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What are the common faults that valve positioners encounter during use? 1. Poor linearity of the valve: After a major overhaul, the valve exhibited fairly accurate zero and range values, but poor linearity. Upon checking the actuator, it was found to have poor balance; after readjustment and calibration, everything returned to normal. Furthermore, the aforementioned phenomenon can also occur when the rotator axis arm (feedback arm) of the locator and the connecting arm are not connected horizontally. 2. The valve does not operate; the air supply line from the actuator to the valve diaphragm head is corroded and leaking air. Process engineers reported that, after a signal was sent to open a control valve, its flow rate did not change. Upon on-site inspection, it was found that the air supply line (copper tube) connecting the actuator to the valve diaphragm head was corroded and leaking air. The issue was resolved by replacing it with a new stainless steel tube. 3. The valve operates slowly due to the small diameter of the hole at the diaphragm head connection, resulting in a low flow rate of air. After maintenance, the valve continued to operate slowly; it was found that there was no problem with the valve itself, and both the air supply and the input signal were normal. During the removal of the positioner for inspection, it was found that the bore size of the connector used in the gas pipeline leading from the positioner to the diaphragm head was too small, resulting in a very low flow rate of gas through it; this caused the valve to operate slowly. After replacing the connector, the valve operated quickly. 4. The valve does not operate; the balance beam is raised. In a manually controlled control valve that was functioning normally, process technicians discovered during a manual adjustment that the valve would not operate. An on-site inspection revealed that the balance beam of the actuator was raised; after restoring it to its proper position, the valve began to function normally. 5. Valve oscillation, amplifier failure: A steam control valve that had been functioning normally suddenly began to oscillate. With the input signal remaining unchanged, the valve continued to oscillate on its own. After on-site inspection and exclusion of issues related to the valve itself, such as overly tight packing, a failure in the valve positioner was suspected. Upon disassembling and analyzing the positioner, it was determined that the problem lay in its amplifier; replacing it restored normal operation. 6. The signal and air supply pressures are normal, but the valve’s stroke does not meet the required specifications. There is a newly installed control valve; the air supply pressure is normal and the input signal is also normal, yet the valve’s stroke never reaches its maximum value – it remains within 70% of the value indicated on its label, and cannot be fully opened. However, tests showed that its zero point was normal; analysis indicated that this was due to the lack of calibration before installation, and it returned to normal after adjusting the baffle assembly. Don’t panic when a fault occurs; analyze the cause of the fault and its severity, then proceed with further repairs! Therefore, it is very necessary to understand the common faults of valve positioning!