Relief valves and relief systems

A relief system is an emergency system for discharging gas during abnormal conditions, by manual or controlled means or by an automatic pressure relief valve from a pressurized vessel or piping system, to the atmosphere to relieve pressures in excess of the maximum allowable working pressure (MAWP).

Components

The relief system may include:

• The relief device
• The collection piping
• Flashback protection
• A gas outlet

A scrubbing vessel should be provided for liquid separation if liquid hydrocarbons are anticipated. The relief-system outlet may be either vented or flared. If designed properly, vent or flare emergency-relief systems from pressure vessels may be combined.

Some facilities include systems for depressuring pressure vessels in the event of an emergency shutdown. The depressuring-system control valves may be arranged to discharge into the vent, flare, or relief systems. The possibility of freezing and hydrate formation during high-pressure releases to the atmosphere should be considered.

Design considerations

There are three main engineering considerations when designing or modifying a relief system:

• Determining the relief requirements of individual pieces of equipment and selecting the appropriate devices to handle the imposed loads.
• Designing a relief header system that will handle the imposed loads or expansion modifications.
• Defining reasonable total relief loads for the combined relief header or disposal system and designing an appropriate disposal system with minimum adverse impact to personnel safety, plant-process system integrity, and the environment.

These considerations are interrelated in such a way that makes it impossible to establish a procedural guideline that would be valid for most cases. The design of one portion of a relief system must be considered in light of its effects on the relief system.

Relief device selection

Determining individual relief loads

There are a number of industry codes, standards, and recommended practices that provide guidance in the sizing, selection, and installation of relief devices and systems. The American Soc. of Mechanical Engineers (ASME) Pressure Vessel Code, Sec. VIII, Division 1, paragraph UG-127, lists the relief-valve code requirements. RP 520, Part 1, provides an overview of the types of relief devices, causes of overpressure, relief-load determination, and procedures for selecting and sizing relief devices. RP 520, Part 2, provides guidance on the installation of relief devices, and RP 521 provides guidance on the selection and design of disposal systems.

Causes of overpressure

The most common causes of overpressure in upstream operations are blocked discharge, gas blowby, and fire. When the worst-case relief load is caused by a control valve failing to open (blocked discharge), the relief device should be sized with full-sized trim in the control valve, even if the actual valve has reduced trim. When the worst-case relief load is caused by gas blowby, the relief device should be sized with full-sized trim in the smallest valve in the liquid-outlet line, even if the actual valve has reduced trim. Many vessels are insulated for energy savings. Thermal insulation limits the heat absorption from fire exposure as long as it is intact. It is essential that effective weather protection be provided so that insulation will not be removed by high-velocity fire-hose streams.

Types of pressure relief devices

The two primary types of relief devices are the relief valve and rupture disk.

Relief valves

The three basic types of pressure-relief valves are conventional spring loaded, balanced spring loaded, and the pilot operated.

• Conventional spring loaded. In the conventional spring-loaded valve (Fig. 1), the bonnet, spring, and guide are exposed to the released fluids. If the bonnet is vented to the atmosphere, relief-system backpressure decreases the set pressure. If the bonnet is vented internally to the outlet, relief-system backpressure increases the set pressure. The conventional spring-loaded valve is used in noncorrosive services and where backpressure is less than 10% of the set point.
• Balanced spring-loaded. The balanced spring-loaded valve incorporates a means to protect the bonnet, spring, and guide from the released fluids and minimizes the effects of backpressure. The disk area vented to the atmosphere is exactly equal to the disk area exposed to backpressure. These valves can be used in corrosive or dirty service and with variable backpressure.
• Pilot operated. The pilot-operated valve is combined with and controlled by an auxiliary pressure pilot. The resistance force on the piston in the main valve is assisted by the process pressure through an orifice. The net seating force on the piston actually increases as the process pressure nears the set point.

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  Fig. 1—Conventional spring-loaded pressure-relief value.

Rupture disk devices

The rupture-disk device is a nonreclosing differential-pressure device actuated by inlet static pressure. The rupture disk is designed to burst at set inlet pressure. The device includes a rupture disk and a disk holder. The rupture disk may be used alone, in parallel with, or in conjunction with pressure-relief valves. They are manufactured in a variety of materials with various coatings for corrosion resistance.

Relief system considerations

The entire relief system must be considered before selecting the appropriate relief device. The relief headers should be designed to minimize pressure drop, thus allowing for future expansion and additional relief loads.

• Conventional spring-loaded-relief-valve considerations. Conventional valves require the relief header backpressure (superimposed plus built up) to be less than 10% of the set pressure of the lowest-set relief valve tied into the header.
• Balanced-spring-loaded-valve considerations. Balanced spring-loaded valves allow the use of smaller relief headers because of the larger pressure drops allowed, under maximum relief-flow conditions, as a result of higher allowable backpressure (40%). Balanced valves and relief headers are designed as a system to operate at a higher backpressure. The balanced valve is more expensive than conventional valves; however, the total cost of the use of balanced valves plus the smaller header system may be lower. Capacity is reduced at the larger backpressure, so it may not be the solution for all backpressure problems. In the bellows model, the bellows is a flexible pressure vessel that has a maximum backpressure limit that is lower in larger valve sizes. Bellows are available in a limited number of materials and may deteriorate rapidly under certain exposure conditions. Bellows should be checked periodically for leakage. A leaking bellow does not provide backpressure compensation, and it allows the relief header to leak to the atmosphere. The balanced valve commonly is used to tie a new low-pressure-relief load into an existing heavily loaded relief header or to protect the relief-valve top works from corrosive gases in the relief header.
• Pilot-operated-valve considerations. Pilot-operated valves should be considered for all clean services within their temperature limitations. They are well suited for pressures below 15 psig and are available with the pilot-pressure sensing line connected to either the valve inlet or to a different point. Pilot-operated valves provide tight shutoff with very narrow margins between operating pressure and set pressure.

Special considerations

When selecting the appropriate relief devices to handle the imposed loads, several issues must be considered.

Set pressure

Relief devices are normally set to relieve at the MAWP. The greater the margin between the set pressure and the operating pressure, the less likelihood there is of leakage. Aside from the requirements to compensate for superimposed backpressure, there is no reason to set a relief device at less than the MAWP.

Backpressure

The backpressure at the outlet of every relief device should be such that the device can handle its design capacity with the calculated backpressure under the design relief conditions.

Dual relief valves

It is common practice to install two relief valves in critical process applications where a shutdown cannot be tolerated. The intent is that if the first relief valve lifts and fails to reseat, a second relief can be switched into service before the first valve is removed for maintenance, without shutting down or jeopardizing the process. This is accomplished by piping the relief valves in parallel and by putting a "car sealed" full-port ball or gate block valve on the inlet and outlet of each relief valve. One set of block valves is sealed open and the other sealed closed. ASME-approved selector valves are available, which simplify relief-valve switching. This provides an interlock of parallel inlet and outlet block valves and ensures full protection for the process equipment.

Multiple relief valves

Multiple relief valves are required when the relief load exceeds the capacity of the largest available relief valve. It is good practice to install multiple relief valves for varying loads to minimize chattering on small discharges. ASME Sec. VIII, Division 1, 3 and RP 520, Part 1, both stipulate a 10% accumulation above the MAWP for a single relief valve and a 16% accumulation above the MAWP for multiple relief valves. The primary relief valve must be set at or below the MAWP. Supplemental relief valves should have staged pressures. The highest pressure may be set no higher than 105% above the MAWP. If different-sized relief valves are used, the smallest relief valve should be set to the lowest pressure.

Sizing the relief device

The most difficult factors for specifying a relief device are determining the limiting cause of pressure relief, determining the relief load and properties of the discharge fluid, and selecting the proper relief device. When the loads are known, the sizing steps are straightforward. RP 520, Part 1, provides formulas for determining the relief-valve orifice area for vapor, liquid, and steam relief.Fig. 2 shows standard orifices available by letter designation, orifice area, and body size. The size of a relief valve should be checked for the following conditions.

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  Fig. 2—Pressure-relief value orifice designations.

Blocked discharge

One design condition for the sizing of a relief valve is to assume that it must handle the total design flow rate (gas plus liquid) into the component. It is possible to isolate a process component or piping segment for maintenance by blocking all inlets and outlets. On startup, all outlet valves could be left closed inadvertently. If the inlet source can be at a higher pressure than the MAWP of the process component, only a properly sized relief valve could keep the process component from rupturing as a result of overpressure.

Gas blowby

On tanks and low-pressure vessels normally receiving liquids from higher-pressure upstream vessels, the maximum flow rate through the relief valve often is determined by gas blowby. This situation occurs when the level controller or level control valve of the upstream vessel fails in the open position or a drain valve from an upstream vessel fails in the open position, allowing liquid and/or gas to flow into the component evaluated. Under blowby conditions, both the normal liquid and gas outlets on the component being evaluated are functioning properly. However, the gas flow into the component could greatly exceed the capacity of the normal gas outlet. This excess gas flow must be handled by the relief valve to keep from exceeding the component’s MAWP. Gas-blowby conditions also can occur when a pressure regulator feeding a component fails in the open position, creating a higher than designed inlet flow rate of gas.

Gas-blowby rate is the maximum that can flow given the pressure drop between the upstream component and the component being evaluated. In computing the maximum rate that can flow because of pressure drop, consideration should be given to the effects of control valves, chokes, and other restricted orifices in the line. A more conservative approach would be to assume that these devices have been removed or have the maximum-sized orifice that could be installed in the device.

Fire or thermal expansion

The pressure in process components exposed to the heat from a fire will rise as the fluid expands and the process liquid vaporizes. For tanks and large low-pressure vessels, the need to vent the liberated gas may govern the size of the vent or relief valve. Fire sizing a relief valve only keeps pressure buildup to less than 120% of the MAWP. If the component is subjected to a fire for a long time, it may fail at a pressure less than the MAWP because a metal’s strength decreases as temperature increases.

On components that can be isolated from the process, it is possible for the process fluid contained in the component to be heated. This is especially true for cold (relative to ambient) service or when the component is heated (such as a fired vessel or heat exchanger). It is also true for compressor cylinders and cooling jackets. The relief valves on such components should be sized for thermal expansion of the trapped fluids. This normally will not govern the final size selected unless no relief valve is needed for the other conditions.

Installation considerations

The installation of a relief device requires careful consideration of the inlet piping, pressure-sensing lines (where used), and startup procedures. Poor installation may render the relief device inoperable or severely restrict the valve’s relieving capacity. Either condition compromises the safety of the facility. Many relief-valve installations have block valves before and after the relief valve for in-service testing or removal; however, these block valves must be car sealed or locked open.

Inlet piping

RP 520, Part 2, and ASME code limit the inlet pressure loss to a PSV of 3% of set pressure where the pressure loss is the total of the inlet loss, line loss, and block-valve loss (if used). Loss is calculated with the maximum rated flow through the relief valve. To minimize the inlet pressure drop to a relief valve, a conservative guideline is to keep the equivalent length-to-diameter ratio of the inlet piping to the relief valve at 5 or less. For pressure-drop limitations and typical piping configurations, refer to RP 520, Part 2.

Discharge piping

The discharge piping should be designed so that the backpressure does not exceed an acceptable value for any relief valve in the system. Piping diameters generally should be larger than the valve-outlet size to limit backpressure. Lift and set pressures of pilot-operated relief valves with the pilot vented to the atmosphere are not affected by backpressure; however, if the discharge pressure can exceed the inlet pressure (e.g., tanks storing low-vapor-pressure material), a backflow preventer (vacuum block) must be used. The set pressure for balanced spring-loaded relief valves will not be as affected by backpressure as conventional spring-loaded relief valves are. Balanced relief valves will suffer reduced lift as backpressure increases.

Reactive forces

On high-pressure valves, the reactive forces during relief are substantial and external bracing may be required. Refer to the formulas in RP 520, Parts 1 and 2 for computing these forces.

Tailpipe considerations

Relief valves that are not connected to a closed relief system should have tailpipes to direct the relieving gases to a safe area away from personnel. The tailpipe should be sized for a maximum exit velocity of 500 ft/s. This ensures that the gas/air mixture is below the lower flammable limit or lower explosive limit at approximately 120 pipe diameters away from the tailpipe. Tailpipes should be supported at the bottom of the elbow. A small hole or a "weep hole" (minimum of ¼ in. in diameter) should be installed in the bottom of the elbow to drain liquids that enter through the tailpipe opening. The weep hole should be pointed away from process components, especially those classified as an ignition source.

Rapid cycling

Rapid cycling can occur when the pressure at the valve inlet decreases at the start of the relief valve flow because of excessive pressure loss in the piping upstream of the valve. Under these conditions, the valve will cycle rapidly, a condition referred to as "chattering." Chattering is caused by the following sequence. The valve responds to the pressure at its inlet. If the pressure decreases during flow below the valve reseat point, the valve will close; however, as soon as the flow stops, the inlet-pipe pressure loss becomes zero and the pressure at the valve inlet rises to vessel pressure once again. If the vessel pressure is still equal to or greater than the relief-valve set pressure, the valve will open and close again. An oversized relief valve may also chatter because the valve may quickly relieve enough contained fluid to allow the vessel pressure to momentarily fall back to below set pressure, only to rapidly increase again. Rapid cycling reduces capacity and is destructive to the valve seat in addition to subjecting all the moving parts in the valve to excessive wear. Excessive backpressure also can cause rapid cycling, as discussed previously.

Resonant chatter

Resonant chatter occurs when the inlet piping produces excessive loss at the valve inlet and the natural acoustical frequency of the inlet piping approaches the natural frequency of the valve’s moving parts. The higher the set pressure, the larger the valve size, or the greater the inlet-pipe pressure loss, the more likely resonant chatter will occur. Resonant chatter is uncontrollable, that is, once started it cannot be stopped unless the pressure is removed from the valve inlet. In actual practice, the valve can break down before a shutdown can take place because of the very large magnitude of the impact force involved. To avoid chattering, the pressure drop from the vessel nozzle to the relief valve should not exceed 3% of the set pressure. RP 520, Part 2 covers the design of relief-valve inlet piping. 5 Pilot-operated relief valves with remote sensing pilots can operate with higher inlet-piping pressure drops.

Isolation (block) valves

There is no industry standard or RP for isolation valves, and practices vary widely. Installed isolation block valves allow the testing of spring-loaded relief valves in place, thus eliminating the need to remove the vessel from service while bench testing the relief valve, and allow the relief device to be isolated from the closed relief system when performing maintenance and repair. The ASME Unfired Pressure Vessel Code allows the use of isolation valves below relief valves. ASME Pressure Vessel Code, Appendix M, describes special mandatory requirements for isolation valves. The ASME Boiler Code prohibits them, and the U.S. Occupational Safety and Health Admin. prohibits them on instrument air receivers. Because improper use of an isolation valve may render a relief valve inoperative, the design, installation, and management of these block valves should be evaluated carefully to ensure that plant safety is not compromised. See RP 520, Part 2, for typical block-valve installations under relief valves.

Relief valve configurations

There is no industry standard or RP that addresses this topic. Some of the more common relief-value configurations are listed here and are shown in Fig. 3.

• Installation of full open isolation (block) valves upstream and downstream of relief valves. Isolation valves should be car sealed open (locked open), and a log should be kept. These valves should be discouraged where the potential overpressure is twice the maximum allowable pressure. A test connection should be provided on all spring-loaded relief valves. The installation of two relief valves (100% redundant) should be considered so that one relief valve can be left in service at all times.
• Installation of pilot-operated valves without isolation valves. This configuration allows for the testing of pilot set pressure only and requires full plant shut-in for relief-valve repair and maintenance.
• Installation of three-way valves with one port open to a tailpipe or a vent stack. This configuration allows for valve maintenance and repair without requiring plant shut-in and ensures a path to the atmosphere if the three-way valve is left in the wrong position.
• Installation of two two-way valves, connected by mechanical linkage, and two relief valves. This configuration provides all the advantages of isolation valves. In addition, it is impossible to isolate a process component by mistake. The only disadvantage of this configuration is the initial cost.
• Installation of a check valve in lieu of an isolation valve. This configuration is not allowed by the ASME Pressure Vessel Code because the check valve may fail or cause excessive pressure drop.

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  Fig. 3—Relief-valve configurations.

Guidelines for determining the number of relief devices

There is no industry standard or RP for determining the number of relief devices, and installations vary widely. Sometimes there are two relief devices (100% standby) on vessels receiving production directly from the wells. The primary relief valve is set at MAWP. If the second relief device is another relief valve, the set pressure of the second relief valve is set 10% above the primary relief valve. If the second relief device is a rupture disk (entirely redundant against all possible relieving scenarios), the pressure is set at 15 to 25% above the primary relief device. This setting ensures that the rupture disk will not rupture when the design primary relieving rate is reached at the set pressure plus 10% overpressure. Primary and standby relief rates are considered adequate for fire sizing.

Some companies install two relief valves on all critical installations so that plant shutdowns are not required during testing and maintenance. If the secondary relief device is being counted on to provide any portion of any required relieving capacity (blocked discharge, gas blowby, fire, etc.), then the secondary device should be set in accordance with the rules of RP 520, Parts 1 and 2, (i.e., ASME Sec. VIII, Division 1, paragraph UG-134).

Liquid discharge considerations

Condensed mists have liquid droplets that are less than 20 to 30 μm in diameter. Testing and experience have shown that with a slight wind, the envelope of flammability for this type of mist is the same as that for a vapor. Liquids will settle to grade, thus presenting a fire and pollution hazard; therefore, the relief device should be installed in the vapor space of process vessels with an LSH that alarms and shuts in flow when activated. The LSH should be set no higher than 15% above the maximum operating level, while the relief valve should be set no higher than the MAWP of the process component. Scrubbers and knockout drums should be installed in flare, vent, and relief lines to separate and remove liquid droplets from the discharge.

Noteworthy papers in OnePetro

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read

External links

API Standards

See also

Safety systems

Recommended methods for safety analysis

Flare and vent disposal systems

PEH:Safety_Systems