Understanding Tank Safety

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UNDERSTANDING TANK SAFETY Storage Tank Venting for Conservation, Safety & Environmental Protection

DEFINITIONS

Atmospheric Tank -A storage tank that has been designed to operate at pressures from atmospheric

through 0.5 PSIG.

Combustible Liquid - A liquid having a flashpoint at or above 100 F.Diaphragm -

The sealing (gasket) material that is part of the pallet assembly and which seals

against the seat surface when the vent is closed.

Design Pressure -The maximum pressure or vacuum that a storage tank can withstand without

damage to its structure.

Flammable Liquid - A liquid having a flashpoint below 100 F.

Flashpoint -The minimum temperature at which a liquid gives off vapor in sufficient

concentration to form an ignitable mixture with air near the surface of the liquid.

Leak Rate - The leakage of vapor from the vent prior to reaching the set point.

Low Pressure Tank -

A storage tank which has been designed to operate at pressures above 0.5 PSIG

but not more than 15 PSIG.

Pallet Assembly -

The weight or spring loaded disc housed within the vent that moves in response

to the tank pressure, allowing flow into or out of the tank. The pallet assembly

covers the vent seat when in the closed position.

Pressure Vessel -A storage tank or vessel which has been designed to operate at pressures above

15 PSIG.

Set Point - The tank pressure/vacuum at which the vent begins to open.

Seat -The machined orifice within the vent housing on which the pallet assemblies sit

when closed.

Tank Vent -

A device intended to provide pressure and/or vacuum relief for atmospheric or

low pressure storage tanks. The set points of the vents may be provided by

weight loading, spring loading or buckling pin.

PRESSURE/VACUUM ACCUMULATION

The use of large capacity tanks and vessels for the temporary storage of flammable or combustible

liquids is a common practice in a wide range of commercial and industrial enterprises. These tanks

provide fixed volume containers to hold liquids transferred (filling and emptying) through

connected piping systems. In any such fixed roof tank, the volume above the liquid level is known

as the vapor space.

Assume that a tank is completely vapor tight and that liquid is being pumped into and out of the

tank. Filling the tank raises the liquid level and causes the vapor space to decrease (vapors arecompressed), with a resulting increase in the pressure in the vapor space. Alternatively, if liquid is

withdrawn from the tank, the vapor space increases (vapors are allowed to expand) and the pressure

in the vapor space decreases.

Now assume that the tank is again completely vapor tight, no liquid is being transferred (the liquid

level does not change), but the liquid in the tank is being heated or cooled. The addition of heat

causes vapors to be generated and evolve into the closed vapor space. The result is an increase in

pressure in the vapor space. Cooling of the liquid leads to contraction of the vapors and a

corresponding pressure decrease in the vapor space.

The scenarios outlined above reflect common hazards associated with the storage of flammable

liquids in fixed roof tanks. Unless the tanks are equipped with properly designed and specified

venting devices, excessive pressure and/or vacuum accumulations in the vapor space can result in

severe tank damage. Protectoseal pressure and vacuum relief vents are specifically designed to

address and eliminate this potentially hazardous situation.
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Normal Venting- In day-to-day tank operations, changes in the liquid level are caused by routine

filling and emptying of the tank. Changes in the temperature of the vapors and liquids in the tank are

the result of variations in the ambient atmospheric temperatures (e.g. higher temperatures during the

day; cooler temperatures at night). Discharging the volume of vapors generated (pressure relief), or

inbreathing the volume of make-up air required (vacuum relief), during such activities is defined as

normal venting (Vents That Provide Normal Pressure/Vacuum Relief).

Emergency Venting- The temperature of the stored liquid and vapors may also increase as a result of

the tank being exposed to an external fire. A significant amount of heat may be transferred throughthe tank shell and the volume of vapors generated as a result of this heat input can be substantial.

Providing a means of discharging this large volume of vapors and prohibiting an increase of

pressure within the tank is defined as emergency venting (Vents That Provide Emergency Pressure

Relief).

EVAPORATION LOSSES

In addition to protecting a tank from excessive pressure and vacuum, Protectoseal vents also play a

key role in the reduction of product evaporation losses and fugitive emissions. The vents are

designed to remain closed until they must open to protect the tanks. Vapors are contained and are not

released into the atmosphere. The reduction in product loss as compared to an open vent pipeline is

significant. The emission of vapors into the atmosphere is minimized. Tank vents are an important

tool in any company's attempts to comply with the Clean Air Act mandates concerning air pollution.

VENT OPERATION

The method of operation of Protectoseal pressure/vacuum vents is straightforward. The vents are

mounted on a nozzle connection that leads to the tank's vapor space. Each vent includes a machined

seat that is closed by a moveable sealing disk (pallet assembly). The pallet assembly is held in its

closed position by weights, springs or buckling pin (depending on the vent style). The amount of

closing force applied determines the set point of the vent. The pressure in the tank's vapor space

pushes against the pallet assembly, in opposition to the closing force. When the tank pressure

reaches the vent set point, the pallet assembly lifts and vapors are allowed to escape from the tank

through the vent. The pressure and/or vacuum in the tank's vapor space is maintained within a safe

range.
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SIZING AND SPECIFICATIONPressure/Vacuum relief vents are available in a range of sizes. Larger size vents provide greater flow

capability than smaller size vents. When choosing a proper size venting device the following

information is significant:

1. THE AMOUNT OF VAPOR/AIR THAT MUST PASS THROUGH THE VENT.

The amount of vapors that must be relieved is usually stated in Standard Cubic Feet of Air per hour

(SCFH). Methods of calculating these volumes for specific normal venting and emergency venting

situations can be found in 29CFR - OSHA 1910.106.

2. THE DESIGN PRESSURE/VACUUM OF THE STORAGE TANK.

Storage tanks are mechanical structures. There are limits as to how much pressure and vacuum they

can withstand before they are damaged. These limits are known as the tank's design pressure andvacuum.

3. ANY OPERATING CHARACTERISTICS OF THE TANK SYSTEM THAT REQUIRE A

SPECIFIED PRESSURE OR VACUUM TO BE MAINTAINED IN THE TANK (MINIMUM

VENT SET POINT).

The relief vent will remain closed until its set pressure is reached. If there is a need to maintain some

pressure in the tank during normal operations, the vent must be set so that it will not open and begin

relieving below that pressure.

4. THE FLOW CAPABILITY OF THE VENT BEING CONSIDERED FOR USE.

Each size and style of vent will flow specific volumes of vapors at a given pressure. These vent flow

capabilities are available from the manufacturer.

The key to sizing a vent for pressure or vacuum relief is to make sure that the vent (with set point)chosen will flow the required amount of vapors at a pressure less than the design pressure of the

tank. This insures that the tank's design pressure or vacuum are never exceeded.


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Although the vent sizing procedure can be done manually, The Protectoseal Company has

automated the calculation and specification process through theProFlow Sizing/Selection

Software.

MATERIALS OF CONSTRUCTION

Protectoseal venting devices are available in a wide range of materials (aluminum, stainless steel,

ductile iron, hastelloy, PVC, FRP, etc.). The material must be compatible with the service

conditions. Improper material choice can lead to contamination of the product being stored or

reduction in the vent's ability to operate safely. Information on the corrosion resistance of materialsunder various service conditions is available in corrosion handbooks and chemical dictionaries.

Understanding / Specifying Flame & Detonation Arresters

DEFINITIONS

Arrester Element -

The portion of a flame arrester or detonation arrester comprised of parallel spaced

plates or crimped metal windings. The element provides the mechanical barrier to

flame passage. The arrester element is mounted in the arrester housing.

Arrester Housing -

The portion of a flame arrester or detonation arrester that houses the arrester

element and that provides the flanged or threaded connection to the pipe/tank

being protected.Flammable Liquid - A liquid having a flashpoint below 100F.

Combustible Liquid - A liquid having a flashpoint at or above 100F.

Confined Deflagration -

A deflagration (see below) propagating in a location where expanding combustion

products are confined. A flame traveling within a pipe may be a confined

deflagration.

Deflagration -A flame front propagating through a flammable gas or vapor at a velocity less

than the speed of sound in that gas or vapor.

Detonation -(Also "Stable Detonation") A flame front propagating through a flammable gas or

vapor at a velocity equal to the speed of sound in that gas or vapor.

Detonation Arrester -An arrester designed to prevent the propagation of unconfined deflagrations,

confined deflagrations, stable detonations and overdriven detonations.

End-of-Line Arrester -

A flame arrester that is mounted at the end of a pipe (flanged or threaded inlet

connection) and which vents directly to the atmosphere. The arrester is designed

to stop unconfined deflagrations.

Explosive Range -The range of values between and including the Lower Explosive Limit (LEL) and

the Upper Explosive Limit (UEL) for any vapor/air mixture.

Flashpoint -The minimum temperature at which a liquid gives off vapor in sufficient

concentration to form an ignitable mixture with air near the surface of the liquid.

Lower Explosive Limit -

(LEL) The lowest volumetric concentration (expressed as a percentage) of

flammable vapor in air that is capable of sustaining and transmitting a flame

throughout the vapor mixture, at a specified temperature and pressure. Mixtures

below the LEL are considered to be too "lean" to burn.

Overdriven Detonation -An unstable flame front that propagates through a flammable gas or vapor at a

speed in excess of the stable detonation velocity.

Stoichiometric Mixture -

The flammable liquid/air mixture where the fuel and oxygen are totally consumed

if the mixture is ignited.

Unconfined Deflagration - A deflagration propagating in a location where the

expanding combustion products are not confined. A vapor cloud ignited in the

open atmosphere is usually an example of an unconfined deflagration.

Upper Explosive Limit -

(UEL) -

The highest volumetric concentration (expressed as a percentage) of flammable

vapor in air that is capable of sustaining and transmitting a flame throughout the

vapor mixture, at a specified temperature and pressure. Mixtures above the UEL

are considered to be too "rich" to burn.
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Vent-Line/In-Line

Arrester -

A flame arrester that may be mounted upstream of a pressure/vacuum relief vent,

or that may be located upstream of a specified maximum length of vent piping to

atmosphere. This arrester is suitable for stopping a confined deflagration that has

propagated through a pipe for some specified maximum distance.

FLAME FRONT GENERATION

If any flammable mixture of vapor or gas comes in contact with an ignition source, a flame front will

develop. This flame will burn through the vapor or gas until:

1. The supply of fuel (vapor or gas) is consumed.

2. The heat necessary to sustain combustion is removed.

3. The oxygen concentration becomes either too high or too low to allow continued burning.

If a flame front is propagating at a speed less than the speed of sound in the vapor, it is known as a

deflagration. A flame front that propagates at a shock wave at the speed of sound in the vapor is known as a

(stable) detonation. An overdriven detonation is a flame front propagating at a speed in excess of the speed

of sound in the vapor. Such an overdriven detonation is a short lived phenomenon and usually occurs as the

flame front is transitioning from a high speed (near the speed of sound) deflagration to a detonation.

A deflagration may develop in the atmosphere as an unconfined deflagration, or in an enclosed area,

typically a piping system, as a confined deflagration. Detonations and overdriven detonations are most

commonly encountered in closed piping systems.

An unconfined deflagration results in relatively low flame speeds and virtually no pressure increase. A

confined deflagration (e.g. - an ignition in a run of pipe) starts at low speed and pressure. As the flame frontpropagates in the pipe, its speed and associated pressure increase. In long or complicated (multiple bends)

pipe runs the flame accelerates until it transitions through an overdriven detonation state into a stable
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detonation. In a 4.3% propane/air mixture the stable detonation velocity is 5800 ft/sec and the associated

pressure is approximately 300-400 PSIG.

Flame arresters and detonation arresters that are designed and tested to withstand and stop these various

categories of flame fronts are available.

FLAMMABLE VAPOR/GAS CLASSIFICATION

Common flammable chemicals have been examined and arranged into groupings on the basis of their

burning and explosion characteristics. In the National Electric Code (NEC) chemicals are categorized in

Group A, B, C or D. Group D contains the least volatile flammable chemicals. Groups C, B and A contain,respectively, chemicals of increased volatility. Similar chemical groupings have been developed by the

International Electrotechnical Commission. Their categories are designated as IIA, IIB, IIC, with IIA

containing the least volatile and IIC containing the most volatile chemicals. In general terms, Group D is

equivalent to Group IIA. Propane/air is a representative Group D (IIA) vapor. Group C is equivalent to

Group IIB. Ethylene/air is a representative Group C (IIB) vapor. Group B is equivalent to Group IIC.

Hydrogen is a representative Group B chemical. Group A contains only acetylene. The classification of the

chemical in the flammable vapor is a significant parameter in the choice of a flame arresting device.

HOW AN ARRESTER FUNCTIONS

Flame arresters and detonation arresters are passive mechanical devices that are mounted to threaded or

flanged connections on a tank or in a process piping system. In normal operation, vapors in the pipe are

directed through the arrester. An arrester consists of a housing and an arrester element.

Arrester elements are available in a number of different configurations (parallel rectangular metal plate,

wound crimped metal, parallel round metal plate). One common feature of all flame arresters is that the

flammable vapor mixture is forced to pass through a series of small openings as it flows through the

arrester. The size of the openings and their length of passage can vary, depending on the arrester style.

If the flammable vapor should ignite, the flame burns towards the arrester/element. As the flame attempts to

pass through the element, it is slowed and cooled by contact with the metal walls of the small passages.

Heat is transferred to the element until combustion cannot be maintained. The flame front is extinguished.
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SIZING AND SPECIFICATION

The primary function of a flame arrester or detonation arrester is to provide protection against an

approaching flame front. In their typical applications, however, they must also allow vapors and/or air to

pass through the openings in their elements so that pressure and vacuum relief may be provided and so that

normal processing of the vapors can be conducted. The resistance to flow through the arresters is based on

their size and configuration. The arrester must be sized to allow the required flow rate at some acceptable

resistance (pressure drop). Although the sizing procedure can be done manually, The Protectoseal Company

has automated the calculation and specification process through the ProFlow Sizing/Selection Software.

The optimum location for the arrester must be determined. End-of-Line flame arresters are mounted on

outlet flanges and they vent directly to atmosphere.Vent-Line/In-Line flame arresters may be installed at

some maximum distance (specified by the manufacturer) from the end of a section of open vent piping.

Detonation arresters are designed so that they may be installed anywhere in a flammable vapors piping

system. Specific information on the restrictions to location of any arrester is available from the

manufacturer.

Flame and detonation arresters are rated for use with chemical vapors of appropriate groups defined by theNational Electric Code (NEC) and the International Electrotechnical Commission (IEC). The suitability of

the arrester for service with a particular vapor group must be verified. The initial pressure and temperature

of the vapors in the system being protected are also significant factors that must be reviewed. The materials

of construction of a flame arrester or detonation arrester must be selected to insure compatibility with the

process vapors being handled. The possibility of corrosion of the arrester components or contamination of

the process materials must be minimized.

APPROVALS AND LISTINGS

The Protectoseal Company has submitted their flame arresters and detonation arresters for inspection and

testing by nationally recognized independent, third party approval agencies. We have been granted

acceptance of our arresters by Underwriters Laboratories, Inc. (UL), Factory Mutual Research (FM), The

United States Coast Guard (USCG), in the United States and by the Federal Institute for Physics andTechnology (PTB), in Germany.

MATERIALS OF CONSTRUCTION

Protectoseal flame arresting devices are available in a wide range of materials (aluminum, stainless steel,

ductile iron, hastelloy, etc.) The material must be compatible with the service conditions. Improper material

choice can lead to contamination of the product being stored or reduction in the flame or detonation

arrester's ability to operate safely. Information on the corrosion resistance of materials under various service

conditions is available in corrosion handbooks and chemical dictionaries.

Tank Blanketing - A Versatile Tool for Fire & Explosion

DEFINITIONS

Blanketing Valve -A device that senses the pressure in the vapor space of a storage tank and controlsthe flow of an inert gas (usually Nitrogen) into the vapor space so that the tank

pressure can be maintained within an acceptable range.

Deadband - The total pressure difference between the blanketing valve opening pressure (or set
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point) and resealing pressure. This applies to the main valve. Some minor leakage

through the pilot will occur above the main valve resealing pressure.

Dome Pressure - In a pilot operated blanketing valve, the pressure in the dome volume.

Dome Volume -In a pilot operated blanketing valve, the chamber between the poppet in the pilot

valve and the piston in the main valve.

Flow Plug -A small cylinder which may installed in the valve to partially block the flow of

inert gas through the valve.Main Valve - The portion of the valve through which the supply gas flows into the storage tank.

Pilot Valve -In a pilot operated valve, the portion of the valve that senses tank pressure and

controls the opening and closing of the main valve.

Poppet -

The component in the valve which moves in response to changes in pressure in the

sensing diaphragm chamber and which, when unseated, allows flow through the

device.

Pressure Balanced

Poppet -

A poppet designed so that the supply pressure will not have an effect on its

opening or closing characteristics. All Protectoseal blanketing valves have pressure

balanced poppets.

Sense Chamber -The space below the diaphragm chamber to which the sense line pressure, from thetank, is directed. The pressure in the sense chamber controls the opening and

closing of the poppet.

Sense Diaphragm -A thin, non-metallic disc in the diaphragm chamber which flexes in response to

changes in the sense line pressure.

Sense Line -A tube running from the tank's vapor space to the sense port of the blanketing

valve. This tube transmits tank pressure to the sense chamber.

Set Point - The pressure at which the main valve opens and flows.

FUNCTION OF A BLANKETING VALVE

A blanketing valve uses a supply of high pressure gas to maintain a blanket of low pressure gas above thestored material in a storage tank. The gas is usually non-flammable and chemically non-reactive when

mixed with the vapors of the stored product. The gas, usually inert Nitrogen, is injected as necessary in

order to maintain a non-flammable atmosphere in the vapor space. The blanketing pressure is usually very

low, less than 1 pound per square inch (PSIG).

Blanketing valves serve several purposes:

Maintain the vapor space of the storage tank within an acceptable pressure range.

Keep the vapors non-flammable by eliminating oxygen-rich air.

Minimize evaporation losses (and product losses).

Reduce product degradation and tank corrosion by keeping contaminants and moisture from

entering the tank.BLANKETING VALVE OPERATION

A blanketing valve is typically mounted on top of a storage tank along

with a pressure/vacuum conservation vent and an emergency pressure

relief vent. Piping from the blanketing gas supply is connected to the

valve inlet, and the valve outlet is piped to the tank. A sense line runs

from a remote location on the tank to the valve's sense port, thus

supplying control pressure for the valve.

The blanketing valve provides primary vacuum relief for the tank. It

opens and supplies gas to the vapor space when pressure decreases to

the valve's set point. When vapor space pressure increases, the valve

reseals. The P/V relief vent (Series No. 8540H) is sized to take care of

overpressure and vacuum conditions brought about by unforeseen

conditions or equipment failures. The pressure setting of the vent is set

at a slightly higher setting than the blanketing pressure in the tank but below the maximum pressure the tank
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can withstand. Similarly, the vacuum pallet is set at a higher vacuum setting than normal operating

conditions bring about and below the maximum vacuum pressure the tank could withstand. Note the

placement of the flame arrester (Series No. 4950) to provide additional protection in the event of inert gas

failure. An emergency relief vent (Series No. 7800) is also placed on the tank, the setting being slightly

above the conservation vent pressure setting.

Pilot Operated Blanketing Valve

A pilot operated blanketing valve (Series No. 20 - Pilot Operated Blanketing Valve) consists of two separate

valves, working in tandem (the main valve and the pilot valve). The main valve inlet connects to the highpressure gas supply source. The valve outlet is piped to the tank vapor space. The piston in the main valve is

held in its closed position by supply line pressure accumulated in the dome volume (the space between the

poppet in the pilot valve and the piston in the main valve). This accumulated pressure is called the dome

pressure.

Opening and closing of the main valve is controlled by the pilot valve. The tank's vapor space pressure is

transmitted, via the sense line, to the diaphragm sense chamber. Decreases in the sensed pressure result in

movement of the pressure balanced poppet in the pilot valve. The poppet unseats and allows gas to flow out

of the dome volume. This results in a reduced pressure in the dome volume and opening of the main valve

piston to allow gas to flow into the tank. Increases in tank pressure cause the poppet to reseal, the dome

pressure to increase and the main valve piston to reseal.

Pilot operated blanketing valves provide very accurate sensing of the

tank pressure and also provide full open flow through the main valve

at a pressure very near to the blanketing valve set point.

Spring Operated Blanketing Valve

A spring operated blanketing valve (Series No. 30 - Spring Operated

Blanketing Valve) functions in a manner similar to a spring loaded

valve. The valve's inlet is connected to the supply gas and the outlet is

connected to the tank. The pressure balanced poppet provides theprimary seal. The tank's vapor space pressure is transmitted, via the

sense line, to the diaphragm sense chamber. Decreases in the sensed

pressure result in movement of the sealing, pressure balanced poppet.

This results in flow through the valve, into the tank. Increases in tank pressure cause the poppet to reseal,

stopping flow into the tank.

Spring operated blanketing valves are often used on smaller tanks

and vessels and in situations where the very small dead band

provided by a pilot operated device is not considered necessary.

SIZING AND SPECIFICATION OF BLANKETING VALVES

Data concerning the flow characteristics of blanketingvalves is available from Protectoseal. This information defines the

maximum flow of gas through the device for a specific supply gas

pressure and a specific set point. This full flow rating through the

valve can be reduced by the use of specially designed flow plugs.

The proper blanketing valve to meet the flow requirements of the tank

system can be determined.

Once the basic valve has been chosen, options that may

enhance or simplify system operations should be reviewed. Among the most common options are:

Optional connections for piping to supply and tank.

Material choice for soft goods (gaskets, o-rings, etc.).

Pressure gauges to accurately record supply and/or sense line pressures

Integral purge system to constantly direct a small volume of supply gas through the outlet and sense

line. This prevents tank vapors from propagating into the valve.

Field test option to allow for checking and changing of set point in the field.
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It is recommended that you contact Protectoseal for full information on the sizing, specification and use of

tank blanketing valves. Fully documented User's Guides and Installation & Maintenance Instructions are

available:

Series No. 20 - Pilot Operated Blanketing Valve User Guide

Series No. 20 - Pilot Operated Blanketing Valve Installation & Maintenance Instructions

Series No. 30 - Spring Operated Blanketing Valve User Guide

Series No. 30 - Spring Operated Blanketing Valve Installation & Maintenance Instructions
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