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Factors affecting the normal operation of valve regulated and Countermeasures
In the chemical control system with higher degree of automation, control valve as the implementation of automatic regulation system terminal device, control signals to achieve the regulation of chemical process to accept. Sensitivity of its action is directly related to the quality control system. According to the actual statistics, about 70% of the failures come from regulating valve. So to sum up the routine maintenance of the security impact of valve operation and its countermeasures.
BlockValves often appears to be blocked, usually occurs in the early new operation system and overhaul operation, as pipe welding slag, rust etc. to medium circulation by causing blockage in the throttle, steering parts, or valve repair the packing too tight, resulting in increased friction, resulting in small signal does not move too large signal operation the phenomenon of. Troubleshooting: you can quickly open, close the vice line or valve, so that dirt or adjust the valve from the line to be washed medium run. Another way to stem tube clamp tight, the pressure in the applied signal case, the positive and negative force rotating the stem, so that spool flash card. If it can not increase the air pressure increases the driving power up and down several times repeated, can solve the problem. If still no action, you need to deal with the disintegration.
Leakage1.Valve leakage, valve stem length does not apply. Open gas valve, valve stem long stem upward (or downward) the distance is not enough, resulting in gaps between the valve core and valve seat, can not be fully engaged, leading to lax customs leakage. The same gas off valve stem is too short, resulting in gaps between the valve core and valve seat, can not be fully engaged, leading to lax customs leakage.
Solution: should be shortened (or extended) valve stem to the length of the right valve, internal leakage so that it no longer.
2.Packing leakage. After packing into the stuffing box, the axial pressure imposed by the gland. Because the plastic filler, to produce a radial force, and close contact with the stem, but this contact is not very uniform. Loose contact with some parts, some parts of tight contact, no contact even on some parts. Regulating valve in the use process, stem with the relative motion exists between the filler, this movement is called axial movement. In the use process, with the effect of high temperature, high pressure and osmotic fluid medium strong, valve packing leak letter is part of the phenomenon more. Main causes of the leakage of the interface packing leakage, but also for textile packing leakage (pressure medium filler along the small gap between the outward leakage).
The interface between the stem and packing leakage is due to the gradual decay of the contact pressure packing, packing their own causes aging, then the pressure medium will be along the contact gap between the valve stem packing leakage.
Solutions: for the filling into the convenience, the chamfered top of stuffing, metal protective ring placed a smaller gap erosion resistance in the stuffing box at the bottom (the contact with the filler can not inclined), in order to prevent the filler is medium pressure introduced. The metal surface in contact with the filler filler letter each part to finishing, in order to improve the surface finish, reduce the packing wear. Use flexible graphite packing, because of its good air tightness, the friction is small, small changes after long-term use, wear of burning a small, easy to repair, re tighten the gland bolts did not change after the friction, pressure and resistance heat well, not the erosion of the internal medium, and the stem and the stuffing does not occur within the contact metal pitting corrosion. In this way, the effective protection of the valve stem packing seal, packing seals to ensure the reliability and long-term.
3.Spool, the valve seat leakage deformation. The main reason for core, valve seat leakage is due to the strengthening of the control valve production process of casting or forging defects can lead to corrosion of the. The corrosion medium through the fluid medium erosion, also can cause valve leakage. Erosion or corrosion mainly exists in the form of cavitation. When the corrosive medium through the valve, it will produce on the spool, the valve seat material erosion and the impact of the spool, the valve seat into an oval or other shapes, with the passage of time, resulting in valve, valve seat is not complete, there is space, is not closed tightly leakage.
Solution: the selection of the key control valve, valve seat material, quality control. Select corrosion resistant materials, Ma, trachoma and other defective products rejected. If the valve, the valve seat deformation is not too serious, can be fine sandpaper grinding, remove traces, improve sealing finish, to improve the sealing performance. If badly damaged, it should be re valve replacement.
4.OscillationInsufficient rigidity spring valve, regulating valve output signal instability can lead to drastic changes in the oscillation valve. Select valve also said the same frequency or frequency channels and systems, the base vibration, the vibration along the valve. Improper selection, adjusting flow resistance, flow rate, there is a sharp pressure a small opening in the valve, when more than the valve stiffness, stability becomes worse, severe oscillation.
Solution: as the oscillation are many reasons, so the specific analysis of specific issues. Slight vibration of the vibration, increased rigidity to eliminate. As a large selection of spring stiffness, use piston actuator structure. Pipeline, severe shock by increasing the support base to eliminate vibration interference; selection valve frequency and system frequency are the same, then switch to a different structure of the valve; work in the small opening caused by the oscillation, it is improper selection of the value of selected large flow capacity C, flow capacity must be re selection or the smaller C sub process control valves or mother in order to overcome the valve working in a small opening.
The valve positioner fault(1).Ordinary locator by mechanical force balance principle, namely flapper nozzle technology, mainly has the following types of fault;
1).By using mechanical force balance principle, the movable parts more easily affected by temperature, vibration, impact, cause valve fluctuation;
2).The flapper nozzle, because the nozzle hole is very small, easy to dust or dirty air is blocked, locator does not work; 3) by using the principle of force balance, spring elasticity coefficient change in bad environment, cause valve nonlinearity leads to the decline in the quality control.
(2).Intelligent positioner is composed of a microprocessor (CPU), A/D, D/A converters and other components, its working principle is different from the ordinary locator. Comparison of a given value and the actual value of purely electric signal, is no longer a force balance. It can overcome the shortcomings of conventional force balance locator. But for emergency parking, such as emergency shut-off valve, emergency vent valve. These valves required in a stationary position, only the event of an emergency, it needs reliable action. Long time stay in a location easy to electrical converter control dangerous small signal does not move. In addition to the valve position sensing potentiometer due to work in the field, the resistance value caused by the change of the small signal does not move, large signal full of danger. Therefore, in order to ensure the reliability and intelligent positioners availability, they must be frequently tested.
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Article tags: Valve,Linear Guideway,Power Transmission.
