Discussion on Technical Problems in Maintenance and Application of Control Valve

In a high degree of automation of the refining and chemical control system, the control valve as the automatic adjustment system of the terminal implementation of the device, to accept the control signal to achieve the refinery chemical process regulation. As the ultimate control of the quality of the process medium and safety indicators for the production of valves, it is stable production, optimal control, maintenance and overhaul cost control plays a decisive role. Take the refinery plant as an example. Along with the high-load operation of the device, problems such as jamming, corrosion, scouring, abrasion, vibration and internal leakage of the control valve occur continuously, which results in shortening the service life of the control valve and lowering the reliability of the work, Resulting in a decrease in the production efficiency of process systems and devices. Therefore, it is a key issue to select and install the regulating valve in the original construction and maintain the regulating valve at a high level in the future. With the actual construction and routine maintenance talk a few personal views for your reference. 1 control valve selection and installation When selecting the control valve, the first to collect the physical properties of the process fluid and control valve operating conditions, the main fluid composition, temperature, density, viscosity, normal flow, maximum flow, minimum flow , The maximum flow and minimum flow of import and export pressure, the maximum pressure and so on. In the technical aspects of the main control and determine the structure of the valve itself, flow characteristics, rated flow coefficient Kv value, caliber size, the process allows the calculation of pressure and actuator selection, materials and installation aspects. The following points should generally be followed when selecting and installing control valves. 1.1 Selection of control valve 1.1.1 Control valve structure type: should be able to meet the media temperature, pressure, mobility, flow direction, the adjustment range and tightness requirements. 1.1.2 regulating valve flow characteristics: should be able to meet the system characteristics of a reasonable compensation. The flow characteristics of the control valve refer to the relationship between the relative flow of the medium flowing through the valve and the relative displacement of the valve stem. (1) The mathematical expression is as follows: Q / Qmax = f (l / L) The relative flow rate is the ratio of the flow rate Q to the full-flow Qmax at a certain opening of the control valve; l / L is the relative displacement, and the ratio of the spool displacement l to the full-open displacement L when the control valve is at a certain opening degree. The general rule of thumb is that the flow characteristics of the regulating valve should be opposite to the characteristics of the regulating object and the characteristics of the regulator so that the overall characteristics of the regulating system approach to linearity. The selection of flow characteristics is usually carried out under the requirements of the process system, but the following actual situation is also to be taken into account. 1) linear flow characteristics of the scope of application: ① differential pressure changes small, almost constant; ② process changes in the main parameters of a linear; ③ most of the system pressure loss distribution in the control valve (change the opening, the valve differential pressure changes Relatively small); ④ small external interference, a small change in the given value, the adjustable range of small requirements. 2) The percentage of the other characteristics of the scope of application: ① actual adjustable range; ② opening changes, the valve pressure changes relatively large; ③ piping system pressure loss; ④ process system load fluctuations; ⑤ regulating valve often small Opened under the opening. 3) In addition to the above two commonly used flow characteristics, there are other flow characteristics of the parabolic and quick opening characteristics of the valve. 1.1.3 Control valve diameter: should be able to meet the requirements of the flow on the process. According to the known fluid conditions, calculate the necessary Kv value, select the appropriate control valve diameter. 1.1.4 control valve material: should be able to meet the media temperature, pressure, pressure, corrosion requirements. (1) the impact of fluid pressure and temperature on the material. (2) fluid corrosive effect on the material. (3) the phenomenon of cavitation fluid or mud fluid on the material. (4) From the structural point of view, whether there is a problem with the material assembly. 1.1.5 Regulating valve actuator output force: should be able to meet the requirements of field use of pressure, the stiffness should meet the requirements of system stability; switch type should meet the requirements of safe operation of the system. 1.1.6 control valve in the choice of, at the same time to consider under special circumstances, through the valve positioner, solenoid valves, limit switches and other auxiliary devices so that the valve should meet the operation time, split-step control and other requirements. 1.2 The installation of regulating valve 1.2.1 In the newly installed control valve put into operation and overhaul put into operation early, pay attention to purging process pipe to take isolation or removal measures to prevent the pipe due to welding slag, rust and other restrictions in the throttle, The leading parts of the blockage caused the poor circulation of the medium, overfilling of the regulating valve overfilling resulted in the increase of the frictional force, which led to the phenomenon that the small signal did not move and the large signal moved too much. 1.2.2 When installing the regulator valve to pay attention to regulating the valve open and gas off, to prevent the valve stem length discomfort, resulting in valve leakage. Gas valve stem is too long and gas off the valve stem is too short, likely to cause a gap between the valve core and valve seat, can not fully contact, leading to lax closure and internal leakage. 1.2.3 In order to prevent cavitation, pay attention to the following points in the original selection and installation: (1) Try to install the control valve in the lowest position of the system, so as to relatively increase the pressure at the inlet P1 and outlet P2 of the control valve. (2) Install a stop valve or orifice on the upstream or downstream of the control valve to change the original installation pressure drop characteristics of the control valve (this method is generally effective for small flow conditions). (3) With special anti-cavitation inner parts can also effectively prevent flash or cavitation, it can change the fluid flow in the control valve changes, thereby increasing the internal pressure. (4) try hard material selection of the control valve, because in the event of cavitation, for such a control valve, it has a certain degree of erosion resistance and abrasion resistance, cavitation can be present under certain conditions, and Will not damage the valve trim. On the contrary, for the soft material control valve, due to its poor erosion resistance and wear resistance, when the cavitation occurs, the internal components of the control valve will soon be worn, which can not be in the case of cavitation Working normally. 1.2.4 Regulating valve should be installed upright, if inclined, it should be supported. 1.2.5 The flow direction of the medium must be consistent with the body arrow. 1.2.6 control valve installation location should facilitate the observation, operation and maintenance. 1.2.7 Enforcement agency's signal management operations have adequate redundancy, which should not hinder the operation of the implementing agencies. 2 factors that affect the safe operation of regulating valve and countermeasures 2.1 regulator valve leakage 2.1.1 valve leakage, stem length discomfort. Valve leakage reasons: open the valve, the valve stem is too long and gas valve stem is too short, the valve stem up (or down) the distance is not enough, resulting in a gap between the valve core and valve seat, can not fully contact, Lead to lax closure and internal leakage. Solution: Should shorten (or extend) the valve stem so that the length of the stem so that it no longer internal leakage. (1) Leakage of packing Leakage of packing causes: ① After the packing is filled with stuffing box, axial pressure is applied to it by gland. Due to the plasticity of the packing, which generates radial forces and comes into close contact with the valve stem, the contact is not very uniform. Some parts of the contact loose, tight contact with some parts, and even some parts of the contact is not on; ② regulating valve in use, there is a relative movement between the stem and packing, with the high temperature, high pressure and permeability of the fluid medium , The valve packing box is also a place where there is more leakage. (3) The main reason for the leakage of packing is the leakage of the interface and the leakage of textile packing (the pressure medium leaks out along the tiny gap between packing fibers ); ④ valve stem and the interface between the filler leakage is due to gradual attenuation of the contact pressure of the filler, the filler itself caused by aging and other reasons, then the pressure medium along the gap between the filler and the valve stem leakage to the outside. To solve the problem: ① In order to facilitate the filling stuffing, chamfering at the top of the stuffing box, placed at the bottom of the stuffing box corrosion-resistant gap smaller metal guard ring (contact with the filler can not be inclined) to prevent the filler by the media Pressure launched; ② filler parts and parts of the filler metal parts of the contact surface to be refined to improve the surface finish, reducing filler wear and tear; ③ filler of flexible graphite, because of its good air tightness, friction, long-term use of change Small, small wear and tear, wear and tear, easy maintenance, gland bolts tighten again after the friction does not change the characteristics. Through the above points to ensure the reliability and long-term packing seal. (2) regulating valve spool, valve seat deformation Leak valve, valve seat leakage is mainly due to the production process of valve casting or forging defects can lead to enhanced corrosion. The corrosive medium through the erosion of the fluid medium can also cause the regulator valve leakage. Corrosion mainly in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, it will produce the valve core, seat material erosion and impact of the valve core, the valve seat into an oval or other shape, with the passage of time, resulting in spool, valve seat is not Supporting, there is a gap, the gap is not strict leak. Solutions: The key to good spool, valve seat material selection, quality control. Select corrosion-resistant materials, pitting, trachoma and other defects of products resolutely removed. If the valve core, seat deformation is not serious, can be fine sandpaper grinding, eliminate traces, improve seal finish, to improve the sealing performance. If the damage is serious, the new valve should be replaced. 2.2 regulating valve wear and tear valve body or valve parts erosion. The main reason for erosion: liquid speed is too high, the liquid particles in the air or flash. Solution: Increase the size of the valve trim or valve to reduce fluid velocity; change the flow direction of the valve body to reduce the impact of the fluid; increase the hardness of the valve body and trim material; change the valve trim structure to reduce the flow rate; To avoid the role of empty belt, switch to low-pressure recovery valve; stainless steel welding. 2.3 Oscillatory oscillation causes: ① control valve spring stiffness is not enough, the electric control valve output signal instability and rapid changes can cause valve oscillation; ② pipeline, the base of the violent vibration, so that the control valve vibration; ③ improper selection, Regulating valve work in a small opening there is a sharp flow resistance, flow rate, pressure changes, when the valve stiffness exceeds the stability of the worse, severe oscillation. Solution: As there are many reasons for the oscillation, so the specific problems of specific analysis. Slight vibration of the vibration, can increase the stiffness to eliminate. ① If the use of large stiffness spring, use the piston instead of the implementation of the structure; ② pipeline, the base vibration by increasing the support to eliminate vibration interference; ③ work in a small opening caused by the oscillation, it is improper selection of flow capacity Kv value selected, must Re-select the flow capacity Kv value smaller or split control to solve. 2.4 valve positioner ordinary positioner mechanical balance principle work, nozzle baffle technology, the main existence of the following types of faults: 2.4.1 due to the use of mechanical force balance principle of work, the moving parts are more susceptible to temperature , The impact of vibration, resulting in fluctuations in the control valve. 2.4.2 The nozzle baffle technology, because the nozzle hole is very small, easily blocked by dirty or dirty gas source, the positioner can not work properly. 2.4.3 using the principle of balance of power, the spring coefficient of elasticity in the harsh changes in the scene, resulting in control valve non-linearity led to decline in quality. 2.4.4 Because the electrical interface is not tight, the water flows into the positioner through the threading pipe, causing the coil to short-circuit or open circuit, the positioner can not work normally. To solve the problem: When installing, the locator should be firmly fixed and the electrical interface should be tightly sealed to improve the quality of the purifying wind and periodically clean the purge air. 2.5 The reasons for the jamming of the regulating valve: During the initial operation of the new commissioning system and overhaul, Internal welding slag, rust, etc. in the throttle, leading to blockage of the media so that the poor circulation, or overhaul the valve overhaul, resulting in increased friction, resulting in small signals do not move over large signal action phenomenon. Solutions: Quickly open, close the deputy line or control valve, so that dirt from the secondary line or regulating valve at the media run. Another way to clamp the valve stem with the pipe clamp, in the case of external signal pressure, positive and negative rotation of the stem, so that the spool flashed over the card office. If you can not increase the pressure of the gas source to increase drive power to move up and down repeatedly several times, you can solve the problem. If still not action, you need to disintegrate. 2.6 regulating valve action failure 2.6.1 valve no action. Reason: There is no lack of gas or air pressure; actuator failure leak; no output valve control signal; supply pipe rupture, deformation, joint damage, leakage; flow direction is not correct, the force is too large, so that the valve off, off Rod; high temperature parts of the valve stuck, damaged. Solution: Check the gas source, disassemble the valve, re-configure the gap between the shaft and sleeve, re-calibration direction, the repair. Valve can not reach the rated stroke. Reason: Locator is not calibrated, stroke adjustment is not appropriate; actuator spring rating is too small; manual mechanism limit position is not allowed. Solution: calibration locator, replace the spring. Valve moves slowly or slowly. Cause: packing friction, deterioration aging; piston actuator friction is too large; bearing friction. Solution: Replace the repair, readjust the damaged parts or clean the grinding cylinder and piston. 4 Conclusion The selection and application of control valve is a highly specialized, involving a wide range of technical systems work, to do this work, not only in the selection and installation of a good off, but also with the actual use of experience to synthesize Analysis and judgment, so that the combination of theory and practice scientifically. Taking proper measures, improving measures, mastering the performance and raising the level of maintenance will greatly reduce the failure rate of the instrument, improve the production efficiency and economic benefits of the process technology, and reduce the energy consumption, which can effectively improve the performance of the control system Quality, thus ensuring long-term operation of the production plant.

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