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determining pressure drop across valve

kahlilj (Mechanical) (OP)
18 Oct 02 14:28
i need to determine the max volume (capacity) of nat gas across a couple of valves.  this seemed simple enough, get the valve constant from mfg and plug into eqn:
Cv = (Q/1360) * [(2*G*T)/(deltaP*(P1-P2))]^0.5

i contacted one mfg and got their valve constant, but the other mfg (Grove) told me there was no constant for their valve!  just use equivalent length of pipe for that diameter pipe.  

that "other" valve is a Grove B5 12" ball valve.  any idea on best way to calculate maximum flow (or pressure drop) across this valve?

my understanding is to assume a max pressure drop of 50% of inlet pressure and then use that to determine flow across valve.  does this seem reasonable? or does anyone have other ideas or corrections?

thanks,
kahlil
TD2K (Chemical)
18 Oct 02 15:00
Umm, using a dP of 50% of the inlet pressure will give you approximately sonic flow through the valve and that will be the maximum flow.  I'm not sure what you are trying to do with this number though.  The maximum dP is more likely going to be set by the piping system the valve is installed in.

For the 12" ball valve, using your approach, the flow would be huge.  The L/D for a ball valve (which is what the manufacturer is suggesting) is about 3 * turbulent friction factor for an full port ball valve.  The upstream and downstream piping for this case is going to set the flow rate, not just the valve (and the upstream/downstream piping is likely going to have a similar effect on the other valve as well).  The manufacturer should have Cv values for this valve, talk to someone else.  At the very least, they should have flow versus dP data and you can calculate the Cv off that.
valgro (Mechanical)
27 May 03 12:08
If you provide me with the pressure class I can provide you with the Cv.
Jim Dietrich
Project Sales Manager - Canada
Dresser Flow Control
kahlilj (Mechanical) (OP)
27 May 03 16:01
thanks jim.  valve is 600#.  i estimated flow to be 20,350,735  scfh.

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