Problems with flow control valves
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In production, we encountered a flow control valve of this type: a 1-inch single-seat valve. When the output is increased from bottom to top, for example to 45%, the flow rate recorded at each 5% increase differs significantly from the flow rate recorded when the output is decreased, for example to 75%; at each 5% decrease, the flow rate differs greatly from the flow rate corresponding to the same output level when it is increased. This is an equal-percentage control valve, and the actuator used is a Siemens PS2 intelligent actuator. We checked that the feedback rod is properly fixed, and the actuator has also been calibrated, but the problem persists. We’re not sure what the cause could be
Equal percentage refers to adjusting the same opening degree, with the change in flow rate relative to the full-open flow rate expressed as a percentage. In other words, a 5% change in the opening angle results in a large change in flow rate, while the change at larger openings is relatively smaller
Look up the definition in a book; it’s a percentage, not a flow value
The definition of equal percentage is that the change in relative flow rate caused by a unit change in stroke is proportional to the relative flow rate at that point
I may not have expressed myself clearly. The problem I’m describing is that, for example, there is a significant difference in flow rate when the valve is opened to 50% compared to when it is closed at 50%. Regardless of the type of valve, I think that when the pipeline pressure is stable, there shouldn’t be any such difference
I’m not quite sure what the original poster’s goal is. For equal percentage valves, the flow rate changes at 45% and 74% should be unequal ； If we look at the deterioration, it should be the traffic when rising to 45% compared to the traffic when falling to 45%. The flow rate under the operating conditions of the control valve changes significantly. Theoretically, the positioning accuracy achieved through the locator is only 2.5% (in static conditions); under actual operating conditions, the positioning accuracy becomes even worse, resulting in a large deviation in the flow rate values. If the actual position of the valve is measured using a dial indicator, the flow rates will be relatively close at the same position, provided that the pressure difference before and after the valve remains constant (this needs to be measured with a differential pressure gauge; using two pressure gauges alone will not yield accurate results). .
What I mean is that when it rises to 45% or falls to 45%, the traffic volume differs too much
How much less? What is the flow rate at 100% opening?
If it can automatically adjust the flow rate within the specified operational range, then this control valve is an ideal choice – there’s no need to worry about it
I roughly understand what you mean. You’re saying that when I reduce the opening from 55% to 50%, the flow rate is A； When the opening is increased from 45% to 50%, the flow rate is B; the difference between A and B is quite large, right? Other parameters are ignored; the flow rate is determined by the velocity, that is, by the pressure difference. Were these two flow tests conducted at the same time? If tested at the same time, it shouldn’t be like that
This is a test carried out during production; changes in flow rate are also related to changes in load. Even with the same valve opening, the flow rate can vary. To measure the flow rate using the method suggested by the original poster, it is necessary, in my opinion, to ensure that the pressure difference before and after the control valve is the same, as well as that the pressure before the control valve remains stable. Only under such conditions is such a comparison meaningful.