Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not
already 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, Engineering Services, Saudi Aramco.
Chapter : Vessels For additional information on this subject, contact
Determining Acceptability of Vents and
Fire Protection Systems For Storage Tanks
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 FOR STORAGE TANK FIRE-PROTECTION SYSTEMS ARE ACCEPTABLE ... 11
Types and Applications of Fire-Protection Systems ... 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 WHETHER CONTRACTOR-SPECIFIED DETAILS FOR STORAGE TANK FIRE-PROTECTION SYSTEMS ARE ACCEPTABLE ... 19
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 or leaves the tank. The vapor pressure also will increase rapidly if there is a fire in the vicinity of the tank, and the fire heats the liquid. Vents are installed on storage tanks to relieve any excess pressure or vacuum that may cause storage tank failure conditions. Figure 1 graphically illustrates the functions that vents perform on a fixed roof storage tank.
Fixed Roof Tanks
Pressure/vacuum relieving devices or vents that are installed on fixed-roof tanks serve as both safety devices and conservation devices. Vents permit the entrance of outside air into a tank and allow vapors that are inside the tank to escape. Vapors must be permitted to move into and out of a tank during normal operation to prevent the occurrence of excessive internal pressure or vacuum conditions. Catastrophic events, such as a fire in or near a tank, can require that the vapor move out of the tank more quickly than is required for the normal operating tank filling rate. Therefore, venting requirements are treated under two categories: normal venting (pressure/vacuum vents) and emergency venting (emergency vents). These two categories are discussed in a later section of this module.
Pressure/vacuum and emergency vents could consist of appropriately sized pipes that are open 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 these losses, pressure/vacuum vents are designed and sized to open at specified internal or external pressures 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 vapors to escape from the tank as the tank "breathes" normally. The pallets open and close to permit only the amount of intake or outlet relief that is necessary for the tank to remain within the permissible working pressure. The escape of hydrocarbon vapors from the tank is prevented when 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. During normal operation, the pallet assembly remains closed and provides a vapor-tight seal. In the event of a fire or other extreme vaporizing event, excessive internal pressure causes the pallet to lift and permits excess vapors to be vented. When the pressure reduces to an acceptable level, the pallet automatically closes and reseals.
Figure 3. Typical Emergency Vent
Pressure/vacuum and emergency vents are installed on vertical, flanged nozzle connections that are located on the fixed roof. These vents typically are shipped with protective covers that are attached to the pallets to prevent excessive movement and potential damage during shipment. These covers must be removed to permit vent operation. Supplementary pallet weights are sometimes necessary for the vent to function at the required pressures. These supplementary pallet weights are often shipped separately and must be installed properly to permit correct vent operation.
Floating Roof Tanks
Floating roof tanks also require vents to prevent excessive pressures. Automatic bleeder vents, 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 formation when the tank is being emptied with the roof landed. The vent opens just before the roof lands and closes when the roof rises. The height at which the vent operates is adjustable and can be set to match the high or low position of the roof legs.
Improper bleeder vent operation can cause severe roof damage. During emptying, the roof can be damaged if a vacuum is created. Structural damage is less likely during filling, but the roof may tilt. If the roof tilt becomes extreme, the roof can jam against the shell, or liquid can flow 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. The vents 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 floating roofs. These rim vents discharge air or noncondensable gas from the annular vapor space that is located between the pontoon and the metallic shoe seal. Without these vents, excessive pressure could develop in the annular vapor space beneath the fabric seal and cause damage to the fabric or the metallic shoe. The vents normally open at approximately 0.2 kPa (0.03 psi). If the pressure-activated rim vents fail to open, pressure will build up in the annular vapor space. This pressure buildup forces the seal hard against the tank shell, results in rapid wear, and may restrict roof travel.
Rim vents are installed at the periphery of the pontoon. A pipe extends between the vent and the rim space area that is to be vented.
Vent Capacity Considerations
The number and size of vents must be determined to ensure that there is adequate capacity to provide 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 in
this course are vertical.)
• Is the tank above or below ground? (All tanks that are discussed in this course are 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 adequate capacity to prevent damage to the tank shell or roof due to excessive internal or external pressure.
The two circumstances that must be considered in determining the total venting requirements for a fixed roof tank are normal operation and emergency conditions. API-2000, Venting Atmospheric and Low-Pressure Storage Tanks (Nonrefrigerated and Refrigerated), provides vent capacity requirements for both normal operation and emergency conditions. Participants are referred to API-2000 for specific vent capacity requirements. Also refer to SAES-D-100 and
Normal Operation - Storage tank vents are openings whose primary purpose is to permit the
tank to "breathe," inhale as well as exhale. The need to inhale, or in-breathing, is caused by negative 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 from the following:
• Vapor expansion and evaporation due to normal increases in ambient temperature.
• Volumetric displacement due to liquid entry.
Vents are sized for the "breathing" cycles that are caused by the normal filling and emptying operations of a tank. The following information is required in order to determine the vent capacity 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 cause excessive tank internal pressure must also be considered. For instance, a fire in or near the tank causes high vapor evolution, and, therefore, a rapid increase in internal pressure. The pressure/vacuum vents that are sized to handle normal venting requirements might be inadequate to handle the abnormally large amount of vapor that would be generated by the heat 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 frangible joint, that will relieve the excessive internal pressure before the tank fails. This device might be additional breather valves, self-closing gauge hatches, hinged manhole covers, or large relief 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 emergency venting capacity that meets API-2000 requirements. The following information is required in order 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 the maximum specified tank filling and emptying rates. This capacity requirement ensures that the 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 to what is used for the vents of fixed roof tanks. However, there is no API procedure for calculating 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 the tank contractor.
Vent Testing Requirements
Vents for fixed roof tanks are typically tested and certified by the manufacturer in accordance with API-2000 requirements. These requirements specify testing procedures that must be followed to determine the relieving capacity of pressure/vacuum vents. Curves that provide rated vent capacity as a function of tank internal or external pressure are developed by the manufacturer for each vent design and size.
GI 447.002, Pressure Relief Valve Program Authorization for Installation, Deletion and Changes, requires that pressure/vacuum vents be tested to confirm their set pressure and proper 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 vent for future tracking purposes.
Capacity tests are not done for either automatic bleeder vents or rim vents. Testing of these vent types typically only involves a check to confirm that they operate freely.
determine WHETHER CONTRACTOR-SPECIFIED DETAILS FOR STORAGE TANK 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:
• Deluge and spray Foam
Foam fire-protection systems are designed to contain storage tank fires by smothering a fire when 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, foam pumps and proportioning system, pipe lines, and foam discharge devices. Fixed foam protection systems require less manpower to operate than either semi-fixed or portable fire-protection systems. Therefore, fixed foam fire-fire-protection systems are preferred in locations where available manpower is limited.
Semi-fixed foam fire-protection systems usually consist of fixed discharge devices and rely on 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 schematic drawing of the main piping for a semi-fixed fire-protection system. The foam lateral starts outside 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-transportable equipment. 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 for both fixed roof and floating roof tanks. For fixed roof tanks, an intermediate back pressure foam maker is located near the base of the tank. The expanded or aerated foam is piped up the outside of the tank through foam riser piping that is attached to the foam lateral. The foam is then piped to a foam chamber that is mounted on top of the tank shell. The foam then flows to a deflector or foam pourer that discharges the foam down the inside of the shell. The foam then spreads across the surface of the liquid.
Figure 7 illustrates a typical foam distribution system for a fixed roof tank. The top of the foam riser is closed by a glass diaphragm to prevent entry of any hydrocarbon, rainwater, or dirt that could block the riser pipe. The diaphragm is broken by the first flow of the foam solution.
Based on SAES-B-007B, Air Foam Systems for Storage Tanks, it is unlikely that fixed roof or covered floating roof tanks would be used to store crude or low-flash point flammable materials. Therefore, an air-foam system would not be necessary. However, if these tank types are used for such storage, the foam system requirements must be approved by the Chief Fire Prevention Engineer.
For floating roof tanks, a low-pressure foam-maker is located near the top of the tank. A floating roof tank needs a lower pressure foam-maker than that required for a fixed roof tank because only the seal area around the roof periphery requires foam coverage. The foam then flows through a foam chamber to a foam deflector and screen, where the foam is directed into the periphery of the floating roof. The foam chamber is mounted to a backboard that extends above the top of the tank shell. This elevated attachment ensures that the foam discharge will be above the highest roof elevation and will not interfere with roof travel. A continuous steel foam dam is welded to the top of the roof pontoon and to keep the foam from spreading away from the seal area.
Figure 8 illustrates a typical foam distribution system for a floating roof tank. For tanks that are less than 76 m (250 ft.) in diameter, a single foam outlet that is located near the top platform is usually sufficient. For tanks that are over 76 m (250 ft.) in diameter, multiple foam outlets are located around the tank circumference at approximately 24 m (80 ft.) intervals.
Deluge and Spray
Deluge and spray fire-protection systems are designed to protect tanks from fires by keeping the 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 be caused by the heat generated from a nearby tank or other equipment that is on fire. A water deluge system consists of permanently mounted piping that provides water to a circular header or to a spray head that is mounted on top of a storage tank. When the system is activated, 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 specified for a storage tank is acceptable. This procedure is based on requirements that are contained in the 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 on approximately 1.8 m to 2.4 m (6 ft. to 8 ft.) centers. In the event of a fire, this bracing helps to prevent the shell from buckling inward for a sufficient period of time to allow the foam to be applied.
SAES-B-007B contains additional design and installation requirements for air-foam systems and associated components. Topics which are covered and appropriate paragraph references are 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 fire protection water systems in oil/gas handling plants and related in-plant facilities. Therefore, this SAES specifies general firewater system requirements that the primary contractor must implement. Requirements that are specifically for storage tanks relate to water flow rates for all 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 and requirements for other equipment types.
SAES-B-007B, Air Foam Systems for Storage Tanks, provides design and installation requirements for air foam fire protection systems for large atmospheric storage tanks.
32-SAMSS-005, Atmospheric Steel Tanks, states that, for external floating roof tanks, foam system 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, the contractor must determine the foam system requirements based on SAES-B-007B and specify these to the tank supplier.
Figure 9 summarizes where requirements that relate to fire protection systems are found within the referenced standards. Work Aid 1 contains a procedure for the evaluation of contractor-specified fire protection systems, in conjunction with the information that is contained in these standards.
Topic SAES-B-007A SAES-B-007B 32-SAMSS-005
Water Flow Rates Para. 4.4 --
--Firewater Monitors Para. 11 --
--Water Deluge System
Para. 12.6 --
--Foam Systems for Fixed Cone Roof Tanks
-- Para. 4.1
--Foam Systems for Covered Floating Roof Tanks
-- Para. 5.1
--Foam Systems for External Floating Roof Tanks
-- Para. 6.0 Appendix C
Work Aid 1: PROCEDURE for Determining WHETHER CONTRACTOR-SPECIFIED DETAILS FOR STORAGE TANK Fire-Protection Systems are Acceptable
Use the copies of SAES-B-007A and SAES-B-007B that are contained in Course Handout 2 as specified in this Work Aid.
32-SAMSS-005 does not contain specific requirements for fire-protection systems. It simply states that foam system requirements for external floating roof tanks are to be specified if required. Foam system requirements are specified by SAES-B-007B for tanks that store crude oil or low flash stocks, or higher flash stocks that are stored at temperatures above or within 8°C (15°F) of their flash point.
1. If a fixed cone roof tank or an internal floating roof tank is used to store crude oil or low flash stocks, confirm that the air foam system design has been approved by the Chief Fire Prevention Engineer.
2. Verify that open-top, floating roof tanks that are over 36 m (120 ft.) in diameter and that 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 a
handrail, a foam dam shall be installed on the roof and a single foam outlet station with two hose connections shall be located at the top of the tank top platform. The hose connections provide solution outlets for the use of hose and foam 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 with a handrail, and for tanks over 76 m (250 ft.) in diameter, a foam dam at least 600 mm (24 in.) high shall be provided. In addition, multiple fixed foam chambers and deflectors located at intervals not to exceed 24 m (80 ft.) must be equally spaced around the entire tank shell.
3. Verify that the foam system installation meets the requirements of SAES-B-007B, Air Foam Systems for Storage Tanks. Specific items to check and appropriate paragraph references within SAES-B-007B are as follows:
• Foam riser, lateral and distribution pipe sizes in accordance with Paras. 6.1.1 and 22.214.171.124.
• Foam makers, foam chambers, and deflectors in accordance with Paras. 6.2 and 6.2.1.
• Laterals shall run independently from each tank and terminate at a road on or outside the dike for the tank involved. Additional requirements for laterals and hose 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 floating roof atmospheric storage tanks are in accordance with Paras. 4.4.1 through 4.4.4 of SAES-B-007A.
5. Verify that any tank that is located within one tank diameter of the dike or spillage area of a refrigerated dome roof tank is provided with adequate cooling.
• If the exposed tank is also a refrigerated tank, verify that the cooling is provided by a top deluge system.
• If the exposed tank is a nonrefrigerated tank, verify that the cooling is provided by hose streams on the upper half of the shell.
annular space The space between the outside diameter of a floating roof pontoon and the metallic shoe seal.
breathe The natural process of allowing air into and vapors out of a storage 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 inside builds above a preset level and that allow air into a tank when the 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 and to open only when pressure safety limits for the tank are being exceeded due to a fire in the vicinity of the tank.
fire water monitor A swivel-mounted, counterweighted nozzle mounted on a stable base, designed so that one person can easily control and direct the vertical and lateral attitude of the nozzle, and therefore the direction of the stream from the nozzle.
floating roof A roof that floats on the surface of the stored liquid. Known as an internal floating roof if the tank also has a fixed roof at the top of the shell. Known as an external floating roof or just floating roof if there is no fixed roof on the tank.
flammable liquid Liquid having a flash point equal to or below 54°C (130°F).
flammable material Flammable liquid, hydrocarbon vapor, or other vapors, such as hydrogen or carbon disulfide, that are readily ignitable when released to the atmosphere.
flash point The lowest temperature at which a flammable liquid gives off enough vapor to form a flammable or ignitable mixture with air near the surface of the liquid or within the vessel being used.
foam fire-protection system A system to extinguish a fire by smothering it with foam that
in-breathe The process of allowing air into a storage tank through vents.
low flash stock Liquids with flash points of 55°C (130°F) or below, such as naphtha and gasoline.
out-breathe The process of allowing vapors out of a storage tank through vents.
pallets Flappers or other physical structures inside a vent that open to allow passage of gases.
pressure/vacuum relieving devices
Vents and other devices that allow air into and vapor out of a storage tank as a part of the normal operation of the tank.