Vents & Fire Protection System for Storage Tanks

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcosemployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Vessels For additional information on this subject, contactFile Reference: MEX20306 J.H. Thomas on 875-2230

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Determining Acceptability of Vents andFire Protection Systems For Storage Tanks

Engineering Encyclopedia Vessels

Determining Acceptability of Vents and Fire ProtectionSystems for Storage Tanks

Saudi Aramco DeskTop Standards

CONTENTS PAGE

STORAGE TANK VENT REQUIREMENTS.................................................................... 1

Pressure/Vacuum Relieving Considerations ............................................................ 1

Fixed Roof Tanks ......................................................................................... 3

Floating Roof Tanks..................................................................................... 5

Vent Capacity Considerations.................................................................................. 8

Fixed Roof Tanks ......................................................................................... 8

Floating Roof Tanks................................................................................... 10

Vent Testing Requirements.................................................................................... 10

DETERMINE WHETHER CONTRACTOR-SPECIFIED DETAILS FORSTORAGE TANK FIRE-PROTECTION SYSTEMS ARE ACCEPTABLE ................... 11

Types and Applications of Fire-Protection Systems .............................................. 11

Foam........................................................................................................... 11

Deluge and Spray ....................................................................................... 16

Fire-Protection System Selection Criteria.............................................................. 16

Considerations for Installation and Piping Support ............................................... 16

Pertinent Sections of SAES-B-007A, SAES-B-007B,and 32-SAMSS-005 ................................................................................... 16

WORK AID 1: PROCEDURE FOR DETERMINING WHETHERCONTRACTOR-SPECIFIED DETAILS FOR STORAGE TANK FIRE-PROTECTION SYSTEMS ARE ACCEPTABLE ............................................................ 19

GLOSSARY ............ ....................................................................................................... 21



Saudi Aramco DeskTop Standards 1

STORAGE tank VENT Requirements

This section discusses the following topics:

Pressure/vacuum relieving considerations

Vent capacity considerations

Vent testing requirements

Pressure/Vacuum Relieving Considerations

The vapor pressure above the liquid in a storage tank will change whenever liquid enters orleaves the tank. The vapor pressure also will increase rapidly if there is a fire in the vicinityof the tank, and the fire heats the liquid. Vents are installed on storage tanks to relieve anyexcess pressure or vacuum that may cause storage tank failure conditions. Figure 1graphically illustrates the functions that vents perform on a fixed roof storage tank.




Figure 1. Vent Functions on Fixed Roof Storage Tank




Fixed Roof Tanks

Pressure/vacuum relieving devices or vents that are installed on fixed-roof tanks serve as bothsafety devices and conservation devices. Vents permit the entrance of outside air into a tankand allow vapors that are inside the tank to escape. Vapors must be permitted to move intoand out of a tank during normal operation to prevent the occurrence of excessive internalpressure or vacuum conditions. Catastrophic events, such as a fire in or near a tank, canrequire that the vapor move out of the tank more quickly than is required for the normaloperating tank filling rate. Therefore, venting requirements are treated under two categories:normal venting (pressure/vacuum vents) and emergency venting (emergency vents). Thesetwo categories are discussed in a later section of this module.

Pressure/vacuum and emergency vents could consist of appropriately sized pipes that areopen to the atmosphere. However, open pipes result in high evaporative losses from the tank,and lead to environmental pollution, increased fire risk, and product loss. To avoid theselosses, pressure/vacuum vents are designed and sized to open at specified internal or externalpressures and will remain closed unless these set pressures are reached.

Figure 2 is a cutaway illustration of a typical pressure/vacuum vent for a fixed roof tank.Pallets that are located in the vent housing allow air to enter the tank and hydrocarbon vaporsto escape from the tank as the tank "breathes" normally. The pallets open and close to permitonly the amount of intake or outlet relief that is necessary for the tank to remain within thepermissible working pressure. The escape of hydrocarbon vapors from the tank is preventedwhen the pallets are closed.




Figure 2. Typical Pressure/Vacuum Vent for Fixed Roof Tank

Figure 3 is a cutaway illustration of a typical emergency vent for a fixed roof tank. Duringnormal operation, the pallet assembly remains closed and provides a vapor-tight seal. In theevent of a fire or other extreme vaporizing event, excessive internal pressure causes the palletto lift and permits excess vapors to be vented. When the pressure reduces to an acceptablelevel, the pallet automatically closes and reseals.




Figure 3. Typical Emergency Vent

Pressure/vacuum and emergency vents are installed on vertical, flanged nozzle connectionsthat are located on the fixed roof. These vents typically are shipped with protective coversthat are attached to the pallets to prevent excessive movement and potential damage duringshipment. These covers must be removed to permit vent operation. Supplementary palletweights are sometimes necessary for the vent to function at the required pressures. Thesesupplementary pallet weights are often shipped separately and must be installed properly topermit correct vent operation.

Floating Roof Tanks

Floating roof tanks also require vents to prevent excessive pressures. Automatic bleedervents, as illustrated in Figure 4, are fitted to single-deck and double-deck floating roofs.These vents discharge tank vapors and air during initial filling and prevent vacuum formationwhen the tank is being emptied with the roof landed. The vent opens just before the rooflands and closes when the roof rises. The height at which the vent operates is adjustable andcan be set to match the high or low position of the roof legs.




Figure 4. Automatic Bleeder Vent for External Floating Roof




Improper bleeder vent operation can cause severe roof damage. During emptying, the roofcan be damaged if a vacuum is created. Structural damage is less likely during filling, but theroof may tilt. If the roof tilt becomes extreme, the roof can jam against the shell, or liquid canflow on the roof. The additional liquid weight on the roof can cause the roof to sink.

Automatic bleeder vents are installed at a selected number of floating roof support legs. Thevents are evenly spaced over the roof in order to provide uniform venting capability.

Rim vents, as illustrated in Figure 5, are fitted only when metallic seals are used on floatingroofs. These rim vents discharge air or noncondensable gas from the annular vapor space thatis located between the pontoon and the metallic shoe seal. Without these vents, excessivepressure could develop in the annular vapor space beneath the fabric seal and cause damageto the fabric or the metallic shoe. The vents normally open at approximately 0.2 kPa (0.03psi). If the pressure-activated rim vents fail to open, pressure will build up in the annularvapor space. This pressure buildup forces the seal hard against the tank shell, results in rapidwear, and may restrict roof travel.

Rim vents are installed at the periphery of the pontoon. A pipe extends between the vent andthe rim space area that is to be vented.

Figure 5. Rim Vent for External Floating Roof




Vent Capacity Considerations

The number and size of vents must be determined to ensure that there is adequate capacity toprovide the needed pressure and vacuum relief.

Fixed Roof Tanks

Several factors must be considered in the design of a tank vent system for a fixed roof tank:

Is the tank orientation horizontal or vertical? (All tanks that are discussed inthis course are vertical.)

Is the tank above or below ground? (All tanks that are discussed in this courseare above ground.)

What is the tank diameter and height?

What pressure and vacuum will the tank withstand?

What are the diameters of the tank inlet and outlet pipes?

What are the maximum tank filling and emptying rates?

What does the tank store?

- What is the flash point of the contents?

- Are the contents volatile or nonvolatile?

The vent opening or openings, and their connecting pipe are sized to provide adequatecapacity to prevent damage to the tank shell or roof due to excessive internal or externalpressure.

The two circumstances that must be considered in determining the total venting requirementsfor a fixed roof tank are normal operation and emergency conditions. API-2000, VentingAtmospheric and Low-Pressure Storage Tanks (Nonrefrigerated and Refrigerated), providesvent capacity requirements for both normal operation and emergency conditions. Participantsare referred to API-2000 for specific vent capacity requirements. Also refer to SAES-D-100and32-SAMSS-005 for additional venting requirements.




Normal Operation - Storage tank vents are openings whose primary purpose is to permit thetank to "breathe," inhale as well as exhale. The need to inhale, or in-breathing, is caused bynegative pressure or vacuum buildup that results from the following:

Vapor compression due to normal decreases in ambient temperature.

Volumetric displacement due to liquid withdrawal.

The need to exhale, or out-breathing, is caused by positive pressure buildup that results fromthe following:

Vapor expansion and evaporation due to normal increases in ambienttemperature.

Volumetric displacement due to liquid entry.

Vents are sized for the "breathing" cycles that are caused by the normal filling and emptyingoperations of a tank. The following information is required in order to determine the ventcapacity that is needed for normal operation:

Maximum rate of liquid withdrawal from the tank

Tank capacity

Flash point of the stored liquid

Maximum rate of liquid entering the tank

Emergency Conditions - Emergency venting requirements for conditions that might causeexcessive tank internal pressure must also be considered. For instance, a fire in or near thetank causes high vapor evolution, and, therefore, a rapid increase in internal pressure. Thepressure/vacuum vents that are sized to handle normal venting requirements might beinadequate to handle the abnormally large amount of vapor that would be generated by theheat of a fire. The large amount of vapor would cause excessive external pressure.Therefore, storage tanks must have some device or form of construction, such as a frangiblejoint, that will relieve the excessive internal pressure before the tank fails. This device mightbe additional breather valves, self-closing gauge hatches, hinged manhole covers, or largerelief valves. The frangible joint is discussed in MEX 203.03.




API-650 requires that a fixed roof tank either have a frangible joint or have emergencyventing capacity that meets API-2000 requirements. The following information is required inorder to determine the emergency venting capacity that is needed:

Tank diameter

Design vapor pressure in the tank

Floating Roof Tanks

Automatic bleeder vents and rim vents are sized to provide adequate venting capacity for themaximum specified tank filling and emptying rates. This capacity requirement ensures thatthe vent size and number are sufficient for the most severe operating conditions.

Automatic bleeder vents are sized by the tank contractor using an approach that is similar towhat is used for the vents of fixed roof tanks. However, there is no API procedure forcalculating bleeder vent capacity. Capacity calculations are normally not done for rim vents.One or two rim vents are normally provided based on tank size and the past experience of thetank contractor.

Vent Testing Requirements

Vents for fixed roof tanks are typically tested and certified by the manufacturer in accordancewith API-2000 requirements. These requirements specify testing procedures that must befollowed to determine the relieving capacity of pressure/vacuum vents. Curves that providerated vent capacity as a function of tank internal or external pressure are developed by themanufacturer for each vent design and size.

GI 447.002, Pressure Relief Valve Program Authorization for Installation, Deletion andChanges, requires that pressure/vacuum vents be tested to confirm their set pressure andproper operation prior to installation on the tank. Form 3099A, Relief Valve Authorization,must also be completed and a control number must be assigned to each pressure/vacuum ventfor future tracking purposes.

Capacity tests are not done for either automatic bleeder vents or rim vents. Testing of thesevent types typically only involves a check to confirm that they operate freely.




determine WHETHER CONTRACTOR-SPECIFIED DETAILS FOR STORAGETANK Fire-Protection Systems are Acceptable

The following topics are discussed in this section:

Types and applications of fire protection systems

Fire protection system selection criteria

Considerations for installation and piping support

Pertinent sections of SAES-B-007A, SAES-B-007B, and 32-SAMSS-005

Types and Applications of Fire-Protection Systems

This section describes the following two types of fire-protection systems:

Foam

Deluge and spray

Foam

Foam fire-protection systems are designed to contain storage tank fires by smothering a firewhen it occurs in the tank. There are three types of foam fire-protection systems: fixed,semi-fixed, and portable.

Fixed foam fire-protection systems consist of storage tanks for the foam concentrate, foampumps and proportioning system, pipe lines, and foam discharge devices. Fixed foam fire-protection systems require less manpower to operate than either semi-fixed or portable fire-protection systems. Therefore, fixed foam fire-protection systems are preferred in locationswhere available manpower is limited.

Semi-fixed foam fire-protection systems usually consist of fixed discharge devices and relyon mobile foam-proportioning trucks or trailer-mounted units to produce the foam solution.A backup truck is used to replenish the foam concentrate as required. Figure 6 is a schematicdrawing of the main piping for a semi-fixed fire-protection system. The foam lateral startsoutside the dike and runs to the base of the tank.




Figure 6. Piping for a Semi-Fixed Fire-Protection System

Portable foam fire-protection systems can be a combination of mobile and hand-transportableequipment. The foam discharge devices can be foam towers, monitor nozzles (either trailer-mounted or hand-carried), and hose line play pipes or foam nozzles.

The semi-fixed system is the most commonly used type of foam fire-protection system forboth fixed roof and floating roof tanks. For fixed roof tanks, an intermediate back pressurefoam maker is located near the base of the tank. The expanded or aerated foam is piped upthe outside of the tank through foam riser piping that is attached to the foam lateral. The foamis then piped to a foam chamber that is mounted on top of the tank shell. The foam then flowsto a deflector or foam pourer that discharges the foam down the inside of the shell. The foamthen spreads across the surface of the liquid.

Figure 7 illustrates a typical foam distribution system for a fixed roof tank. The top of thefoam riser is closed by a glass diaphragm to prevent entry of any hydrocarbon, rainwater, ordirt that could block the riser pipe. The diaphragm is broken by the first flow of the foamsolution.

Based on SAES-B-007B, Air Foam Systems for Storage Tanks, it is unlikely that fixed roof orcovered floating roof tanks would be used to store crude or low-flash point flammablematerials. Therefore, an air-foam system would not be necessary. However, if these tanktypes are used for such storage, the foam system requirements must be approved by the ChiefFire Prevention Engineer.




Figure 7. Fixed Roof Tank Foam Distribution System




For floating roof tanks, a low-pressure foam-maker is located near the top of the tank. Afloating roof tank needs a lower pressure foam-maker than that required for a fixed roof tankbecause only the seal area around the roof periphery requires foam coverage. The foam thenflows through a foam chamber to a foam deflector and screen, where the foam is directed intothe periphery of the floating roof. The foam chamber is mounted to a backboard that extendsabove the top of the tank shell. This elevated attachment ensures that the foam discharge willbe above the highest roof elevation and will not interfere with roof travel. A continuous steelfoam dam is welded to the top of the roof pontoon and to keep the foam from spreading awayfrom the seal area.

Figure 8 illustrates a typical foam distribution system for a floating roof tank. For tanks thatare less than 76 m (250 ft.) in diameter, a single foam outlet that is located near the topplatform is usually sufficient. For tanks that are over 76 m (250 ft.) in diameter, multiplefoam outlets are located around the tank circumference at approximately 24 m (80 ft.)intervals.




Figure 8. Floating Roof Tank Foam Distribution System




Deluge and Spray

Deluge and spray fire-protection systems are designed to protect tanks from fires by keepingthe tank temperature below the point at which the contents of the tank are likely to ignite.These systems protect a tank that is not on fire by reducing the risk of ignition that could becaused by the heat generated from a nearby tank or other equipment that is on fire. A waterdeluge system consists of permanently mounted piping that provides water to a circularheader or to a spray head that is mounted on top of a storage tank. When the system isactivated, the water cascades down the top and sides of the tank and provides cooling.

Fire-Protection System Selection Criteria

Work Aid 1 provides a procedure to determine if the fire-protection system that is specifiedfor a storage tank is acceptable. This procedure is based on requirements that are contained inthe following Saudi Aramco Standards:

SAES-B-007A, Firewater System Design

SAES-B-007B, Air Foam Systems for Storage Tanks

32-SAMSS-005, Atmospheric Steel Tanks

Considerations for Installation and Piping Support

The foam riser piping should be rigidly braced to the shell at every shell course onapproximately 1.8 m to 2.4 m (6 ft. to 8 ft.) centers. In the event of a fire, this bracing helpsto prevent the shell from buckling inward for a sufficient period of time to allow the foam tobe applied.

SAES-B-007B contains additional design and installation requirements for air-foam systemsand associated components. Topics which are covered and appropriate paragraph referencesare contained in Work Aid 1.

Pertinent Sections of SAES-B-007A, SAES-B-007B, and 32-SAMSS-005

SAES-B-007A, Firewater System Design, provides the minimum design requirements for fireprotection water systems in oil/gas handling plants and related in-plant facilities. Therefore,this SAES specifies general firewater system requirements that the primary contractor mustimplement. Requirements that are specifically for storage tanks relate to water flow rates forall storage tank types, and to deluge system requirements for refrigerated dome roof tanks.Other items that are contained in SAES-B-007A relate to general system requirements andrequirements for other equipment types.




SAES-B-007B, Air Foam Systems for Storage Tanks, provides design and installationrequirements for air foam fire protection systems for large atmospheric storage tanks.

32-SAMSS-005, Atmospheric Steel Tanks, states that, for external floating roof tanks, foamsystem requirements will be specified on the Tank Data Sheet or in the purchase order.Nothing is stated regarding fixed roof or internal floating roof tanks. Therefore, thecontractor must determine the foam system requirements based on SAES-B-007B and specifythese to the tank supplier.

Figure 9 summarizes where requirements that relate to fire protection systems are foundwithin the referenced standards. Work Aid 1 contains a procedure for the evaluation ofcontractor-specified fire protection systems, in conjunction with the information that iscontained in these standards.

Topic SAES-B-007A SAES-B-007B 32-SAMSS-005

Water Flow Rates Para. 4.4 -- --

Firewater Monitors Para. 11 -- --

Water DelugeSystem

Para. 12.6 -- --

Foam Systems forFixed Cone RoofTanks

-- Para. 4.1 --

Foam Systems forCovered FloatingRoof Tanks

-- Para. 5.1 --

Foam Systems forExternal FloatingRoof Tanks

-- Para. 6.0 Appendix C

Figure 9. Locations of Fire Protection System Requirements




Summary




Work Aid 1: PROCEDURE for Determining WHETHER CONTRACTOR-SPECIFIED DETAILS FOR STORAGE TANK Fire-ProtectionSystems are Acceptable

Use the copies of SAES-B-007A and SAES-B-007B that are contained in Course Handout 2as specified in this Work Aid.

32-SAMSS-005 does not contain specific requirements for fire-protection systems. It simplystates that foam system requirements for external floating roof tanks are to be specified ifrequired. Foam system requirements are specified by SAES-B-007B for tanks that storecrude oil or low flash stocks, or higher flash stocks that are stored at temperatures above orwithin 8C (15F) of their flash point.

1. If a fixed cone roof tank or an internal floating roof tank is used to store crude oil orlow flash stocks, confirm that the air foam system design has been approved by theChief Fire Prevention Engineer.

2. Verify that open-top, floating roof tanks that are over 36 m (120 ft.) in diameter andthat store crude oil or low flash stocks have air foam systems installed as follows:

For tanks up to 76 m (250 ft.) in diameter with a wind girder equipped with ahandrail, a foam dam shall be installed on the roof and a single foam outletstation with two hose connections shall be located at the top of the tank topplatform. The hose connections provide solution outlets for the use of hose andfoam nozzles for coverage of the dam area away from the platform.

For tanks up to 76 m (250 ft.) in diameter without a wind girder equipped witha handrail, and for tanks over 76 m (250 ft.) in diameter, a foam dam at least600 mm (24 in.) high shall be provided. In addition, multiple fixed foamchambers and deflectors located at intervals not to exceed 24 m (80 ft.) must beequally spaced around the entire tank shell.

3. Verify that the foam system installation meets the requirements of SAES-B-007B, AirFoam Systems for Storage Tanks. Specific items to check and appropriate paragraphreferences within SAES-B-007B are as follows:

Foam riser, lateral and distribution pipe sizes in accordance with Paras. 6.1.1and 6.1.2.1.

Foam makers, foam chambers, and deflectors in accordance with Paras. 6.2 and6.2.1.




Laterals shall run independently from each tank and terminate at a road on oroutside the dike for the tank involved. Additional requirements for laterals andhose connections in accordance with Paras. 6.3.1 through 6.3.8.

Foam backboards and dams in accordance with Paras. 6.4 and 6.4.1.

Ring supply piping installation in accordance with Para. 6.5.

Wind girder handrail installation in accordance with standard drawing AD-036211.

Foam system performance testing in accordance with Para. 6.6.

The top stair platform shall be oriented upwind.

4. Verify that firewater system flow rate capabilities in the area of cone roof and floatingroof atmospheric storage tanks are in accordance with Paras. 4.4.1 through 4.4.4 ofSAES-B-007A.

5. Verify that any tank that is located within one tank diameter of the dike or spillagearea of a refrigerated dome roof tank is provided with adequate cooling.

If the exposed tank is also a refrigerated tank, verify that the cooling isprovided by a top deluge system.

If the exposed tank is a nonrefrigerated tank, verify that the cooling is providedby hose streams on the upper half of the shell.

Refer to Para. 4.4.5 of SAES-B-007A for required water flow rates.




GLOSSARY

annular space The space between the outside diameter of a floating roofpontoon and the metallic shoe seal.

breathe The natural process of allowing air into and vapors out of astorage tank, required because of tank emptying and filling,and changes in ambient temperature.

breather valves Valves that allow vapor out of a tank when the pressure insidebuilds above a preset level and that allow air into a tank whenthe vacuum inside builds above a preset level.

deluge and spray fire-protection system

A system to keep a tank cool and reduce the risk of it catchingfire when an adjacent tank is on fire.

emergency vents Vents designed to remain closed during normal operations andto open only when pressure safety limits for the tank are beingexceeded due to a fire in the vicinity of the tank.

fire water monitor A swivel-mounted, counterweighted nozzle mounted on astable base, designed so that one person can easily control anddirect the vertical and lateral attitude of the nozzle, andtherefore the direction of the stream from the nozzle.

floating roof A roof that floats on the surface of the stored liquid. Knownas an internal floating roof if the tank also has a fixed roof atthe top of the shell. Known as an external floating roof or justfloating roof if there is no fixed roof on the tank.

flammable liquid Liquid having a flash point equal to or below 54C (130F).

flammable material Flammable liquid, hydrocarbon vapor, or other vapors, such ashydrogen or carbon disulfide, that are readily ignitable whenreleased to the atmosphere.

flash point The lowest temperature at which a flammable liquid gives offenough vapor to form a flammable or ignitable mixture withair near the surface of the liquid or within the vessel beingused.

foam fire-protection system A system to extinguish a fire by smothering it with foam thatprevents oxygen from reaching the flame.




in-breathe The process of allowing air into a storage tank through vents.

low flash stock Liquids with flash points of 55C (130F) or below, such asnaphtha and gasoline.

out-breathe The process of allowing vapors out of a storage tank throughvents.

pallets Flappers or other physical structures inside a vent that open toallow passage of gases.

pressure/vacuum relievingdevices

Vents and other devices that allow air into and vapor out of astorage tank as a part of the normal operation of the tank.

volatile Quickly and easily evaporated at ordinary temperatures.

Vents & Fire Protection System for Storage Tanks

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