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1. Overview
The ball valve is a device used to cut off, distribute and change the flow direction of the medium. Its opening and closing part is a sphere, which is opened and closed by rotating the sphere at an angle of 90 around the axis of the valve stem. During the operation of the gas transmission pipeline, the internal leakage of the ball valve will bring great trouble to the process operation, especially the internal leakage of the shut-off valve of the gas transmission trunk pipeline will seriously affect the hot work of the pipeline. The common methods for online detection of internal leakage of the gas transmission ball valve include valve cavity venting, process judgment, valve cavity pressure detection, instrument detection, etc., but the above methods have various shortcomings. The method of detecting the internal leakage of the ball valve realizes the intuitive display and accurate measurement of the internal leakage of the ball valve.
 
2. Online detection of internal leakages of ball valves
2.1 Valve cavity venting
For a ball valve with a valve cavity with a drain port (drainage port or vent port or both), you can close the valve and open the valve cavity drain port to vent to judge whether the valve is leaking. If the gas in the valve cavity can be emptied to normal pressure at one time, it means that the valve has no internal leakage; if the gas in the valve cavity can be emptied to normal pressure at one time, but the gas will be discharged after closing the valve cavity for a short time, it means that the valve has a slight internal leakage; if the gas in the valve cavity cannot be discharged all the time, it means that the valve is leaking internally or the valve limit is not correct. At this time, you can first adjust the valve limit and then observe the internal leakage of the valve. This is the most commonly used online detection method for internal leakage of valves. Although the operation is simple, it lacks quantitative detection and analysis, and there are certain risks in on-site sewage discharge or natural gas venting.
 
2.2 Process Judgment
For normally closed valves, it can be judged whether the valve is internally leaking according to the pressure change of the processing system at the back end of the valve; for normally open valves, if the process conditions permit, the leakage of the valve can be judged by venting the processing system upstream or downstream of the valve after closing the valve.
 
The internal leakage rate Q of the valve can be calculated by the differential pressure and volume of the processing system connected to the valve.
 
In the formula, Q is the internal leakage rate of the valve.
P1 is the initial pressure of the valve cavity, the pipeline behind the valve or the pressure vessel.
P2 is the pressure of the valve cavity, pipeline after valve or pressure vessel during inspection.
V0 is the volume of the valve chamber, pipeline after valve or pressure vessel.
T is the time.
D is the nominal diameter of the pipeline.
 
This method can realize qualitative and quantitative detection at the same time, but the calculation formula of the internal leakage rate is difficult for production operation and maintenance personnel to understand and master. The calculation involves many parameters, which are not easy to obtain, and the on-site operability is poor.
 
2.3 Instrument detection
Use an ultrasonic detector, acoustic emission detector, infrared thermal imaging camera and ultrasonic phased array instrument to conduct qualitative or quantitative detection of internal leakages of ball valves. However, such instruments cannot detect buried globe valves, and the price is relatively high, so it is not easy to popularize.
 
2.4 Pressure detection of valve chambers
For a ball valve with a relief port in the valve cavity, a pressure gauge can be installed at the relief port to observe the pressure rise rate of the valve cavity or the pressure difference of the valve cavity to judge whether the valve leaks internally and calculate the leakage of the valve. According to the ideal gas state equation, the volume V0 of the valve cavity gas in the standard state can be calculated. The influence of temperature can be ignored.
 
This method can quantitatively detect the internal leakage of the valve, but the formula for quantitative calculation is not easy to be understood and mastered by the majority of production and maintenance personnel, and the parameters involved in the calculation, such as the water volume at the closed position of the valve cavity, are not easy to obtain, and the on-site operability is poor.
 
2.5 Flowmeters
For a ball valve with a valve cavity with a discharge port (drainage port or vent port or both), you can judge whether the valve internally leaks or not by installing a flow meter at the discharge port to measure the gas flowing out of the valve cavity. This method realizes the quantification of internal leakage measurement of valves, and the equipment is portable and easy to operate.