Process parameters of gravity flow control valve
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Have anyone used control valves to regulate flow rate under gravity flow conditions? How to accurately provide the pressure before and after the control valve? In my personal opinion, it’s easier to increase the pressure before the valve, but it’s more difficult to do so after the valve. I would appreciate your guidance. Thank you!
Based on the hydrostatic head height h of the two sections of the pipe, the pressure drop is equal to h minus the resistance loss in the piping system; the valve is placed further downstream to prevent the liquid behind it from vaporizing.
Agreed; if the control valve is installed very close to the downstream equipment, the pressure behind the valve will be equal to the pressure at the equipment. If, due to layout reasons, the control valve is installed in the middle or at an upper position, the pressure behind the valve may be negative, making it difficult to calculate accurately.
P2 is calculated inversely based on the system’s end pressure (such as tower pressure). You cannot control the flashing behind the valve, because the P2 pressure is controlled by the downstream system; when the pressure in that downstream system decreases, flashing occurs behind the valve, and the control valve merely reduces the pressure drop further. With P1 and other physical property parameters, it is possible to estimate whether flashing will occur inside the valve; this is the task of the instruments, not something that the process needs to worry about. Place the valve at the lowest point of the pipeline to ensure that P1 is as high as possible. There’s no need to consider the diameter either; the control valve is the same size as the process pipe, so select the highest CV setting. Even if you reduce the caliber by one stage, how much can the opening be changed? Under such operating conditions, it wasn’t necessary to install a control valve; the process simply required a slight pressure drop to regulate the flow rate. . .
Under these operating conditions, the equipment upstream and downstream of the control valve is at equal pressure, with no pressure variations within the equipment further downstream. According to Bernoulli’s equation, once a continuous flow is established, the closer the control valve is to the upstream side, the lower the negative pressure generated behind the valve, and the greater the flow rate. Therefore, for gravity flow pipes that can generally form a continuous flow, it is incorrect to use the empirical value of 0.5 m/s.
Please consider my question again: “The upstream and downstream devices are voltage-balanced.” Is then the flow rate controlled by the pressure drop of the control valve? For example, if I manually reduce the valve opening (increasing the resistance), the flow rate should decrease, right? “After a continuous flow is established, the closer the control valve is to the upstream side, the lower the negative pressure formed behind the valve, and the greater the flow rate. ” If the same valve is used with the same opening degree (i.e., the same resistance coefficient), will the outlet pressure P2 at the upstream side of the valve be lower than that at the downstream side?