Field Testing and Maintenance of Control Valves

Control valves play a vital role in industrial production processes. They act as precise "valve managers", controlling parameters such as fluid flow rate and pressure to ensure the stable operation of the entire production system. However, the performance of control valves is not constant. Over time and influenced by the working environment, their performance may deviate. Therefore, regular on-site testing and maintenance of control valves are essential for ensuring production safety and improving production efficiency.

On-site Testing of Key Performance Indicators of Control Valves

Control valves have numerous performance indicators, but in actual on-site testing, several key indicators need to be tested and calibrated to ensure that the control valves can work normally and efficiently.

1. Control Valve Basic Error Testing

The basic error is a key indicator of the accuracy of control valve performance. During testing, a signal ranging from 20 to 100 kPa is steadily increased or decreased and input into the air chamber (or positioner). The travel values corresponding to each point are measured. By calculating the differences between the "signal-travel" relationship and the theoretical values at each point, the maximum value is identified as the basic error. To ensure the accuracy of the test, the test points should be set at 0%, 25%, 50%, 75%, and 100% of the signal range. Additionally, the basic error of the measuring instrument should be limited to one-fourth of the basic error limit of the tested valve to ensure the reliability of the test results.

2. Control Valve Hysteresis Testing

Hysteresis refers to the maximum difference between the forward and reverse travel of the control valve under the same input signal. The testing method is the same as that for basic error testing. The presence of hysteresis can cause lag or instability in the actual operation of the control valve, so it needs to be strictly controlled. During the testing process, the forward and reverse travel data at different signal points should be carefully recorded, and the differences are calculated to obtain the specific value of hysteresis.

3. Control Valve End-Point Deviation Testing

End-point deviation refers to the basic error at the upper limit (start point) and lower limit (end point) of the signal. This indicator reflects the performance stability of the control valve under extreme working conditions. The same testing method mentioned above is used to measure the travel values at the upper and lower signal limits and calculate the deviations from the theoretical values to obtain the end-point deviation. Testing for end-point deviation helps to promptly identify any abnormal conditions that may occur when the control valve is in the fully open or fully closed state, allowing for timely adjustments.

4. Control Valve Leakage Testing

Leakage testing is an important part of evaluating the sealing performance of control valves. The test medium is usually room-temperature water. When the valve's pressure difference is less than 350 kPa, the test pressure is set at 350 kPa; when the valve's working pressure difference is greater than 350 kPa, the allowable pressure difference is used. During the test, the test medium should enter the valve in the specified direction, and the valve outlet can be directly connected to the atmosphere or connected to a low-head measuring device with an outlet to the atmosphere. Only after confirming that the valve and downstream connections are completely filled with the medium can the leakage rate be measured. For main valves, a high-pressure test is also required to ensure their sealing performance under high-pressure conditions. For valves equipped with positioners, on-site commissioning should be carried out before installation and operation to ensure their compatibility and overall performance with the control valve.

On-site Maintenance of Control Valves

Control valves come into direct contact with the process medium, and their performance not only affects the quality of the system but can also cause environmental pollution. Therefore, regular maintenance and periodic inspection of control valves are crucial, especially in harsh working conditions and important applications where maintenance work should be given more attention.

1. Key Inspection and Maintenance Areas

For control valves used in high-pressure difference and corrosive medium environments, the inner walls of the valve body and the diaphragm are often subjected to the impact and corrosion of the medium. The pressure resistance and corrosion resistance of these parts should be carefully inspected. The threads used to fix the valve seat are susceptible to corrosion, which can cause the valve seat to loosen. This is another area that requires close inspection. For valves operating under high-pressure differences, the valve seat sealing surface should be checked for erosion and cavitation. The valve core is the core component of the control valve and is most severely affected by the erosion and corrosion of the medium. During maintenance, it is important to carefully check for corrosion and wear, especially in high-pressure difference situations where cavitation can cause more severe wear on the valve core. In addition, the diaphragm, "O" ring, and other sealing gaskets should be inspected for cracking or aging. Attention should also be paid to the aging of polytetrafluoroethylene packing and sealing lubricating grease, as well as whether the mating surfaces are damaged. If necessary, these components should be replaced in a timely manner.

2. Common On-site Issues and Solutions

During the use of control valves, some common problems may occur, such as inability to close tightly, inability to open, large hysteresis, large leakage, vibration, and oscillation. There are corresponding solutions for these problems:

Valve core cannot close tightly: For air-closing valves, the solution is to increase the air source pressure or loosen the spring preload (i.e., reduce the starting pressure of the air chamber). For air-opening valves, the solution is to increase the spring preload while also increasing the air source pressure.

Slow or no action of the push rod: The diaphragm, rolling diaphragm, and gasket should be inspected for aging or rupture that may cause air leakage. If any problems are found, the damaged parts should be replaced promptly.

Large hysteresis: Check whether the push rod is bent, whether the packing gland is pressed too tightly (especially for graphite packing), and whether there are any injuries on the valve core guide. The solutions include replacing the valve stem and packing, increasing the guide clearance, or replacing the actuator with a higher force.

Insufficient full travel of the valve: Loosen the valve stem connection nut, rotate the valve stem outward or extend it inward to make the full travel deviation exceed the allowable value, and then tighten the nut.

Poor stability at small opening of the valve: First, check on-site whether the flow direction is installed in reverse or whether the valve is oversized. The solution is to change to a flow-opening installation or reduce the valve core size.

Unstable valve action: This may be due to a positioner malfunction, air leakage in the output pipeline, insufficient stiffness of the actuator, or insufficient thrust caused by changes in fluid pressure. The solutions include repairing the positioner and pipeline and using an actuator with higher stiffness.

Large leakage rate: First, check whether there are any injuries on the sealing surface, whether the connection thread between the valve seat and the valve stem is loose, and whether the pressure difference when the valve is closed is greater than the output force of the actuator. The solutions include replacing the sealing surface, tightening the valve seat, and replacing the actuator with higher output force.

Oscillation phenomenon: Oscillation is usually caused by the valve operating at a small opening or having a flow-closing type. The solution is to avoid operating at a small opening and change to a flow-opening type.

Importance of On-site Testing and Maintenance

On-site testing and maintenance of control valves are not only technical tasks but also important responsibilities related to production safety and economic benefits. Regular testing can timely detect deviations in the performance of control valves, allowing for early adjustments to prevent production accidents caused by control valve failures. At the same time, good maintenance can extend the service life of control valves, reduce equipment replacement costs, and improve production efficiency. In actual work, a comprehensive testing and maintenance system should be established, specifying the testing items, cycles, and maintenance procedures to ensure that control valves are always in good working condition.

In addition, on-site staff should possess certain professional knowledge and skills to operate testing equipment proficiently, accurately assess the performance of control valves, and take effective maintenance measures. Companies can improve employees' professional quality through regular training and technical exchanges, providing strong human support for the on-site testing and maintenance of control valves.

In summary, the on-site testing and maintenance of control valves is a systematic task that requires the joint efforts of companies, technical personnel, and on-site staff. Only by attaching great importance to and effectively carrying out this work can the stable and reliable performance of control valves be ensured, providing a solid guarantee for the smooth progress of industrial production.