shutoff valves, etc., as well as specific response to eachtype of event. Ensure outside responders are familiar with the response plan.

2.3 Protection

2.3.1 Indoor Tanks

1. Provide automatic sprinkler protection in the tank room/vault designed as specified in Table 11 over theentire tank room/vault area.

2. Provide sprinklers below tanks that are elevated and have greater than 3 ft (1 m) diameter or a plan areaof 10 ft2 (0.9 m2), or encase all tank supports in 3-hour fire-rated concrete.

3. Where pumps are present, extend a sprinkler down to within 2 ft of the pumps.

4. Provide an allowance of at least 500 gpm (1900 L/min) for hose stream use.

Table 11. Sprinkler Density for Storage Tank Rooms, gpm/ft2(mm/min)

Ceiling height ≤ 15 ft (4.5 m) > 15 ft (4.5 m) up to 30 ft (10 m)Flash point ≤ 200 °F (93 °C)No drainage, no foam Not permitted Not permittedWith drainage, no foam 0.3 (12) 0.3 (12)Foam with or without drainage As required by foam Approval* As required by foam Approval*Flash point > 200 °F (93 °C)No drainage, no foam 0.3 (12) 0.4 (18)With drainage, no foam 0.2 (8) 0.2 (8)Foam with or without drainage As required by foam Approval* As required by foam Approval*

*See Approval Guide listing for ″foam water sprinklers″.

2.3.2 Outdoor Tanks

The basic protection for tank farms is hose streams along with adequate spacing and containment as specifiedin Section 2.1.2. This will generally limit fire involvement to all tanks within a common dike or three largetanks that are individually diked. For large tanks or tanks farms, manual fixed foam protection may be appro-priate (automatic foam is rarely justified).

Automatic water-spray protection is of value mainly for exposure protection of buildings where tanks arelocated too close. An alternative protection method is fire-rated construction for the building.

1. Provide hydrants within 200 ft (60 m) of tanks so they can be reached by hose streams or monitor nozzlesfrom outside the dike.

2. Locate hydrants so every tank can be reached by hose or monitor streams from at least two directions.

3. Provide each hydrant with a minimum of two outlets controlled by individual valves.



4. Provide FM Approved combination straight stream/water spray nozzles for each hose. A straight streamdischarge can cool exposed tanks or facilities, while a high-velocity spray discharge can control or extinguishfires in liquids with flash points above 200°F (93°C).

5. Provide foam monitor nozzles or foam hose streams for exterior protection and spills in the dikes wherethere are tanks that contain stable liquids with flash points below or equal to 200°F (93°C) or unstable liquidsof any flash point.

6. Provide fixed foam outlets and supply piping to a remote point outside the dike installed in accordancewith DS 4-7N on vertical cone-roof tanks storing stable or unstable liquids with flash points below or equalto 200°F (93°C) when one or more of the following conditions exist:

a) The tank capacity exceeds 50,000 gal (190 m3) or there are multiple tanks in the same dike whoseaggregate capacity exceeds this value.

b) The tanks present a serious exposure to important buildings, process equipment, or utilities due toinadequate spacing.

c) The tank-to-tank spacing and containment is deficient compared to the requirements of this standard.

d) The tank contents are of considerable value or are essential for continued operations and are notreadily replaceable. The contents can be readily salvageable after foam contamination.

e) Other unfavorable situations that cannot be corrected.

7. Where spacing between tanks and nearby buildings is inadequate (not in accordance with Section 2.1.2)provide one of the following:

a) Provide building construction in accordance with DS 1-20 using guidelines for yard storage and considerthe tanks as high-hazard occupancy.

b) Provide deluge water spray (installed in accordance with DS 4-1N) on the exposed wall at a rate of0.3 gpm/ft2 (12 mm/min) of exposed wall using the criteria in DS 1-20 to determine the extent of theexposed wall. Include water supply duration for 2 hours and at least 500 gpm (1,900 L/min) for hosestreams.

8. Where spacing between adjacent tanks is inadequate (not in accordance with Section 2.1.2), providedeluge water spray (installed in accordance with DS 4-1N) on all exposed tanks at a rate of 0.3 gpm/ft2 (12mm/min) of tank surface. Include water supply duration for 2 hours and at least 500 gpm (1,900 L/min) forhose streams.

9. Where spacing to rail or truck load/unload stations is inadequate (i.e., not in accordance with Section 2.1.2)provide deluge water spray (installed in accordance with DS 4-1N) for the load/unload station (vehicle andpumps) at a rate of 0.3 gpm/ft2 (12 mm/min) of tank surface. Include water supply duration for 2 hours and atleast 500 gpm (1,900 L/min) for hose streams.

2.3.3 Water Supply

1. Calculate the water demand as the sum of the following:

a) The hose stream demand for tanks storing all classes of liquids as determined from Table 12 andsupplied at a minimum pressure of 50 psi (345 kPa).

b) Water required for fixed foam equipment, when provided, supplied at minimum Approved pressure.For purposes of estimation, see Table 13.

c) Provide water supply duration for a minimum of 4 hr for liquids with a flash points below 140°F (60°C)and 2 hour for liquids with a flash points above or equal to 140°F (60°C).



Table 12. Hose Stream Demand for TANKS Storing Flammable Liquids 1

Flash point of liquidLargest Tank Involved in Fire Largest Exposed Tankgpm L/min gpm L/min

< 140°F (60°C) 10002 3,8002

5002 1,9002

≥ 140°F (60°C) 750 2,800 250 9501 Required flows may be reduced by half for horizontal tanks.2 Add 250 gpm (950 L/min) for each 100 ft (30 m) increase in tank diameter above 100 ft (30 m).

Table 13. Estimated Water Demand for Fixed Foam Protection for a full Surface Fire.

Tank Diameter Water Demandft m gpm L/min50 15 200 750

100 30 800 3,000150 45 2,000 7,500200 60 3,200 12,100250 75 5,000 19,000300 90 7,100 27,000

2.4 Operation and Maintenance

1. Implement a formal mechanical integrity program, as described in DS 7-43, Loss Prevention in Chemi-cal Plants, for all flammable-liquid storage tanks.

2. Conduct monthly visual inspections of aboveground and indoor tanks for the following (where appli-cable):

a) Leaks, corrosion, settlement

b) Condition of attached piping, piping supports, gauging, level control systems, alarms, emergency andbreather vents, instrumentation, grounding, ladders, accessways

c) General housekeeping, water accumulation, and vegetation in dikes

d) The physical condition of berms/dikes/containment

e) Clear and operable drainage systems along with accessibility of any applicable valves

f) Operation of inerting systems

3. Conduct annual recorded inspections of tank vents, vent pipes, screens, and flame arresters and keepthem free from obstructions (e.g., stones, dirt, insect nests, polymerized material, etc.) that could preventproper operation and possibly overpressurization of the tank.

4. Conduct recorded inspections of detonation arresters in manifolded piping systems for damage and accu-mulations of debris caused by polymerization, condensation, corrosion, etc., which could impair operabil-ity. Replace damaged units (or repair if the damage does not affect their functionality) and removeaccumulations.

a) Conduct inspections at least quarterly during the first year and as experience dictates thereafter, butat least annually (where practical).

b) Conduct inspections at least quarterly where powders are added to the system. (Arresters in this ser-vice are particularly susceptible to accumulations and may be inappropriate for use.)

5. Conduct annual recorded inspections of the performance of cathodic protection (CP) systems by quali-fied persons for the attainment of satisfactory CP criteria, proper functioning of equipment and that the levelof CP applied is properly controlling corrosion. Criteria for determining the effectiveness of CP include NACERP0169, 0285 and API 651.



2.4.1 Repair, Reconditioning, and Abandonment

Prior to working on any tank that has contained flammable liquids, take the following precautions as appro-priate:

1. Drain any residual liquids remaining in the tank.

2. Purge flammable-liquid tanks with steam or warm air before repairs are made or before the tanks arereused. Route displaced flammable vapors to a safe location. Avoid excessive pressure or vacuum. (SeeDS 7-59, Inerting and Purging OF Equipment)

3. Use an FM Approved flammable vapor indicator to determine whether vapors have been eliminated. Makeadditional tests at frequent intervals.

4. Remove all remaining scale and sludge with nonferrous scrapers.

5. Fill the tank with an inert gas, such as carbon dioxide, or maintain positive continuous air movement throughthe tank if cutting or welding torches are used on the outside of the tank.

6. Use a hot work permit system to control welding operations on a tank. (see Data Sheet 10-3, Hot Work Man-agement)

7. Supervise workers and provide sufficient ventilation when welding is being done inside a tank. Station atleast one person outside near the manhole to watch the welder and assist in an emergency.

8. Remove, repair, or recondition underground flammable-liquid tanks that are no longer of any use. Priorto removal, inert the tank. If removal of the tank is not possible, it may be left in place after doing the follow-ing:

a) Remove all of the liquid from the tank.

b) Purge the tank of flammable vapors.

c) Remove all suction, inlet, gauge, and vent lines.

d) Fill the tank with a solid inert material (e.g., sand, diatomaceous earth, perlite, etc.).

e) Cap all remaining underground piping.

f) Re-bury the tank and fittings.

2.5 Ignition Source Control

1. Install electric lights and other fixed or portable electrical equipment near storage tanks containing liq-uids with flash points below or equal to 100°F (38°C) or liquids with higher flash points heated to within 25°F(14°C) of their flash point in accordance with the following:

a) Provide electrical equipment suitable for Class I, Division 1 or Zone 1 hazardous locations as definedin Article 500 of the National Electrical Code when located within 3 ft (1 m) of vents.

b) Provide electrical equipment suitable for Class I, Division 2 or Zone 2 hazardous locations as definedin Article 500 of the National Electrical Code when located between 3 and 5 ft (1 to 1.5 m) of vents.

c) Provide electrical equipment suitable for Class I, Division 2 or Zone 2 hazardous locations when locatedwithin 10 ft (3 m) of any other tank openings or when located within a diked area.

d) Provide electrical equipment suitable for Class I, Division 1 or Zone 1 hazardous locations when tanksare located in a room or vault.

e) Refer to Data Sheet 5-1, Electrical Equipment in Hazardous (Classified) Locations, and Data Sheet5-7, National Electrical Code, for additional information regarding electrical installations.

2. Provide static grounding connections on tanks that are out of contact with the earth if piping is ungroundedor nonconductive. (Ordinarily, special electrical grounding connections will not be needed. Adequate ground-ing for a tank is provided by its own contact or the contact of its connected piping with the earth.) (See DS5-8, Static Electricity)



3. Electrically bond all tank plates, internal structural members, fittings and isolated metal parts or pipe sec-tions on tanks containing liquids with flash point less than or equal to 100°F (38°C) or liquids with higherflash points heated to within 25°F (14°C) of their flash point to reduce the danger of internal sparks from light-ning or charged liquid. (See Data Sheet 5-8, Static Electricity for further information on grounding and bond-ing.)

4. Prohibit the discharge of liquids with flash points below or equal to 100°F (38°C) or liquids with higherflash points heated to within 25°F (14°C) of their flash point above the liquid level in the tank (usually called’’splash filling‘‘) as it creates the possibility of static buildup and spark discharge to grounded components.

5. Prohibit hot work, maintenance, repair, or modification in or near (see Table 14) tanks, pumps, and otherhandling equipment, tank truck or railcar loading and unloading, or fume-collection systems where flam-mable vapors could be present until the tank or system is isolated, drained, and purged or blanketed withan inert gas. Use a hot work permit system to control the progress of such work. (See Data Sheet 10-3, HotWork Management.)

6. Prohibit smoking or open flames in or near (see Table 14) tanks, pumps, and other handling equipment,tank truck or railcar loading and unloading, or fume-collection systems where flammable vapors could bepresent. Provide designated safe areas for such activity.

Table 14. Safety Distances for Hot Work, Open Flames, Maintenance, Repair or Modification

Safety distances for hot work, open flames, maintenance, repair or modification*, ft (m)Flash point = 100°F (38°C) or heated to within

25°F (14°C) of their flash point> 100°F (38°C)

Tanks outdoors 50 (15) 35 (10)Within dikes or tank rooms Not allowedTruck or railcar loading/unloading 75 (22.5) 35 (10)Pumps or other handling equipment 75 (22.5) 35 (10)

* allowed after hot work permit process is completed

3.0 SUPPORT FOR RECOMMENDATIONS

3.1 Background information

Tanks containing gasoline, alcohol, benzene, and other flammable and combustible liquids have beeninvolved in serious fires. The contents of a large tank can cause extensive damage if released during a fire.The design and construction of such tanks needs to ensure a high degree of confinement and reliability.

3.1.1 Hazards

Flammable and combustible liquids are classified by various US and international regulatory bodies for thepurposes of packaging, transportation, and handling. The various definitions can make the application ofstorage standards across a broad spectrum difficult. For the most part, this document limits differentiationby using a breakpoint of 140°F (60°C) for spacing criteria and 200°F (93°C) for protection.

Crude oil (not addressed in this standard) and other liquids containing components with a wide range of boil-ing points, and some free water, present the additional hazards of boil-over, slop-over, or froth-over. Boil-over is a phenomenon that may occur spontaneously during a fire in an open-top tank of crude oil that hasbeen burning for an extended period of time. In time, a sudden expansion of a steam-oil froth beneath the liq-uid surface can occur, resulting in a sudden explosion of hot residual oil from the tank. Generally, four con-ditions have to exist for a boil-over to occur:

1. The tank must contain free water or a water-oil emulsion near the tank bottom. This is a normal condi-tion in crude-oil storage tanks as well as in some tanks storing heavier, residual oils.

2. The tank must be open-top. Experience indicates that fire in an open-top tank will result if an explosionblows the roof off or if the pan or deck in a floating-roof tank sinks.

3. The oil must be capable of forming a heat wave of 300°F (145°C) or more. The heat wave is createdwhen lighter components in the liquid (e.g., pentane, hexane, etc.) distill off and burn at the liquid surface leav-ing a residue of higher density than the liquid just below it. This residue has a temperature in excess of 300°F



(145°C) and, if it sinks at a rate substantially faster than the rate of regression of the liquid surface, the heatwave is formed. The heat wave is created by convection (within the stored liquid) not conduction.

4. The oil must contain sufficient heavy ends to produce a persistent froth of oil and steam.

The boil-over tendencies of the oil can be evaluated by small-scale tests. While all crude oils are not sus-ceptible to boil over, successive storages may exhibit boil-over potential. Thus, always design, install, and pro-tect tanks storing crude oil recognizing the possibility of boil over.

Other liquids can exhibit slop-over or froth-over tendencies. Slop over occurs when a water stream is appliedto the surface of a burning viscous oil. The resultant frothing and ejection of liquid is generally much lesssevere than a boil over because only the surface of the liquid is involved. It could present a hazard to fire fight-ers. Froth over occurs when a hot viscous liquid, such as asphalt or oil, floats on a water layer in a tank.In time, the water is superheated and erupts, ejecting liquid from the tank. Unlike boil over or slop over, thereis no fire. Froth overs have occurred with sufficient violence to blow off tank roofs and spread the tank con-tents over a large area.

3.1.2 Types of Tanks

3.1.2.1 Atmospheric Tanks

Atmospheric tanks are used to store large quantities of liquids at pressures ranging from atmospheric to1.0 psig (7 kPa). The following are the principal types of atmospheric tanks:

Cone roof tanks are the most widely used for flammable liquid storage. They are usually welded and mayhave either weak roof or weak shell-to-roof seams designed to fail preferentially to the tank shell in the eventof a fire or internal explosion. Their major disadvantage is the vapor loss caused by breathing (the normalexpansion and contraction of the tank contents with atmospheric changes). The normal operating range of thetank is ±11⁄2 in. of water (± 370 Pa).

Floating roof tanks are constructed with a roof floating on the liquid surface. The roof may be of double-deck or pontoon-type construction (Figs. 9 and 10). By eliminating the vapor space, breathing losses becomenegligible, and the fire and explosion hazard is greatly reduced. The seal provided between the roof edgeand the tank wall allows the roof to move freely within the shell. Drainage facilities are provided to prevent theaccumulation of water on the roof surface.

Covered floating roof tanks are similar in construction to cone roof tanks, except for a metal pan (or, occa-sionally, a double or pontoon internal roof) that floats on the liquid surface (Fig. 11). Since the floating coveris protected from the weather, no provision for drainage or for rain or snow loading is required. Vents are pro-vided around the periphery of the tank.

Lifter or expansion roof tanks resemble cone roof tanks, except the entire roof assembly has limited free-dom to move up and down within the shell. A vapor-tight liquid seal, which maintains a slight pressure onthe contents of the tank, provides a seal between the roof assembly and the shell. The moving roof mini-mizes normal breathing losses. An expansion roof tank is occasionally used with a group of fixed roof tanksto take up their composite vapor change.

Breather roof tanks are used where the liquid storage is not frequently disturbed. The horizontal flexible dia-phragm, or roof, is attached to the top edge of the tank shell and maintains a variable vapor space by mov-ing up and down. The roof, by confining the vapor, exerts a slight pressure upon the liquid, reducingevaporation losses.

Vapordome tanks employ a dome containing a plastic diaphragm, which is free to move with the expan-sion of vapor in the tank. This is an effective method of reducing vapor loss from the top of the tank.

Cylindrical tanks are used for small quantities of liquids. Heads may be dished or flat. The long axis maybe either horizontal or vertical and the tank buried or aboveground.

3.1.2.2 Low-Pressure Tanks

Low-pressure tanks have a maximum working pressure of 15 psi (103 kPa). They are used to store vola-tile liquids, such as those with flash points below 73°F (23°C) and boiling points below 100°F (38°C) (Class



IA), under their own vapor pressure. Such tanks may be spheres, spheroids, or cylinders. In general, therequirements applicable to atmospheric storage tanks apply to low-pressure storage tanks, with some modi-fications in construction, venting, and spacing.

Fig. 9. Open top double deck

Fig. 10. Open top pontoon



3.1.3 Indoor Tanks

Putting large quantities of a flammable liquid inside an important building is not recommended. Tank stor-age of flammable liquids creates the potential for many fire scenarios, including overflow during filling, over-pressurization when exposed to fire, leak in a discharge line, or tank failure (a very low likelihood event,but one that has the potential for significant consequences).

The main goals of the recommendations are to isolate the tank from non-flammable liquid occupancies, pro-vide adequate protection for most fire scenarios and to ensure adequate access to the tank room for fire-fighters. Since all of the fire scenarios in a tank room involve a liquid release, adequate isolation must includeprovisions for containment and emergency drainage.

In cases where tanks are not only inside a building, but are also located either below or above grade, addi-tional safeguards are needed. Access to these tanks for manual firefighting will be very limited. The over-all severity of a liquid release and fire involving the tank will be entirely dependent on what was provided foractive and passive protection around the tank. In buildings where the potential loss is significant, there isa need to ensure any potential flammable liquid release/fire is contained to the tank room. The only reli-able way to accomplish this is through the use of a 3-hour fire rated vault with only limited openings for freshair. This combination will limit the fire severity and help ensure survival of the room regardless of the sizeof the liquid release.

The design goal for pumping and transfer systems is to ensure the liquid stays in the piping system andcan be shut down when necessary (e.g., leak or fire). The best way to accomplish this is to use welded steelpiping, positive displacement pumps and safety shut-off valves. There will always be several potential leak-age sources in this type of system that can produce a liquid release and fire. The most likely source of leak-age is the pump. Pump rooms must be isolated from other occupancies. Since the pumping system createssimilar hazards as the storage tank, it may be cost effective to locate the pumps in the tank room/vault. Asmall fire at the pump can grow because the initial fire will produce additional failures. Sprinklers that areextended from the ceiling to within 2 ft (0.6 m) of the fuel pumps can help to prevent those additional fail-ures.

A second potential leakage source is flanged or threaded pipe joints/unions. Welded piping systems requirethe use of flanged joints to permit equipment maintenance and repair. Leaks at flanged joints can be causedby poor maintenance or fire exposure to a gasket that can melt. Threaded joints are inherently weakerbecause the pipe wall thickness has been reduced. Locate flanged or threaded joints/unions in rooms thatare properly isolated and protected for a flammable liquid fire exposure.

Fig. 11. Pan-type covered tanks



3.1.4 Tank Spacing and Containment

Tank spacing criteria were developed by comparison with existing standards and by analysis of pool firesimulations for all three classes of liquid assuming a 20 mph wind during the event. The final simplified cri-teria were based primarily on heat flux predictions rather than existing standards.

3.1.5 Manifolded Vents

Environmental regulations have increased the use of emission control systems on tanks. The emission controlsystems can include carbon bed adsorbers, scrubbers, condensers, incinerators, etc. (Fig. 6). In some cases,the system could be handling vapors within the flammable range. An ignition at one point in the system couldcause a flame front to propagate throughout with damaging results. The ignition source could be static, light-ning, an incinerator flame, etc. Proper design of the system can prevent such a situation.

Flame propagation is not possible in the manifold piping and connected vessels if the vapor-air mixture isout of the flammable range. This is most often achieved by an inert gas system to decrease the oxygen toan acceptable level. To accept such a system in lieu of arresters, it must be reliable. The criteria in Data Sheet7-59, Inerting and Purging of Tanks, Process Vessels, and Equipment, will provide this reliability as longas open manway operations do not occur.

The propagation velocity in a piping system containing a flammable mixture depends on the inherent turbu-lence in the system caused by flow, bends, valves, and fittings as well as the turbulence of the combus-tion process itself. It has been recognized that a deflagration flame front can transit to detonation velocitieswith significant increase in the pressures within the piping and the potential for failure of the piping. Transi-tion to detonation in pipe lengths of 50 to100 diameters are typically reported. Flame-arresting devices thatsuccessfully stop the deflagration fail to stop the detonation or even a ’’fast‘‘ deflagration. Detonation arrest-ers (Fig. 12) can stop detonation fronts, and test procedures are available to Approve/list these devices. Deto-nation arresters are rated for a specific gas or class of gases. Some classifications are based on NationalElectrical Code groupings (A, B, C, D), while others are based on minimum experimental safe gap (MESG)required to quench a flame.

Detonation arresters are normally bidirectional; that is, they will stop a detonation front approaching fromeither direction. Since it is not possible to ensure the direction of flame approach, use of unidirectional arrest-ers usually is not appropriate.

Fig. 12. Detonation arrester



Testing has demonstrated that a detonation arrester is likely to fail if installed in piping whose diameterincreases within a distance of 120 pipe diameters from the detonation arrester in either direction.

This data sheet, NFPA 30, Flammable and Combustible Liquids Code, and other standards specify therequirements for installation of flame arresters on tanks. Although some FM Approved flame arresters areequipped with flanges at both ends for short pipe-aways (Figs. 13 and 15) of the released vapors, they can-not be used in extended piping systems. The testing organization’s listing will detail limits between the openpipe end (to atmosphere) and the arrester. They are based on the test conditions, and additional length couldpermit a deflagration flame front to increase velocity, even up to a detonation front, and result in failure of thedevice to stop the flame.

Conservation vents are installed on many low pressure tanks to minimize the release of vapors during tankidle times while permitting release of pressure or vacuum created during filling or emptying. This data sheet,NFPA 30, and other codes accept these devices in lieu of flame arresters where vented directly to atmo-sphere. The pressure setting (typically 3⁄4 in. water gauge [190 Pa]) and the device design create local veloci-ties in excess of the propagation velocity of ordinary combustion flames, thus preventing flashback into thetank. These velocities and the general construction are insufficient for stopping detonation propagation.These are not acceptable alternatives to detonation arresters in manifolded piping systems .

Using rupture disks on elbows, or direction changes in the piping system, to provide explosion venting isnot considered effective in halting the progress of a flame front. Venting the piping in this manner will at least

Fig. 13. Storage tank with flame arrester

Fig. 14. End-of-line flame arrester



temporarily relieve the pressure but may not stop the flame front, which could continue on to intercon-nected vessels. The flame front will continue down the pipe and, if it is not vented at regular intervals (50to 100 diameters), it could transit to detonation velocity. Other methods of explosion isolation are available.These include fast-acting valves, rapid discharge extinguishing (blocking) systems and flame-front divert-ers or backflash interrupters (Fig. 16). None of these devices presently are FM Approved and therefore arenot discussed in detail. There is limited information available on installation criteria and applicability limits.All are designed to interrupt deflagrations, not detonations.

3.1.6 Asphalt Tanks

Loss history shows a disproportionate number of events involving tanks containing asphalt. Factors in theseevents include:

• Tanks often operate at temperatures near the flash point.

• Material can condense on tank roof surfaces, overloading the roof.

Fig. 15. End-of-line flame arrester with pipe-away flange

Fig. 16. Backflash interrupter



• Condensed material can be pyrophoric, iron sulfides, or carbonaceous.

• Water can accumulate on the bottom and froth over on addition of hot materials.

• Operating procedures are not always followed.

3.1.7 Fire Protection

The severity of flammable liquid tank fires requires that fire protection be provided. Normally, only hydrantprotection is required. Fixed or portable foam-making equipment or water spray systems may be required tocontrol fires where the quantities of liquids stored or tank sizes are unusually large. Special precautions arerequired for the storage of crude oil and other liquids subject to boil over. Storage tank fires involving liq-uids with flash points of 100°F (38°C) or lower are difficult to control and extinguish and frequently burn fordays.

Do not consider provision of fixed foam or water spray systems as a substitute for adequate spacing of tanksfrom important plant facilities.

Fixed foam systems have been effective in extinguishing fires in cone roof tanks, but have sometimes failedfor the following reasons:

a) Fires in tanks storing liquids with flash points of 100°F (38°C) or lower have originated with an explo-sion in the vapor space of the tank, damaging one or more foam distribution devices.

b) The roof support members have fallen into the liquid, preventing formation of a uniform foam blanketover the liquid surface.

Even under these adverse circumstances, fixed foam systems may provide partial control until manual fire-fighting can be organized. Subsurface application could improve the operating experience of fixed foam extin-guishing systems for fires in cone roof tanks. Some standards consider subsurface foam a requirement forsuccessfully extinguishing fires in tanks exceeding 200 ft (60 m) in diameter.

Floating roof tanks are less susceptible to serious fire loss than cone roof tanks. Seal fires in floating rooftanks can be readily extinguished with either portable extinguishing equipment or fixed foam extinguishingsystems, depending upon the size of the tank.

For new installations, do not consider fixed foam systems as equivalent to adequate spacing and diking, selec-tion of proper tank construction, or provision of exposure protection where needed.

3.2 Loss History

3.2.1 Storage Tanks

Loss history was developed for storage tanks handling flammable liquids for the period 1984 – 2004. A totalof 303 events at FM Global client locations resulted in a gross loss of US$280 million (all figures indexedto 2005 dollars). Of these events, a total of 115 exceeded $100,000 gross loss, accounting for about 70% ofthe total loss. The losses by industry group are shown in the Table 15. Metal working, plastic, wood andpaper and office, retail, and warehouses had the highest number of events. The largest single event in thisclass was a tank explosion caused by cutting and welding at a paper mill.

Table 15. Losses over US$100,000 by Occupancy Class

Occupancy NumberNone listed 5

Textiles 2Metal Working 34

Plastic, Wood & Paper 20Food & Beverage 5

Chemical & Pharmaceutical 7Power Generation 9

Office, Retail, Warehouses 22Residential 2

Misc Properties 9Grand Total 115



Table 16 provides a breakdown of the engineering peril associated with the 115 losses.

Of the 115 losses reported over US$100,000 gross, losses caused by leakage and contamination were themost common. There were a total of 57 incidents with a total gross loss of US$31 million.

There were a total of 29 fires and explosions with a total gross loss of US$133 million with fires accountingfor US$118 million. Of these, improper cutting and welding was often the cause.

“Oil” was the most common liquid involved in events, but asphalt was next most common with 12 eventsand a total gross loss of about US$10 million.

Table 16. Losses over US$100,000 by Engineering Peril

Engineering Peril NumberFire 16

Explosion 13Escaped Liquid Damage 27Riot & Civil Commotion 1

Collapse 2Water-Liquid Damage 3

Implosion 2Mechanical Breakdown 1

Impact 1Miscellaneous 49

Total 115

3.2.2 Manifolded Vents

There has been at least one FM Global loss involving manifolded vapor recovery systems on storage tanks.In addition, a recent study in the province of Alberta, Canada, showed flame arrester failure in sour gas flar-ing operations (related to crude oil production) was responsible for 10 to 20 oil storage tank explosions peryear.

4.0 REFERENCES

4.1 FM Global

Data Sheet 1-57, Plastic in Construction

Data Sheet 4-7, Low Expansion Foam Systems

Data Sheet 5-1, Electrical Equipment in Hazardous (Classified) Locations

Data Sheet 5-7, National Electrical Code

Data Sheet 5-8, Static Electricity

Data Sheet 6-10, Process Furnaces

Data Sheet 7-14, Fire & Explosion Protection for Flammable Liquid, Flammable Gas, & Liquefied Flam-mable Gas Processing Equipment & Supporting Structures

Data Sheet 7-29, Flammable Liquid Storage in Portable Containers

Data Sheet 7-30, Solvent Extraction Plants

Data Sheet 7-32, Flammable Liquid Operations

Data Sheet 7-43/17-2, Loss Prevention in Chemical Plants

Data Sheet 7-51, Acetylene

Data Sheet 7-59, Inerting and Purging of Tanks, Process Vessels, and Equipment

Data Sheet 7-78, Industrial Exhaust Systems

Data Sheet 7-83, Drainage Systems for Flammable Liquids



Data Sheet 10-3, Hot Work Management

4.2 NFPA

NFPA 11, Standard for Low, Medium and High Expansion Foam (2005)

NFPA 30, Flammable and Combustible Liquids Code (2003)

In section 4.4.2, reference is made to testing criteria for the integrity of secondary containment tanks. Belowis that reference:

4.4.2.3 Underground secondary containment tanks and horizontal aboveground secondary containmenttanks shall have the primary (inner) tank tested for tightness either hydrostatically or with air pressure atnot less than a gauge pressure of 20 kPa (3 psig) and not more than a gauge pressure of 35 kPa (5 psig).The interstitial space (annulus) of such tanks shall be tested either hydrostatically or with air pressure ata gauge pressure of 20 to 35 kPa (3 to 5 psig), by vacuum at 18 kPa (5.3 in. Hg), or in accordance withthe tank’s listing or manufacturer’s instructions. The pressure or vacuum shall be held for not less than 1hour or for the duration specified in the listing procedures for the tank. Care shall be taken to ensure thatthe interstitial space is not over pressured or subjected to excessive vacuum.

4.4.2.4 Vertical aboveground secondary containment–type tanks shall have their primary (inner) tanktested for tightness either hydrostatically or with air pressure at not less than a gauge pressure of 10 kPa(1.5 psig) and not more than a gauge pressure of 17 kPa (2.5 psig). The interstitial space (annulus) ofsuch tanks shall be tested either hydrostatically or with air pressure at a gauge pressure of 10 to 17 kPa(1.5 to 2.5 psig), by vacuum at 18 kPa (5.3 in. Hg), or in accordance with the tank’s listing or manufac-turer’s instructions. The pressure or vacuum shall be held for 1 hour without evidence of leaks. Care shallbe taken to ensure that the interstitial space is not over pressured or subjected to excessive vacuum.

NFPA 70, National Electric Code

4.3 Others

American Petroleum Institute, API 620, Design and Construction of Large, Welded, Low-Pressure StorageTanks, Tenth Edition, 2002(American Petroleum Institute, API 650, Welded Steel Tanks for Oil Storage, Tenth Edition, 1998)

American Petroleum Institute, API 2000, Venting Atmospheric and Low Pressure Storage Tanks, Fifth edi-tion, 1998

American Petroleum Institute, ANSI/API 2610, Design, Construction, Operation, Maintenance, and Installa-tion of Terminal and Tank Facilities, Second edition, 2005

American Petroleum Institute, ANSI/API 651, Cathodic Protection of Aboveground Petroleum Storage Tanks,Second edition, 1997

American Petroleum Institute, API Standard 2015, Requirements for Safe Entry and Cleaning Petroleum Stor-age Tanks

American Petroleum Institute, API Recommended Practice 2016, Guidelines and Procedures for Enteringand Cleaning Petroleum Storage Tanks

American Petroleum Institute, API Recommended Practice 2023, Guide for Safe Storage & Handling ofHeated Petroleum Derived Asphalt Products & Crude Oil Residua

American Society of Mechanical Engineers (ASME), Boiler and Pressure Code, Section VIII, Unfired Pres-sure Vessels, latest edition

American Society of Mechanical Engineers (ASME), B31.3, Chemical Plant and Petroleum Refinery Piping,latest edition

ASTM International, ASTM D4206, Standard Test Method for Sustained Burning of Liquid Mixtures Usingthe Small Scale Open-Cup Apparatus, 2001

Code of Federal Regulations, 33 CFR, Part 154, Appendix A, Guidelines for Detonation Flame Arresters

Code of Federal Regulations, 49 CFR, Chapter I, Subchapter C, Parts 171 – 180 Department of Transpor-tation, Hazardous Materials Regulations



International Standards Organization, ISO 2592, Determination of flash and fire points — Cleveland opencup method, 2000

National Association of Corrosion Engineers, NACE RP-0169, Control of External Corrosion on Under-ground or Submerged Metallic Piping Systems

National Association of Corrosion Engineers, NACE RP-0285, Corrosion Control of Underground StorageTanks System by Cathodic Protection

APPENDIX A GLOSSARY OF TERMS

Conservation vents: These devices have both vacuum and pressure relief capacity. Vents usually openwhen the positive or negative pressure in the tank reaches 3⁄4 to 1 in. water column (185 to 250 Pa). Theyare normally closed and vent pipes equipped with conservation vents do not need flame arresters. The veloc-ity through the openings is normally sufficient to prevent flashback. A typical conservation vent is shown in Fig-ure 17.

Exposed Wall Categories:

Combustible Wall: A wall made of any combustible material, including overhanging wood eaves, any metalfaced plastic insulated sandwich panels that are not FM Approved, and any wall with single pane, annealed(not tempered) glass windows. Increase separation by 25% for asphalt-coated metal walls.

Noncombustible Wall: Materials include FM Approved Class 1 insulated, steel, or aluminum faced sand-wich panels with thermoset plastic insulation; EIFS assemblies having noncombustible insulation and gyp-sum board sheathing, and aluminum or steel panels that are uninsulated or insulated with noncombustibleinsulation such as glass fiber, mineral wool, or expanded glass. It also includes cementitious panels orshingles over steel or wood. There can be no overhanging wood eaves. Any windows should be multi-pane or tempered glass.

Fire Rated Wall: The wall should meet the required fire rating per FM Global Loss Prevention Data Sheet1-21, Fire Resistance of Building Assemblies. Any openings should be protected with a comparably fire-rated door. Any windows should be fire rated to match the rating of the wall.

Fig. 17. Typical conservation vent



Flammable Liquid: Any liquid rated as NFPA Class I, II or III or DOT/UN Class 3.

FM Approved: References to ‘‘FM Approved’’ in this data sheet mean the product or service has satisfiedthe criteria for FM Approval. Refer to the Approval Guide, a publication of FM Approvals, for a complete list-ing of products and services that are FM Approved.

Intermediate Bulk Container (IBC): Any container that has a volumetric capacity of not more than 793 gal-lons (3,000 L) and not less than 119 gallons (450 L) as defined and regulated by the U.S. Department ofTransportation in CFR Title 49, Part 178, subpart N, and the United Nations Recommendations on the Trans-port of Dangerous Goods, chapter 6.5.

IBCs can be constructed of metal, plastic or a metal-plastic composite. In the UN and US DOT regulations,metal IBCs are designated 31A, 31B, and 31N (for liquids, and the letter code is for steel, aluminum, andother metals, respectively.), rigid plastic are designated by the codes 31H1, 31H2, and composite are 31HZ1,31HZ2.

Listed: Equipment or materials included in a list published by an organization that maintains periodic inspec-tion of production of listed equipment or materials and whose listing states that either the equipment or mate-rial meets appropriate designated standards or has been tested and found suitable for a specified purpose.

Stable liquid: Any liquid not defined as unstable.

Tank

Aboveground tank: A tank that is installed above grade, at grade, or below grade without backfill.

Atmospheric tank: A storage tank that has been designed to operate at pressures from atmospheric througha gauge pressure of 1.0 psig (6.9 kPa) measured at the top of the tank.

Double-skinned tank: See Secondary Containment Tank, a term used in European Union (EN) standards.

Floating roof tank: An atmospheric tank intended for storage of high vapor pressure liquids such as crudeoil and gasoline with vapor pressure exceeding 15 psig (103 kPa or 1 bar gauge) with a roof floating on theliquid surface. (Floating roof tanks are not covered by this standard.) Design according to the criteria in API650, Appendix C or H, or other recognized equivalent standard.

External floating roof: A roof that sits directly on the liquid surface, usually on pontoons with a sealattached to the roof perimeter to cover the annular space between the roof and the shell. Design crite-ria are in API 650, Appendix C. This type has inherent buoyancy and are difficult, though not impos-sible, to sink.

Internal floating roof: A roof similar to the external floater but with a fixed roof above, intended for weatherprotection or quality assurance. The internal floater is often a simple pan or plastic membrane floatingdirectly on the liquid surface with little or no inherent buoyancy and is subject to sinking. Design criteriaare in API 650, Appendix H. Pontoon type roofs similar or identical to external floaters are possible but notcommon. Unless the internal floater has the inherent buoyancy of a pontoon type, treat the tank as acone roof tank.

Low-pressure tank: A storage tank designed to withstand an internal pressure of more than 1 psig (6.9kPa) but not more than 15 psig (103 kPa or 1 bar gauge) measured at the top of the tank.

Portable tank: Any closed vessel having a liquid capacity over 60 gal (230 L) and not intended for fixed instal-lation. This includes intermediate bulk containers (IBCs) as defined and regulated by the U.S. Departmentof Transportation in CFR Title 49, Part 178, subpart N, and the United Nations Recommendations on theTransport of Dangerous Goods, chapter 6.5.

Protected aboveground tank: An aboveground storage tank that is listed in accordance with UL 2085, Stan-dard for Protected Aboveground Tanks for Flammable and Combustible Liquids, or an equivalent test pro-cedure that consists of a primary tank provided with protection from physical damage and fire-resistiveprotection from exposure to a high-intensity liquid pool fire.

Secondary containment tank: A tank that has an inner and outer wall with an interstitial space (annulus)between the walls and that has a means for monitoring the interstitial space for a leak.

Storage tank: Any vessel having a liquid capacity that exceeds 60 gal (230 L), is intended for fixed installa-tion, and is not used for processing.



United Nations Recommendations on the Transport of Dangerous Goods: Model Regulations directedat providing safe packaging criteria but only related to the transport of all types of dangerous solids, liq-uids, and gases. Hazard class 3 addresses flammable liquids.

Unstable liquid: A liquid that, in the pure state or as commercially produced or transported, will vigorouslypolymerize, decompose, undergo condensation reaction, or become self-reactive under conditions of shock,pressure, or temperature. A liquid with an NFPA instability hazard rating of 2 or greater in accordance withNFPA 704, Standard System for the Identification of the Hazards of Materials for Emergency Response.

Vent, normal: Pressure relief opening on a tank to permit the intake and discharge of air during emptyingand filling operations and to permit expansion and contraction of vapor due to temperature changes. Some-times called breather vent.

Vent, emergency relief: Pressure relief opening on a tank to prevent overpressurizing the tank in the eventof fire exposure.

Weak seam roof (weak shell-to-roof joint construction): The attachment of the roof to the shell forms afrangible joint that, in the case of excessive internal pressure, will rupture before rupture occurs in the tankshell joints or the shell-to-bottom joint. Design criteria can be found in UL 142 or API 650.

APPENDIX B DOCUMENT REVISION HISTORY

October 2011. The reference in Table 10, note 1 was corrected from 2.2.2-5 to 2.1.2-5.

September 2010. Changes were made in Table 3, Spacing for Flammable Liquid Tank Containment Dikes.

March 2009. Minimum spacing requirements in Table 3, Spacing for Flammable Liquid Tank ContainmentDikes, were modified.

May 2008. Minor editorial changes were made for this revision.

January 2008. Minor editorial changes were made for this revision.

May 2007. Corrections were made to Table 2.

April 2007. Minor editorial changes were made to January 2007 version.

January 2007. The following changes were made:

• Removed the recommendation against bottom connections on FRP tanks to be consistent with refer-enced standards.

• Extended the recommendation for containment to all tanks with flash points below 200°F (93°C). Previ-ously, no containment was required for tanks of less than 15,000 gal (57 m3) except to protect buildings.

• Simplified the spacing and diking recommendations.

• Upgraded the recommendation for indoor tanks, including automatic fire and leak-detection systems.

• Added recommendations for secondary containment tanks.

• Eliminated the exemption to the recommendations for cutoffs for small fuel oil tanks serving heating appli-ances.

• Added recommendations for IBCs when supplying flammable liquids to a process.

• Added earthquake recommendations, including seismic shutoffs for indoor tanks.

• Added section on asphalt storage tanks.

• Added recommendations for monitoring tanks during fill operations, level-gauging, and high-level alarmsto an attended location.

• Moved information on carbon disulfide to Data Sheet 7-23N, Hazardous Chemical Data.

• Added section on asphalt storage tanks.

• Added recommendations for monitoring tanks during fill operations, level-gauging, and high-level alarmsto an attended location.

• Moved information on carbon disulfide to Data Sheet 7-23N, Hazardous Chemical Data.



September 2000. This revision of the document has been reorganized to provide a consistent format.

October 1994. Added information on manifold vents in systems, and detonation arrestors.

July 1976. Miscellaneous revisions and updating.

May 1971. Miscellaneous revisions and updating.

November 1967. Updated and consolidated material from handbook.

1959. original guideline in Factory Mutual handbook of Industrial Loss Prevention.

APPENDIX C HYDROCARBON FIRE DURATION

Spills in defined areas (e.g., curbed area in a room, tank contained by dike, etc.):

See Table B1 below.

1. Determine if the dike will contain the largest expected spill:Volume = (Depth of Dike) x (Area of Dike)

2. Determine the depth of the spill in the confined area:Depth of Fuel = (Volume of Spill) / (Area of Dike)

3. Determine the liquid fire duration:Fire Duration = (Depth of Spill) x [(7 minutes) / (1 in.)] (English)

Or

Fire Duration = (Depth of Spill) x [(7 minutes) / (2.5 cm)] (metric)

Spills in undefined areas:

Assume an average spill depth of 1⁄16 in. (1.5 mm) for a relatively flat surface and use these equations to cal-culate the area of the spill. Thermal damage will occur to everything touched by the spill. The duration ofthis type of liquid spill fire will be limited. See Table 17 below.

The spill area can be calculated as follows:

Area of Spill = (Volume of Spill) / 1⁄16 in. (English)

Or

Area of Spill = (Volume of Spill) / 1.5 mm (metric)

Table 17a. Relationship Between Fuel Volume, Pool Size, and Fire Duration (English)

Liquid Volume,gal

Spill Area, ft2 for1⁄16 in depth

Spill Depth (in.) for Liquid Pools ofDefined Area

Fire Duration (min) For Liquid Poolsof Defined Area

1000 ft2 2000 ft2 1000 ft2 2000 ft2

100 2600 0.2 0.1 1.1 0.6200 5100 0.3 0.2 2.3 1.1300 770 0.5 0.2 3.4 1.7400 10300 0.6 0.3 4.5 2.3500 12800 0.8 0.4 5.6 2.8600 15400 1.0 0.5 6.7 3.4700 18000 1.1 0.6 7.9 3.9800 20500 1.3 0.6 9.0 4.5900 23100 1.4 0.7 10.1 5.01000 25700 1.6 0.8 11.2 5.6



Table 17b. Relationship Between Fuel Volume, Pool Size, and Fire Duration (metric)

Liquid Volume,liters

Spill Area, m2 for1.5 mm depth

Spill Depth (mm) for Liquid Pools ofDefined Area

Fire Duration (min) For Liquid Poolsof Defined Area

93 m2 190 m2 93 m2 190 m2

380 240 5 3 1.1 0.6760 480 8 5 2.3 1.1

1,100 720 13 5 3.4 1.71500 950 15 8 4.5 2.31900 1200 20 10 5.6 2.82300 1400 24 13 6.7 3.42600 1700 29 15 7.9 3.93000 1900 33 15 9.0 4.53400 2100 37 18 10.1 5.03800 2400 41 20 11.2 5.6

Continuous Spills

Depending on the spill rate, a flammable liquid may be fully consumed before it reaches the floor or it will cre-ate a burning pool on the floor. The pool diameter is controlled by the rate at which the liquid is being con-sumed in the fire and the rate at which it is being released. The pool diameter will stop growing when thesetwo rates are equal. Table 18 below provides some expected pool sizes, heat release rates, and flame heightsfor various flow rates of kerosene. Diesel fuel will produce similar results. Since even small spill rates will pro-duce sizable fires, the key issue in deciding if building steel will be damaged is the fire duration. The dura-tion of this type of fire is controlled by the volume of fuel available to be spilled and the rate at which it is spilled.

The spill fire duration can be calculated as follows:

Fire Duration = (Volume of Fuel) / (Spill Rate)

Table 18a. Flow Rate, Pool Diameter, Heat Release Rate, and Flame Height for a Flowing Kerosene Fire (English)

Flow Rate(gpm)

Pool Diameter(ft)

Pool Area(ft2)

Heat Release Rate(MW)

Flame Height(ft)

1 3 9 2 132 5 17 4 163 6 26 6 194 7 34 8 215 7 43 10 23

10 10 86 20 3015 13 128 30 3420 15 171 40 3825 17 214 50 41

Table 18b. Flow Rate, Pool Diameter, Heat Release Rate, and Flame Height for a Flowing Kerosene Fire (Metric)

Flow Rate(l/min)

Pool Diameter(m)

Pool Area(m2)

Heat Release Rate(MW)

Flame Height(m)

3.8 1.0 1 2 3.97.6 1.4 2 4 5.011.4 1.7 2 6 5.815.2 2.0 3 8 6.519.0 2.2 4 10 7.038.0 3.2 8 20 9.157.0 3.9 12 30 10.576.0 4.5 16 40 11.795.0 5.0 20 50 12.6



APPENDIX D HAZARDS

The National Fire Protection Association and various US federal and state regulations use the following liq-uid classifications:

1. Flammable liquids are defined as liquids having closed cup flash points below 100°F (38°C) and vaporpressures not exceeding 40 psia (276 kPa) at 100°F (38°C). Flammable liquids are referred to as Class I liq-uids, and are subdivided as follows:

a) Class IA liquids are those having flash points below 73°F (23°C) and boiling points below 100°F (38°C).

b) Class IB liquids are those having flash points below 73°F (23°C) and boiling points at or above 100°F(38°C).

c) Class IC liquids are those having flash points at or above 73°F (23°C) and below 100°F (38°C).

2. Combustible liquids are defined as liquids having closed cup flash points at or above 100°F (38°C). Com-bustible liquids are referred to as either Class II or Class III liquids and are subdivided as follows:

a) Class II liquids are those having flash points at or above 100°F (38°C) and below 140°F (60°C).

b) Class IIIA liquids are those having flash points at or above 140°F (60°C) and below 200°F (93°C).

c) Class IIIB liquids are those having flash points at or above 200°F (93°C).

The U.N. Recommendations on the Transport of Dangerous Goods has only defined flammable liquids (haz-ard class 3) as follows:

Flammable liquids are liquids, or mixtures of liquids, or liquids containing solids in solution or suspension(for example, paints, varnishes, lacquers, etc., but not including substances otherwise classified on accountof their dangerous characteristics) which give off a flammable vapor at temperatures of not more than 60.5°C(141°F), closed-cup test, or not more than 65.6°C (150°F), open-cup test, normally referred to as the flashpoint. This class also includes:

(a) Liquids offered for transport at temperatures at or above their flash point; and

(b) Substances that are transported or offered for transport at elevated temperatures in a liquid state andwhich give off a flammable vapor at a temperature at or below the maximum transport temperature.

Liquids meeting the above definition, with a flash point of more than 35°C (95°F) which do not sustain com-bustion need not be considered as flammable liquids for the purposes of these Regulations. Liquids are con-sidered to be unable to sustain combustion for the purposes of these Regulations (i.e., they do not sustaincombustion under defined test conditions) if:

(a) They have passed a suitable combustibility test (see SUSTAINED COMBUSTIBILITY TEST pre-scribed in the UN Manual of Tests and Criteria);

(b) Their fire point according to ISO 2592 — 2000 is greater than 100°C (212°F) or

(c) They are water miscible solutions with a water content of more than 90% by mass.

The UN system applies to materials in transport.

The current U.S. Department of Transportation Code 49CFR 171 defines hazard class 3 slightly more broadly,as follows:

(a) Flammable liquid. For the purpose of this subchapter, a flammable liquid (Class 3) means a liquid hav-ing a flash point of not more than 60.5°C (141°F), or any material in a liquid phase with a flash point ator above 37.8°C (100°F) that is intentionally heated and offered for transportation or transported at orabove its flash point in a bulk packaging, with the following exceptions :

(1) Any liquid meeting one of the definitions specified in § 173.115 Class 2, Divisions 2.1 (flammablegas), 2.2 (non-flammable, nonpoisonous compressed gas—including compressed gas, liquefied gas,pressurized cryogenic gas, compressed gas in solution, asphyxiant gas and oxidizing gas), and 2.3(gas poisonous by inhalation).

(2) Any mixture having one or more components with a flash point of 60.5°C (141°F) or higher, thatmake up at least 99 percent of the total volume of the mixture, if the mixture is not offered for trans-portation or transported at or above its flash point.



(3) Any liquid with a flash point greater than 35°C (95°F) that does not sustain combustion accordingto ASTM D 4206 or the procedure in appendix H of this part.

(4) Any liquid with a flash point greater than 35°C (95°F) and with a fire point greater than 100°C (212°F)according to ISO 2592.

(5) Any liquid with a flash point greater than 35°C (95°F), which is in a water-miscible solution with awater content of more than 90 percent by mass.

(b) Combustible liquid.

(1) For the purpose of this subchapter, a combustible liquid means any liquid that does not meet the defi-nition of any other hazard class specified in this subchapter and has a flash point above 60.5°C (141°F)and below 93°C (200°F).

(2) A flammable liquid with a flash point at or above 38°C (100°F) that does not meet the definitionof any other hazard class may be reclassed as a combustible liquid. This provision does not apply totransportation by vessel or aircraft, except where other means of transportation is impracticable. Anelevated temperature material that meets the definition of a Class 3 material because it is intention-ally heated and offered for transportation or transported at or above its flash point may not be reclassedas a combustible liquid.

(3) A combustible liquid that does not sustain combustion is not subject to the requirements of this sub-chapter as a combustible liquid. Either the test method specified in ASTM D 4206 or the procedurein appendix H of this part may be used to determine if a material sustains combustion when heatedunder test conditions and exposed to an external source of flame.

The DOT system applies to materials in transport.

Finally, the European Union (EU) has the Classification, Packaging, Labeling and Notification of Danger-ous Substances Regulations S.I. 116, 2003 with the following liquid flammability definitions:

Extremely flammable – liquid substances and preparations which have a flash point lower than 0°C (32°F)and a boiling point (or in case of a boiling range the initial boiling point) lower than or equal to 35°C (95°F).

Highly flammable – liquid substances and preparations having a flash point below 21°C (70°F) but whichare not extremely flammable.

Flammable – liquid substances and preparations having a flash point equal to or greater than 21°C (70°F),and less than or equal to 55°C (131°F).

This system covers only identification methods for these substances. Other regulations would apply to stor-age or transport.



Documents

FMDS0788-Flammable Storage Tank