Factors and Measures Affecting the Operation of Control Valves

1 Introduction

In the chemical control system with a high degree of automation, the regulating valve is used as the terminal execution device of the automatic regulating system, and accepts the control signal to realize the regulation of the chemical process. Its action sensitivity is directly related to the quality of the regulation system. According to actual statistics on site, about 70% of the faults are caused by the regulating valve. Therefore, the factors that affect the safe operation of the regulating valve and their countermeasures are summarized and analyzed in daily maintenance.

2. Jam

The frequent problem of regulating valve is jamming, which often occurs in the new commissioning system and the initial stage of overhauling. Due to welding slag, rust, etc. in the pipeline, clogging at the orifice and guiding part makes the medium flow poor, or the regulating valve is overhauled. The middle packing is too tight, resulting in increased friction, leading to the phenomenon that the small signal does not act and the large signal acts excessively.

Troubleshooting: The auxiliary line or the regulating valve can be quickly opened and closed, so that the dirt can be washed away by the medium from the auxiliary line or the regulating valve. Another way is to use a pipe wrench to clamp the valve stem. When the signal pressure is applied, rotate the valve stem positively and negatively so that the valve core flashes through the card. If not, increase the air source pressure to increase the driving power and move it up and down several times to solve the problem. If it still doesn’t work, it needs to be disassembled.

3. Leakage

3.1 The valve has internal leakage and the length of the valve stem is not suitable. When the valve is opened by air, the valve stem is too long and the upward (or downward) distance of the valve stem is not enough, resulting in a gap between the valve core and the valve seat and insufficient contact, resulting in loose closing and internal leakage. Similarly, the valve stem of the air shutoff valve is too short, resulting in a gap between the valve core and the valve seat, which cannot be fully contacted, resulting in loose closing and internal leakage.

Solution: shorten (or extend) the valve stem of the regulating valve to make the length of the regulating valve suitable, so that it no longer leaks.

3.2 Packing leakage.

After the packing is loaded into the stuffing box, the axial pressure is applied to it through the gland. Due to the plasticity of the packing, it generates radial force and is in close contact with the valve stem, but this contact is not very uniform. Some parts are in loose contact, some parts are in tight contact, and even some parts are not in contact. During the use of the regulating valve, there is relative movement between the valve stem and the packing, and this movement is called axial movement. In the course of use, with the influence of high temperature, high pressure and strong permeability fluid medium, the stuffing box of the regulating valve is also a part of the leakage phenomenon. The main cause of packing leakage is interface leakage. For textile packing, there will also be leakage (pressure medium leaks outward along the tiny gaps between packing 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 will leak outward along the contact gap between the packing and the valve stem.

Solution: In order to make the packing easy to install, chamfer the top of the stuffing box, and place an erosion-resistant metal protection ring with a small gap at the bottom of the stuffing box (the contact surface with the packing cannot be an inclined surface) to prevent the packing from being pressured by the medium roll out. The metal surface of each part of the stuffing box in contact with the packing should be finished to improve the surface finish and reduce the wear of the packing. The filler is made of flexible graphite, because it has good airtightness, low friction, small changes after long-term use, small wear and tear, and easy maintenance. After the gland bolts are re-tightened, the friction does not change, pressure resistance and heat resistance It has good performance and is not corroded by the internal medium, and the metal in contact with the valve stem and the stuffing box does not undergo pitting or corrosion. In this way, the sealing of the valve stem stuffing box is effectively protected, and the reliability and long-term nature of the sealing of the packing are ensured.

3.3 The valve core and valve seat are deformed and leaked.

The main reason for the leakage of the core and valve seat is that the casting or forging defects in the production process of the regulating valve can lead to increased corrosion. The passage of corrosive media and the erosion of fluid media can also cause leakage of the regulating valve. Corrosion mainly exists in the form of erosion or cavitation. When the corrosive medium passes through the regulating valve, it will cause erosion and impact to the valve core and valve seat material, which will make the valve core and valve seat oval or other shapes. As time goes by, the valve core and valve seat will not Matching, there is a gap, and leakage occurs due to lax closure.

Solution: The key is to control the selection and quality of the material of the valve core and valve seat. Choose corrosion-resistant materials, and resolutely eliminate defective products such as pitting and trachoma. If the deformation of the valve core and valve seat is not too serious, it can be ground with fine sandpaper to eliminate traces and improve the seal finish to improve the sealing performance. If the damage is severe, the valve should be replaced with a new one.

4. Oscillation

The spring stiffness of the control valve is insufficient, and the output signal of the control valve is unstable and changes sharply, which can easily cause the control valve to oscillate. It is also said that the frequency of the valve selection is the same as the system frequency or the pipe and the base vibrate violently, causing the regulating valve to vibrate. Inappropriate selection, the regulating valve has sharp changes in flow resistance, flow rate, and pressure at a small opening. When the valve stiffness exceeds the valve stiffness, the stability becomes worse, and oscillation occurs when it is severe.

Countermeasures: As the causes of oscillations are multifaceted, specific problems are analyzed in detail. Slight vibration can be eliminated by increasing stiffness. If a high-stiffness spring is used, the piston actuator structure is used instead. The violent vibration of the pipeline and the base can eliminate the vibration interference by increasing the support; the frequency of the valve selection is the same as the system frequency, then replace the valve with a different structure; the oscillation caused by the working at a small opening is due to improper selection of the flow capacity C value selection, It is necessary to re-select the type with a smaller flow capacity C value or adopt split-range control or sub-mother valve to overcome the small opening of the regulating valve.

5. Valve positioner failure

5.1 Ordinary positioners work on the principle of mechanical force balance, that is, nozzle baffle technology, and there are 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, causing fluctuations in the regulating valve;
• 2) Adopting nozzle baffle technology, because the nozzle hole is small, it is easy to be blocked by dust or unclean air source, so the positioner cannot work normally;
• 3) Adopting the principle of force balance, the elastic coefficient of the spring changes in harsh scenes, which causes the nonlinearity of the regulating valve and reduces the control quality.

5.2 The intelligent positioner is composed of a microprocessor (cpu), A/D, D/A converters and other components, and its working principle is completely different from that of ordinary positioners. The comparison between the given value and the actual value is purely an electric signal, not a force balance. Therefore, the shortcomings of the force balance of the conventional positioner can be overcome. But when used in emergency parking lots, such as emergency shut-off valves, emergency vent valves, etc. These valves are required to be stationary in a certain position, and only need to operate reliably when an emergency occurs. Staying in a certain position for a long time will easily cause the electrical converter to lose control and cause a dangerous situation where the small signal does not act. In addition, because the position sensing potentiometer used for the valve works in the field, the resistance value is prone to change, causing the small signal to not operate, and the large signal is fully open. Therefore, in order to ensure the reliability and availability of smart locators, they must be frequently tested.

6. Concluding remarks

By analyzing the cause of the failure of the regulating valve and adopting appropriate treatment and improvement methods, it will greatly increase the utilization rate of the regulating valve, reduce the failure rate of the instrument, and play an important role in the improvement of the production efficiency and economic benefits of the process and the reduction of energy consumption. , Can effectively improve the quality of the adjustment system, thereby ensuring the long-term operation of the production device.