Selection of cut-off valves
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I would like to ask everyone how to select pneumatic or electric shut-off valves – is it necessary to calculate the CV value, and is it required to know the flow rate? It is generally believed that when selecting a shut-off valve, knowing the pressure difference is sufficient; there is no need to calculate the CV or to know the flow rate. Is this statement correct?
The diameter of cut-off valves (gate valves, butterfly valves, ball valves) is generally the same as that of the pipeline. Only on/off is required; flow control is not necessary. So there is no need to calculate CV.
According to the process design, for special valves such as those used for venting in hydrogenation units that require flow limitation, it is necessary to calculate the flow rate. Generally, it is not required; refer to SH3005
The diameter of cut-off valves (gate valves, butterfly valves, ball valves) is generally the same as that of the pipeline. Only on/off is required; flow control is not necessary. So there is no need to calculate CV
Isn’t CV a parameter related to traffic? The cut-off valve is just 0 and 1, so it’s not necessary
1. Is throttling required? Pipelines that are in constant flow are equipped with shut-off functions. Since the process design does not want excessive pressure drop when the shut-off valve is fully open, valve bodies with low pressure loss are selected, such as ball, butterfly, or gate types. Their resistance drop in the fully open state is very low, on the order of KPA; if pipes of the same diameter are chosen, there is no need to calculate the CV value. To achieve flow control (throttling), a pressure drop across the valve must be assigned in order to regulate the flow rate; therefore, when choosing a seat valve of the throttling type, it is necessary to calculate the CV value. 2. Assume a high-pressure relief valve that requires manual operation; obviously, the piping system must absorb the pressure drop. Option A: Throttling seat valve – calculate the CV value and select an appropriate diameter. Option B: Ball valve/butterfly valve of the same diameter – no need to calculate the CV value, but a throttling orifice is installed after the valve to absorb the pressure drop. For valves with on/off switching functions, consult the process engineering team: should the pressure drop be kept as low as possible when the valve is fully open? A certain voltage drop still has to be absorbed, and in the vast majority of cases it is the former.