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Saturday, March 22, 2008

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A pressure relief valve (PRV) in "Ready-to-operate" mode and "relieving" mode during plant operation will expose to different type of pressure. "Ready-to-operate" mode is the PRV's disc keeping PRV in closed position when inlet pressure (Pi) is lower than or equal to PRV set pressure (Ps). "Relieving" mode is PRV disc away from seat allowing fluid passing the PRV valve nozzle when the inlet pressure (Pi) is higher than PRV set pressure (Ps).

Defintion of "backpressure"
As PRV may be in Ready-to-operate and Relieving mode, the "backpressure" exist at different modes will vary. In many event, understanding of this "backpressure" creates a lot of confusion among engineers in operation, design, vendor, manufacturer, etc. It is important to make the defintion clear prior to any discussion.

Superimposed backpressure is the static pressure that exists at the outlet of a pressure relief device (PRD) at the time the device is required to operate ("Ready-to-operate" mode). Superimposed backpressure is the result of pressure in the discharge system coming from other sources i.e. Pressure control valve (PCV), pressure relief valve (PRV), etc. Superimposed backpressure may be constant or variable.

Built-up backpressure is the increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device (PRD) opens ("relieving" mode).

Backpressure is the pressure that exists at the outlet of a pressure relief device (PRD) as a result of the pressure in the PRD discharge system. Backpressure is pressure result of both superimposed backpressure caused by other sources and built-up backpressure due to the relief flow during "relieving" mode. Backpressure is the sum of the superimposed and built-up backpressures.

How "backpressure" affect Conventional PRV ?
"Backpressure" will affect the performance of a Conventional spring loaded Pressure Relief Valve (PRV). "Backpressure" in quotation is common name which includes Backpressure, Superinposed backpressure and built-up backpressure. How "backpressure" affect Spring loaded PRV ? Infact, there are at least three aspects a "backpressure" affecting a conventional spring loaded PRV.

i) Maximum allowable accumulated pressure of protected vessel.
A pressure contained vessel made from any pressure vessel code (e.g. ASME VIII div 2), a PRV shall be provided to protect the vessel from overpressure. Per ASME code, maximum allowable accumulated overpressure is 10% of it maximum allowable working pressure (MAWP). A PRV is normally set at the MAWP (typically). In this case a conventional PRV maximum allowable built-up backpressure shall be 10% of set pressure.

A conventional type PSV protecting a pressure vessel with maximum allowable overpressure of 10% MAWP and exposed to built-up back pressure higher than 10% of MAWP, (e.g. 15% of MAWP) it will results higher accumulated pressure in the vessel (i.e. 115% of MAWP) and exceeded the maximum allowable working pressure (MAWP) with 10% allowable overpressure. Thus one shall always remember that built-up back pressure of conventional spring loaded PRV shall always equal or lower than maximum allowable overpressure.

ii) PRV Capacity
Increased in backpressure (total of superimposed and built-up) below 10% of set pressure is typically does not affect the conventional PRV relieving capacity. As it exceeded 10% of set pressure, the conventional PRV relieving capacity will reduce significantly. See following image.

iii) PRV Stability
Backpressure (total of superimposed and built-up) also affecting the stability of PRV. This can be seen from the study carried out by Thornton (1979) followed by Sterland (1984) and Lai (1989) has shown that a PRV may experience chatting and instability operation when the backpressure is exceeded 10% of set pressure for conventional spring loaded PRV. See the following image.

The definition are as follow :

L = Lift of disc
D = Diameter of Inlet Bore
Pb = Absolute Back pressure
Pa = Atmospheric pressure
Pb-Pa = Gauge Back pressure
Ps = Gauge set pressure

Above has briefly discussed the impact of backpressure to a conventional Spring loaded PRV. Do you aware of any other impact ?

Updated : March 02, 2008

Related Post


posted by Webworm, 4:31 PM


Anonymous Paul McLean said...

I don't agree with point i). The effects of a backpressure greater then 10% on the accumulation above the 10% above MAWP would be due in reality to points ii and iii. An increase in back pressure that causes the relief valve not to be at critical flow will decrease the capacity of the relief valve and hence increase the accumulation in the vessel.
Also instability/chattering of the relief valve will also cause potential flow problems and increase the accumulated pressure in the vessel.
For these reasons is the effect on accumulation so point i) should really be removed as it makes a far too simplistic correlation.


August 13, 2008 at 5:34 AM  
Blogger Webworm said...

A maximum allowable built-up back pressure may be used for allowable overpressures greater than 10% provided the built-up back pressure does not exceed the allowable overpressure...read more in API 521 section


August 13, 2008 at 10:37 AM  
Anonymous Anonymous said...

Could you please precise what is Pb-Pa in the abscissa (on figure at the end of the article? Is it the differential set pressure (or net spring setting)?

October 28, 2008 at 1:00 PM  
Blogger Webworm said...

The article has been updated to include the definition.

October 29, 2008 at 3:52 PM  
Anonymous Anonymous said...

API 520 section 3.5
Back pressure:
pressure that exists at the outlet of a pressure-relief device as a result of the pressure in the discharge system
NOTE The back pressure is the sum of the superimposed and built-up back pressures

Could you please then make it clear in the article which one you use in each part i), ii) and iii)?


November 1, 2008 at 10:57 AM  
Blogger Webworm said...

Back pressure affect all three parts

November 2, 2008 at 11:53 PM  

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