SCFM info needed

[MechTeg]

What is the standard calculation to determine SCFM relative to an SRV without a known orfice size? The service in question is air service with the vessels mawp @ 250psig. The compressor is capable of suppling 510 SCFM. There are other varibles available, but just need some insight as to how to figure SCFM. Any help is appreciated!

MechTeg
1/28/2006.

 

[Latexman]

What is the standard calculation to determine SCFM relative to an SRV without a known orfice size?

I don't think there is one. That's like asking how much stuff can you put in a bag IF you don't know what size bag you got?

What are you doing? Sizing a new SRV since the orifice size of the existing one is unknown, or trying to find the capacity of what you got? A little more explanation please.

Good luck,
Latexman

 

[MechTeg]

Basically, have an old vessel that was being used to store chlorine in the past. The vessel has undergone some updates and now will be used as "air" service, being used as a accumilation tank for emergency air. We had 250# Crosby JQ valves in the old service, but obviously they would not work in the newer service nor would it be an wise choice. So, I need a new srv for this service. Basically you have a 28 (length)x 10.5 (dia.) bullet tank that is horizontal. Vessels MAWP is 250#. I understand that the compressor would not exceed 510 SCFM @ 250#. The vessel has a 2" 300# RF Flanges on the top of the vessel.

We have converted tanks of the same size and sources in the past and have come up with 2100 SCFM, but I was not involved at that time. Is the 2100 SCFM the flowrate seen at the orfice of the SRV?

MechTeg

 

[Latexman]

First you need to understand what the "standard conditions" are in this 510 SCFM number for the compressor. Back in high school and college "standard conditions" was 1 atmosphere and 273.15 K. In the oil and gas industry it is 1 atm and 15 C a lot of the time. I've seen a lot of references to 14.7 psig and 60 F in the chemical industry. Standard conditions are not so standard! Different industries use different standard conditions.

Once you know Ts and Ps, you can use the ideal gas law and the molecular weight of air to calculate the mass flow rate of air.

PV = nRT --> n = PV/RT

m = n x MW

Then you can use the mass flow rate *at* the conditions of your scenario, not at STP, to size the relief. Once you know what size relief you need, it will flow a certain amount of air at the set pressure and at the scenario conditions. *This* maximum flow rate can be converted to standard conditions and that will be the SCFM of that PSV at that set pressure in that scenario.

Is the 2100 SCFM the flowrate seen at the orifice of the SRV? I have no idea. Is the 2100 SCFM the *true* flowrate seen at the orifice of the SRV? Absolutely not! The inlet of the orifice is going to be close to the set pressure, not the standard pressure.


Good luck,
Latexman