1-25 Process Tanks and Silos

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January 1980

Revised May 2003

Page 1 of 14

PROCESS TANKS AND SILOS

Table of ContentsPage

1.0 SCOPE .................................................................................................................................................. 3

1.1 Changes .......................................................................................................................................... 3

2.0 LOSS PREVENTION RECOMMENDATIONS ....................................................................................... 3

2.1 Concrete Tanks ................................................................................................................................ 3

2.1.1 Operation and Maintenance .................................................................................................. 3

2.1.2 Equipment and Processes .................................................................................................... 3

2.2 Metal Tanks ...................................................................................................................................... 3



2.2.3 Construction and Location .................................................................................................... 4

2.3 Wood Tanks ..................................................................................................................................... 42.3.1 Operation and Maintenance .................................................................................................. 4

2.4 Concrete Silos ................................................................................................................................. 4


2.5 Bolted Steel and Aluminum Silos .................................................................................................... 4



2.5.3 Construction and Location .................................................................................................... 5

2.6 Wood Silos ....................................................................................................................................... 5

3.0 SUPPORT FOR RECOMMENDATIONS ............................................................................................... 5

3.1 General ............................................................................................................................................. 5

3.2 Loss History ..................................................................................................................................... 5

4.0 REFERENCES ....................................................................................................................................... 5

4.1 FM Global ........................................................................................................................................ 5

4.2 Other ................................................................................................................................................ 5

APPENDIX A GLOSSARY OF TERMS ....................................................................................................... 5

APPENDIX B DOCUMENT REVISION HISTORY ....................................................................................... 5

APPENDIX C SUPPLEMENTARY INFORMATION ..................................................................................... 6

C.1 General .......................................................................................................................................... 6

C.1.1 Foundations .......................................................................................................................... 6

C.1.2 Mechanical Equipment ......................................................................................................... 6

C.1.3 Tank Venting ......................................................................................................................... 6

C.1.4 External Corrosion of Metal Tanks and Silos ....................................................................... 7

C.2 Process Tanks ................................................................................................................................. 9

C.2.1 Concrete Tanks ..................................................................................................................... 9

C.2.1.1 Inspection ................................................................................................................ 10

C.2.1.2 Venting .................................................................................................................... 10

C.2.2 Metal Tanks ......................................................................................................................... 10

C.2.2.1 Open Metal Tanks ................................................................................................... 10

C.2.2.2 Metal Tank Construction-Welded ............................................................................ 11

C.2.2.3 Corrosion ................................................................................................................. 11

C.2.2.4 Closed Metal Tanks ................................................................................................. 11

C.2.3 Wood Tanks ........................................................................................................................ 12

C.2.4 Plastic Tanks ....................................................................................................................... 12

FM GlobalProperty Loss Prevention Data Sheets 1-25

©2003 Factory Mutual I nsurance Company. All rights reserved. No part of this document may be reproduced,stored in a retrieval system, or transmitted, in whole or in part, in any form or by any means, electronic, mechanical,photocopying, recording, or otherwise, without written permission of Factory Mutual Insurance Company.

Valid Through January, 2004



C.3 Silos .............................................................................................................................................. 12

C.3.1 Concrete Silos .................................................................................................................... 12

C.3.1.1 Reinforced Concrete Silos ...................................................................................... 12

C.3.1.2 Concrete Stave Silos .............................................................................................. 13

C.3.2 Steel Silos ........................................................................................................................... 13

C.3.3 Aluminum Silos ................................................................................................................... 13C.3.4 Wood Silos .......................................................................................................................... 14

List of FiguresFig. 1. Flat bottom tank supported on concrete or sand (seldom more than 500,000 gal.). ........................ 6

Fig. 2. Flat bottom tank supported on legs (seldom more than 500,000 gal.). ............................................. 7

Fig. 3. Cone bottom tank supported on legs (seldom more than 50,000 gal.). ............................................ 8

Fig. 4. Horizontally mounted tank (seldom more than 20,000 gal.). ............................................................. 8

Fig. 5. Tank roof vent through building roof. .................................................................................................. 9

1-25 Process Tanks and SilosPage 2 FM Global Property Loss Prevention Data Sheets

©2003 Factory Mutual Insurance Company. All rights reserved.




1.0 SCOPE

This data sheet provides inspection and maintenance guidelines for process tanks and silos. These guide-

lines include advice in recognizing and correcting conditions which could lead to collapse or similar failure.

(Fire, explosion and overpressure recommendations are covered in other standards.)

This data sheet discusses various types of tanks and silos. Emphasis is placed on ‘‘process’’ tanks and silosfor which loss experience has been appreciable. However, this data sheet may also be used, where

applicable, for holding and storage tanks. Fire water tanks, tanks containing flammable liquids or gases,

underground tanks, portable tanks and code pressure vessels are not included and are generally covered

by other standards.

1.1 Changes

May 2003. Minor editorial changes were made for this revision.

2.0 LOSS PREVENTION RECOMMENDATIONS

2.1 Concrete Tanks

2.1.1 Operation and Maintenance

2.1.1.1 Circular tanks of concrete that have tensioned wires should be periodically examined to determine

the condition of the wires. If corrosion is discovered, the wires should be replaced by a qualified contractor.

If this is not feasible, consideration should be given to placing a steel shell of sufficient strength around the

tank and/or lowering the operating level.

2.1.2 Equipment and Processes

2.1.2.1 When constructing a prestressed concrete tank, the gunite should always be placed at least to the

designer’s specified thickness — usually on the order of 1 in. (25 mm).

2.2 Metal Tanks


2.2.1.1 The interior walls, floor, and roof of metal tanks that are accessible should be inspected annually.

This is particularly important when the tank is subject to corrosive or abrasive influence. Corrective action

should be taken when any parts or areas are thinned more than 25%.

2.2.1.2 Fasteners securing the joints of bolted steel tanks should be examined semiannually. They should

be tightened as needed. If minor corrosion is found, the fasteners should be sprayed or coated with a rust-

resistant compound. Fasteners whose cross-sectional area has been reduced by corrosion more than 25%

should be replaced. Tank draining will be necessary.

2.2.1.3 Tank roofs should be kept clear of dust accumulation. Steel supports, particularly for tanks contain-

ing corrosive liquids, should be examined periodically for corrosion. Any leaks should be sealed, the support

sandblasted and then treated with a corrosion resistant paint or coating.

2.2.1.4 Tank linings should be routinely inspected. The lining should be patched or replaced in the event

cracks or openings are discovered.

2.2.1.5 When sticky materials are being handled, vacuum breakers should be inspected monthly to deter-

mine that the seats have not become stuck together. When handling other materials, semiannual inspection

of the breakers may be sufficient.

2.2.1.6 When tanks are steam cleaned, the vacuum breakers should be inspected monthly and their working

condition maintained.


2.2.2.1 Open tank vent capacity should be one cu ft/hr of free air for every cu ft/hr (one cu m/hr for every

cu m/hr) of discharge or fill rate, whichever is larger. If the vent is unrated, the flow capacity should be checked

by the formula shown under C.1.3. If screens are needed, the area should be three times the vent pipe area

Process Tanks and Silos 1-25FM Global Property Loss Prevention Data Sheets Page 3





(Fig. 5). The vent screen should be above the highest expected snow level on the tank roof. Also, the low-

est part of the vent should be higher than the overflow or highest level of liquid in the tank. Vent pipes should

be periodically inspected and any obstructions found should be removed.

2.2.2.2 Vacuum breakers normally should have a flow capacity of one cu ft/hr for every cu ft/hr (one cu m/hr

for every cu m/hr) of discharge or fill rate, whichever is larger.Exception: When a tank receives steam for cleaning, the vacuum breaker should be designed by a special-

ist, taking into consideration the possibility of a sudden vacuum inside the tank caused by rapidly condens-

ing steam.

2.2.3 Construction and Location

2.2.3.1. When tanks hold corrosive liquids that will destroy considerable property if released, and the potential

loss justifies, a dyke or barrier that will contain the liquid, or floor drains and piping to holding tanks should

be provided.

2.2.3.2 Access manholes or hatches are needed in tanks under the following conditions.

1. The tank interior is subject to corrosion.

2. The tank needs periodic maintenance to, or installation of a liner.

3. Deposits accumulate inside the tank or vent pipe that need removal.

4. The tank has mechanical equipment inside that needs inspection and maintenance.

2.3 Wood Tanks


2.3.1.1 Hoops or bands encircling all wood tanks should be examined on a yearly basis for evidence of

deterioration. This is particularly important when the tank is subject to a corrosive atmosphere or when any

protective coating has outlived its useful life. When appreciable deterioration is discovered, the hoops or

bands should be replaced. If corrosion has already begun, remove the corrosion and coat the metal with

corrosion-resistant paint.

2.3.1.2 The staves of older wood tanks should be periodically examined for signs of deterioration andreplaced as necessary.

2.3.1.3 Special consideration should be given to large wood tanks containing liquids having considerable

value.

2.3.1.4 Liquid damage to adjacent property should be considered in the event of tank collapse.

2.4 Concrete Silos


2.4.1.1 Inspection of both the interior and exterior of concrete silos should be conducted semiannually. The

silo builder should be consulted if evidence of serious cracking, spalling, crushing of the concrete or silo lean-

ing is observed.

2.4.1.2 Iron hoops or bands securing all concrete staves in silos should be examined on a yearly basis for evi-dence of deterioration. This is particularly important when the silo is subject to a corrosive atmosphere or

when galvanizing has worn off. When appreciable deterioration is discovered, the hoops or bands should be

replaced. If corrosion has already begun, remove it and coat the metal with corrosion resistant paint.

2.5 Bolted Steel and Aluminum Silos


2.5.1.1 All joints, hatches and other penetrations should be examined annually. Defective gaskets and worn

or missing bolts should be replaced. The silo should be maintained so that rainwater will not enter.







2.5.2.1 Silo venting capacity, when bridging is not likely to occur, should be one cu ft/hr for every cu ft/hr

(one cu m/hr for every cu m/hr) of discharge or fill rate, whichever is larger. Air velocity through vents should

not exceed 20,000 ft/hr (6100 m/hr).

2.5.2.2 When a silo contains over 100,000 bushels (125,000 ft3, 3,500 m3) and the nature of the con-tained material along with the diameter are such that bridging of the material may occur, additional venting

area should be provided (in the central part of the silo).

2.5.2.3. Vent openings should be constructed to minimize icing of the screens. The screened area should

be three times the vent area.

2.5.2.4 When fans are used to create a vacuum inside a silo, consideration should be given to interlocking

the fan motor with a vacuum measuring device. The fan will then be shut down if vacuum inside exceeds

design values. (Not needed when the venting area is in accordance with this standard and vent screens are

not subject to freezing over or plugging.)

2.5.3 Construction and Location

2.5.3.1 Adequate structural reinforcement should be provided around all silo penetrations, unless the

penetration itself provides the reinforcement.

2.6 Wood Silos

Recommendations under 2.3 Wood Tanks apply.

3.0 SUPPORT FOR RECOMMENDATIONS

3.1 General

Refer to Appendix C, Supplementary Information, for general comments on recommendations.

3.2 Loss History

This data sheet places emphasis on process tanks and silos for which loss experience has been appre-

ciable. Three examples of past loss history include: 1) In July 1994, a process vessel imploded due to exter-nal corrosion under insulation. 2) In June 1995, a mastic coated, tile lined, open top process tank ruptured

due to corrosion under a mastic coating. 3) In August 1995, a starch silo collapsed due to corrosion of a

mechanical fastener under insulation.

4.0 REFERENCES

4.1 FM Global

Data Sheet 7-49/12-65, Emergency Venting of Vessels

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

Data Sheet 9-13, Evaluation of Flood Exposure

Data Sheet 17-1, Nondestructive Examination

4.2 Other

Standard D 100, American Water Works Association

Standard 313-91 (or latest edition), The American Concrete Institute

Standard SNT-TC-1A, American Society of Nondestructive Testing

APPENDIX A GLOSSARY OF TERMS

This document does not have any defined terms.

APPENDIX B DOCUMENT REVISION HISTORY

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






APPENDIX C SUPPLEMENTARY INFORMATION

C.1 General

Process tanks usually hold liquids or slurries, but in some cases can contain bulk or granular materials or

low pressure gases. The word ‘‘silo’’ is normally associated with the handling of bulk storages such as farmproducts, wood chips, coal and plastic molding compounds.

C.1.1 Foundations

Adequate foundations for support of tanks and silos are extremely important. Flat bottoms of large tanks

with a capacity range of 100,000-500,000 gallons (379-1890 m3) or more normally can rest directly on a thick

concrete slab or on a sand cushion supported by strong soil below. In the latter case, the tank shell or wall

has a concrete ring wall directly under it. It is also acceptable to place the tank on a stone berm extend-

ing at least 3 ft (1 m) beyond the tank wall and a few feet (about 1 m) higher than the adjacent grade. The con-

tents of flat bottom tanks (Fig. 1) are fluid enough to be discharged through the wall of the tank.

Smaller flat bottom tanks (Fig. 2) and cone bottom tanks (Fig. 3) sometimes have raised bottoms sup-

ported on legs to facilitate discharge. Foundations for such tanks can be an individual concrete footing per

leg, a large concrete ring supported on soil or piles, or the legs can rest directly on a thick concrete slab.

C.1.2 Mechanical Equipment

Pumps are normally used to fill and empty tanks. Agitators are used for mixing. Fans are frequently used

to facilitate discharge for large silos. Small tanks in the 1,000-20,000 gallon (3.79-7.58 m3) range can be

mounted horizontally to ensure a more even head on the discharge equipment and to facilitate filling (Fig. 4).

C.1.3 Tank Venting

When material is removed or added to a tank or silo, provision for air to enter or be discharged is needed

to prevent excessive vacuum or pressure within the vessel. The device used for this purpose is termed a tank

or atmospheric vent and the size of the vent is a function of the tank discharge or filling rate (whichever is

larger). When the vent is an ‘‘off the shelf’’ type, designed by a specializing firm, the flow capacity in cu ft per

unit of time is usually imprinted on it. When the unit is fabricated in a welding shop as an individual unit,

the capacity of the vent may be unknown. In such a case, the capacity may be checked by the following

formula:

Q = CAv

Where Q = flow capacity in cu ft/hr (m3 /hr)

A = vent area in sq ft (m2)

v = air velocity through vent (20,000 ft/hr max, 6100 m/hr max)

C = an orifice constant, approx. 0.9.

Fig. 1. Flat bottom tank supported on concrete or sand (seldom more than 500,000 gal.).






If dusts, noxious vapors, etc. are expected to rise in the vent, the vent should go through the roof of the build-

ing. In this case, the top of the vent should be equipped with a hood to keep out rain or snow and with screens

to prevent entry by birds or vermin (Fig. 5). In cold climates, vent screens can be covered with ice or snow

which obstructs air passage and may result in a vacuum being pulled inside the vessel. It is necessary that

the screened areas be considerably larger than the vent pipe area in the event of partial icing and to account

for air flow obstruction by the screen wires. Also, vent pipes have become plugged with crust formed from

froth or suds prevalent in processes such as fermentation and soap-making.

C.1.4 External Corrosion of Metal Tanks and Silos

Insulated process vessels (tanks and silos) are subject to corrosion under insulation any time a fluid or vapor

penetrates the weather or vapor jacket of the insulation or is released (leaked) inside the jacket. Uninsulated

process vessels and tanks having a corrosion resistant external coating are similarly subject to thinning if

the coating is compromised. Corrosion of external stiffening rings and mechanical fasteners can signifi-

cantly reduce the pressure or vacuum containing capacity of a vessel.

Vessels in chemical process, pulp and paper and mining and ore processing industries are more likely to expe-

rience external corrosion. Vessels in damp or humid environments or environments tending to be alkaline,

acidic or containing chlorides are susceptible to external corrosion. Operating temperatures of 160 ° to 220°F

(71° to 104°C) are known to rapidly accelerate corrosion.

In addition to planned internal inspections, during which thickness readings may be taken to reveal either inter-nal or external thinning, an external inspection should be completed at a maximum five-year interval. Use

a testing procedure qualified to a recognized standard such as SNT-TC-1A. Eliminate any doubt that the ves-

sel condition is suitable for continued operation.

Failure of tanks and silos due to corrosion under external coating or insulation can be avoided or miti-

gated. Facilities with susceptible tanks or silos should have a planned inspection program to identify com-

promise of the coating or insulation jacket, identify leakage under the coating or jacket, confirm thickness of

the pressure containing material, and promptly correct any deficiencies.

Fig. 2. Flat bottom tank supported on legs (seldom more than 500,000 gal.).






Fig. 3. Cone bottom tank supported on legs (seldom more than 50,000 gal.).

Fig. 4. Horizontally mounted tank (seldom more than 20,000 gal.).

U.S. gal. meters 3

20,00 76

50,000 189

500,000 1890






C.2 Process Tanks

Process tanks are constructed of concrete, metal, wood, glass fiber-reinforced plastic or combination of these

materials.

C.2.1 Concrete Tanks

Concrete tanks are usually circular but some are rectangular. Circular tanks are built of poured-in-place

reinforced concrete, precast sections assembled at the site or prestressed concrete. Concrete for rectangu-

lar tanks is usually poured-in-place. The prestressed method of constructing circular tanks is economically

feasible when large tanks in the 250,000-500,000 gallons (946-1, 890 m3) range or more are needed in envi-

ronments where corrosion would be a problem if the tank were steel.

The prestressed concrete tank is the only type of concrete tank that has had any significant loss history.

The first step in construction is the pouring-in-place of a circular reinforced concrete wall. After proper cur-

ing, hundreds of fine strands of galvanized reinforcing wires are wrapped around the wall and tensioned to

approximately 100,000-160,000 lb/in.2 (7,030-11,250 kg/cm2). This action creates compressive forces in the

wall that will absorb the ring tension caused by the outward thrust of the tank contents. When the atmo-

sphere is corrosive to steel, a layer of concrete (sometimes called gunite) approximately 1 in. (25 mm) thick

is sprayed over the wires to protect them. Other materials known to provide equivalent protection may be

considered.

The guniting method of preventing the atmosphere from penetrating to the wires is not always effective. The

gunite may be sprayed too thinly or small cracks may develop in it. Several major collapses have recently

occurred in which the strands of wire were found to be severely corroded. In one case, the tank contained

bleached wood pulp; another collapsed tank contained cement slurry. Prestressed concrete tanks are eco-

nomically feasible only when the tank capacity is in the range of 250,000-500,000 gallons (946-1,890 m3) or

more. Therefore, collapse of such a large tank leads to substantial loss to the tank and its contents. Liquid

damage and business interruptions are also possible.

Fig. 5. Tank roof vent through building roof.






Only qualified, experienced personnel should design or construct concrete tanks. Organizations such as

The Portland Cement Association and The American Concrete Institute have excellent technical data on the

design of concrete tanks.

C.2.1.1 Inspection

Corrosion of the tensioned wires can sometimes be detected by close examination of the exterior tank sur-

face. Evidence of the protective coating expanding, cracking or spalling, or bare wires exposed in a corro-

sive atmosphere should be considered a danger signal. Chipping off a few small areas of gunite to the bare

wires is a feasible method of inspection; and if no corrosion is found, the areas can subsequently be patched.

If any of the above symptoms or corrosion of the strands is found, a specialist should be consulted to deter-

mine appropriate action.

C.2.1.2 Venting

FM Global loss records do not reveal any venting problems with concrete tanks.

C.2.2 Metal Tanks

Metal tanks can be classified into two groups: (1) open, in which the tank has an open pipe or vent through

its roof to the atmosphere; (2) closed, in which the contents cannot be exposed to the outside atmospherefor reasons of contamination. The latter type of tank should be equipped with a vacuum breaker.

C.2.2.1 Open Metal Tanks

Corrosive influences on the floor or walls of metal tanks are probably the most important factor in selecting

the proper metal from which to construct the tanks. Carbon steel is used widely. For certain corrosive pro-

cesses, nickel or copper alloy steel or special metals may be needed.

Nickel and chromium alloy (stainless) steel tanks are widely used in the pulp and paper and food process-

ing industries. Aluminum and copper are some other metals used to build tanks. Metal tanks other than steel

are normally not more than 50,000 gallons (189 m3) and are seldom that large. Metal tanks are some-

times lined with glass, brick, natural or synthetic plastics, or corrosion-resistant metal cladding that will pre-

vent the structural tank material from chemically reacting with the tank contents. It is important that linings

stay adhered to the tank. If not, protection to the substrate material will be lost, the process material may be

contaminated by particles of the liner material or piping and pumps may be clogged.

Collapse and implosion losses have occurred mainly to steel tanks having capacities greater than 100,000 gal-

lons (379 m3). Loss history has been satisfactory for non-ferrous tanks. There have been several instances

where tanks holding corrosive materials have collapsed due to rusting of the steel supports.

Problems associated with steel tank failures other than internal corrosion are (1) joint failures, (2) abrasive

contents (such as cement slurry) thinning the sides of the tank until failure occurred, (3) tank dismantled and

reconstructed at another location. (In the latter case, if the new use for that tank is to store a liquid of higher

density than design, the operating level should be lowered.) Also, great care is needed to reestablish either

bolted or welded joints to their original strength. When the tank is dismantled, the seams are burned with a

torch, leaving slag and rough edges on the plate. Such edges are difficult to properly reweld and need to

be ground smooth and even before rewelding is attempted.

Roofs of large steel tanks have sometimes collapsed and fallen into the tank. This can be caused by corrosion,

heavy dust accumulations, wind and earthquake or a combination of these factors.Restricted air passage through tank roof vents due to freezing over or plugging of screens can cause an

implosion during discharge of the tank contents.

For protection of a tank or vessel due to overpressurization from heat application, steam or chemical reaction,

see Data Sheet 7-49/12-65, Emergency Venting of Vessels.

If a tank is subject to flooding it may float, if empty. If these conditions are possible, consideration should

be given to the anchoring of the tank to its foundation. See Data Sheet 9-13, Evaluation of Flood Exposure

.






C.2.2.2 Metal Tank Construction-Welded

Most tank plates are joined by butt welding in which a narrow space is left where the plates meet that is sub-

sequently filled with appropriate weld metal. The molten weld metal fuses with the plates to be joined, cre-

ating continuous metal at the joint, which should be as strong as the plates themselves. This type of joint weld

is termed a ‘‘full penetration butt weld.’’ When penetration is not complete, the weld is improper. Some-times an unfused or void space is left in the middle of the joint that cannot be detected visually. When welds

are improper or incomplete, the tank can collapse on its first filling, or may collapse later on after corro-

sion has attacked and further weakened the already weak welds.

A radiographic test procedure (utilizing x-rays or gamma rays) for examination of butt-welded plates is out-

lined in the latest edition of Standard D 100, American Water Works Association.

Bolted tank joints have not been as great a problem as welded joints because (1) they are not used nearly

as often, (2) the bolts, nuts and holes that make up the joint can be visually examined.

Any flammable or toxic vapors need to be purged from the tank and ventilation maintained for purposes of inte-

rior inspection. (Data Sheet 7-59, Inerting and Purging of Tanks, Process Vessels and Equipment may be

referred to where applicable.)

C.2.2.3 Corrosion

There is a nondestructive test method known as the ultrasonic test (Data Sheet 17-1, Nondestructive Exami-

nation ) that will determine the tank wall thickness when either filled or empty. If the tank is empty, it will be

easier to drill 1 ⁄ 4 in. (6 mm) holes in the wall and measure the thickness. The hole can be tapped and plugged

for future measurements. Any widespread reduction in wall thickness beyond 25% is serious enough to con-

sider either replacement of the whole tank, reconstruction of part of the tank or a lowered level of the tank

contents. The actual decision to reconstruct should be based upon the stress in the thinnest cross section of

the tank wall.

Local corroded areas such as pits, fissures, gouges etc., can be filled with weld material if small or patched

with plate if larger. Sandblasting or wire brushing to bright metal is necessary before any patching work is

done.

A local tank builder can very often provide competent assistance in refurbishing a badly corroded tank.

C.2.2.4 Closed Metal Tanks

Closed tanks are normally not large, the majority usually being less than 50,000 gallons (189 m 3). These

tanks are frequently used in the food or drug industries. Collapse due to corrosion is rare. By far the great-

est hazard to closed tanks is implosion caused by inside vacuum. This has generally happened when the

tank contents are being discharged and the vacuum breaker is either too small in area, malfunctions or the

line to it plugs or freezes.

The handling of certain materials such as bulk or liquid sugar, soybean oil, etc., can result in the sticking of

vacuum breaker seats which can lead to an implosion.

Tanks that receive steam for cleaning or for condensate purposes, etc. are regularly exposed to a vacuum

caused by the condensing steam. Implosion of such tanks has occurred because the vacuum breaker was

undersized and the external air could not enter the tank quickly enough to relieve the vacuum. (See

Recommendations.)

Before installing vacuum breakers in tanks receiving steam, major consideration should be given to thepossibility of opening the tank to the atmosphere.

Certain tanks are constructed with some vacuum resistance. The amount of vacuum the tank can with-

stand will be stated on the nameplate. If not stated, the tank may take very little vacuum and the vacuum

breaker should be extremely sensitive.

Freezing of condensation in the line to a vacuum breaker may be prevented by locating the breaker in an

area not subject to freezing or by having it heat traced.






C.2.3 Wood Tanks

New wood tanks and tubs are generally purchased for use in certain specialized industries such as paper

manufacturing, mining and metallurgical, leather tanning, spirit distillation, soap making and food process-

ing. Wood tanks are also widely used for the storage of various acids as well as bases such as calcium and

magnesium hydroxides. To prevent damage from very corrosive materials, plastic bag-type linings can eas-ily be placed in wood tanks. The tanks will then withstand solutions having a pH value as low as 2 or as

high as 11, depending on the plastic. Also, linings are sometimes placed in tanks subject to minor dripping

or leakage, as the liner can easily bridge small cracks or openings.

Wood tanks have had a satisfactory loss record, although they may contain products of high value. In one

instance, a screw device holding on a wood manhole cover failed, allowing 10,000 gallons (37.9 m 3) of wine

to be lost.

Leakage can be due to loose hoops, long service life or allowing the tank to go dry, or partially dry (fol-

lowed by refilling). Hoop tightening and caulking or installation of a liner will usually remedy leaking, provid-

ing the tank is structurally sound.

The greatest hazard to a wood tank is the corroding of the encircling metal bands or hoops. These are tight-

ened, compressing the staves along their edges which prevents leaking at their joints. The bands also take

the outward thrust of the liquid contents.

The strength of the staves in wood tanks more than 30 years old can be appreciably reduced simply because

of ‘‘old age.’’ A rupture and loss of contents may occur if the condition is either unknown or ignored.

C.2.4 Plastic Tanks

Fiber-reinforced plastic tanks are used for certain processes in the handling of materials likely to corrode

most metals. Adequate design of such tanks to contain corrosive chemicals under possible high tempera-

ture conditions can only be accomplished by experienced designers working closely with the plastic manu-

facturer and tank constructor.

The major problem associated with plastic tanks in the 20,000-30,000 gallon (75.7-114 m 3) range has been

failure at or near manholes or other openings in the wall. There also have been failures of tanks in the 50,000-

100,000 gallon (189-379 m3) range where the wall is joined to the tank floor. Chemicals such as sodium

chlorate and magnesium silicate have been lost due to failure of reinforced plastic tanks.

Plastic tanks usually collapse suddenly. Any leaking or weeping is a danger signal. As they normally hold cor-

rosive or hazardous materials, considerable damage can occur to nearby property. This may be greatly

reduced by the construction of barriers or drains around these tanks. When the tank is erected outdoors, a

dyke to contain the spilled material or a ground slope so it will flow away from buildings or other property

may be advisable.

C.3 Silos

The material to be stored in a silo normally is dumped in through the top by conveyor and removed by means

of openings or chutes in the bottom. Certain granular materials flow directly from the silo by gravity. Other

materials need agitating and/or pneumatic exhausting equipment to facilitate removal. The action is some-

times used to transport the material through pipes or ducts to the next step in the process.

Silos are constructed of concrete, steel, aluminum or wood.

C.3.1 Concrete Silos

Concrete silos are of two basic types: (1) reinforced concrete, in which construction is normally by the slip-

form method (these silos are usually found in clusters), (2) concrete stave silos, in which the silo is con-

structed in a manner similar to a wood tank except the staves are a special type of lightweight concrete

compressed in the usual manner by tightening metal bands or hoops.

C.3.1.1 Reinforced Concrete Silos

Only qualified, experienced personnel should design or construct concrete silos. It is extremely important

that the horizontal reinforcing steel rings within the wall are designed sufficiently strong to resist the out-

ward thrust of the silo contents, that the rings are lapped sufficiently where the ends join so they will not pull






out of the concrete, that the concrete cures to the specified strength, and that it is properly placed in the

form. For design purposes, Standard 313-91 (or latest edition) by the American Concrete Institute is

recommended.

Hairline cracks in concrete silos are usually harmless. Vertical cracks in silo walls up to 1/16 in. (1.58 mm)

can be tolerated; however, wider cracks or the widening of any crack may be a danger signal and the crackwill need careful monitoring.

C.3.1.2 Concrete Stave Silos

These normally are built by firms who are specialists in their design and construction. Experience has shown

that the only problem of any significance that has occurred with this type of silo has been corrosion of the

metal bands, or the lugs that connect them.

C.3.2 Steel Silos

Large steel bolted type silos have been the most vulnerable to forces that cause collapse. Some of these

structures are rather large, containing 500,000 or more bushels (622,000 ft3, 17,620 m3).

Most collapses of large silos appeared to be an implosion caused by a vacuum inside, as some of the plates

were observed to be buckled inward. The discharge fans or other equipment were usually running when

the collapses occurred. In one case, an ice storm sealed over the screened vents and the fans created a

vacuum inside. In another case, the material was being discharged too rapidly.

A condition known as ‘‘bridging’’ can also lead to damage. FM Global loss records show two incidents in

which bridged soybean meal collapsed bolted steel silos. Bridging occurs mainly during discharging. The

material above that being removed arches and hangs to the silo sides. When that material suddenly falls, a

vacuum is created in the space where the material had been. Outside air pressure will then force air into

the silo through the vent openings or around plate joints and hatch covers (if not completely tight). If the air

cannot enter quickly enough because of insufficient flow capacity, the external pressure may force the sides

of the silo inward (implosion). Discharge fans or similar equipment, if running, can add to the vacuum inside.

Bridging in the food products industry occurs mainly in meal or ground grains. Soybean meal is one example

of a product that will bridge. The kernels of unground grains such as barley, oats and corn tend to roll off

the sides of the silo. The diameter of the vessel plus moisture content of the material are factors that con-

tribute to bridging. In the above mentioned silo implosions, the silo diameters were 25 ft (7.62 m) and their

heights were 45 ft (13.7 m) and 87 ft (26.5 m).

Bridging can usually be prevented by mechanically agitating or vibrating the stored material. On new designs,

a greater silo diameter may be in order. Ample venting areas will reduce the chance of implosion.

Bridging can also occur to stored, fine particle materials such as portland cement and sulphur.

Adequate reinforcement around the sides of openings such as hatches and manholes is necessary to pick

up and transfer loads from the interior of the opening. When the opening is a chute (or pipe) for the purpose

of filling or discharging, the chute itself sometimes acts as reinforcement.

Bolted joints, if not tight, are vulnerable to moisture penetration. This can cause material such as grain and

soybeans to ‘‘swell’’ and increase pressure on the silo sides. Rain water may also enter through access open-

ings, vents, etc. that are not adequately sealed.

Relocation of silos is not always successful. In one case, a silo whose panels were bolted with high strength

bolts was relocated. Some of the bolts became lost in the move and were substituted with weaker bolts,which resulted in a collapse.

Extremely cold weather can create additional forces on silo tension rings. Contraction of the rings com-

presses the silo contents, placing additional stress on the rings. If provision was not made in the design for

these added stresses, failure of the ring can occur.

C.3.3 Aluminum Silos

Bolted aluminum silos are considerably smaller than steel silos and loss experience has been more favor-

able. Problems similar to those of bolted steel silos may also be encountered in aluminum silos and the same

recommendations apply.






C.3.4 Wood Silos

Supplementary information under C.2.3 Wood Tanks apply. Recommendations under ‘‘Wood Tanks’’ apply.

Note: There is no comparable NFPA standard.

FMELPC Nov 1979



Documents

1-25 Process Tanks and Silos