To determine whether a gate valve has internal leakage, one can follow the principle of starting from the simple to the complex and from the surface to the interior, and make a comprehensive judgment by combining multiple methods. Here are several effective detection methods: 

Initial sensory judgment 

This method is suitable for rapid screening, but its accuracy is relatively low. It is usually used as the first step. 

Auditory Inspection Method: In a quiet environment near the valve, place the stethoscope or the pointed end of a long-handled screwdriver against the valve or the downstream pipeline, and rest the other end against your ear. If you hear a continuous "clicking" sound or a turbulent sound of fluid flow, it may indicate that there is an internal leakage in the valve. 

Feel and Observation: For low-temperature media, if frost forms on the downstream pipeline of the valve or the temperature drops significantly; or for valves that discharge water to a non-pressurized funnel, and there is still continuous water flow from the funnel after the valve is closed, these situations can be preliminarily judged as internal leakage. 

Temperature measurement method 

This is the most commonly used and relatively reliable method in industrial sites, and it mainly employs infrared thermometers. 

Basic operation: After the valve is closed for 4 to 6 hours, use an infrared thermometer to measure the metal temperature of the valve stem (near the valve body) or the pipeline about 150mm downstream of the valve. 

Judgment criterion: If the measured temperature is higher than 70℃, it is generally determined that there is internal leakage in the valve. 

Notes:

High-temperature interference: If the downstream connection of the valve is a pressurized steam drainage main, or there are other high-temperature steam sources nearby, even if the valve is tight, the temperature may still exceed 70℃. At this time, the temperature of the upstream pipe wall or the valve stem should be measured for comparison. If the downstream temperature is significantly higher than the upstream temperature, it can be confirmed that there is an internal leak. 

Temperature difference comparison: A more rigorous method is to compare the temperature difference between the valve inlet and outlet. If the difference in temperature between the downstream and upstream is too small after the valve is closed (for example, less than 30-50℃, depending on the temperature of the medium), it also indicates a serious internal leakage. 

Acoustic detection method 

The detection is carried out using ultrasonic waves or acoustic principles, which is highly accurate and particularly suitable for locating leakage points. 

Ultrasonic detector (four-point method/three-point method): 

Principle: By using a highly sensitive sensor, the ultrasonic turbulent signals generated when the fluid passes through tiny gaps are captured. 

Operation: The measurement is conducted using the "four-point method" or "three-point method". In simple terms, it involves measuring the ultrasonic signal strength at the upstream side of the valve (point A), the point immediately adjacent to the valve at the downstream side (point B), and a further downstream location (point C). 

Judgment: If the signal strength at point B is significantly higher than that at points A and C, it can be accurately determined that there is an internal leakage in the valve. This method can effectively eliminate the noise interference from other equipment upstream and downstream. 

Parameter analysis method 

It is determined indirectly by analyzing the changes in the operating parameters of the process system. 

Energy consumption and yield analysis: Under the same conditions of raw materials and process parameters, if the energy consumption of the device significantly increases or the product yield decreases, and after ruling out other process reasons, it can be suspected that there is a leakage inside the valve. 

Pressure monitoring: After closing the valve, isolate the downstream system and observe the pressure downstream. If the pressure does not decrease but instead increases after a period of time, or fails to reach the theoretical value, it indicates that there is internal leakage in the valve. 

Flow monitoring: In the pipe section where the valve should be completely closed, if the flow meter still shows a continuous flow, or if the total system flow is abnormally high, it could also be caused by internal leakage of the valve. 

Professional instruments and standard tests 

Suitable for maintenance inspection or in situations where high sealing requirements are necessary. 

Machine learning model: Some advanced industrial facilities have begun to apply machine learning models that integrate pressure, temperature, and flow data. Through algorithms, these models predict valve flow, thereby accurately determining whether there is internal leakage. 

Standard pressure test: According to national standards such as GB/T 13927, the valve is subjected to a sealing test in an offline state. By applying a specified pressure to the valve body and observing whether there is any leakage or if the pressure drop exceeds the limit within the specified time, the test is conducted to determine the result. This is the most authoritative method for judgment. 

Summary of recommendations: In practical operation, it is recommended to first use the temperature method for preliminary screening. For valves that are suspected to have problems, an ultrasonic detector should be used for precise diagnosis. If conditions permit, cross-validation with pressure or flow parameters can be conducted to obtain the most reliable conclusion.