Valves are an indispensable part of petrochemical refining equipment. They use many kinds of valves and large quantities. The mediums in chemical production are mostly corrosive, toxic or flammable and explosive. When the valve leaks, it will not only cause serious damage. The waste of raw materials, energy and products, but also has a serious impact on the environment, and even cause serious safety accidents. Therefore, we must also pay attention to the leakage of valves in the petrochemical production process. The following is a discussion on the causes of valve leakage in petrochemical plants and the countermeasures and methods to prevent and eliminate leakage.
2 analysis of the cause of valve leakage
2.1 Classification of valves
Valves are an indispensable control device in petrochemical production facilities. There are many types of valves, and the variety of valves continues to increase with the continuous improvement of various types of equipment. According to their different uses, they can be divided into the following categories:
(1) Gate valves, globe valves, plug valves, ball valves and butterfly valves for cutting or closing the medium in the pipeline; (2) regulating valves for regulating the pressure and flow of the medium in the pipeline (both pneumatic and electric) Type), pressure reducing valve, throttle valve, butterfly valve, balance valve, etc.; (3) check valve for preventing medium backflow and ensuring one-way flow of medium; (4) for over-temperature and over-pressure safety protection, Discharge excess medium to prevent temperature and pressure from exceeding the specified value, thus ensuring safe operation of the equipment and piping system, relief valve, etc.; (5) for changing the direction of medium flow in the pipeline, and distributing the distribution of the distributing medium Valve, three-way or four-way plug valve, three-way or four-way ball valve, etc.; (6) sampling valve for sampling analysis of medium; (7) other special purpose steam trap, drain valve, vent valve, slag valve Wait.
2.2 Analysis of the causes of valve leakage [1]
There are two types of valve leakage, namely external leakage and internal leakage. In most cases, the consequences of external leaks are often more severe than internal leaks. In industrial production, the leakage of valves not only causes waste of raw materials and energy, but also directly pollutes the environment, and even causes major accidents that endanger life and safety such as fire, explosion, poisoning, and cause serious losses to the national economy.
(1) Analysis of the causes of leakage of valves
External leakage is common in the connection between the valve body, the valve stem, the stuffing box and the valve body.
1) Causes of valve body leakage The valve body is usually cast, which is easy to form casting defects such as blisters. The sand hole on the valve body will cause leakage of the medium. This leakage is generally manifested as leakage, and the flow rate is small. Can be found. 2) Causes of valve stem leakage Due to improper design and material selection, the valve stem of the valve may cause the valve stem to be stuck at a certain position, so that the valve cannot be closed or closed, resulting in leakage of the medium. Such leaks tend to have large flows, which can easily cause serious damage to the production equipment and the surrounding environment. 3) Reasons for the leakage of the stuffing box The stuffing box is composed of a stuffing box, a packing, a packing pad and a packing pressing mechanism. The stuffing box is placed on the valve body or bonnet to accommodate the packing. The packing mat is placed at the bottom of the stuffing box to support the packing. The filler is divided into two types: soft sealing filler and forming filler. The forming filler is made of elastic material and is formed into various annular sealing rings by molding or turning. In contrast, the forming packing has compact structure, good sealing performance and strong applicability. The medium leaks at the sealing packing because the packing gland is loose, the sealing packing is not tight, the type or quality of the packing is not satisfactory, the packing is aged or worn by the valve stem. 4) Cause of leakage at the valve body joint The seal at the valve body connection is the seal between the valve body and the valve cover. In general, the flange connection seals, when the nominal diameter of the valve is small, it is a threaded connection seal. If the type, material or size of the gasket does not meet the requirements, the processing quality of the flange sealing surface is poor, the connecting bolt is improperly fastened, and the excessively added load is generated at the joint due to the unreasonable piping configuration, the valve body connection portion can be caused. leakage.
(2) Analysis of the causes of leakage inside the valve
The leak that is not tightly closed is an internal leak, which often occurs on the seat sealing surface. Analyze the reason:
1) There are problems with the design and manufacturing process of the valve, resulting in a leaky valve that causes leakage of the medium, mostly for leakage or continuous discharge of small flow. 2) The deformation of the valve plate or sealing surface causes the sealing to be not strict, which causes leakage of the medium, and is generally discharged continuously or leaking. 3) In the process of manufacturing, transportation, inspection, installation and use of the valve, the sealing surface of the valve is damaged, so that the sealing is not strict, resulting in leakage of the valve. This leakage also manifests as a small flow of leakage. 4) The solid impurities contained in the medium cause the valve to be closed tightly, causing the medium to leak. This leakage may be a small flow of leakage or a large flow.
2. 3 loss caused by valve leakage
The economic losses caused by the above-mentioned leakage and the degree of pollution to the environment vary depending on the characteristics of the medium. If it is a gaseous medium, it is easy to spread and spread in the air, and its pollution radius will be large. The liquid medium will pollute the ground through the infiltration of the ground, or it may flow through the open channel or the dark ditch to pollute rivers, lakes and other environments. In the early morning of December 3, 1984, a highly toxic liquid in a storage tank containing a 45t liquid poisonous sulphuric acid methyl vinegar in the Bhopal Pesticide Plant in India broke through the valve and turned into a thick smoke. In the densely populated city of Bhopal, in just a few days, the poison gas claimed 2,800 lives, and more than 520,000 people were poisoned to varying degrees, of which more than 100,000 were disabled for life. The Chernobyl nuclear power plant in the former Soviet Union caused an unprecedented disaster due to a valve problem, which not only caused major casualties, but also caused direct and indirect economic losses and environmental damage. Statistics show that about 20% of the valves in industrial plants have leakage problems, and the direct economic losses caused by them account for about 5% of the total losses. In the petrochemical industry, a set of 5000 kt/a atmospheric and vacuum units is taken as an example. Under normal circumstances, there are about 5,000 valves. The economic loss caused by leakage of 20% of the valves and the pollution caused by the environment cannot be underestimated. If the whole petrochemical industry is concerned, the situation will be more serious.
3 countermeasures
3.1 Optimization design selection [2,3]
Whether the degree of valve leakage can be minimized and the service life is the longest depends on whether the valve design is reasonable, the selection of the sealing form, the quality of the valve, the installation and the production operation are in conformity with the specifications, and to a large extent. The top is optimized depending on the valve design selection.
The optimization of valve design and selection involves various aspects such as the choice of valve form, the design and manufacture of the valve itself, and the selection of valve materials. When selecting the valve form, it should be fully optimized from the perspective of the requirements of the process conditions and the requirements of the design specifications. The use of the valve, the temperature of the medium, the pressure, the flow rate, the pressure drop, and the corrosiveness of the medium all directly affect the selection of the valve, and the material used to manufacture the valve is selected according to the temperature and corrosiveness of the medium. According to the construction and actual operation experience, in addition to meeting the relevant process requirements and design specifications, the selection of valves should also fully consider various specific conditions, so as to match the operating conditions as much as possible to meet the requirements of use to the utmost.
For liquid, gas or two-phase fluids, the use of gate or shut-off valves can have significantly different effects than ball valves. The ball valve is more airtight than the gate and shutoff valves, and the opening and closing speeds are much faster, but the operating temperature range is much smaller than the gate and shutoff valves. In general, most ball valves cannot be used at high temperatures. The performance of the gate valve and the shut-off valve is relatively stable under high temperature conditions. The pressure loss of the ball valve is smaller than that of the gate valve and the shut-off valve. In contrast, the pressure loss of the shut-off valve is greater than the pressure loss of the gate valve, which is determined by the inherent construction of the valve. For different material flow rates and the effect of regulating the flow rate, the effect of using different types of valves will be quite different. In the case of high-speed flow, the shut-off valve is generally not suitable because the pressure loss of the shut-off valve is the highest in the case of high-speed flow, but the regulating effect of the shut-off valve on the flow is much better than that of the gate valve and the ball valve. When selecting the valve type, the material used to make the valve is also considered based on the temperature and corrosivity of the medium. The material used for the valve must meet the process conditions of the valve operating point. According to the process requirements of the working point, the material of the valve core and the valve body can be reasonably selected to prolong the service life of the valve without greatly increasing the cost. Simply pursuing a high-grade material will only result in unnecessary waste of materials.
3.2 stuffing box leakage countermeasures
Due to the leakage of the seal of the valve stuffing box and the leakage of the seal at the joint of the valve body, it is the key to the leakage of the valve, so special attention is required.
The traditional soft packing seal is based on the axial pressure of the packing gland, which causes a certain radial contact stress between the valve stem and the packing and the packing and the side wall of the stuffing box to achieve sealing. Therefore, the axial force of the gland must be quite large, which causes the friction torque between the packing and the valve stem to increase, the wear increases, and the soft sealing packing wears quickly. Therefore, it is necessary to tighten the gland bolt or replace the packing frequently to ensure Better sealing effect.
The use of a suitable packing seal and packing seal combination can improve the reliability of the valve and prolong its service life. For example, the combination of flexible graphite ring packings is better than the sealing effect of only flexible graphite ring packing. At present, there are many cases of using a single flexible graphite ring packing in the country; while in foreign countries, the use of a flexible graphite ring packing combination has become popular and has achieved good results.
3.3 Eliminate the leakage of the valve body connection
The valve body joint is sealed and is a static seal in terms of its sealing properties, which should meet the following requirements:
(1) can adapt to sudden changes in temperature and pressure; (2) multiple disassembly without damaging the sealing element; (3) simple and compact structure, low metal consumption; (4) insensitive to vibration and shock loads; ) can meet the requirements of various working media.
The connecting part of the valve body is usually sealed by a birch groove or a concave-convex flat gasket. In recent years, the "O"-shaped sealing ring has been sealed and has been widely used.
The Zen trough type flat gasket seal is to install the flat gasket in the closed groove. This structure can produce a high sealing specific pressure on the sealing surface, which usually far exceeds the yield limit of the gasket material, thus ensuring Reliable sealing. It is suitable for medium and high pressure valves with a pressure greater than or equal to 4.OMPa. The disadvantage of this type of sealing structure is that it is difficult to remove the gasket from the sealing groove when the valve is removed, and the gasket is often damaged if it is taken out rigidly.
The embossed flat gasket seal is to install the flat gasket on the sealing surface of the concave and convex flange. Compared with the grooved flat gasket sealing structure, it has the following advantages:
(1) When the valve is disassembled, the gasket is easy to take out; (2) Since the sealing groove is stepped, the processing performance is good.
The material of the flat gasket can be selected from aluminum, copper, 1Cr18Ni9Ti and rubber asbestos according to the process parameters and fluid properties. Fluoroplastics are also commonly used gasket sealing materials, but due to their cold flow properties, if the sealing structure is not properly designed, it will lead to adverse consequences.
"O"-shaped sealing ring has a simple structure and is easy to manufacture. As long as the sealing structure is designed reasonably, sufficient radial deformation can be generated after assembly, and the sealing can be achieved without axial loading. Therefore, it is used as a flange. The connection seal reduces the structural size of the flange and thus the weight of the valve.
For low-pressure small-diameter valves, in order to reduce the size and weight of the valve body structure, the valve body is generally connected by internal threads, and the sealing elements at the joints may also be flat gaskets or "O"-shaped seal rings.
3.4 stem leakage countermeasures
The valve stem is an important force component in the valve. The valve stem material must have sufficient strength and toughness to withstand the corrosion of the medium, atmosphere and filler, scratch resistance and good process performance. In order to improve the corrosion resistance and wear resistance of the valve stem surface, the surface should be reinforced. At present, the material for manufacturing the valve stem is mostly made of martensitic stainless steel in China, but the stainless steel has weak crevice corrosion resistance, which is due to the action of acidic substances, causing local corrosion caused by the destruction of the passivation film on the valve stem surface. Therefore, it is better to use austenitic-ferritic duplex stainless steel to enhance the crevice corrosion resistance of the valve stem, so that the leakage at the valve stem can be effectively controlled.
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