Changing the set pressure of a safety valve . 2

[Mohammad Saad]

I want to ask if any standard discusses how much is the allowable limit of changing the set pressure from the design set pressure. To be clear, I understand the 3% plus or minus tolerance but I am not referring to that. If a psv has design set pressure of 6 kg/cm[sup]2[/sup], and I have to install it at a service that requires the set pressure to be, let's say, 3 or 9 kg/cm[sup]2[/sup], am I allowed to adjust the spring accordingly? I have only found one related text in the Crosby Pressure Relief Valve Engineering Handbook which says:

"The spring in a safety valve or safety relief valve shall
not be set for any pressure more than 5% above or below
that for which the valve is marked, unless the setting is
within the spring design range established by the valve
manufacturer or is determined to be acceptable to the
manufacturer"

Thank you in advance for your help.
Regards.

 

[LittleInch]

You've answered your own question....

"unless the setting is within the spring design range established by the valve manufacturer"

Springs come listed with a min and max range.

You also need to change the set point stamped on the valve.

Ask the vendor or look up the technical details on their website.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[The Obturator]

In changing the set pressure, you are also changing the rated capacity of the Pressure-relief Valve. It is strongly recommended the valve is resized with the new set pressure value. If you are changing to a higher set pressure, expect a smaller size, lower set pressure will require larger size. You may even find that the valve is now oversized or undersized. Other factors may also be affected


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Mohammad Saad]

Thank you @LittleInch and @The Obturator for your responses.

 

[gguliye]

If u are worried about liquid chattering that may be result of oversizing, please choose modulating pilot valve.

 

[georgeverghese]

In almost all cases, changing the set pressure on a PSV is not permitted without going through an MOC approval process within the Engg Dept in your Operating Company. Several key design documents may need to be revised. Part of the approval process may most likely involve a formal mini HAZOP also. All you can do now is to fill up a form requesting for this change and hand it over to the Plant Manager for his review and action.