Common Fault Locations and Causes of Electric Control Valves

The performance of the regulating valve will directly affect the working quality of the entire regulating system. Because the regulating valve is in direct contact with the media being adjusted in the field, the working environment is very harsh, and therefore it is easy to produce various failures. In the production process, besides eliminating these faults at any time, regular maintenance and regular maintenance must be carried out. Especially for the use of a particularly bad control valve, it should pay more attention to maintenance and regular maintenance. Different types of regulating valves have different faults and their causes. Taking the straight-stroke electric control valve produced by the Sichuan Instrument Factory as an example, the general failure and maintenance methods of the electric control valve will be described.

2.1 Servo Amplifier When the servo amplifier is operating normally:

(1) When there is no input signal, there should be no output voltage.

(2) open-loop dead-zone current ≤ 160? A (type II is 100? A).

(3) When the input signal is >240?A (Type II is 150?A), the output load voltage is 205 to 220V AC.

(4) The output voltage is basically symmetrical.

If the servo amplifier is not working properly, there may be several situations in each part:

(1) Preamplifier

1 When no signal is input, the output of the two-shot magnetic amplifier can be adjusted to zero through the potentiometer W101.

If you can't zero it may be:

B. Welded or de-welded resistors R110 and R111 and electrolytic capacitors C101 and C102.

C. Welded or damaged diodes D105 to D108.

D. The offset current is abnormal. E. AC winding asymmetry.

2 There is an input signal, but no output or output asymmetry may be:

A. Transformer B301 provides asymmetrical secondary-side voltage for AC winding currents.

B. The resistance of the resistors R110 and R111 changes, and the electrolytic capacitors C110 and C111 are damaged.

C, AC excitation winding short circuit.

D. Some of the diodes D105 to D108 are damaged or have been welded.

(2) When the trigger has an input, one side of the trigger should have a pulse signal on the oscilloscope screen, change the polarity of the input signal, then the other side of the trigger has a pulse signal. The number and amplitude of the two sets of trigger pulses should be basically the same. Otherwise there are the following situations.

1 There is no input signal (preamplifier is in normal operation) and the output has a trigger pulse, which may be:

A. There is damage to the triode on the trigger pulse output side, or c, e extremely cold.

B, resistance R204, R205, R208, R209 resistance changes.

2 There is an input signal, the output has no trigger pulse, may be:

A. Transistor and single junction tube on the side without output pulse are damaged.

B. The primary or secondary open or short circuit of the pulse transformer on the side of no output pulse.

C. The diode on the output pulse side (D207 or D208) is short-circuited.

3 The output trigger pulse is not normal with the following phenomena:

A. When there is no signal, the two sets of triggers have pulse output. After adding the signal, they are all normal. It may be electrolytic welding of C202 or C203.

B. The output of both sides of the trigger is normal. After working for a period of time, the output pulse gradually disappears and there may be problems with the single junction tube.

C. Positive and negative pulses appear on the output. The diodes D207 and D208 may be broken down or soldered.

(3) After the above inspection and maintenance, the servo amplifier should be able to work normally if there is no fault in the control circuit. Otherwise there are two situations:

1 When there is no input signal, the output voltage (voltage close to 220VAC) may be:

A, thyristor SCR1, SCR2 is damaged.

B. The capacitors C301, C302 are broken down.

2 When there is an input signal, no output signal or output voltage is low, it may be:

A. There are breakdowns in the diodes D301 to D308.

B. The thyristor SCR1 and SCR2 are damaged.

2.2 Regulating valve When the 4 to 20 mA signal is input, the valve stem of the regulating valve can move freely up and down within the rated stroke as the signal changes. If it fails to operate properly, the following problems may occur: 1 The operation hand wheel feels abnormal

A. It feels too light. It may be the pinch pin off or broken.

B. Feeling too heavy or spinning. Foreign material stuck in the reducer; the valve plug and the bushing or the valve seat are stuck; the valve stem is seriously bent.

2 Input 4 to 20 mA signal, the regulating valve does not operate.

A. The system wiring is incorrect.

B. The motor phase separation capacitor CD is damaged.

C, motor wiring open circuit.

D. The valve core is detached or the valve stem is broken (at this time, the actuator has an action)

3 Input 4 ~ 20 mA signal, adjust the valve can not complete the rated travel.

A. The position of the differential transformer is not adjusted.

B. Some of the diodes D701 to D712 are damaged.

C. The transistor BG701 is damaged.

(4) Other failures at the site.

1 The valve oscillates and buzzes.

A. The sensitivity is adjusted too high and the actuator is oscillating.

B. Fluid pressure changes too much and actuator thrust is insufficient.

C. Large selection of regulating valve, valves often work at small opening degree.

D. The direction of flow of the medium is the same as the direction of valve closing.

E. The influence of other nearby earthquake sources is not stable.

F. The valve plug and bushing wear badly.

2 The valve is dull.

A, medium viscosity is too large, there is a blockage or coking phenomenon.

B, filler aging, packing pressure too tight.

3 large amount of leakage.

A. The valve plug or valve seat is corroded and worn.

B. The valve seat is loose or the thread is corroded.

C. There are foreign objects on the valve seat and spool.

D. The starting point of the valve (electrically open) or the end point (electrically closed) is not adjusted.

4 The packing and the upper and lower bonnet connections leak.

A. The packing gland is not pressed.

B, aging PTFE filler deterioration.

C. The stem is damaged.

D. Loosen the fastening nut.

E. The gasket is damaged.