Bellows design PSVs selection 1

[Nasim12345]

If the back pressure is more than 10% of set pressure, bellows design psv need to be used(instead of conventional design), such statement is available in any API or ASME standard? Can i have the paragraph reference.

 

[The Obturator]

To Clarify;-
Built up back pressure can be as high as overpressure value, with conventional, spring operated pressure-relief valve.
For superimposed variable back pressure, a balanced spring operated pressure relief valve is necessary.
Ref. API Standard 520 Part I (Oct 2020). Section 5.3 Backpressure

The '10% backpressure' statement is largely misunderstood. Generally you can use a conventional spring pressure relief valve with up to 10% BUILT UP backpressure, where overpressure is 10%.
ANY variable, superimposed backpressure, requires a balanced PRV.

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

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

 

[LittleInch]

Oh that's interesting.

so if the back pressure when fully open is >10% of set pressure whether by static pressure plus friction or friction alone you need a balanced bellows valve, its what the valve sees which is important?

So e.g. your set pressure is 10 bar and the static back pressure from e.g. a flare header is 0.5 barg, you only have 0.5 bar left for the frictional back pressure when at full flow?

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

 

[MortenA]

@Littleinch,

Yes thats how i would interpetate it. Notice when @the_obturator says with regards to SP ad super imposed back pressure. If you have a superimposed BP it will in effect "move" the SP for a spring loaded valve up. In some low pressure applications this might be critical.

--- Best regards, Morten Andersen

 

[The Obturator]

You need to understand the difference between 'built up' and 'superimposed' back pressures.

Built up back pressure develops from the point of initial PRV opening - in these cases, where there is no other back pressure specified, you are permitted to use a conventional design spring operated pressure relief valve up to a maximum built up back pressure of 10 % (of set pressure), in cases where the overpressure is 10%.

API-520 permits built up back pressure equal to the overpressure for conventional PRV designs.

Superimposed back pressure of a variable nature, is always present, and so will affect a spring operated pressure relief valve opening/set point. In such cases a balanced design PRV is always necessary.

For superimposed constant backpressure, a conventional spring operated pressure relief valve is used. Here the PRV set pressure is compensated in the workshop by subtracting the constant back pressure value from the set pressure to get the nett spring setting.


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

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