Valve seal principle and leakage analysis of petrochemical equipment
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Key words: petrochemical plant; valve sealing principle; leakage cause
Introduction In the overall structure of a petrochemical plant, the valve is one of the most critical units and plays the role of fluid flow control and is responsible for protecting the pipeline and ensuring the safety of the plant. In the production of petrochemical plants, most of their products are toxic or corrosive, and there is also the danger of explosion. If there is a problem with the sealing of the valve, there will be a danger of product leakage, and the surrounding environment will also cause greater pollution. Petrochemical companies have always invested a lot of money in the maintenance and replacement of valves, trying to achieve a good seal of the valve and prolong the service life of the valve. Therefore, the selection of the valve should be cautious.
1 Sealing of valve of petrochemical device 1.1 Inner sealing of inner sealing valve refers to the part that contacts the opening and closing element. This part often suffers from corrosion of the residual product medium, and at the same time, it is subject to the wear of the opening and closing element and is prone to leakage. Case. The main function of the inner seal is to cut off the flow of the medium of the petrochemical product in the pipeline and ensure that the amount of permeation leakage does not exceed the standard. The form of the inner seal is mainly determined according to the condition of the sealing surface, and its form is mainly three kinds: plane, taper, and sphere.
Fig. 1 shows the effect of the cone sealing of the petrochemical device's valve. The contact area between the valve and the related component is small, and it is easy to achieve the sealing effect. It has great advantages in terms of leakage prevention, but it will cause greater wear on the valve seat. In the construction, the valve seat should be properly treated according to the specific conditions to enhance its hardness and life. The figure a is used in a conventional sealed valve, and the sealing method in figure b is only applicable to a valve with a needle surface in the sealing surface. The c drawing shows the effect of the cone surface sealing.
1.2 External seal The external seal refers to the seal of the connection between the petrochemical device valve and the valve cap, and also includes the seal between the position of the valve stem and the stuffing box. The sealing of the joint is mainly performed at the bonnet. It should meet the changing conditions of temperature and pressure. It can be easily disassembled without damaging other components. The consumption of metal materials should be minimized. The most important thing is to withstand the medium. (Petrochemical products) erosion.
The sealing packing of the valve stem mainly includes the sealing of the bellows material and the diaphragm. The packing type sealing is further divided into a soft packing (the axial force mainly through the gland to produce the packing) and a shaped packing (common materials include graphite and plastic. , metal and carbon fiber). Its sealed form is mainly composed of three structures: external, internal and bi-directional compressive soft packing.
The material of the sealing surface is generally non-metal materials and metal materials. The commonly used materials are rubber and plastic, as well as bearing alloys with good wear and corrosion resistance, and other copper alloys and chromium nickel suitable for petrochemical products. Stainless steel and so on.
2 Causes of valve leakage in petrochemical plants Valve leakage in petrochemical plants is divided into internal and external leakage. Under normal circumstances, in the leakage caused by petrochemical plants, external leakage will cause more serious impacts, including the burning and explosion of petrochemical products, as well as the environmental pollution caused by proliferation, and will also cause great harm to the health of nearby residents. .
2.1 Causes of External Leakage of the Valve Leakage caused by the valve at the outside is mainly caused by the seals at various locations.
The first is the leakage of the valve body, its external performance is generally the valve body leakage, resulting in less leakage; the valve stem is mainly when the valve falls and the contact surface has a larger gap, or the valve stem is stuck to cause the valve There is a problem with the closure. The leakage here will result in the loss of a large amount of petrochemicals, and the pollution to the environment will be more serious. The leakage is found where the valve body is connected, mainly because the size or type of the sealing member does not meet the sealing requirements, and the sealing surface There is a problem with the quality of the material or the relevant configuration of the pipeline is unreasonable.
2.2 The causes of internal leakage The first is the valve design and manufacturing, there is a gap between the valve design and the actual requirements, resulting in leakage of the media; in the manufacturing quality and process problems, in the course of the use of deformation, in the tightness There is a big problem with the valve, there is a gap between the valve, making the leakage of petrochemical products from the gap; the valve is damaged during the process of manufacture and transportation, or sealing problems during installation and use, petrochemical device Leakage and loss of media will occur; the final issue is petrochemical products, mainly manifested in solid medium in the media between the valve and the contact surface, so that the valve does not close the problem of leakage, resulting in leakage [2].
Leakage of petrochemical products will cause different effects in the form of media. If the relevant medium is in a liquid state, it will leak into or leak into the land or river, causing land pollution and pollution of river water sources. It will pose a serious threat to the health of nearby residents; if it is a gaseous medium, it will follow. The circulation of air will cause serious pollution to precipitation, domestic water and land in a certain area.
3 Measures to Prevent Valve Leakage in Petrochemical Plants 3.1 Optimizing Valve Performance Valve performance optimization involves the selection of valves, design and manufacturing, and material selection. In the selection of valves, from the point of view of the manufacturing process and valve design specifications, comprehensive consideration should be given to petrochemical products such as temperature, corrosion, and pressure, and materials that meet the requirements should be selected to achieve optimal valve performance and meet its requirements. The requirements to adapt it to the operating specifications.
In addition, the choice of valve form is also very important, and the appropriate valve should be selected for different forms of petrochemical products. In addition to conventional liquid media, petrochemical companies will also produce gaseous products. In the flow of pipelines, the effectiveness of valves (such as flat and needle-shaped valves) that meet the requirements of liquid media will be greatly reduced. Leak-proof effect will be worse. For a pipeline in which a gaseous medium circulates, it is very important to select the corresponding valve. In general, the spherical valve has a good blocking effect on the gaseous medium because of its good airtightness and the speed of opening and closing. Fast, but its applicable temperature range is smaller [3].
3.2 Stem Leakage Prevention Measures The main cause of leakage at the stem is the material selection of the stem. The main role of the valve stem is to control the opening and closing of the valve, so it will be subject to more wear, which will lead to leakage of the stem; if the material is poor, the wear will be very serious in the course of frequent use It will lead to leakage of the medium and even affect the opening and closing of the valve, resulting in a large loss of medium. In order to solve this problem, starting from the corrosiveness of the media, different materials are used for different media. The common method is to select suitable materials and perform surface treatment. The surface treatment of the valve stem is generally performed by surface heat treatment. , Or directly on the surface of gold plating, this method can better improve the degree of wear of the stem, but the cost will exceed the budget.
3.3 Anti-leakage Measures for Valve Cover Connections Leakage at the connection parts of the valve bonnet is mainly due to the size or type of the sealing member not conforming to the requirements of the seal, and the unreasonable configuration of the piping, and the use of gaskets and bolts that are not standardized. For these problems, we must first pay attention to the size and type requirements in the construction process, to achieve a good sealing of the valve cover connection, in the treatment of the sealing surface, to avoid scratches or surface defects, to ensure its surface Flat to prevent media leakage from the sealing surface. In addition, when the sealing performance is enhanced, the material of the soft filler is rigorously screened to ensure the tightness of the joint and prevent leakage of the medium [4].
4 Conclusion The petrochemical plant has very strict requirements on the sealing performance. Once a sealing problem causes leakage of the medium, serious consequences will result. This article has carried on the detailed analysis to the valve seal, separately starts from the internal and the external seal, in-depth analysis summarizes the reason of the medium leakage, and proposes the related leakage prevention countermeasure to the part which appears the seal problem, aims at provides the petroleum chemical installation protection Leakage methods and measures to deal with leakage problems.
references:
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[2] Su Xiang, Li Qiang, Guo Jin. Research on valve sealing and leakage problems in petrochemical plants [J]. China New and Innovative Products, 2016(4):186-186.
[3]Ji Xiaoshan. Valve sealing principle and leakage treatment of petrochemical plants[J]. Chemical Engineering Design Communications, 2017, 43(3): 67-67.
[4] Wu Pengchao. Analysis and countermeasures of valve leakage in petrochemical pipeline stations [J]. Engineering Technology: Full Text, 2016(5):311-312.