Factors Affecting Normal Operation of Regulating Valves and Countermeasures

1 Introduction

In a chemical automation system with a relatively high degree of automation, the regulating valve serves as a terminal execution device for an automatic regulating system and receives control signals to realize adjustment of the chemical process. Its motion sensitivity is directly related to the quality of the regulating system. According to the actual statistics of the site, about 70% of faults come from the regulating valve. Therefore, the factors that affect the safe operation of the regulating valve and its countermeasures are summarized and analyzed during daily maintenance.

2, stuck

The regular problem of regulating valves is jamming, which often occurs during the initial operation of the new commissioning system and overhaul. Because the welding slag, rust, etc. in the pipeline cause blockage at the orifices and guide points, the circulation of the medium is not smooth, or the adjustment valve is overhauled. If the packing material is too tight, the friction force increases, causing the phenomenon that the small signal does not move and the large signal moves too much.

Troubleshooting: You can quickly open or close the secondary line or control valve, so that dirt from the sub-line or control valve is washed away by the media. Another method uses a pipe wrench to clamp the valve stem. When the signal pressure is applied, the valve stem is rotated forward and backward to allow the spool to flash over the card. If not, increase the air pressure and increase the drive power by moving it up and down several times to solve the problem. If it still does not work, it needs to be dismantled.

3, leak

3.1 The valve leaks, the length of the stem does not apply. Air-opening valve, stem too long The stem's upward (or downward) distance is not enough, resulting in a gap between the valve plug and the valve seat that does not allow sufficient contact, resulting in lax shut-off and internal leakage. Also, the air valve stem is too short, resulting in a gap between the valve plug and the valve seat that cannot be fully contacted, resulting in lax shut-off and internal leakage.

Solution: The valve stem should be shortened (or lengthened) so that the length of the regulator valve is appropriate so that it is no longer leaking.

3.2 Filler leakage. After the packing is installed in the stuffing box, axial pressure is applied to it by the glands. Due to the plasticity of the packing, it generates a radial force and is in close contact with the valve stem, but this contact is not very uniform. Some parts of the loose contact, some parts of the contact tight, and even some parts of the contact. During the use of the regulating valve, there is a relative movement between the valve stem and the filler. This movement is called axial movement. In the process of use, with the influence of high temperature, high pressure and strong fluid medium, the valve packing box is also the site where more leakage occurs. The main cause of the leakage of the filler is the interface leakage, and leakage occurs to the textile filler (the pressure medium leaks outward along the tiny gap between the filler fibers). The interface leakage between the valve stem and the packing is caused by the gradual attenuation of the packing contact pressure and the aging of the packing itself. At this time, the pressure medium leaks outward along the contact gap between the packing and the valve stem.

Solution: In order to make the packing easy to install, chamfer at the top of the stuffing box. Place a metal ring with less erosion resistance at the bottom of the stuffing box (the contact surface with the packing cannot be beveled) to prevent the packing from being stressed by the medium. roll out. The metal surfaces of the parts of the stuffing box that are in contact with the packing must be finished to improve the surface finish and reduce filler wear. Filler is made of flexible graphite, because of its good airtightness, low friction, small changes after long-term use, small wear and tear on the wear, easy maintenance, no change in friction force after re-tightening the gland bolts, pressure resistance and heat resistance Good, free from internal media, no pitting or corrosion of the metal in contact with the stem and stuffing box. In this way, the sealing of the stem packing box is effectively protected, ensuring the reliability and long-term sealing of the packing.

3.3 Spool, seat deformation leakage. The main reason for the leakage of cores and seats is due to the strengthening of corrosion due to casting or forging defects in the production of regulating valves. The passage of corrosive medium, the flushing of the fluid medium can also cause leakage of the regulating valve. Corrosion mainly occurs in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, it will produce erosion and impact on the valve core and the seat material so that the valve core and the valve seat become elliptical or other shapes. Over time, the valve core and the valve seat are not Supporting, there is a gap, leaks occur lax.

Solution: The key to a good valve plug, valve seat material selection, quality off. Choose corrosion-resistant materials and strongly reject defective products such as pits and trachoma. If the valve core and the valve seat are not deformed seriously, they can be ground with fine sandpaper to eliminate the marks and improve the sealing finish to improve the sealing performance. If the damage is serious, the new valve should be replaced.

4, oscillation

The spring rate of the regulating valve is insufficient, and the output signal of the regulating valve is unstable and drastic changes easily cause oscillation of the regulating valve. It is also said that the frequency of the selector valve is the same as the system frequency or the piping and the base are violently vibrated, causing the control valve to oscillate accordingly. Improper selection, there is a sharp flow resistance, flow velocity, and pressure changes in the valve when the valve is operated at a small opening. When the valve stiffness is exceeded, the stability is deteriorated, and the oscillation occurs when the valve is severe.

Solution: Because the causes of oscillation are many, specific problems are analyzed. For vibrations with slight vibration, stiffness can be increased to eliminate it. If a large stiffness spring is selected, use a piston to perform the structure. The violent vibration of pipes and bases eliminates the vibration interference by increasing the support; if the frequency of the valve selection is the same as the system frequency, the valves of different structures are replaced; when the oscillation caused by the small opening degree is selected, the C value of improper selection circulation capacity is selected. Must re-select the smaller flow capacity C value or the use of split control or sub-valve to overcome the valve in the small opening.

5, valve positioner failure

5.1 Ordinary locators work using the principle of mechanical force balance, that is, nozzle baffle technology, mainly the following types of failures:

1) Due to the principle of mechanical force balance, there are many movable parts, which are easily affected by temperature and vibration and cause the fluctuation of the regulating valve;

2) nozzle baffle technology, because the nozzle hole is very small, easily blocked by dust or dirty air source, the locator can not work;

3) Using the principle of force balance, the elasticity coefficient of the spring changes under bad conditions, resulting in non-linearity of the control valve and deteriorating the control quality.

5.2 Smart Locator consists of microprocessor (cpu), A/D, D/A converter, and other components. Its working principle is very different from that of ordinary positioners. The comparison between a given value and the actual value is purely electrical and is no longer a force balance. It is therefore possible to overcome the disadvantages of the force balance of conventional positioners. However, when used for emergency parking, such as emergency shut-off valves, emergency vent valves, etc. These valves are required to stand still at a certain position, and only need to be reliably operated when an emergency occurs. Staying in a certain position for a long time makes it easy for the electrical converter to run out of control and cause a dangerous situation where the small signal does not move. In addition, the position sensing potentiometer used for the valve is operated in the field, and the resistance value is liable to change, causing the small signal to not move and the large signal to be fully opened. Therefore, in order to ensure the reliability and availability of smart locators, they must be tested frequently.