Is the pressure regulation of a self-acting control valve related to flow rate?
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Main pipeline + self-acting control valve / Branch pipeline 1 + self-acting control valve 1 -- Burner 1 \ Branch pipeline 2 + self-acting control valve 2 -- Burner 2. All three self-acting control valves are used for pressure stabilization downstream of the valves; however, the pressure downstream remains fluctuating. Valve regulation results in surging, and the valve stem keeps moving up and down without stopping. The designed minimum flow rate for the main pipeline is 430, but the actual value is 150. Could the surging be related to the fact that the valves chosen are too large? It is always said that self-acting control valves only regulate pressure and not flow, but does the amount of flow affect pressure regulation?
The use of this type of valve is to adjust the pressure difference within a certain range. A certain pressure should be established between the control valve and the combustion nozzle; it is best if this pressure falls within the operating range of the control valve. The current fluctuation may be caused by selecting a valve with an excessively large diameter. With high flow rate, the relative pressure decreases. If this increase in flow is not sufficient to reduce the pressure, then the choice of valve becomes more critical. Because a slight increase in pressure over this distance to the nozzles is sufficient to cause the valve to close. For reference.
Whether the regulation is stable is related to whether the load is stable. Large fluctuations in the load mean large changes in flow rate, which in turn leads to significant pressure fluctuations; as a result, the regulation becomes unstable.
It’s related to traffic. (Just like a pressure relief orifice plate, the pressure behind the flow control plate must change.) The burner generally exhibits a linear P-Q relationship, while the check valve has a linear (or quick-opening) characteristic. At low opening degrees, the actual behavior of the valve is such that it opens quickly; the flow rate increases rapidly upon any movement, but the burner cannot handle that much flow, so the pressure rises. As a result, the PCV valve closes again, leading to this repetitive cycle. You can try tightening the manual valve before the PCV. It is best to obtain the load curve (pressure-flow) of the burner, and select a self-acting valve based on this curve; since the burner’s load does not change suddenly, the PCV does not need to be very large. Compared to nitrogen-sealed PCVs, it has higher dynamic requirements; after all, the large volume of the tank results in a large time constant. However, burner PCVs need to adjust quickly to match the load.
For a system like this, it is recommended that you set one of the branches to manual mode; otherwise, fluctuations in one branch will interfere with the others!
I’ve learned something! We are also considering trying to reduce the size of the process shut-off valve in front of the self-acting control valve, in order to minimize the amplitude of fluctuations. Back-pressure stabilization after the valve, using nitrogen sealing, is also widely employed; however, pressure fluctuations after the valve and valve surge phenomena do occur. What’s more interesting is that when the nitrogen pressure was reduced from 0.5 MPa to 5 KPa, and after initial installation and commissioning, no issues were observed in the process. However, once the gas-phase water seal was installed in the tank as part of the operational setup, the pressure behind the self-acting control valve could only reach 12–13 KPa; no changes occurred regardless of attempts to adjust it. The manufacturer was consulted, and it was suspected that the valve core might be damaged. At present, it’s not possible to inspect the valve offline, so the globe valve before the self-acting valve has to be adjusted to a position that maintains a pressure of 5 KPa behind the valve. Yet, it’s likely that the flow rate resulting from this setup will not meet the actual requirements.