Probe into the methods of valve leakage detection
Compared with other industries, the chemical industry and oil industry have more serious environmental pollution, so relevant departments and personnel must pay more attention to it. At present, China has clearly stipulated the discharge standards of waste residue, waste gas and waste water and taken relevant measures to intervene, but at present there is a lack of attention to pollution sources, such as valve leakage. Valves are the most critical components used in petrochemical smelting plants. Due to the large number of valves and the large number of valves in petrochemical production, most of them use flammable, explosive, toxic and corrosive substances as the main medium. In practical applications, the probability of valve failure is high, which will lead to more serious waste of products, energy and raw materials, and aggravate the adverse impact on the environment, Therefore, this paper discusses and analyzes the detection methods of valve leakage.
Valves play an indispensable role in oil and gas storage and transportation. Valve leakage is one of the common problems. If measures cannot be taken to solve the problem as soon as possible, serious consequences will occur. Valve leakage mainly includes internal and external shackles. If external leakage occurs, the sound of air flow can be heard for a long time. If the valve has internal leakage, it is not easy to detect, and the concealment is strong, which is very easy to lead to the formation of safety hazards. This paper analyzes the main factors of valve leakage, understands its harm to production and the environment, starts from the situation of petrochemical production, analyzes the valve detection technology and mechanism, and puts forward targeted suggestions for valve leakage detection in petrochemical production.
1. Analysis of valve leakage factors
During the actual production research, the main problem of valve leakage is quality. Because valve leakage mainly includes valve internal leakage and valve external leakage, valve internal leakage and external leakage are generally caused by the failure of valve sealing pair. The external leakage of the valve refers to the leakage of fluid from the outside of the valve, and the internal leakage refers to the leakage caused by the loose sealing of the valve. After the valve is closed, some fluid still flows inside the pipeline. Although there are differences between the two leakage phenomena, they are related to the quality of the valve itself and the application of the valve. In general, the adverse consequences caused by external leakage are more serious than those caused by internal leakage. In production, once the valve leaks, it will lead to serious energy waste and waste of raw materials, and even pollute the environment, resulting in poisoning, explosion or fire, endangering life and causing loss of national economy .
1.1 Valve leakage factors
External leakage often occurs at the valve body, stuffing box, valve rod and valve body connection. The leakage of valve body is mostly caused by casting, which is very easy to cause sand hole formation and induce casting defects. There will be sand hole in the valve body, which is very easy to cause medium leakage. The general condition is leakage, with small flow, which can be found by hydraulic test. The causes of valve stem leakage are mostly due to the valve stem being stuck at a certain position due to the material selection or the unreasonable valve stem design during the valve stem design, which causes the valve to be unable to close normally, affects the tightness of valve closing, and has a negative effect on the production device and the surrounding environment. The stuffing box is mainly composed of packing press, packing pad, packing and stuffing box. The packing box needs to exist on the bonnet or valve body, and its main function is to contain the packing. A packing pad is placed at the bottom of the stuffing box to play the role of packing support. The seasoning mainly includes two types of molding filler and soft sealing filler. The elastic material is the applied raw material of molding filler. It mainly uses turning processing and compression molding to make the annular sealing ring. In contrast, the molding filler has a more compact structure, better sealing performance and stronger adaptability . The main factors of leakage in the application of stuffing box are loose packing gland, lack of tightness of sealing, inconsistent packing type or quality with requirements, aging of packing and wear of valve rod. Leakage factor of valve body connection position. The tightness of valve body connection position refers to the tightness between valve bonnet and valve body. In general, the sealing is mostly made of flange. The gasket size, gasket material and type do not conform to the requirements, the flange sealing surface processing quality is abnormal, the connecting bolts are not tightened properly, and the connection position is subject to large additional load due to different pipe configurations, It is easy to cause the leakage of the valve body connection.
1.2 Factors causing valve internal leakage
The partition type of medium mainly depends on the sealing of the valve seat surface and the valve core. The loss of the sealing effectiveness of the valve seat and the sealing effectiveness of the valve core will lead to the internal leakage of the valve. The reason is that there are defects in the manufacturing process and design of the valve, which is very easy to lead to the lack of tightness of the valve seal and induce the medium leakage. Most of them are small flow discharge or leakage. In the process of manufacturing, transporting, inspecting, installing and applying valves, the sealing surface of the valve is seriously damaged and its sealing performance is affected. Even if the leakage is small, it is easy to cause large flow leakage; There are solid impurities in the medium, which is very likely to have adverse effects on the valve closing, leading to the occurrence of medium leakage. This leakage can be small flow leakage, and if it is serious, there may also be large flow leakage .
A comprehensive study of valve leakage factors can be concluded that it is very easy to form leakage problems in the process of manufacturing, designing and transporting valves, and reliability control research is carried out relying on the effective inspection form in the early stage. In the process of valve installation and application, leakage is easy to occur, mainly due to the damage of valve stem stuffing box seal or flange seal at the valve connection, resulting in the loss of the effectiveness of valve seat sealing surface, leading to the formation of external or internal leakage. According to the statistical results, four fifths of the valve leakage problems are mostly caused by the damage of the sealing surface. In the process of petrochemical production, because the internal leakage is not easy to detect, there are often accidents such as medium pollution, poisoning or fire.
2. Valve leakage detection method
2.1 Visual observation method
Patrol the device according to experienced operators, and analyze whether there is leakage by smelling, listening and looking. This form cannot carry out continuous leak detection, so it has poor real-time detection.
2.2 Mass balance method
From the difference between the mass value and the volume value of the incoming and outgoing valve media, it is analyzed whether there is a leakage problem. Its application advantage is that it has high reliability and can test the small flow leakage problem, but there are also defects in its application, because the application has poor timeliness and can only detect the leakage of the valve .
2.3 Pressure point analysis method
The pressure point analysis method is the most important method for valve leakage detection. The leakage generated above the operating valve will increase the upstream flow, and the pressure value in the downstream and upstream will also decrease. Leakage will lead to the largest data of pressure drop at the leakage point, and it will gradually decrease towards the head and tail of the leakage section. If the inlet flow or pressure value changes greatly, it indicates that there may be valve leakage. This method can detect large leakage problems.
2.4 Acoustic emission method
In materials, after the local source energy is released rapidly, the transient elastic wave phenomenon will be formed, which refers to acoustic emission, also known as stress wave emission. In the process of valve inspection, if the leakage problem is caused by the poor sealing of the valve, the material inside the valve body will be sprayed out from the gap of the sealing surface, resulting in the formation of turbulence. This turbulence has an impact on the sealing surface, causing the elastic wave to be excited, which is the leakage signal. The strength of the signal is related to the flow speed, flow rate, fluid physical properties, pipeline layout, valve pressure drop and valve front pressure between the fluids. The elastic wave will spread from the valve body to the position of the material surface, which will induce the change of the detection surface position of the acoustic emission sensor. The detector will be affected by the piezoelectric effect, and the material will undergo vibration transformation to form an electrical signal, which will then be amplified, processed and recorded. Depending on the acoustic emission signal recording, analysis and identification form, the valve leakage can be effectively controlled .
2.5 Negative pressure wave method
If the leakage problem occurs, the local liquid density will be reduced due to the loss of fluid material at the leakage location, resulting in the instantaneous pressure drop. As the source of pressure relief wave, it can rely on the fluid medium and pipeline to spread at a certain speed upstream and downstream of the leakage point. If the pressure value before leakage is taken for reference, the pressure relief wave generated during leakage is also called negative pressure wave. The setting of the sensors at both ends of the leakage point can start from the change of the pressure signal and combine with the time difference of the negative pressure wave generated in the leakage to transmit it to the upstream and downstream, so as to determine the location of the leakage. This method has strong sensitivity in detection and does not need to build a mathematical model. Its application principle is simple and has strong applicability. Since the most critical problem in the application of the negative pressure wave method is the setting of the number of pressure transmitters. A large number will induce the problem of resource waste, and a small number will induce the problem of leakage and false alarm. Therefore, the self-learning function and sample selection function can be carried out in combination with the current advanced RBF neural grid method in foreign countries to optimize the number of pressure transmitters.
To sum up, in the production and operation of petrochemical enterprises, valve leakage will cause major accidents. It is an important issue that must be paid attention to in oil production, which will damage the economic benefits of enterprises, affect the safety of enterprise operation, and affect the effect of energy conservation and environmental protection. Analyzing the current valve management mode of petrochemical enterprises, simple inspection means are the main form of valve management, which must be regularly overhauled in enterprises, which seriously affects the service life of valves and the safety of equipment operation. Therefore, to eliminate the valve leakage problem, it is necessary to optimize its structural design, select reasonable types, gradually optimize the online leakage detection technology, carry out predictive maintenance, and apply negative pressure wave method and RBF neural network to carry out valve leakage detection, so as to ensure the improvement of valve application safety.
Author: Li Dong’an
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.sfutube.com)
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