Influence control valve operation factors and countermeasuress
1. preface
In higher automatization chemical control system, the regulator as automatic adjustment system terminal actuators and accept control signal to achieve the chemical process adjustment. Its movements sensitivity relates directly to the control system quality. According to actual statistics have 70% fault from regulator. Therefore in the daily maintenance, summary analysis affecting regulator safe operation factors and countermeasures.
2. block
Regulator often appears question is the blocking, often appear in new substation system and overhaul powerstation early days, due to pipe welding slag, rust inside in the throttled position, oriented part to cause medium circulation impeded, or regulator maintenance packing overtighten, causing friction increases, cause small signal don't act or too large signal action phenomenon.
Fault handling: can rapidly opening or closing vice lines or regulator, let the dirt from vice lines or regulator to rush by medium. Another way is to use a pipe wrench clamping stem, plus signal pressure cases, positive and negative force turn the stem, let valve spool flash blocking place. If can not, then increase air pressure increases driving power move up and down repeatedly several times, can solve the problem. If still do not move, it is required to handle disintegration.
3. leakage
3.1 valve inner leakage, stem length is unwell. Gas open valve, the valve stem is too long upward (or downward) distance is not enough, there is space between causing valve spool and seat, cannot fully contact, lead to shut the lax and internal leakage. Same gas shut stem is too short, led to the space between valve core and seat, cannot fully contact, lead to lax shut and internal leakage.
Solution: should be shortened (or extend) regulator valve stem makes regulator appropriate length, making its no longer leakage.
3.2 packing leakage. After packing load in stuffing box, through gland to exert its axial pressure. Due to packing plastic, make it produced radial force, and close contact with stem, but the contact is is not very evenly. Some parts contacted loosenly, some parts contacted tightly, even some places no contact. Regulator in use process, there exists relative movement between stem and packing, the movement called axial movement. In using process, with the influence of high temperature and high pressure and strong permeability, the regulator stuffing box is the part with more fluid leak phenomenon. The main reason of causing packing leakage is interface leak, for spinning filler, still can appear leakage (pressure medium along the tiny gaps between packing fiber to external leakage). The interfaces leakage between stem and packing is due to packing contact pressure gradual reduced, stuffing aging causes and pressure medium will along with the gap between stem and packing to external leakage.
Solutions: to make packing into convenience, in the stuffing box top chamfering, in the stuffing box bottom placed corrosion proof clearance smaller metal protecting ring (and packing contacting surface can not be cant), in order to prevent the packing to push by medium pressure. Stuffing box parts and packing contacting metal surface to finish machining, in order to improve surface finish, reduce packing wear. Choose flexible graphite packing, due to its gas tightness, friction small, after long-term use, changes small, wear loss is small, easy maintenance, gland bolts tighten the friction does not change, pressure resistance and heat-resistance are good, not affected by the internal medium erosion and stem and stuffing box internal contact metal will not occur corrosion. So, effectively protect the stem stuffing box sealing, guarantee the packing sealing reliability and long-term.
3.3 Valve disc, seat deformation leakage. Core, seat leakage mainly because regulator production process casting or forging defects can lead to corrosion strengthening. And the corrosive medium through fluid scouring can also cause regulator leakage. Corrosion mainly in the form of cavitation erosion and existence. When the corrosive medium through the regulator, will produce erosion and shock to valve disc, seat materials and makes core, seat to an oval or other shapes, with the time pass, leading to valve disc, seat don't match, existing gap, and can not tight closed and occur leakage.
Solution: The key is to select good valve core, seat materials type and quality. Choose corrosion resistant material, to pit, defect product firmly rejects. If the valve core, seat deformation is not very serious, cam through sand grinding, eliminate trace, improve sealing finish, in order to improve the sealing performance. If damaged, should replace the new valve.
4. oscillation
Regulator spring stiffness is insufficient, regulator output signal instability and easy cause regulator oscillations. Other said choose valve frequency and system frequency is same or piping, pedestal violent vibration, makes the regulator subsequently vibration. Selection is undeserved, regulator working in small open degree exist sharply flow resistance, velocity and pressure changes, when more than valve stiffness and stability becomes poor, when serious, produce oscillations.
Solutions: due to produce oscillation reasons are many, so the concrete analysis. To slight vibration, can increase stiffness to eliminate. If choose big stiffness spring, convert the piston execution structure. Piping, pedestal strenuous vibration through increasing support eliminate vibration interference, Choose valve frequency and the system frequency are equal, replacing different structural valves, working in small open degree caused vibration, must be improper selection, flow capacity C value is too large, must reselect valve type with smaller C value or using process control or portion valve to overcome regulator working in small open degrees.
5. Valves locator fault
5.1 ordinary locator adopts mechanical force balance principle work, namely jet flapper technology, mainly exist in the following fault types:
1） because by mechanical force balance principle works, its moving parts more, easily influenced by temperature, vibration, the fluctuation caused by regulator,
2） using jet flapper technology, because the nozzle aperture is small, susceptible to dust or unclean air blocking, making locators can not work normally,
3） using force balance principle, springs elastic coefficient changed in bad scene, causing regulator nonlinear lead to control quality decreased.
5.2 Intelligent locator is composed by microprocessor (CPU), A/D, D/A converter and other parts, its working principle and common locators are quite different. The given value and the actual value compasion is electric signals, is no longer force balance. Therefore overcome the conventional locators force balance faults. But for emergency parking timeliness, such as emergency cut-off valve, emergency vent valves, etc. These valves required stationary stay in one position, only an emergency occurs, they need to reliably action. Long time stay in one position easy to make the electrical signal converter runaway small signle without moving dangerous conditions. Besides the sensor used in valve position because working at the scene, electric resistance value is easy changed and cause small signal do not move, in order to ensure the reliability and intelligent locator availability, must doing frequent testing to them.
6. conclusion
Through the analysis of failure causes of regulator and take the appropriate ways to improve the treatment methods, will greatly improve the efficiency ultilization ratio, reducing the instrument regulator fault rate, playing important roles in the production efficiency in process and improve economic benefits, reduce the energy consumption, which can effectively improve the quality of regulating system, to ensure production device long period running.
