A. The regulating valve does not operate, and the possible faults and reasons are as follows:
1. No signal, no gas source
①The air source is not turned on, ②Due to the water content in the air source and freezing in winter, the air duct is blocked or the filter and pressure reducing valve are blocked and malfunctioned, ③The air compressor is faulty; ④The air source main pipe leaks.
2. With gas source, no signal
①The regulator is faulty, ②The signal tube leaks; ③The positioner bellows leaks;
3. Positioner without air source
①The filter is clogged; ②The pressure reducing valve is faulty; ③The pipeline is leaking or blocked.
4. Positioner has air source, no output
Orifice of positioner is clogged.
5. The positioner output is normal and the regulating valve does not act
①The valve core falls off, ②The valve core and the valve seat are stuck; ③The valve stem is bent or broken; ④The valve core is frozen or scorched and dirty; ⑤The spring of the actuator is rusted due to long-term use.
B. The action of the regulating valve is unstable. The symptoms and causes of the failure are as follows:
1. Unstable air source pressure
①The capacity of the compressor is too small; ②The pressure reducing valve is faulty.
2. Unstable signal pressure
①The time constant (T=RC) of the control system is not appropriate; ②The output of the regulator is unstable.
3. The air source pressure is stable, the signal pressure is also stable, but the action of the regulating valve is still unstable
①The amplifier in the positioner is worn or dirty, and the output shock will occur when the air consumption is particularly increased; ②The nozzle baffle of the amplifier in the positioner is not parallel, and the baffle cannot cover the nozzle; ③The output pipe and line leak;④ Stem friction resistance is large.
C. The regulating valve oscillates, and the fault phenomena and reasons are as follows:
1. Regulating valve oscillates at any opening
① The support is unstable; ② There is a vibration source nearby; ③ The valve core and bushing are severely worn; ④ Air leakage from the locator; ⑤ The packing is too tight; ⑥ The packing is biased;
2. Regulator valve oscillates near fully closed position
①The regulating valve is selected to be large, and it is often used at a small opening; ②The medium flow direction of the single-seat valve is opposite to the closing direction.
D. The action of the regulating valve is slow. The symptoms and causes of sluggishness are as follows:
1. The valve stem is only sluggish when acting in one direction
①The diaphragm in the pneumatic diaphragm actuator is damaged and leaked;
② "O" type seal leakage in the actuator.
2. The valve stem is sluggish when it reciprocates
① There is sticky matter in the valve body;
② PTFE filler deteriorates and hardens or graphite-asbestos filler lubricating oil dries up;
③The packing is pressed too tightly, and the frictional resistance increases;
④The frictional resistance is large due to the non-straightness of the valve stem;
⑤Pneumatic control valve without positioner will also cause delay in action
E. The leakage of the regulating valve increases. The reason for the leak is as follows:
1. Large leakage when the valve is fully closed
①The spool is worn and the internal leakage is serious.
②The valve is not properly adjusted and closed tightly.
2. Valve does not reach fully closed position
①The pressure difference of the medium is too large, the rigidity of the actuator is small, and the valve is not closed tightly;
② There are foreign objects in the valve;
③The bushing is sintered.