Calculate Max Flow Through Failed PCV
Calculate Max Flow Through Failed PCV
(OP)
This is a continuation of thread1203-205528: Calculate Max Flow through failed PCV which was closed.
I was given the following equation from a speciality gas vendor. I was wondering if anyone could identify the source:
For Critical Flow:
Qg = Cv * P1
-------
2 * SQRT(Sg)
Where:
Qg = gas flow in SCFM (60°F and 14.7 psia)
Cv = flow coefficient for regulators and valves
P1 = Inlet pressure in psia
For sub-critical flow:
Qg = Cv * SQRT(deltaP * P2)
----------------------
SQRT(Sg)
deltaP = P1 - P2
P2 = Outlet pressure in psia
Where did these formulas come from? I have my doubts that the gas company came up with them. They seem too simple to me. (I realize they ignore geometry factors.)
I was given the following equation from a speciality gas vendor. I was wondering if anyone could identify the source:
For Critical Flow:
Qg = Cv * P1
-------
2 * SQRT(Sg)
Where:
Qg = gas flow in SCFM (60°F and 14.7 psia)
Cv = flow coefficient for regulators and valves
P1 = Inlet pressure in psia
For sub-critical flow:
Qg = Cv * SQRT(deltaP * P2)
----------------------
SQRT(Sg)
deltaP = P1 - P2
P2 = Outlet pressure in psia
Where did these formulas come from? I have my doubts that the gas company came up with them. They seem too simple to me. (I realize they ignore geometry factors.)
Onwards,
Matt





RE: Calculate Max Flow Through Failed PCV
Thanks.
Onwards,
Matt
RE: Calculate Max Flow Through Failed PCV
ht
RE: Calculate Max Flow Through Failed PCV
RE: Calculate Max Flow Through Failed PCV
RE: Calculate Max Flow Through Failed PCV
I'm wondering about the source of the equation and whether or not it can be universially applied to gas flow sizing when a Cv is given.
I appreciate the link to the PDF. However, it looks like it is just a rehash of the ISA equations which are also given in Fisher's Control Valve Handbook. The equation does not appear in either publication that I can see.
I have never seen the above equation in any publication. That's why I'm trying to find it. It is quite simple and, if correct, would be very easy to use.
Anyone know the source?
Onwards,
Matt
RE: Calculate Max Flow Through Failed PCV
Good luck,
Latexman
RE: Calculate Max Flow Through Failed PCV
RE: Calculate Max Flow Through Failed PCV
scfh = (520 / (SG * T))^0.5 * P1 * Cg * sin (blah blah blah).
I left my notes at the office so be gentle with me
The sin factor goes to 1 for critical flow so it goes away.
Cg is approximately 30 * Cv for many globe style valves.
scfh = scfm * 60
60 * scfm = (520 / (SG * T))^0.5 * P1 * 30 * Cv
scfm = [(520 / (SG * T))^0.5 * P1 * 30 * Cv] / 60
scfm = [(520 / (SG * T))^0.5 * P1 * Cv] / 2
If we assumed the speciality gas vendor assumed their products would be used at room temperature then T1 = 520R and we end up with
scfm = P1 * Cv / 2 * SG^0.5
Can't help you with the second equation.
RE: Calculate Max Flow Through Failed PCV
VOGT for flow has scfm = 22.6 * Cv * ((dP * P1)/T1 * Sg))^0.5
If I assume T1 is supposed to be at 60F as previously, the square root of 520 is 22.8. Round off to 1, change P2 to P1 and you have your equation.
RE: Calculate Max Flow Through Failed PCV
Start with equation 3-20 for scfm q'm
Set Y = 1.0 (that's one thing that bothered me with VOGT's equation, an increasing dP didn't account for expansion effects)
Substitue Cv for K (equation 2-5 on pg 2-8). If you rearrange it, you get K = 894 * d^4 / Cv^2
If you substitute it into the equation for q'm, you get
scfm = 22.6 * Cv * (dP * P1 / (T1 * Sg))^0.5 which is VOGT's equation except P2 is replaced with P1.