choked flow and positive displacment pump
choked flow and positive displacment pump
(OP)
We are testing a positive displacement pump with water, applying pressure with a control valve. Pressure drop is very high - P1=1500 psig, P2=10 psig. By calculation of choked flow conditions, the max allowable pressure drop is around 1100 psi. Max flow is calcluated around 250 gpm. Actual flow from the pump is 350 gpm. We can hear the cavitation. FL value of valve is .56.
I'm trying to get my head around what is happening. Obviously the flow is not limited at 250 gpm. Do the Cv calculations become inappropirate at conditions exceeding max deltaP? The choked flow calcs are based on the calculated Cv position - is it possible the actual position is different than calculated (unfortunately we don't have valve position data)?
I'm trying to get my head around what is happening. Obviously the flow is not limited at 250 gpm. Do the Cv calculations become inappropirate at conditions exceeding max deltaP? The choked flow calcs are based on the calculated Cv position - is it possible the actual position is different than calculated (unfortunately we don't have valve position data)?





RE: choked flow and positive displacment pump
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: choked flow and positive displacment pump
What type of valve trim is it, linear, =%? Have you got the valve Cv curve from open to closed? I would see how much % you are talking about for the 250 gpm flow you are calculating the valve should pass versus the 350 gpm the pump should be putting out. Can you do a drawdown test on the suction tank to see what the pump is moving as a check against the two different numbers?
RE: choked flow and positive displacment pump
TD2K, we are measuring flow with a coriolis meter. The valve is plug and seat type (see attached curve). I'm not suprised at cavitation either. I guess I'm really trying to figure out what the max dP and max flow in Crane's really means. I said "we are testing," but actually it was a long time ago. In looking at valves for another application and doing the calculations I'm thinking back to that testing (which was for the pump, not the valve so it didn't concern me at the time) and wondering what exactly was going on. My numbers above were based on wrong fluid SG... Using Fp=1 I get 475 psi for max dP and 198 gpm for max flow at the valve position of the calculated Cv (9.1).
RE: choked flow and positive displacment pump
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: choked flow and positive displacment pump
Ted
RE: choked flow and positive displacment pump
RE: choked flow and positive displacment pump
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: choked flow and positive displacment pump
I don't see any specific discussion on choked flow of compressible fluids, but it's basically defined in the discussion on how an incompressible fluid becomes choked.
In the sizing and selection section for incompressible fluids after the initial Cv is calculated they say to check for the possibility of choked flow using equations for Qmax and delta-Pmax. It says these are the max flow and max differential pressure at which choking occurs at the valve position of the calculated Cv.
What I'm getting from my calcs is that if I put the valve in the position of the calculated Cv and turn the pump on, then I will quickly pop the relief valve. Of course that's not how we did the testing of the pump - we started with full open valve and slowly closed until we achieved the pump test pressure. I guess we achieved the pressure at a larger valve opening than calculated and that at that position our flow rate was somewhere between the incipient cavitation point and choked flow conditions. In this region the flow rate deviates from the Cv curve. That's my current theory anyway.
RE: choked flow and positive displacment pump
you did not say the temperature of the water, i guess 20C Deg?
choked flow for liquid seems just a range between incipient bubbling and fully vapourization. for the gas choke flow is pretty clear defined as "sonic velocity in that fluid".
you are lucky you guys did not test that pump with closed valve, otherwise, the pump will be damaged in a few second! I think your valve is choked or not , depends on the vapour pressure (temperature ) of downstream line. still 350gpm flow through the valve, it means not fully vaporized inside of valve. mostly, close to severe cavitation condition.
choked flow noise sounds like jet plane but less vibration, cavitation noise is not so sharp but more vibration on the valve body and downstream line.
Get help and return with helps!!.