Pressure Relief/Safety Valves (PRV / PSV)

Process equipment is designed to operate at a pressure at least 10% above maximum allowable working pressure (MAWP), and at temperatures much higher than normal operating temperatures.  To ensure equipment integrity, a range of pressure relieving devices are available.
19.1    Pressure Relief/Safety Valves (PRV / PSV)
All pressure vessels are equipped with pressure relief or safety valves (PRV / PSV) or rupture discs that are designed to open and relieve the pressure if it reaches 10% above the design pressure.  PRV / PSV’s are also sized to release the total fluid inlet flow to the vessel to prevent pressure continuing to increase after the PSV has opened.
The descriptions “relief” and safety” are commonly applied to all types of valves designed to protect process systems and vessels for excessive pressure.  There is, however, a general difference between the two.
Relief valves are designed to relieve excessive pressures in systems containing  incompressible fluids (liquids), where there is no chance of explosion under over pressure.  Not rapid opening.  Safety valves are characterized by rapid full opening or pop action.  Safety-Relief valves are full opening valve capable of being used either as a safety valve or a relief valve, depending on the application.
Whilst operating characteristics may differ in detail, both safety valves and relief valves are of similar basic design, normally direct action spring type.  Lever and weight or dead-weight types are obsolete but may still be found in use.  Alternative designs, such as torsion bar types, have limited application.
Spring loaded safety valves normally have their set pressure adjustable by means of a screw in the top of the bonnet and are normally fitted with an external lever for checking their action (or such a lever is available as an option).  Relief valves may be similarly adjusted for set pressure, but do not usually have a relief-checking lever.
The purpose of a safety relief valve is to discharge a given amount of gas, vapor or liquid to prevent an increase in system pressure from exceeding a safe level.  The safe pressure level is set below the maximum safe working pressure of the pressure vessel or equipment and associated pipe work into which the safety valve is fitted to protect.  In order to understand how safety valves function and how they are constructed, it is helpful to become familiar with the following terms.
19.2      Relief Valves
Relief valves are installed in flow lines and liquid service pipelines as a protection against too high a pressure caused by liquid expansion.  Relief valves are commonly installed on positive displacement pumping systems and heat exchanger equipment.
NOTE :  Relief valves are used primarily in liquid service and must not be used in gas or vapor service.  They are not usually provided with a lifting lever to operate the valve manually.

19.3      Safety Valves
Safety valves are automatic spring-loaded pressure relieving devices actuated by the static pressure upstream of the valve and characterized by rapid full opening or pop action.
These valves are usually constructed with the tension spring outside and isolated from the valve body or casing.  This arrangement protects the spring from contact with the relieving fluid.  The valve moves in an adjustable sleeve which regulates the lifting height of the valve lid and therefore the flow (volume) of relieved gas.  The sleeve is adjusted by two rings which are secured by locking bolts.  Safety valves are fitted with protective hoods which are locked and sealed to prevent unauthorized persons from altering the adjustment of the spring via the adjusting bolt.
Note :   It is strictly prohibited for unauthorized persons to open or to break the lock on the protective hood.  This may be done by authorized persons only.
19.4    Safety Relief Valves
Safety relief valves are automatic spring loaded pressure relieving devices, actuated by static upstream pressure.  These valves are characterized by rapid full opening or pop action for gases or a non pop action for liquids.  They are similar to the safety valve described in the previous section.  A blow-down ring is used to control the pop action of the valve  Safety relief valves are designed for use in process service where the gas, vapor or liquid discharged is known to be flammable, toxic or corrosive and is therefore routed to a remote area for disposal (flare or vent) via a closed relief system.  This is a dual purpose, safety/relief type valve.  See Figure 16.  There are two types of safety relief valve, conventional and balanced.

 Conventional Safety Relief Valves
This valve consists of a housing containing a directly loaded spring that forces a valve disc onto a valve seat.
The pressure of the disc on the seat is adjusted by means of an adjusting screw located on top of the tension spring.  The adjusting screw and locking nut are protected from dirt and dust by a screw cap.
When the fluid pressure exceeds the spring tension, the valve disc is lifted from its seat and the excess pressure is released, through the exhaust port into the relief line or relief header.

19.5      Pressure-Vacuum Safety Valves (PVSV)
Pressure-Vacuum safety valves are commonly installed on hydrocarbon liquid storage tanks and are used to relieve either excessive pressure or vacuum conditions.  Excessive pressure (over pressure) can lead to tanks and equipment bursting and vacuum to tanks collapsing.
A typical tank safety valve (Figure 19) comprises weighted pallets which normally keep the valves closed.  The weights can be adjusted to suit a pre-set pressure or vacuum condition.  When either of these conditions occur the pallet is forced off its seat to allow:
The escape of excessive pressure in a system,
The entry of atmospheric pressure to reduce an excessive vacuum condition.
After operating, the weights automatically re-seat the valve to close the port at the pre-set pressure or vacuum condition.

19.6      Pilot Operated Safety Relief Valves
Figure 20 shows a sectional view of a pilot operated safety relief valve which is suitable for air, gas and liquid service.
This type of safety relief valve consists of a main valve which is operated by a separate pilot valve.  The pilot valve, which is mounted on the main valve body, is connected through a pressure tight tubing in such a way that, the pilot valve senses system pressure and operates the main valve disc by automatically controlling its position.
Since the top of the piston is larger than the bottom there is a net force holding the piston down.  When the set pressure is reached, the pilot opens and partially or totally vents the dome, thus reducing the load on the top of the piston to the point where the upward force on the main valve seat can overcome the downward loading.  This causes the piston to lift and there is a flow through the main valve.  When the system blow-down pressure is reached, the pilot valve closes and full system pressure is diverted to the dome with the main valve closing.

19.7      Rupture Discs (PSE)
The rupture disc or pressure safety element is not a valve because it cannot be opened or closed; however it is a fail-safe type of relief device.  These devices are fitted to spur lines from certain pipelines and vessels.  They are additional relief devices to the normal safety valve.
A rupture disc (a form of diaphragm) is made from materials such as aluminum, stainless steel or monel.  It is fitted between two companion flanges called a holder, and is used to contain the fluid in a system but allow relief above a set pressure rating.
Discs of different thickness are available for most diameter pipes.  While normal pressures exist in a process system the disc acts as a blind.  If excessive pressure is not relieved quickly enough by the pressure safety valve, the disc ruptures to assist the safety valve.  So a rupture disc is a designed weak point in a system that will self destruct (like a fuse in an electrical system) to protect the equipment.
Rupture discs are safety devices which protect process vessels, pipelines and tanks which would be damaged by excessive pressure.  A ruptured disc allows the pressure to be relieved quickly and is the final safety device installed on vessels.
Rupture discs are generally made in two forms, tension loaded and re-buckling.  Figure 21 shows one type of rupture disc.  The downstream clamping flange includes several equally spaced knife blades arranged in a shallow pyramid with a sharp point at the apex.  At predetermined system pressure, the disc will suddenly reverse bulge and be pierced by the knife point and segmented by the knife to provide full-flow opening.  This type is called a knife edge re-buckling disc.  A variation of this type is the jaws re-buckling disc.  Tension loaded disc have no scored device but rely on system pressure to push through the metal.

Rupture disc require protection from the environment to reduce corrosion of the metal surface which could downgrade the pressure rating.  They need to be correctly positioned as incorrect positioning could raise the rupture pressure.  Constant pressure change can cause flexing or movement of the disc.  This could lead to metal fatigue so discs are pre-bulged to reduce this problem.
The rupture disc can be used as a primary relief device or for secondary relief in conjunction with a relief valve.  When used with a safety relief valve, the rupture disc pressure is higher than the safety relief valve pressure but less than the maximum allowable working pressure.  On rupturing the disc is simply replaced but a system of periodic inspection is usually carried out to check and/or replace discs.