The issue of pressure drop in the control valve during the flashing process
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If there is a liquid before the control valve and flashing occurs after the valve, resulting in a two-phase flow, then can’t we calculate the pressure drop across the control valve by subtracting the static pressure behind the valve from the static pressure before the valve? In this case, how should the pressure drop of the control valve be calculated? Is it necessary to determine the frictional resistance loss as fluid passes through the control valve? I hope I can discuss this with everyone.
Personally, I think this pressure drop is determined by the flash tank behind your control valve. Once the pressure of the liquid before the valve is determined, it becomes clear what pressure is required for flashing, as well as what pressure should be maintained in the flash tank behind the valve; thus, the pressure drop across the control valve can be calculated. So, what you said is not wrong; it’s just reversed. The pressure drop of the control valve should be the static pressure before the valve minus the static pressure after the valve.
Yes, what I mean is the static pressure before the valve minus the static pressure after the valve; I said it backwards. The pressure of the liquid before the valve is fixed, and the pressure in the flash tank is also fixed; however, for pipelines with a height difference, it seems to me that the pressure before the valve minus the pressure in the flash tank does not represent the total pressure drop along the entire pipeline. Especially when it comes to problems involving two-phase flow, things become even more complex. Returning to the question I asked: there is no height difference before and after the valve. For single-phase flow, the pressure drop across the control valve is indeed equal to the static pressure before the valve minus the static pressure after the valve. But what about the case where two-phase flow occurs after the valve? It should be different, right?
The control valve should be located near the flash tank. As you mentioned, after flashing, a mixture of gas and liquid exists; if the pipeline from the control valve to the flash tank is too long, it is necessary to calculate the flow rate or even increase the diameter of the pipeline. Therefore, in the case of a setup close to the flash tank, the pressure behind the valve can be considered to be the pressure of the flash tank. Of course, you can also determine the pressure behind the valve by using the pressure in the flash tank, and calculate the pressure loss based on the state of the gas-liquid mixture after flashing.
What are your insights on the pressure drop calculation for control valves where flashing occurs downstream of the valve?