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REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

(OP)
Can the fail open capacity as defined in ASTM F1795 be determined from regulator performance curves?
The regulator manufacturer lists a Cv but the calculated compressible gas flow is much larger (4x) than the 'Flow Performance Curves' with similar inlet and outlet pressures.
I suspect the the 'regulated' flows are not a conservative indication of fail open capacity but I wonder if the Cv conversion is too conservative.

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

Hi Bambie.

I am dealing with the same issue at the moment. I have, for now, asked for the regulator vendors to provide the "wide open" discharge coefficient. For a gas regulator, in the absence of superseding data, for P1/P2 > 2, I assume sonic flow and Cd = 0.72.

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

(OP)
Snorgy, would you mind sharing your calculations with me?

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

Search out Fisher's website. In their regulator section I believe they have technical information that will guide you through the calculation.

Also, your vendor may be able to supply this calculation to you.

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

Bambie,

Tomorrow, what I have, I will share.

Tonight, I regret to admit that I spilled too much beer on it...

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

(OP)
Snorgy,

Without an area (A) or coefficient of discharge (Cd) for the regulator, I calculated 314 SCFM N2 using Crane 410 and Cv = .2, P1 = 2000 psig and P2 = 100 psig. The Manufacturer suggested that 205 SCFM is more accurate but I haven't seen their numbers.

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

Bambie,

I believe you have forgotten to use the fact that the flow will choke at the "critical pressure", Pc. So, dP = 2000 - Pc. Also, is Crane's equation you are using for compressible flow? It needs to be. I don't recall Crane having a Cv based equation for compressible flow, but I am 5000 miles from my copy at the moment. That's why I suggested you search Fisher's web site. I know they have the right equations for the problem.

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

Bambie:

Sorry, I was away from home this weekend.

The equation I use (or resort to) is per the link attached. You need some idea of the orifice size and, as I mentioned, the discharge coefficient.

For your conditions, my initial calculation for a 1/4" port (orifice) was on the order of 2300 SCFM nitrogen *at the onset of flow*. But, I assumed a 1/4" hole and C=0.72 in the above.

In any event, the 312 SCFM at that pressure differential sounds low to me.

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

Full Disclosure: I work for the local Fisher Representative. The full open flow coefficient for a specific regulator (used so you can do the flow calculation - either choked or not) is not a calculated number but actually comes from empirical data from the flow lab at Fisher. So you will find two different regulator models with the same flow coefficient may have a different wide open flow coefficient. So that you cannot assume what the full open coefficient is, you should get this from the manufacturer.

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

(OP)
Steamdog,
ASTM F1795 only states that fail-open capacity must be established accurately and defined clearly - not that it must be empirically established.
Snorgy and Latexman,
I re-worked my calculations in accordance with CGA E-4-2006 for sonic flow conditions and the actual mixture under consideration (90%Argon 10%Methane).
I would appreciate any comments, thanks.

RE: REGULATOR FAIL OPEN CAPACITY PER ASTM F1795

Looks good! And, it's close to the manufacturer's number. Not bad, for a sparky. Run with it!

Good luck,
Latexman

Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers

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