Guidelines for Pressure Relief Design
10. Relief Device Types and Operation
A. Terminology –the terms pressure relief valve and safety relief valve are commonly used interchangeably.
B. Pressue Relief Valve
i. Direct Spring Pressure Relief Valves-are commonly classified as either conventional or balanced bellows
1. Conventional Relief Valve
a. Both the super-imposed back pressure and the built-up back pressure in the discharge piping must be accounted for in the selection of the valve style and sizing of the associated piping.
b. Back pressure acts on the top of the disk assembly and thus adds to the spring force.
2. Balanced Bellows Relief valve
a. Back-pressure effects are minimized
b. Center portion of the top side of the disk assembly is isolated from the back pressures by a bellows or others means and open to atmospheric pressure.
ii. Pressure Relief Valve Characteristics
1. Inlet Piping loss
a. Chatter- if the flow resistance of the associated inlet piping is excessive, the amplitude of the oscillations can increase until the disc impacts the nozzle seat (See Fisher 1995 for Example problems) the valve body and /or the piping can fail if harmonic resonance occurs.
b. The API 3% Rule is currently accepted as the criterion for the upper limit on inlet losses to pressure relief valves. This rule requires that the non-recoverable (friction) losses be less than 3% of set pressure when the valve is operating at nameplate capacity and corrected for the properties of the flowing fluid.
i. Some designer and valve manufactures follow the more conservative practice of using the best estimate flow rate at 10% over-pressure for the loss calculation. This flow is about 10% higher than the relieving capacity.
ii. When a pressure relief valve is adjusted to correct for service conditions of constant superimposed back pressure, the nameplate shows the actual test pressure (cold differential test pressure) and the set pressure in service. Common practice in this case is to compute the inlet loss using the calculated relieving capacity for the stamped set pressure rather than relieving capacity under test conditions.
iii. The non-recoverable pressure loss from the vessel to the valve is less than the pressure drop, since the pressure drop includes the change in velocity head from vessel to valve. This velocity head is recoverable (part of the lifting force on the disk) and is not included in the determination of the inlet loss.
iv. Hydrostatic head of either single or two-phase fluids in the inlet piping is typically not included in the pressure loss for either conventional or balanced bellows valves.
1. Assumes that head remains constant throughout cycling.
2. Back Pressure in Discharge Piping
a. Balanced Valves-are designed to minimized sensitivity to back pressure in discharge piping.
i. Gradual loss after 30% of set pressure
ii. Use manufacture Kd to determine effects of back-pressure.
b. Conventional Valve – are very sensitive to back-pressure
i. Pressure increase adds directly to the closing force on the disc of a conventional valve and results in capacity loss and chatter if excessive.
ii. Back pressure should be limited to 10% of set pressure for conventional valves (ASME BPV Code, Appendix M-7(c)) when flowing at stamped capacity or nameplate capacity and corrected for the properties of the given fluid.
3. Body Bowel Choking /Outlet flange Flow Limitation
a. On certain occasion, the designer can encounter a dilemma when sizing a discharge pipe to meet the built-up back pressure limitation for conventional pressure relief valves. Calculation for a discharge of the same size as the outlet flange can say that the built-up pressure exceeds the allowable limit even if the length is reduced to zero.
i. Usually arise for valves with low ratios of outlet to nozzle area and high set pressure.
1. Air flow 8T10 valve with set pressure above 150
2. For two-phase flashing flow at lower area ratios and lower set pressures
3. Backpressure will not be reduced by increasing the discharge pipe size
4. Computational problems arises only if the chocked flow pressure exceeds the allowable back pressure limit for the particular valve body bowl outlet flange.
4. Liquid Relief-there are three basic types for liquid service:
a. Standard Trim- Pressure relief valves with provisions for making adjustments to provide pop action for gas service or more proportional action for liquid service.
b. Liquid trim – pressure relief valve with trim designed for liquid service
c. Valves designed specifically for only liquid pressure relief service.
i. Referred to as thermal relief. May or may not be ASME certified.
5. Pressure Relief Valve Specification –do not mix ASME and API orifice sizes and discharge coefficients.
i. Uses actual orifice area and certified valve of Kd. Certified valve is 90% of actual
i. Uses 90% of actual area and actual Kd
6. Pilot Operate Pressure Relief Valves
a. Held shut by system pressure action on piston, which typically has a larger area than the seating surface
b. Opens at set pressure and vents the piston chamber and at the same time sealing off the flow to the pilot from the system
c. Many variation of pilot operated pressure relief valves are available and offer feature such as:
i. Snap-action (fully open at very low pressures)
ii. Modulating operation (opening proportionate to over-pressure)
iii. Low operating margin (seat leakage does not increase even at pressures above about 95% of set pressure
iv. Inlet line loss compensation: the pilot tube connection can be at the vessel instead of in the valve inlet (remote sensing), so that the operating stability will not be affected by pressure losses in the inlet piping. (See section 2.5.2 for applicable ASME BPV code requirements for inlet piping with losses greater than 3% of set pressure.
v. Back pressure compensation (set pressure and main valve lift not affected by back pressure)
vi. Flowing vs non-flowing pilot system
d. Principle disadvantages of pilot valves are the greater complexity and the requirement that the sensing line to the pilot remain open. (see CCPS Guidelines for Pressure Relief 184.108.40.206 for further detail)
11. Rupture Disc Devices
A. Forward acting or tension loaded designs are pressurized on the concave side of the dome and the bursting or breaking elements fail in tension or a combination of bending and tension. Flat or no-domed rupture disk also fall under the classification of forward acting. The following rupture disc fall into this category:
i. Conventional pre-bulged: relies on material thickness to determine burst pressure. Fragmenting design with random opening characteristics, see Figure 2.4-5a, requires a special holder for mounting between flanges. Requires a vacuum support at lower pressures to withstand vacuum conditions
ii. Forward acting composite: Burst pressure and opening are mechanically controlled by a fabricated load carrying top section. Most commonly use a fluorocarbon seal member. Fragmenting or limited fragmenting design.
iii. Graphite: Machined from solid resin impregnated graphite. Fragmentation design. Most designs install directly between pipe flanges with no additional holder required. Vacuum supports generally causes a significant reduction in flowing area for this type of disk. The material is quite corrosive resistant. Disc can be supplied with different specified burst pressures in the two directions.
iv. Forward acting scored: uses scoring or coining to control burst pressure and opening pattern. Non- fragmenting design commonly used for pressure relief valve isolation. Typically used for lower set pressure and requires a support to withstand vacuum conditions. Generally requires a special holder although some lower performance configurations install directly between pipes.
B. Reverse acting- Compression loaded designs ae pressurized on the convex side of the dome and the bursting or breaking elements are subjected to primarily compressive stresses during the initial buckling of the disc. The following rupture disc fall into this category.
i. Reverse acting scored: Uses scoring or coining to control opening pattern. Non-fragmenting design commonly used for pressure relief valve isolation. Generally, more resistant to fatigue than other types.
ii. Reverse acting with knife blade: relies on knife blade in holder or sharp teeth in attached ring to cut the disc to achieve opening. Generally, not suitable for liquid service. Some of these designs represent older technology that has fallen out of favor due to problems with the corrosion or dulling of knife blades.
C. Rupture Disc Specifications
i. Burst tolerance –defined as the allowable accuracy range of disc actual burst pressure verse the specified burst pressure at the specified temperature.
1. Burst pressure tolerance at the specified disk temperature - +/- 2 psi at marked burst pressure < or = 40 psig: +/- 5% of marked burst pressure > 40 psig.
ii. Specified disk temperature: the temperature of the rupture disc at the time it is expected to burst. Depending on the overpressure scenario (s) this could be the normal operating temperature, the maximum operating temperature, or some other temperature. Is not necessarily the flowing temperature during discharge and may even be close to the temperature of the local environment
iii. Manufacturing design range: Defined as the range of pressure within which the marked burst pressure must fall to be acceptable for the particular requirement as agreed upon between the rupture disc manufacturer and the user or his agent. Rupture disks can be ordered with zero (+ /-), or +/- 10 percent tolerances. Both burst tolerances (+/-) must be taken below the vessel MAWP.
iv. Actual burst pressure range: MAWP + Plus Burst tolerance. MAWP – (minus Burst Tolerance +/- Manufacturing Ranges).
v. Operating margin: Operating pressure of 70, 80, 90, 95 percent of specified rupture disc Burst Pressure dependent upon the type and model of rupture disc specified.
vi. Operating ratio: The required differential between maximum normal operating pressures and the burst pressure typically represented as a ratio. At marked burst pressure < or = 40 psig the operating ratio = maximum operating pressure / (marked burst pressure – rupture tolerance).
vii. Fluid state: Different types of rupture disc respond differently when burst in compressible vs incompressible fluids due to the time duration of the event and the amount of stored potential energy. Opening characteristics such as opening or no-opening, changes to KR (flow resistance) and fragmentation are all influenced by the fluid state at the time of burst.
viii. Cyclic conditions: This is related to the operating margin. The greater the magnitude and number of cycles the greater chance for the metal fatigue and premature failure of the rupture disc.
ix. Fragmentation: many rupture disc types discharge metallic fragments during the bursting process. If these fragments can cause damage or blockage to downstream equipment or be discharged into the populated areas when discharging to atmospheric, then no-fragmenting should be specified.
x. Flow resistance: Many rupture disc device designs have been tested to quantify typical opening characteristics in compressible and or incompressible service in terms of a unitless, velocity head loss parameter KR. The opening characteristic for most rupture disc devices are influenced by the state of the process media against the disc at the time of burst. The rated KR values have corresponding designations for compressible service (KRG), incompressible service (KRL), or either (KGL).
12. Relief Device Combination
Relief device combinations are normally used to take advantage of or combine specific characteristics that are not found in individual devices. The following are the most commonly used combinations:
A. Rupture disc upstream of a pressure relief valve: This is by far the most common combination used in the process industries. The benefits include:
i. Prevention of leakage through the relief valve
ii. Prevention of product build-up in the valve nozzle
iii. Isolation of valve components from corrosive process
iv. In-place pop testing of the relief valve (the value of this practice is not universally accepted in industry and only a few rupture disk types are suitable for this activity.
B. Rupture disk downstream of a pressure relief valve: This is typically accomplished with a low-pressure rupture disc. The primary benefit is to prevent corrosive vapors in common headers from entering the discharge side of the relief device.
C. Rupture Disc in series: These can be factory supplied double disc devices or individual devices installed in series. These are typically specified for one of two reasons:
i. Highly corrosive or otherwise aggressive environment that are prone to disk failure with the goal of having the second disk remain intact to provide time for a safe shutdown without a discharge event
ii. To isolate the process side disc from variable back-pressure in a common header
Note: Virtually all of these combinations have a common disadvantage in that most of these devices are affected by the pressure differential across them and therefore the space between them has to be monitored and/or vented appropriately to ensure safe operation.
13. Low Pressure Relief Valves & Vents
Such devices are not included in the scope of her ASME BPV Code and are not ASME certified. These devices can be specified to protect low pressure tanks from unexpected over or under- pressure conditions or in many cases serve to allow normal in and out breathing in pressure to compensate for changes in pressure due to normal ambient temperature changes.
A. Weight loaded (Pallet-type) devices- are reclosing valves. They can be used for pressure entering or vacuum relief depending on the connection scheme. Two devices can be combined to provide both pressure and vacuum relief.
i. These devices typically rely on the weight on a pallet to control set pressure.
B. Spring loaded devices – are similar to weight loaded devices except they use a spring to provide the closing force in lieu of weighs. These are typically used at higher pressures than weight loaded designs.
C. Pilot operated devices – use tank pressure acting against a diaphragm instead of weights or springs. These devices are generally capable of higher performance (lower leak rates and greater capacity at lower overpressures) than traditional weight or spring- loaded devices.
D. Weight loaded emergency relief device (manhole covers) – are non-reclosing types of devices. These covers have the advantage of providing large venting areas with available vessel openings. The cover can be hinged or simply restrained by a cable.
Note: See Appendix C of API Standard 2000 for the characteristics of these and other low-pressure devices. Since the opening characteristics of these devise are not standardized, manufacturer’s recommendation should be used to predict the venting capacity.
E. Rupture disc devices- are also available for low pressure application. These are typically the composite type disk. Information on such devices is available from the manufactures.
F. Tanks can be fabricated with a weak-seam roof to assure that the roof will detach before the tanks fail below the liquid level. Standards apply only to steel tanks over 30 feet in diameter (refer to the current edition of API Standard 650).
14. Miscellaneous Relief System Components
A. Shut off valve-are commonly used between connected vessels and /or between vessels and relief devices to isolate connected areas for maintenance purposes. The ASME BPV Code appendix M places certain restrictions on the use of shut off valves
i. Such valves are permitted if they can be locked or sealed open and not be closed except by an authorized person who remains stationed there while the vessel is in operation.
1. Can be avoided with the use of Three-way selector valve or transflo- valve, that have an open flow path regardless of the valve position. However, they can cause pressure drop to exceed API 3% Rule.
B. Vent stacks –may require some weather protection to avoid the entrance of excessive amounts of rain, etc. Goose neck bends should be used with caution because these tend to impede plume dispersion.
15. Selection of Pressure Relief Devices
i. Safety relief valves are the most widely used valves because they can be adjusted to operate as a relief valve for liquid service or a pop-action safety valve for gas/vapor service.
ii. Pilot operated valves are recommended only where a clean, non-corrosive fluid flows to the pilot valve and the top of the piston.
iii. Conventional safety relief valves can only handle 10% back-pressure, based on set pressure.
iv. Balanced bellows valves can maintain performance with up to 40-55% back-pressure
v. ASME “Code” requires valve manufactures to de-rate relief flows by 10% as a safety factor.
vi. The difference between an API orifice size and an ASME orifice size is that API de-rates the actual orifice size while ASME de-rates the discharge coefficient (Kd).
vii. Graphite rupture disc are highly recommended over metal disc since metals have tendency to corrode. Metal disc will also require a holder.
viii. Metal disc are recommended at times because they can be selected in many different metals like aluminum, 316 stainless steel, Inconel, nickel, silver, monel, and they can be pre-scored to burst without fragmentation.
ix. A new “energy Absorbing Disk” (EAD) assembly has been developed by BS&B safety systems. This is a double disc assembly. If the lower disc fails prematurely due to fatigue, or improper installation, the upper disc will not fail due to the resulting pressure shock. The maximum operating pressure is 85%of the set pressure.
B. Device Advantages and Disadvantages
i. Spring loaded Pressure relief Devices
a. Relief valve recloses and retain vessel contents after the pressure subsides to the blowdown pressure level during an episode
a. Basic disadvantage to the use of pressure relief valves over typical non-reclosing devices are their relative complexity and associated cost of both purchase and maintenance
b. Reliability less than for non-reclosing due to complexity
c. Effects of back-pressure on capacity
i. Use of balanced bellow to ease this constraint
1. Bellows failure will cause the set pressure of the valve to increase by the amount of superimposed back pressure, which is generally unacceptable.
ii. Pilot Operated Valves
a. Can be designed to overcome the piping design constraints of spring-loaded valves.
i. Special provisions for back pressure compensation are not usually required
ii. Frictional losses can be accommodated by connecting the pressure-sensing line to the vessel instead of the valve inlet (remote sensing).
iii. Valve characteristics can be tailored for snap action, proportional opening etc.
iv. Total weight of pilot vale in larger sizes can be substantially less than an equivalent spring-loaded type
v. Advantage over spring loaded valves for application requiring bubble tight seal and resistance to corrosion and fouling.
a. Greater complexity
b. Lower temperatures
c. Higher cost
d. Sensing line must remain unobstructed
i. Filter can be provided to prevent solids from settling out
e. Cannot be used with fouling fluids (such as polymerizable liquids)
iii. Rupture Disc Devices
a. Remain open; pressure is reduced to minimum
b. Mechanically simple; lower maintenance and replacement cost
c. Provide a bubble tight seal
d. Much lower cost for corrosion-resistant construction
e. Much higher relieving capacity for a given vessel nozzle diameter
f. No design constraints on associated piping with regard to operating stability
a. Greater loss of vessel contents compared to devices that recloses
b. some styles subject to fatigue in service, resulting in premature opening or other failure
c. Require careful handling and installation to avoid mechanical damage
d. Most types require special holders that are not interchangeable among manufactures
e. Must be careful to avoid installing upside down
f. Burst pressure is sensitive to temperature
C. Valve Choice for Two-Phase Service
If a pressure relief valve system is chosen over another type of relief device, specifying the proper valve type for a particular fluid service is important. The ASME BPV Code is clear for single-phase fluids. Use valves designed or set up as relief valves for liquid service and use valves designed or set up as safety valves for gas/vapor service. The question of valve selection for two-phase fluids is not addressed in the Code.
The designer must choose between a pressure relief valve with standard trim set up for vapor service or with liquid trim designed for liquid service. The nozzle of the valve is the same for either valve. Principal features of the two types are:
i. Standard Trim Valve
1. Can be adjusted for either compressible (gas) or incompressible (liquid) service by changing the setting of the blowdown ring.
2. Not operable (unstable) for liquid service if blowdown ring set for gas and not recommended for gas (or two- phase) service with the blowdown ring set for liquid.
ii. Liquid Trim Valve:
1. Capacity can be certified for gas as well as liquid service
2. No blowdown specifications for certification
3. Blowdown in gas service typically much higher than with standard trim.
A reasonable recommendation is to use the standard (vapor) trim if the flow is predicted to choke within the nozzle. Otherwise select the liquid trim. If the use of a liquid relief valve is indicated, only certified liquid trim valves should be used.