Pressure drop vs. Valve opening in Critical flow

[keunsoop]

Hi, I am struggling in modeling of multiphase flow valve in oil and gas production pipeline.

I am using Hydro long model, and trying to find C_T value (Throttling coefficient) to be matched with the measured pressure drop.

One interesting thing I found is for critical flow pressure drop decreases as decreasing valve opening (C_T value), while the pressure drop increases as decreasing valve opening for subcritical flow case.

I am looking for some theoretical background why pressure drop decreases as decreasing valve opening in critical flow. Is there anyone who knows why? Or my result is completely wrong?

Thanks.

Best regards,
Keunsoo

 

[zdas04]

It sounds like the model drops into a phase-change subroutine in the critical flow case and stays out of that subroutine in the sub-critical case.

It may or may not be valid, but critical flow in a multiphase line is absolutely the most chaotic condition that we see in fluid mechanics--gas flow is choked, but liquid flow is not, at what point does the liquid see very low pressure, what is the JT component, and a thousand other unknowables. Any assumption will be wrong at least as often as it is right.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[keunsoop]

Thank you very much for the answer.

Either subcritical or critical flow, my model includes phase change - mass fraction is from the pressure and temperature measurement in three positions of valve: 1) valve inlet, 2) valve outlet, 3) downstream of valve when flow reattach the wall.

Then can we find more clear answer why pressure drop decreases as closing valve more..?