Calculate flow through valve using Cv value

[GasDistEng]

I am trying to calculate the flow rate of natural gas through an upstream valve on a block and bleed set up on a large boiler. The purpose of this is to determine the flow rate so that I can size the vent line off the bleed-off.

Here is what I am working with:

P_atm = 14.7 PSIA
P_base = 14.7 PSIA
P_inlet = 2.0 PSIG
Known Cv = 32
SG = .6
T_base = 60 F
Q = iterated (9730 SCFH)


I used the Flow Calculations for Gases through Valves method as described at

http://www.ces.clemson.edu/chemeng/undergraduate/uolab/valve6.htm

You need to go to the bottom of the page at this link to see it.

Step #1 Determine if choked or Sub Critical Flow

Choked = (P_inlet+P_atm)>or=2((P_inlet-0.5)+Pb) = FALSE

Sub-Critical = P_inlet+P_atm)<2((P_inlet-0.5)+Pb) = TRUE

Step #2 Choose sub-critical Cv equation for compressible flow:

Cv = [Q[((SG)(T_base+459.67))^0.5)]/816(P_inlet+P_atm]^0.5

For a Q of 9730 SCFH I calculate a Cv of 32. I iterated Q until I got a Cv of 32.

Any suggestions?

 

[davefitz]

If all you want to do is size the vent line, then size it for 10% of soundspeed ( in natural gas at 1 atm).

For flow thru a valve that is not choked, use the simple formula:

W(lb/hr) = 63.3 Cv SQRT(DP/ sv,i)
DP= press drop, psid
sv,i =inlet spec volume, ft3/lb

 

[GasDistEng]

Thanks... Do you think the method used in my original post is O.K.?

 

[GasDistEng]

Oops! made an error this sould read:

Choked = (P_inlet+P_atm)>or=2((P_atm) = FALSE

Sub-Critical = P_inlet+P_atm)<=2((P_atm) = TRUE


Sorry...