The issue of pressure drop in control valves
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The pressure in D-301 is 0.3, the pressure in the nitrogen main line is 0.8, and the pressure in the vent tank is 0.03. Is it correct to measure the pressures before and after the valve in this way? Does the pressure drop of the pipeline need to be considered?
Typically, step control is used to regulate the tank head pressure. It depends on the length of the pipeline later on; logically, pipe friction loss should be taken into consideration. This jar can handle quite a bit of pressure. Why are we all at low pressure?
Whether it is filling with gas or releasing gas, the purpose is to maintain the pressure inside the tank; therefore, it makes sense to have the measurement points on the tank. By using a control valve, there is no need to consider the pressure drop in the pipeline, as the valve operates based on changes in the pressure within the tank, rather than the pressure behind (or in front of) the valve. If an external pressure-taking self-acting valve is used (with the pressure sensing port on or near the equipment), the pressure drop must be taken into consideration, as the pressure before/after the valve \"controls\" the degree of opening of the valve; this is especially true in situations where the tank pressure is very low, such as a few KPA, as the ratio of pipeline pressure drop to the set value becomes large.
It is recommended to set the valve selection parameters at 20101B 0.8/0.25 and 20101A 035/0.03, in order to allow some variation in the tank’s pressure levels and thereby reduce the number of times the control valve needs to be activated
No problem; after all, it’s a regular valve, so the impact is minimal.
A P&ID diagram is a schematic diagram, not a piping diagram. It is unknown what the specific piping route is, and how many elbows, reducers, and expansions are present along that route How long is the piping path? What is the pipe diameter? What kind of pipe? What is the roughness of the inner wall of the tube? What is the viscosity of the medium? These all determine the pipeline pressure drop. If the path is short, there is no necking/expansion, and there are few bends, the pressure drop can be ignored; otherwise, the pipeline pressure drop must be taken into account.
Is it reasonable to use one pipe port for nitrogen filling and pressure relief?
The design for the pressure stages is fine; the problem lies in the data you provided. (This is a control valve – how did you arrive at those values? Consider that the maximum value is 0.8, so the maximum pressure before the relief valve will not be low either.)
Personally, I think there’s nothing wrong with this requirement; it’s a typical example of step-by-step control