Does Atmospheric Pressure Affect PRV ? 1

[IdanPV]

Hello all,

My company is about to install a pressure vessel at altitude of 8000feet, where the atmospheric pressure is 92.5kPa.
PRV set pressure is 2.8bar(g).

I wonder if the low atmospheric pressure will affect the valve operation.

 

[MortenA]

No not really. The PRV is there to protect against over pressure. The vessel "sees" the same over pressure as your valve does. So while it will lift at a lower absolute pressure when comparing to a location at sea level it still lifts at the same over pressure and thus serves its purpose.

--- Best regards, Morten Andersen

 

[The Obturator]

Your sizing calculation should take into account the atmospheric pressure of the installation.

ie., When computing the Absolute Inlet Pressure for use in API 520 calculations, which for gases and vapours is Set Pressure (PSIG) + Overpressure (%) + 14.7 = PSIA, the 14.7 PSIA Normal Atmospheric Pressure should be changed to reflect your stated atmospheric pressure of 92.5 kPa(a) (or 13.4125 PSIA).

In the normal sense, set pressure of 2.8 barg = 40.6 PSIG x 1.10 (10% overpressure) + 14.7 = 59.36 PSIA
But atmospheric pressure is 92.5 kPa(a), therefore;-
2.8 barg = 40.6 PSIG x 1.1 + 13.4125 PSIA = 58.07 PSIA

Some manufacturers sizing programs allow the user to change the atmospheric pressure setting.

The manufacturer should also be consulted about how he compensates the setting (CDTP) of the valve if tested at different atmospheric pressures.




*** 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]

I get 8000 ft normal air pressure to be 75.3 kPa or 10.9 psia....

https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html

It does impact the setting, but the amount is quite low and morten is correct. The pressure inside the vessel will be impacted the same as it is really the differential pressure which is important.


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[IdanPV]

That's true,
I got confused with converted into SI units (2500meter) and I were looking at 2500feet.
10.9 psia that is.

 

[The Obturator]

10.9 PSIA Will certainly influence your valve sizing. Make sure you use it in the calculation.


*** 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]

It's an interesting question as it also means the MAWP of the vessel is in guage pressure, i.e. it is relative to the external pressure. so in space, 2.8 barg would be the same as 2.8 bara. At 17,500 ft - 0.5 bara atm pressure, 2.8 barg would be 3.3 bara

The issue is also how you correct for this on instruments, especially those that use a near perfect vacuum.

You need to zero all readings, guages or transmitters to the local atmospheric pressure. Same with the relief valves? So setting the pressure at sea level to 2.8 bar g would result in a higher pressure at 8000 ft unless you revised the set point locally.

At higher pressures the difference is essentially negligible, but at 2.8 barg, you're not out of the woods on that one.

Have I got that right?

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

 

[IdanPV]

LittleInch and The Obturator,

Thanks for your comments, this is interesting and I couldn't find answer by myself.
So thanks!

As for the MAWP - ASME refers to gage pressure, so 2.8bar is 2.8bar doesn't matter if it is on space or at 20,000 ft.
But you raised a very good question - does a recalculation (for vessel strength) is needed ?

As for readings, guages or transmitters, that is true, and we will re calibrate all of these.

 

[LittleInch]

Don't think it needs a recalculation, but does need all the protection devices to be reset to local atmospheric pressure otherwise it could run at a higher gauge pressure if the TX or guage thinks it is at sea level....

So in your case I think all it needs is for the PRV to be set whilst on a bench at 8000ft

Or if you want to do it a sea level then ship it you need to allow for the 0.25 bar difference and set it at 2.55 barg at sea level. I think.

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