With the rapid development of my country's industry, the application of control valves in metallurgy, petrochemical andother fields has become more and more extensive, and its stability and reliability have become more and more important.ts working status will directly affect the automatic control process., his article wil elaborate on the use and maintenanceof electric controlvalves.

1.Precautions forinstallation and use
For the newly designed and installed control system, in order to ensure that the regulating valve can work normally whiledriving, and to make the system operate safely, before installing the new valve, first check whether the name plate markon the valve conforms to the design requirements.At the same time, the following items should be debugged.Basic error limit; fullstroke deviation; return difference; dead zone; leakage (in strict requirements). If the control valve in the original system has been overhauled, in addition to checking the above items, the stufing boxand connections of the old valve should also be checked for tightness. in the field use of regulating valves, many of them are not caused by the quality ofthe regulating valve itself, but causedby the improper installation and use of the reaulating valve. such as the installation environment, improper installation position and direction or unclean pipelines, Therefore the following aspects should be paid attention to when installingand using electric controlvalves:
(1)The regulating valve is a field instrument, and the ambient temperature should be in the range of -25-60%C and therelative humidity should be 95%. If it is installed in the open air or high temperature, waterproof and cooling measuresshould be taken. Keep away from the vibration source or increase anti-vibration measures where there is a seismic source
2) The regulating valve should generally be installed vertically, and it can be tilted under special circumstances. Forexample, when the tilt angle is arge or the pavion itselfis too heavy, the valve should be protected by supporting parts.
3) Generally, the pipeline for installing the regulating valve should not be too high from the ground or the floor. When thepipeline height is greater than 2 m, the platform should be set as far as possible to facilitate the operation of the handwheeland facilitate the maintenan ce.
4)The pipeline should be cleaned before installation of the regulating valve to remove dirt and welding slag. Afterinstallation, in order to ensure that no impurities remain in the valve body, the valve should be cleaned again, that is, allvalves should be opened when the medium is introduced to prevent impurities from getting stuck. After using thehandwheel mechanism, it should be restored to the original neutral position.
5) ln order to enable the production process to continue in the event of failure or maintenance of the reaulating valve. abypass pipeline should be added to the regulating valve.At the same time, special attention should be paid to whether the installation position of the adjustment read meets therequirements of the process.
(6) The electrical part ofthe electric control valve should be installed in accordance with the construction requirementsof the relevant electrical eguipment, Ifit is an explosion-proof product, it should be installed in accordance with therequirements of the installation Specification for Electrical Equipment in Explosive Hazardous Locations. For example,the field wire adopts SBH type or other six-core or eight-core rubber installation cable with an outer diameter of about11.3mm. During use and maintenance, it is strictly forbidden to open the cover for maintenance and pry on theexplosion-proof surface in explosive places. At the same time, do not knock or scratch the explosion-proof surface duringdisassembly and assembly, and restore to the original explosion-proofrequirement state after maintenance.
(7)Pay attention to lubrication after disassembly and repair ofthe reducer of the actuator. Generally, do not disassemble.wash and refuel the low-speed motor.After assembly, check whether the valve position is consistent with the valve position openingindication.

2.Common faultlocations and cause analysis
The performance ofthe regulating valve will directly affect the working quality of the entire regulating system. Becausethe regulating valve is in direct contact with the adiusted medium on site, the working environment is very harsh, sovarious failures are prone to occur, ln the production process, in addition to troubleshootina these faults at any time.harsh operating environment, more attention should be paid to maintenance and regular overhaul.
Different types of control valves have different faults and their causes. Taking the straight-stroke electric control valveproduced by Sichuan instrument Factory as an example, the genera faults and maintenance methods of electric controvalves are explained.
2.1 Servo amplifier
When the servo amplifieris working normally:
(1) When there is no input signal, there should be no output voltage.
(2) The open-loop dead zone current is less than or equal to 160 A (type ll is 100 uA).
(3) When the input signalis >240 uA (type ll is 150 uA), the output load voltage is 205-220V AC
(4) The output voltage is basically symmetrical.
Ifthe servo amplifier does not work normally, there may be the following situations in each part
(1) Pre-magnetic amplifier
1 When there is no signal input, the output of the dual-beat magnetic amplifier can be adjusted to zero through thepotentiometer W101.lfzero adjustment is not possibleit may be: A.Transformer W101is desoldered or damaged.
B.Resistance R110, R111 and electrolytic capacitors C101,C102 are soldered or unsoldered.
C.The diodes D105-D108 are soldered or damaged.
D.The offset current is abnormal.
E.The AC winding is asymmetrical
2 There is input signal, but no output or output asymmetry may be:
A. The voltage on the secondary side of the transformer B301 for AC winding current is asymmetrical.
B,The resistance of the resistors R110 and R111 has changed, and the electrolytic capacitors C110 and C111 are damaged
C. Short circuit of AC excitation winding
D.Some of the diodes D105-D108 are damaged or soldered.
(2) When the trigger has input, one of the triggers should have a pulse signal on the oscilloscope screen.lf the polarity ofthe input signalis changed, the other side ofthe trigger has a pulse signal. The number and amplitude ofthe two sets oftrigger pulses should be basically the same. Otherwise, there are the following situations. 1)There is no input signal (the pre-magnetic amplifier is in normal working condition), and the output terminal hastrigger pulsewhich may be: A. The transistor on the side where the trigger pulse is output is damaged, or the c and e poles are weldedof resistors R204,R205,R208,and R209 have changed. B. The resistance values
2).There is an input signaland no trigger pulse at the output, which may be: A. The triode and single junction tube on the side without output pulse are damaged. B. The primary or secondary ofthe pulse transformer on the side without output pulse is open or short-circuitedC The diode (D207 or D208) on the side without output pulse is short- circuited.
3.)The abnormaloutput trigger pulse has the following phenomena:
A. When there is no sianal. the two sets oftriagers have puse output,After adding the sianal,they are all normal, lt mabe the electrolytic capacitor C202 or C203.
B. At the beginning,the output on both sides of the trigger is normal. After working for a period of time, the output pulsegradually disappears, and there may be a problem with the single junction tube. C. Positive and negative pulses appear in the output. lt may be that the diodes D207 and D208 are broken down or soldered
(3) After the above inspection and maintenance, the servo amplifier should be able to work normally under the condition
that the controlloop is not faulty.Otherwise, there are two situations:
1) When there is no input signal, there is an output voltage (voltage close to 220VAC), it may be:
A.SCRand SCR2 are damaged.
B.The capacitors C301 and C302 are broken down. 2) When there is an input signal, there is no output signal or the output voltage is low, it may be:
A.Some ofthe diodes D301- D308 are broken down. B.SCR1and SCR2 are damaged.
2.2 Requlating Valve
When a 4-20 mA signal is input, the valve stem of the regulating valve can move up and down freely with the change of the
signal within the rated stroke, lfit cannot operate normally, the following failures may occur:
1.Abnormalfeeling ofoperating handwheel
A. lt feels too light, it may be that the handwheel pin is off or broken.
B.Feeling too heavy or unable to spin. There is a foreign body stuck in the reducer; the valve core and the bushing or the
valve seat are stuck; the valve stem is severely bent.
2Input 4-20 mA signalthe controlvalve will not operate.
A.The system wiring is incorrect.
B.The motor split capacitor CD is damaged.C.The motor wiring is open.D. The spool falls off or the stem is broken (the actuator is in action at this time)3Input 4-20 mA signal, the regulating valve can not complete the rated stroke.A. The position of the differential transformer is not adjusted properly.B.Individually damaged diodes D701-D712.C.The transistor BG701 is damaged.
(4)Other failures during on-site use
1The valve oscillates and buzzes.
A.lf the sensitivity is adjusted too high, the actuator will oscillate.
B. The fluid pressure changes too much, and the actuator thrust is insufficient.
c. The choice of regulating valve is large, and the valve often works at a small opening.
DThe flow direction of the medium is consistent with the closing direction of the valve
E. The support is unstable due to the influence ofother sources nearby.
F.The valve core and bushing are seriously worn
(2The valve action is slow.A.The medium is too viscous, clogging or coking.B.The packing is aging and the packing is too tight
3 The leakageislarge.
A.The valve core or valve seatis corroded and worn.
B.The valve seatisloose or the threads are corroded.
C.There are foreign objects on the valve seat and valve core.
D.The starting point (electrically open) or end point(electrically closed) ofthe valve is not adjusted properly.
4. Leakage at the connection between the packing and the upper and lower valve covers
A.The packing gland is not tightly pressed.
B.The PTFE filler is aged and deteriorated.
C.The valve stem is damaged.
D.The tightening nut isloose.
E.The gasket is damaged.

3 Conclusion
The correct use and maintenance of the reaulating valve can not only improve the reliability ofthe process control, but also increase the service life of the regulating valve, which has considerable economic benefits for the energy saving and consumption reduction of the enterprise