Standard for determining internal leakage of valves 
The method for determining valve internal leakage is as follows: After the valve is closed for 4-6 hours, use an infrared thermometer to measure the temperature of the valve stem (near the valve body) or the metal temperature 150mm downstream of the valve body. If the temperature is greater than 70℃, it is considered "internal leakage". This judgment method is applicable to most valves with internal leakage. 
However, in actual work, we have encountered some special situations:

(1) Due to the installation position of the pipeline, some valves have high-temperature steam disturbances before and after them, such as drain valves or discharge valves connected to pressurized drain and water header pipes. Even if these valves are tightly sealed and leak-proof, their valve stem temperature will exceed 70℃.

(2) Drain valves or discharge valves connected in parallel to the drain and water header pipes. When the position of the last valve is close to the header pipe, as long as any branch valve in the pipeline leaks internally, the temperature of other valves will increase and exceed 70℃, such as boiler drain valves and superheated steam drains. Therefore, other methods should be used to determine the internal leakage of these valves. Generally, the pipe wall temperature in front of the valve or the valve stem temperature in front of the primary valve is measured to determine the internal leakage situation.

2. If the operation personnel confirm or suspect valve internal leakage, they must notify the maintenance personnel to arrive for confirmation. After joint identification and confirmation with the maintenance personnel that it is internal leakage, the defect can be registered, and the names of the maintenance and identification personnel should be recorded in the defect information.

II. Precautions for valve operation:

1. For various manual drain and water valves in the thermal system, they should be tightened again half an hour after operation to eliminate the valve gap caused by thermal expansion and contraction.

2. For thermal pipelines with primary and secondary valves, the operation sequence of the valves is as follows: When opening, open the primary valve first, then the secondary valve; when closing, close the secondary valve first, then the primary valve.

3. For gate valve operation, it can only be fully closed or fully opened, and a half-open and half-closed state is not allowed to reduce valve wear and tear.

III. Precautions for valve management:

1. Before the unit starts, test various electric and pneumatic valves. During the test, a dedicated person should monitor the operation of the valves and dampers on site. Check that the connecting rods and pins of the actuators are not loose, bent, or detached. Check that the DCS opening indication is consistent with the on-site indication. Electric valves with intermediate stops should test the normal intermediate stop. The adjustment valve switch direction is correct, the operation is flexible, and those with interlock circuits should conduct interlock tests with relevant professionals. The pneumatic regulating device should operate flexibly, without leakage or abnormal phenomena. If any abnormality occurs during the test, the test should be stopped, and the maintenance personnel should be notified immediately for processing.

2. All drain and water valves should be checked and closed as required. 6 hours (within 10 hours) after closing each drain and water valve, for drain valves with temperature indication, the valve internal leakage should be judged based on the temperature indication of the downstream pipeline. For drain valves without temperature indication, an infrared thermometer should be used to measure the temperature of the valve's inlet and outlet pipe walls, or manual feeling should be used to determine whether the valve is leaking. If an electric or pneumatic valve is leaking, the manual isolation valve in front of the electric or pneumatic drain valve should be closed. If there is no manual isolation valve in front of the electric or pneumatic drain valve, and the leakage is large, the electric valve should be manually closed and a defect should be reported, and maintenance should be contacted for processing. If the manual valve is leaking, a defect should be reported, and maintenance should be contacted for processing.

3. Before each unit shutdown, a comprehensive check of valve defects (leakage) should be carried out. Leaking valves should be reported as defects and the relevant maintenance department should be notified.

4. During the start-up and shutdown process or the process of draining water under pressure, after the drain valve is opened, the temperature behind the valve or throttle hole (drain) should be checked to ensure that it can play a drainage role and prevent water accumulation and corrosion of the thermal pipeline due to blockage of the valve, pipeline, or throttle hole (drain).