Regarding the calculation of control valves
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In the calculation formula for control valves, the greater the pressure difference before and after the valve, the greater the flow rate. But that means that as the valve opening decreases, the resistance increases, the pressure difference increases, and thus the flow rate also increases – isn’t this contrary to common sense? If the valve is fully closed, the pressure difference before and after is at its maximum; wouldn’t the flow rate then be at its highest?
If the opening remains constant, the greater the pressure difference, the greater the flow rate. If the pressure difference remains constant, the greater the opening, the greater the flow rate.
So how is a blocked flow determined? Isn’t it the case that the valve opens smaller and smaller, to the point where flow obstruction may occur?
You’ve misunderstood; it’s in two stages. 1. During the design phase, the lower the pressure drop required by the process, and the higher the flow rate, the larger the flow area required of the valve, which in turn means a larger valve diameter. 2. Operation status: When the valve is closed, the flow area decreases, and at the same time the differential pressure across the valve increases. However, the square root of the pressure difference is linearly proportional to the flow rate (the flow area is also linearly proportional), so the actual flow rate that passes through will be reduced. In the case of a choked flow, flow rate has no relation to pressure difference, but there is still a linear proportional relationship with the flow area. @jiaguoyun
It’s clear now. Can the first point be understood as meaning that this pressure drop is an inherent characteristic of the valve – each type of valve has its own corresponding pressure drop?
No, this pressure drop was proposed by the process systems team. When calculating the pressure drop in the pipeline, he reserved some of that drop for the control valve, to use as a means of regulating the flow rate in the pipeline; since only the control valve can be moved in the pipeline, while manual valves generally do not move. The inherent characteristic of a valve refers to the relationship between the valve’s opening degree and the flow rate, under the condition that there is only the pressure drop caused by the control valve in the pipeline, with no other resistance losses. This is an ideal situation; it’s as if you own 100% of the shares, meaning it’s you who determines the stock price (i.e., the flow rate). But in reality, due to the pressure drop caused by the pipes (or other equipment such as filters and heat exchangers), the control valve cannot achieve 100% opening; therefore, the actual opening degree must be higher than that in the \"ideal situation\". Increasing the flow area is necessary to meet the desired flow rate. The pressure drop of the control valve/total pipeline pressure drop is defined as S. Typically, valve software calculates based on S=1; the smaller the value of S, the larger the valve diameter you need to choose. However, the calculation software for butterfly valves takes S into account, because the value of S for butterfly valves is generally low, resulting in significant flow distortion, so this factor must be considered. @jiaguoyun
I’ve learned it; thanks for the thorough analysis