Stability of pneumatic control valves
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In the ideal scenario, the steam control valve located before the primary system enters the heat exchanger is controlled using a transmitter that monitors the temperature of the fluid in the heated pipes of the secondary system; once the operator on the control screen sets a temperature, that temperature is achieved. For example, at 120 degrees Celsius, the control valve should be closed or its opening should be adjusted with slight fluctuations. Maintain stable temperature in the two circuits. Actual situation: Once the temperature in the two circuits reaches the set value, the valve continues to open and close frequently, fluctuating back and forth with very short intervals (several times in a few minutes), which causes the equipment on site to keep shaking. This not only poses a risk but may also shorten the valve’s lifespan and affect the production of process-related products. Part of the reason has been identified: after the control valve was closed, internal leakage occurred, causing the secondary circuit to continue being heated. (The internal leakage has been verified separately, and it indeed exists.) There is a question here: logically, even if the valve has internal leakage, once the set temperature is reached, the valve should receive an instruction to close; at least it should remain in the closed position. Why then does it move back and forth? I don’t quite understand. I would appreciate it if the experts and professionals could provide guidance and answer my questions. (It can be determined that the internal PID parameters of the control valve were not set correctly from the very beginning, after it was transported and installed.) Additional information: The control valve is of type ZJHM-16C; the pipe diameter is DN125, the pressure before the valve is 0.5 Mpa, and the steam temperature is 140°C; the mechanical positioner used is HEP-15. I hope everyone can help clarify this; I am extremely grateful. (I’m now planning to buy another control valve.)
In my opinion: 1. To maintain the temperature of the secondary circuit, heat loss must be taken into account, and this control valve should be set to have a minimum opening degree. 2. When controlling the opening degree of the control valve based on the temperature measured in the secondary circuit, the PID parameters must be set according to the actual conditions on site; the range of adjustment for the response time also needs to be set appropriately to prevent large fluctuations or frequent full-opening and full-closing operations.
Go back and discuss with the engineering team regarding the minimum opening. I don’t have a good signal generator at the moment, and I’m not sure whether a signal generator can be used to operate this valve for testing purposes. Now the internal procedures set by his manufacturer are also rather chaotic – which temperature should be chosen? As a sequential action of it, it has not yet been clarified
Most signal generators can be used to tune control valves; a 4-20ma signal is sufficient for this purpose.
I feel this post should be posted in Self-Control. 1/ The control valve cannot be used as a shut-off valve. 2/ Cascade control can be considered. For reference only.