Control Valve Leakage: Testing and Standards
In industrial production, control valves play a crucial role. They are accurately controlling the flow of fluids to ensure the stable operation of the entire production process. However, like any precision equipment, the performance of control valves is affected by various factors, among which leakage is a key indicator. Today, we will take an in-depth look at control valve leakage, including its testing methods and standards, to help everyone better understand and apply this important concept.
Leakage, as the name suggests, refers to the flow of fluid through the valve when it is in the closed position under specified test conditions. This definition may seem simple, but it involves complex technical requirements. First, the test conditions must be strictly defined, including ensuring that the actuator has sufficient thrust, the valve seat and plug fit tightly, a certain pressure differential is maintained upstream and downstream, and the entire test is conducted at room temperature. All of these conditions are indispensable. Only leakage measured under such standardized conditions has practical reference value.
In practical applications, control valve leakage is extremely important. In some scenarios, we want the valve to have minimal leakage when closed; in other special process conditions, the control valve is required to have absolutely no leakage. For example, in chemical production, the leakage of certain toxic or hazardous gases or liquids may cause serious safety accidents; in the food processing industry, leakage may lead to product contamination, affecting product quality and safety. Therefore, to achieve low or zero leakage, engineers have designed a variety of structures and used different materials, such as soft sealing materials like rubber or PTFE. These materials have good flexibility and sealing performance, effectively reducing leakage.
Control valve leakage is not a fixed value and is influenced by multiple factors. First, the magnitude of the seating force is a key factor. If the seating force is insufficient, the valve seat and plug cannot fit tightly, resulting in increased leakage. Second, the material of the sealing surface is also important. Different materials have different sealing performance, and choosing the right material can effectively reduce leakage. In addition, the wear of the sealing surface cannot be ignored. Over time, the sealing surface may wear due to friction or other reasons, affecting sealing effectiveness. Moreover, the stress and deformation of internal valve components can also affect leakage. If internal components are subjected to excessive force or deformation, the valve's sealing structure may change, leading to increased leakage.
To ensure that the control valve's leakage meets requirements, rigorous testing must be conducted. The main testing methods are: testing with water (or kerosene) and testing with air (or nitrogen).
When testing with water, the actuator must first provide sufficient thrust to ensure that the valve seat and plug fit tightly. Water is then pumped into a pressure-stabilizing cylinder via a centrifugal pump, and with the regulation of a pressure regulator and control valve, the upstream water pressure is maintained at the specified level. The water pressure can be read using a pressure gauge. After the valve is closed and the test water pressure stabilizes, the leakage can be measured downstream using a graduated container, and the measurement time is recorded. If the leakage is large, a larger container can be used for measurement.
When testing with air, the test medium is changed to gas, and the test pressure is set using a high-capacity regulator. A rotor flow meter is used to measure gas leakage. During testing, it is also necessary to ensure that the actuator has sufficient thrust so that the valve seat and plug fit tightly. After the valve closes and the test pressure stabilizes, leakage measurement can begin.
Before conducting leakage tests, certain preparations must be made. First, the pressurized fluid (water or air, etc.) introduced upstream of the valve is used to flush away dirt, dust, and oil on the valve seat sealing surface to ensure test accuracy. Then, after the actuator or valve positioner is pressurized, the valve is cycled several times to close it. Only when the upstream test pressure reaches stability can leakage measurement begin.
Standard testing of control valve leakage is a systematic process that must follow strict procedures. First, the pressure of the test medium forms the basis of the test. For control valves, the pressure should be maintained at 3.5 kg/cm². When the allowable differential pressure of the control valve is less than 3.5 kg/cm², the specified allowable differential pressure is taken as the standard. The test medium should be clean liquids (water or kerosene) and gases (air or nitrogen) at 10–50°C. Due to different working conditions, other media may produce errors; therefore, it is recommended to use the recommended medium to complete the test.
During the test, the medium must enter the valve body in the specified flow direction. The valve outlet can be open to the atmosphere or connected to a low-pressure-loss measurement device. Once it is confirmed that all downstream connected piping is completely filled with the medium, the leakage of the control valve can be measured. For shut-off type control valves, the test medium pressure is set as the maximum working differential pressure of the valve.
When testing the actuator of a control valve, a certain signal pressure should be applied to ensure that the control valve is closed. For pneumatic actuators, the handling of air-to-open and air-to-close types differs. For air-to-open, the signal pressure should be zero, ensuring the valve is fully closed; for air-to-close, the signal pressure should be the maximum input signal pressure minus a certain lower limit, ensuring the valve closes properly and reliable leakage measurement is possible.
During leakage testing, measurement accuracy is critical. The accuracy of test medium pressure measurement should be ±2%, and leakage measurement should be ±5%. Only by ensuring measurement accuracy can the test results be reliable.
The standards for control valve leakage vary depending on application requirements and working conditions. Common standards include:
This is a valve sealing standard established by the American Petroleum Institute (API) and is suitable for most industrial applications. API 598 provides detailed specifications for valve leakage, including different test conditions and allowable leakage ranges. In the petroleum and chemical industries, API 598 is widely adopted as an important benchmark for valve sealing performance.
An international standard established by the International Organization for Standardization (ISO) covering various types of valves and testing methods. ISO 5208 provides unified testing and evaluation standards for valve manufacturers and users, helping to improve product quality and market competitiveness.
A valve leakage standard established by the American National Standards Institute (ANSI). ANSI B16.104 primarily targets the U.S. market, strictly regulating valve leakage. In North America, it is an important reference for evaluating valve sealing performance.
In practical applications, attention should be paid to the following when testing and applying standards for control valve leakage:
Selecting the Appropriate Test Medium: The choice of test medium is critical under different conditions. It is recommended to use the recommended medium for testing to avoid errors caused by different media. For example, in chemical production, if the medium is corrosive, a corresponding corrosion-resistant test medium should be selected to ensure test accuracy.
Ensuring Consistency of Test Conditions: When performing leakage tests, test conditions must be consistent. This includes the pressure, temperature, and flow direction of the test medium, all strictly following the standards. Only under the same test conditions can leakage of different valves be accurately compared and evaluated.
Following Relevant Industry Standards: Different industries have different requirements for control valve leakage. Therefore, in practical applications, it is essential to follow relevant industry standards for testing and acceptance. For example, in the food processing industry, valve sealing performance requirements are extremely high, and valves must be completely leak-free when closed to prevent contamination.
Control valve leakage is an important technical indicator in industrial production, directly affecting safety and efficiency. Through rigorous testing and the application of standards, the sealing performance of control valves can be ensured. In practical applications, it is necessary to understand the definition of leakage, influencing factors, testing methods, and standards, and to strictly follow the prescribed procedures. Only in this way can control valves fully perform their functions and ensure the smooth operation of industrial production.
In conclusion, testing and standard application of control valve leakage is a complex and important process. This article aims to help everyone better understand and apply this knowledge, improving industrial production quality and safety.
