Electric control valves are more and more widely used in metallurgy, petrochemical and other fields, and their stability and reliability are becoming more and more important. The quality of their working conditions will directly affect the automatic control process. This article will elaborate on electric control. Use and maintenance of valves.
1 Precautions for installation and use
For the newly designed and installed control system, in order to ensure the normal operation of the regulating valve during driving and the safe operation of the system, before installing the new valve, first check whether the nameplate mark on the valve conforms to the design requirements.At the same time, the following items should be debugged.
Basic error limit; full stroke 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 stuffing box and 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 of the regulating valve itself, but caused by improper installation and use of the regulating valve, such as the installation environment, improper installation position and direction, or unclean pipelines.Therefore, the following aspects should be paid attention to when installing and using the electric control valve:
(1) The regulating valve is a field instrument, and the ambient temperature should be in the range of -25～60℃, and the relative humidity should be ≤95%.If it is installed in the open air or high temperature, it should be adoptedwaterproof, Cooling measures.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 can be tilted under special circumstances. For example, when the tilt angle is large or the valve itself is too heavy, the valve should be protected by supporting parts.
(3) Install the regulating valvepipelineGenerally do not leave the ground orfloorToo high, when the height of the pipeline is greater than 2m, the platform should be set as far as possible to facilitate the operation of the handwheel and facilitate the maintenance.
(4) The pipeline should be cleaned before installation of the regulating valve to remove dirt and welding slag. After installation, in order to ensure that no impurities remain in the valve body, thevalveFor cleaning, that is, all valves should be opened when the medium is introduced to prevent impurities from getting stuck.After using the handwheel mechanism, it should be restored to the original neutral position.
(5) In order to enable the production process to continue in the event of failure or maintenance of the regulating valve, a bypass pipeline should be added to the regulating valve.
At the same time, special attention should be paid to whether the installation position of the regulating valve meets the requirements of the process.
(6) The installation of the electrical part of the electric control valve should be carried out in accordance with the construction requirements of the relevant electrical equipment.If it is an explosion-proof product, it should be installed in accordance with the requirements of the "Specification for Installation of Electrical Equipment in Explosive Hazardous Locations".If the field wire adopts SBH type or other six-core or eight-core, the outer diameter is Φ11.3
Install the cable with rubber around mm.During use and maintenance, it is strictly forbidden to open the cover for maintenance and pry on the explosion-proof surface in explosive places.At the same time, do not knock or scratch the explosion-proof surface during disassembly and assembly, and restore to the original explosion-proof requirement state after maintenance.
(7) Pay attention to lubrication after disassembly and repair of the 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 opening indication.
2 common fault locations and cause analysis
The performance of the regulating valve will directly affect the working quality of the entire regulating system.Since the regulating valve is in direct contact with the regulated medium on site, the working environment is very harsh, so various failures are prone to occur.In the production process, in addition to troubleshooting these faults at any time, regular maintenance and regular overhaul must also be carried out.Especially for control valves with extremely harsh operating environments, more attention should be paid to maintenance and regular overhaul.
2.1 Servo amplifier
When the servo amplifier is 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 160μA (type II is 100μA).
(3)输入信号＞240μA(Ⅱ型为150μA)时，输出负载电压为205～220VAC。
(4) The output voltage is basically symmetrical.
If the servo amplifier is not working properly, there may be the following situations in each part:
(1) Pre-magnetic amplifier
① When there is no signal input, the output of the double-beat magnetic amplifier can be adjusted to zero through the potentiometer W101.If zero adjustment is not possible, it may be:
A. Transformer W101 is 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.
②There 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 values of the resistors R110 and R111 have changed, and the electrolytic capacitors C110 and C111 are damaged.
C. The AC excitation winding is short-circuited.
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. If the polarity of the input signal is changed, 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 (the pre-magnetic amplifier is in normal working condition), and there is a trigger pulse at the output end, which may be:
A. The triode on the side where the trigger pulse is output is damaged, or the c and e poles are welded.
B. The resistance values of resistors R204, R205, R208, and R209 have changed.
②There is an input signal, but there is no trigger pulse at the output end, it may be:
A. The triode and single junction tube on the side without output pulse are damaged.
B. The primary or secondary of the pulse transformer on the side without output pulse is open or short-circuited.
C. The diode (D207 or D208) on the side without output pulse is short-circuited.
③The abnormal output trigger pulse has 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 that the electrolytic capacitor C202 or C203 is soldered.
B. At the beginning, the output on both sides of the trigger is normal. After working for a period of time, the output pulse gradually disappears. There may be a problem with the single junction tube.
C. Positive and negative pulses appear in the output, which may be caused by breakdown or virtual welding of diodes D207 and D208.
(3) After the above inspection and maintenance, the servo amplifier should be able to work normally under the condition that the control loop is not faulty.Otherwise, there are the following two situations:
① When there is no input signal, there is an output voltage (voltage close to 220VAC), it may be:
A. The SCR1 and SCR2 are damaged.
B. Capacitors C301 and C302 are broken down.
②When there is input signal, there is no output signal or the output voltage is low, it may be:
A. Some of the diodes D301 to D308 are broken down.
B. The SCR1 and SCR2 are damaged.
2.2 Regulating valve
When a 4-20mA 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.If it fails to operate normally, the following faults may occur:
① Feeling abnormal when operating the handwheel
A. It 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 4～20mA signal, the regulating valve will not operate.
A. The system wiring is incorrect.
B. The motor split-phase capacitor CD is damaged.
C. The motor wiring is open.
D. The valve core falls off or the valve stem is broken (the actuator is in action at this time)
③Input 4～20mA signal, the regulating valve cannot complete the rated stroke.
A. The position of the differential transformer is not adjusted properly.
B. Some of the diodes D701 to D712 are damaged.
C. The transistor BG701 is damaged.
(4) Other failures during on-site use.
① The valve oscillates and screams.
A. If 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.
D. The flow direction of the medium is consistent with the closing direction of the valve.
E. The support is unstable due to the influence of other earthquake sources nearby.
F. The valve core and bushings are severely 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.
③Large leakage.
A. The valve core or valve seat is corroded and worn.
B. The valve seat is loose 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) of the 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 has deteriorated due to aging.
C. The valve stem is damaged.
D. The tightening nut is loose.
E. The gasket is damaged.
3: Concluding remarks
The correct use and maintenance of the electric control valve can not only improve the reliability of process control, but also increase the service life of the electric control valve, which has considerable economic benefits for the energy saving and consumption reduction of the enterprise.