Acceptable to Size PSV on U/S Orifice Size?

[JSD]

I am sizing a PSV and trying to determine the required relief flowrate. The system consists of an air bottle at 200 barg, regulator then damper. I am basing it on the upstream regulator failing open. This has a certain orifice size. The flow through the orifice from 200 barg to 0 barg (worst case) will give the required relief flow. The equation I have gives it in volumetric flowrate.

To convert to mass flowrate should I use the upstream conditions i.e. density at 200 barg or can i use the downstream/set pressure conditions?

Without actually doing the PSV sizing calculation can i specify the required PSV orifice size to be at least the same size as the regulator fail open orifice size? The PSV should be able to relieve all the flow through the regulator if done this way as this orifice is the restriction.

If so this orifice size ties in with using the downstream conditions to convert to mass flow.

 

[Latexman]

Gas volumetric flow rates don't mean a damn thing unless one knows the T and P of the flowing gas, so you need to know whether your equation is giving flow rates at upstream, downstream, or standard conditions. Once you know that, use those same conditions to calculate a density and convert the volumetric flow rate to mass flow rate.

"Without actually doing the PSV sizing calculation can i specify the required PSV orifice size to be at least the same size as the regulator fail open orifice size?"

Probably not. The PSV may have a different differential pressure and/or resistance to flow than the regulator.

Downstream of the regulator (upstreanm of the PSV) will not be 0 barg, because the PSV needs a set pressure and some driving force to flow to atmosphere.

Good luck,
Latexman

 

[maddocks]

No, you can not simply specify identical orifice sizes - it would be a lucky chance if you guessed correctly. You have to to do the calculations for regulator failure to determine flows and pressures...

 

[MortenA]

JSD:

If you did as suggested by yourself - then you could end up having the bulk of the dP over the PSV - you would thus have 200 barg (minus the dP over the pressure regulator) in your downstream system and i will assume thats too high?

Calculate the mass flow through the valve at max p=200 barg and downstream=your PSV set point + 10%

Size the valve for the mass flow but at the PSV set point + 10%

Best regards

Morten

 

[mbt22]

The downstream pressure would, I think, be the set pressure of the relief valve, not atmospheric.

 

[Latexman]

200 barg -----> Regulator -----> ? barg -----> PSV -----> Atmospheric

? barg = PSV Set Pressure + 10%

Good luck,
Latexman

 

[mbt22]

Sorry, I should have been clearer. I was referring to the pressure drop across the orifice to calculate the relief case. I forgot about the 10% accumulation, but on further reflection, I'm not sure I would take credit for this.

I suspect it is a moot point anyway, as the required area is likely to be far below that of the smallest valve you can find.

 

[Latexman]

I include the 10%, but I do look closely at the maximum upstream pressure (the 200 barg) which may be the set pressure of a different PSV + 10%. I think that is common practice.

Good luck,
Latexman

 

[CHD01]

Those advising you are correct. Downstream orifice pressure for calculating the required relief flow should be based on the downstream relief valve set pressure plus 10%.

Dp across upstream valve orifce may be based on either downstream or up-stream density if less than 10% dp across the valve orifice; and average density if drop across the valve orifice is between 10% and 40%. If drop is greater than 40% you need to include an expansion factor and allow for sonic flow if compressible flow exists. The CRANE flow of Fluids Manual discusses this and is an inexpensive and excellent resource that fluidflow engineers should be informed of.

The more you learn, the less you are certain of.

 

[CHD01]

Those advising you are correct. Downstream orifice pressure for calculating the required relief flow should be based on the downstream relief valve set pressure plus 10%.

Dp across upstream valve orifce may be based on either downstream or up-stream density if less than 10% dp across the valve orifice; and average density if drop across the valve orifice is between 10% and 40%. If drop is greater than 40% you need to include an expansion factor and allow for sonic flow if compressible flow exists. The CRANE flow of Fluids Manual discusses this and is an inexpensive and excellent resource that fluidflow engineers should be informed of.

The more you learn, the less you are certain of.

 

[JoeWong88]

JSD,
I agree with Latexman & Maddock. NO. There is possibility of having small regulator orfice but large PSV orfice especially when you have low PSV setpressure.

mtb22,
200 barg -----> Regulator -----> ? barg -----> PSV -----> Atmospheric

? barg = PSV Set Pressure + 10%

i) Agreed with Latexman and others... The accumulation of 10% is allowed (designed per ASME 8)

ii) ? barg could be much lower than critical pressure. DP across regulator may consider 200-crticial pressure. But when sizing the PSV, it should base on PSV set pressure +10%.


CHD01,
"Dp across upstream valve orifce may be based on either downstream or up-stream density if less than 10% dp across the valve orifice; and average density if drop across the valve orifice is between 10% and 40%. If drop is greater than 40% you need to include an expansion factor and allow for sonic flow if compressible flow exists. The CRANE flow of Fluids Manual discusses this and is an inexpensive and excellent resource that fluidflow engineers should be informed of."

This is applicable to line frictional loss estimation to certain degree of accuracy. I doubt if this can be used here.


JoeWong
Chemical & Process Technology