7 replies to this topic

[vibhs]

Hi all,
Can somebody tell me the governing factors while deciding the PSV Set Pressure ?

Like :-
1. what can be the maximum/ mimimum PSV Set Pressure in a system ?
2. Its dependency on the system normal operating pressure, system design pressure and equipment MAWP.
3. And anything else if applicable?

[breizh]

Hi VIBHS,
Consider these resources .

Hope this helps

Breizh

[fallah]

vibhs,

Maximum set pressure : MAWP of the equipment subject to protection.

Minimum set pressure : Should be specified such that there would be adequate margin between maximum operating pressure and set pressure minus PSV blowdown value in order to avoid any loss of inventory due to continuation of PSV relief at or below maximum operating pressure.

Fallah

[AlertO]

i try to give more detail to you.

minimum set pressure
like Fallah said; however, some typical standard may give you a minimum value although you are operating the plant nearly to ATM pressure.

maximum set pressure
in fact, you can set to MAWP as the maximum value, but actually you have to see the system design pressure. one system always has one design pressure,but MAWPs of each equipment in the system usually be higher than the design pressure and always not the same valvue depending on the vendor standard.

hope my post can help you

[vibhs]

Thank you very much breizh, fallah and devilchicken for your prompt responses.........The information provided is really useful.
Apart from this I would like to know the reasons for setting PSV set pressure near to minimum limit or maximum limit ?

[fallah]

I would like to know the reasons for setting PSV set pressure near to minimum limit or maximum limit ?

vibhs,

It is not being applied in all cases, but:
-After that the vessel/drum has left a considerable part of its life time, set pressure may be considered near to min limit as per required derate performing
-When set pressure is near to max limit it would result in smaller size of PSV for the same relief load

Fallah

Edited by fallah, 27 September 2011 - 06:17 AM.

[vibhs]

Thank u Fallah.........

Vibhs

[deltaChe]

ASME Boiler and Pressure Vessel Code states (at paragraph UG-125) that all pres-
sure vessels “shall be protected by a pressure relief device that shall prevent the
pressure from rising more than 10% or 3 psig, whichever is greater, above the
maximum allowable working pressure except as permitted” in two subsequent
sections. The amount of pressure rise permitted by the relevant design code is the
“allowable overpressure”. Allowable overpressures vary by code, equipment type,
and cause of overpressure. The most common allowable overpressure is 10% of
maximum allowable working pressure (MAWP).

The phrase “design pressure” will be used below to refer to the maximum set pres-
sure permissible by the relevant design code for a single relief device on the equip-
ment item in question. For the ASME Boiler and Pressure Vessel Code, this is the
MAWP; for the ASME Piping Code, this is the design pressure; for a pump or
compressor, this is normally the pressure rating of the casing.