On-site instrument failure analysis (one question per week): Question 12

1. The control valve oscillates at any opening degree: ① Unstable support； ②There is a vibration source nearby ； ③The valve core and bushing are severely worn. 2. Oscillation of the control valve when it is near the fully closed position: ① The control valve selected is too large, and it is often used at a low opening degree ； ②In a single-seat valve, the flow direction of the medium is opposite to the closing direction. 3. Oscillation in the positioner output ； 4. Improper setting of PID parameters in the control system of the control valve.

1. Check the locator output for oscillation using a wire, and test by repeating the action. 2. Changes in process temperature may cause the metal to expand, resulting in greater friction between the valve core and the valve cage, as well as the bushings and packing. 3. If the control valve is selected to be too large, it operates at a low opening degree, resulting in valve oscillation.

1. The valve positioner has excessive sensitivity. 2. The flow coefficient C value is set too high, causing the control valve to operate at a low opening degree. 3. High friction in the valve stem leads to sluggish oscillations. 4. Insufficient stiffness of the actuator results in oscillations throughout its full range of motion. 5. Insufficient spring pre-tensioning causes oscillations at low positions. 6. Vibration in the pipes or base

1. Selection issues for control valves: a. Large pressure difference before and after the fluid in the pipeline； b. The control valve is used at a low opening degree. 2. Issues with air supply to the actuator: a. Unstable air source pressure ； b. Air filter failure ； c. Valve positioner failure, such as issues between the nozzle and the baffle, amplifier throttle hole 3; process-related problems: large fluctuations in fluid pressure within the pipeline.

The reasons are: first, oscillations in the output air pressure of the locator. (1) Air leakage from the air filtration pressure regulator causes unstable air supply. (2) The constant-throttling orifice of the valve positioner is dirty, and there is blockage in the air circuit. (3) The feedback rod connecting the valve positioner and the control valve is not properly connected. II. Control valve failure. (1) Due to the excessively large selected diameter, the control valve often operates at a low opening degree, resulting in reduced control quality. (2) Oscillations are caused by air leakage in the diaphragm of the control valve. (3) The adjustment curve is deformed due to severe wear of the valve core and valve seat. III. Process-related reasons: the pressure difference before and after the control valve exceeds the rated value.

There are many reasons for oscillation in self-regulating valves. If the 4–20mA signal is normal, it indicates that there are no issues with the control mechanisms or PID parameters; the problem lies either with the self-regulating valve itself, the electrical valve actuator, or the air supply. 1. First, check whether the air supply pressure is stable ； 2. If the air supply pressure is stable, then check the electrical valve positioner: whether the gap between the nozzle and the baffle is too small, whether the throttle holes are blocked, whether the feedback rod is loose, whether there are any loose parts in the mechanical components, and whether the interior of the amplifier is dirty or contains impurities. Most of the oscillations in self-regulating valves are caused by a faulty actuator. 3. If the locator cannot identify the cause of the problem, another locator can be tried. If the issue persists, it indicates that there is a problem with the self-regulating valve itself: whether the diaphragm is leaking air, or whether the movement of the valve stem is smooth. Wear on the valve core and seat should not cause oscillations in the valve.

Since 4~20mA is normal, therefore: 1. Check whether the air supply pressure is normal； Whether it contains water, and whether there is frosting. 2. If the air supply pressure is normal, check whether the travel of the valve positioner follows the specified pattern. 3. If the valve has not vibrated before, then it is not due to PID vibration. If it is a new valve, consider PID tuning. 4. Finally, consider issues inside the valve: whether the packing is too tight, whether it is operating at a low opening degree, and whether corrosion of the lining causes friction, etc.

When the electrical signal is normal, it can be checked from the following aspects: 1. Whether the air path is unobstructed. Check everything from the gas supply main interface to the output of the locator ； 2. Is there any damage to the membrane of the actuator? ； 3. The pressure (flow rate) of the medium in the process pipeline under consideration fluctuates ； 4. The PID parameter settings are inappropriate ； 5. Is there any sticking of the valve core during operation? ； 6. There is a discrepancy between the calculation and selection of the control valve and the process requirements.

1. In addition to faults in the valve itself, it is necessary to consider whether there are any disturbances affecting the controlled variable, as well as changes in the operating variable caused by large amplitude variations. 2. The pressure of the process medium is too high, preventing the valve from enabling fine-flow regulation