technical notes of 6th session

7/29/2019 Technical Notes of 6th Session

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Technical Notes – 5th Session M. Ardinur Sridarnanto

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PRESSURE RELIEF DEVICE: actuated by inlet staticpressure and designed to open during emergency or abnormal conditions to prevent a rise of internal fluidpressure in excess of the specified design value, e.g.:pressure relief valve, non-reclosing pressure relief device,or vacuum relief device. Pressure relief devices protect avessel against overpressure only)

PRESSURE SAFETY VALVE (PSV): a valve which isused to handle overpressure (abnormal condition) onequipment (pressurized vessel, etc). Pressure relief valvemust open at a predetermined set pressure, flow a ratedcapacity at a specified overpressure, and close when thesystem pressure has returned to a safe level. This term isused primarily for compressible fluids (steam, gas & vapor)

PRESSURE RELIEF VALVE: a valve which is used tohandle overpressure (abnormal condition) on equipment(pumps, etc). This term is used primarily for

incompressible fluids (liquid). SeePressure Safety Valve(PSV)

RUPTURE DISC: A non-reusable and non-reclosingoverpressure relief device (which ruptures when it’sexposed at designated pressure rating). Use of rupturedisc and other non-reclosing overpressure relief deviceshall be minimized. And also, rupture disc shall be avoidedin services in which pressure is pulsating and in liquidservice. Rupture disc is disposable, so rupture disc mustbe isolated, removed, and replaced after each relief.Margin between maximum operating pressure and burst

pressure on rupture disc protected system may need to beconsiderably greater than 10% but shall not exceed 105%of MAWP.Rupture disc shall be considered in applications where:- very rapid built-up pressure can not be handled by the

slower response time of a pressure relief valve- in series with a pressure relief valve if valve leakage

would be a problem, and a soft-seated, tight shut-off pressure relief valve is not acceptable

- in series with pressure relief valve for protection of thepressure relief valve from excessive corrosion or fouling, unless purging can be used

- in parallel with pressure relief valve only if where 2-stage relief is needed. Pressure relief valve shall haveadequately lower set pressure and shall be able tohandle most relief cases without bursting the rupturedisc

DETAILED INFO ABOUT PRESSURESAFETY/RELIEF VALVE:Pressure safety/relief valve must open at a predeterminedset pressure, flow a rated capacity at a specified

overpressure, and close when the system pressure hasreturned to a safe level. 1) relief valve: opens normally inproportion to the pressure increase over the openingpressure. And it is used primarily for incompressible fluids(liquid). 2) safety valve: rapid opening or pop action. It isused primarily for compressible fluids (steam, gas &vapor). 3) safety relief valve: combination.

The primary purpose of a pressure relief/safety valve is protection of life and property by venting fluid from anover-pressurized vessel or system. An overpressure eventrefers to any condition which would cause pressure in avessel or system to increase beyond the specified designpressure or Maximum Allowable working Pressure(MAWP). Proper sizing, selection, manufacture, assembly,test, installation, and maintenance of a pressurerelief/safety valve are all critical to obtaining maximumprotection. Pressure relief/safety valves must be designedwith materials compatible with many process fluids from

simple air and water to the most corrosive media. Theymust also be designed to operate in a consistently smoothand stable manner on a variety of fluids and fluid phases.

Back Pressure Considerations: pressure relief/safetyvalves on clean non-toxic, non-corrosive system may bevented directly to atmosphere. Pressure relief/safetyvalves on corrosive, toxic or valuable recoverable fluidsare vented into closed systems. Valves that vent toatmosphere, either directly or through short vent stacks,are not subjected to elevated back pressure conditions.For valves installed in a closed system, or when a long

vent pipe is used, there is a possibility of developing highback pressure.Back pressure is pressure which exists at the outlet of PSV, and it may affect the operation of PSV. Magnitude of the back pressure--after PSV has opened—is the total of superimposed back pressure andbuilt-up back pressure.Built-up back pressure is back pressure which developedin the discharge system after the PSV opens, whilesuperimposed back pressure is back pressure whichpresent at the outlet of PSV while the valve is closed.The back pressure on a pressure relief/safety valve mustalways be evaluated and its effect on valve performance

and relieving capacity must be considered. If pressure inthe valve outlet varies while the valve is closed, the valveset pressure will change. If back pressure varies while thevalve is open and flowing, valve lift and flow rate throughthe valve can be affected

Effect of Back Pressure on PSV: #variations in openingpressure, #reduction in flow capacity, #instability, or combination of all three.




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1) Spring Loaded PSV (Standard): a conventionalPSV show unsatisfactory performance when excessive backpressure developed during the relief incident.Built-up backpressure should not exceed 10% of the set pressure at10% allowableoverpressure.2) Balanced PSV: is used if the built-up back pressure istoo high for conventionalPSV or if thesuperimposed back

pressure varies widely compared to the set pressure. Itcould be applied if total back pressure (superimposed &built-up) doesn’t exceed approximately 50% of the setpressure. It utilizes bellows/piston to minimize/eliminatethe effect of superimposed back pressure on set pressure.3) Pilot-Operated PSV: it’s not affected by back pressure(back pressure correction factor = 1.0). Pilot-operatedPSV have pilots which are vented to atmosphere or balanced(vented to main valve) to maintain presence of the setpressure because of variable superimposed backpressure. ThesePSV should be considered if thesuperimposed back pressure is variable. However, if the

variation amount of the superimposed back pressure issmall, a conventional PSV could be used. Pilot-operated PSV shall be considered:- for spheres, spheroids, and low pressure storage

tanks on which maximum storage pressure may resultin less margin below set pressure than normallyrequired

- for low pressure applications below approximately 10psig set pressure

Permissible/Allowable Overpressure (from ASME SectVII, Div 2, AR-150): 1) For all pressure vessels should be

limited not more than 10% or 3 psi (20.6 kPa). 2) For vessels which provided with multiple pressure relievingdevices should be limited not more than 16% or 4 psi (28kPa) . 3) For pressure relief devices which discharging for the conditions such as exposure to fire or other unexpected sources of external heat

Margin above maximum operating p ressure: Pressurerelief valves shall have an adequate margin between setpressure and the maximum operating pressure at thepressure relief valve location, to minimize operation and/or leakage of pressure relief valve. Normal margin shall be at

least 10% of set pressure, but not less than 20 psi (for some COMPANY)

Set Pressure: the inlet gauge pressure at which thepressure relief device is set to open under serviceconditions.For a single relief device (from ASME SectVII, Div 2, AR-140): shall be set at a pressure which notexceed design pressure of the vessel at operatingtemperature (tolerance ± 2 psi (13.8 kPa) for ≤ 70 psi (483kPa), and 3% for > 70 psi).For multip le relief device:

only one device need to be set at a pressure which notexceed design pressure of the vessel, the additionaldevice(s) might be set at a higher pressure but not exceed105% of design pressure of the vessel.For pumps &compressors: set pressure plus 10% overpressure shallnot exceed pressure that would cause electrical motor driver to trip current overload protection and/or mechanical

overload or high operating temperature damage

Materials of Construction: materials must be chosenwith sufficient strength to withstand the pressure andtemperature of the system fluid. Materials must also resistchemical attack by the process fluid and the localenvironment to ensure valve function is not impaired over log periods of exposure.

Installation: Pressure relief valves shall be accessiblefrom grade or platform. Pressure relief valves shall belocated as close as practical to vessel that they protect to

minimize length of inlet piping. Inlet piping shall be sizedsuch that the total of all non-recoverable frictional pressurelosses between the protected equipment and the pressurerelief valve inlet is not more than 3% of set pressure. Thisis to prevent loss of relieving capacity and/or valvedamage as the result of the pressure relief valve rapidlycycling open and closed. The only exceptions to the 3%requirement if:- pilot operated pressure relief valves have a remote

pressure sensing line from a separate vessel nozzleto the pilot

- Nonreclosing pressure relief devices such as rupture

disc and rupture pin relief device- An existing installation (not new construction) of abalanced bellows pressure relief valve may have up to5% inlet frictional pressure losses, providing the valveblowdown is adjusted to 9% of set pressure

Vent o f Closed Bonnet: Closed bonnet of a pressurevalve shall have a vent connection. Conventional pressurerelief valve bonnet vent shall be closed by installing athreaded plug in vent connection. Balanced bellowspressure relief valve bonnet vent shall be vented to closedflange system or to atmosphere. If vented to atmosphere,

vent shall have a bugproof screen.

SPRING LOADED PSV: is a conventionalPSV. It hasbeen developed to meet the need for a simple, reliable,system actuated device to provideoverpressure protectiondevice. This design consists of a valve inlet or nozzle mounted on the pressurized system,a disc held againstthe nozzle to prevent flow under normal system operatingconditions,a spring to hold the disc closed, andabody/bonnet to contain the operating elements. The




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spring loaded is adjustable to vary the pressure at whichthe valve will open

BALANCED PSV: balancedPSV utilize bellows/pistonto minimize/eliminate the effect of superimposed backpressure onset pressure

PILOT-OPERATED PSV: Pilot-operatedPSV havepilots which are vented to atmosphere or balanced (ventedto main valve) to maintain presence of theset pressure because of variablesuperimposed back pressure

SET PRESSURE: the inlet gauge pressure at which thepressure relief device is set to open under serviceconditions

BACK PRESSURE: pressure which exists at the outletof PSV, and it may affect the operation of PSV. Magnitudeof the back pressure--after PSV has opened—is the total

of superimposed back pressure andbuilt-up backpressure. The back pressure tends to reduce the liftingforce which is holding the valve open

BUILT-UP BACK PRESSURE: back pressure whichdeveloped in the discharge system after thePSV opens. Itoccurs due to pressure drop in the discharge system asthe result of flow fromPSV after it has opened. Excessivebuilt-up back pressure can cause the valve to operate inan unstable manner, e.g.Chatter & Flutter

SUPERIMPOSED BACK PRESSURE: back pressure which is present at the outlet of PSV while the valve isclosed. In most cases, superimposed back pressure willvary as a result of changing conditions in the dischargesystem

OVERPRESSURE: is pressure in equipment in excessof the maximum allowable working pressure.Overpressure can be caused by an input source that willdevelop pressure in excess of equipment’s maximumallowable working pressure if inflow exceeds outflow.Inflow may exceed outflow if an upstream flowrate controldevice fails, if there are restrictions or blockage in theequipment’s outlets, or if overflow or gas blow-by fromequipment occurs. Overpressure can also be caused bythermal expansion of fluids within equipment if heat isadded while the inlets and outlets are closed. Primaryprotection from overpressure in equipment should beprovided by a PSH to shut off inflow. If a vessel is heated,the PSH should also shut off the fuel or source of heat.Primary protection for atmospheric equipment should beprovided by an adequate vent system. Secondary

protection from overpressure should be provided by aPSV. Heat exchangers & similar vessels should beprotected against overpressure in case of an internalfailure (usingPressure Safety Valve (PSV)).Cases: FireCase 21%, Normal Case 10%, & Steam Case 5%.Secondary protection for atmospheric equipment shouldbe provided by a second vent. The second vent may be

identical to the primary vent.

technical notes of 6th session

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