China's valve industry is steadily increasing. Correspondingly, North America and the European Union as the traditional market for valve products, while still occupying a large market share in the global valve market, will tend to slow down in terms of growth rate. According to the analysis report, the export growth rate of China’s leading products for valves will average around 25% in the next three years. Enterprises should seize this historic opportunity to achieve leapfrog development and help push China's valve manufacturing industry from being a manufacturing country to a manufacturing powerhouse!

Loss caused by valve leakage

The leakage of valves in the petrochemical industry produces economic losses and pollution to the environment that vary with the characteristics of the medium. If it is a gaseous medium, it will easily spread in the air and its pollution radius will be larger. The liquid medium will pollute the ground, and it may also flow through the open drain or underdrain to drain rivers, lakes and other environments. In the early morning of December 3rd, 1984, a highly toxic liquid in a 45t liquid highly toxic methine vinegar at the Bhopal Pesticide Plant of India leaked out of the valve and slopped into a dense smog. In the densely populated urban area of ​​Bhopal, in just a few days, the poisonous gas took the lives of 2,800 people and caused more than 520,000 people to be poisoned to various degrees, of which more than 100,000 were disabled for life. The Chernobyl nuclear power station in the former Soviet Union caused an unprecedented disaster due to the problem of one valve. This 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 installations have leakage problems, and the direct economic losses caused by them account for about 5% of the total losses. 0 In the petrochemical industry, a set of 5000kt/a-scale atmospheric and vacuum installations is taken as an example. Under normal circumstances, there are about 5,000 valves. The economic loss caused by the leakage of 20% of the valves and the pollution caused to the environment cannot be underestimated. If we look at the entire petrochemical industry, the situation is even more serious.

solutions

Optimized design selection

Whether the degree of valve leakage can be reduced to the minimum and the service life can reach the longest depends on whether the design of the valve is rational, the selection of the sealing form, the product quality of the valve, the installation construction and the production operation are in compliance with the regulations, etc. The above is based on the optimization of the valve design selection.

The optimization of valve design selection involves many issues such as the choice of valve type, the design and manufacture of the valve itself, and the selection of valve materials. When choosing the type of valve, comprehensive optimization should be considered 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 also select the materials used for manufacturing the valve according to the temperature and the corrosivity of the medium. According to the construction and actual operation experience, in addition to satisfying the relevant process requirements and design specifications, the valve should also fully consider various specific conditions to make it match with the operating conditions as far as possible to meet the requirements of use to the maximum.

For a liquid, gas, or two-phase fluid, the use of a gate valve or globe valve can be significantly different from the use of a ball valve. The airtightness of the ball valve is better than that of the gate valve and the shut-off valve, and the speed of opening and closing is also much faster, but the use temperature range is much smaller than the gate valve and the cut-off valve. In general, most ball valves cannot be used at high temperatures. The performance of gate valve and globe valve is stable under high temperature conditions. The pressure loss of the ball valve is smaller than that of the gate valve and the stop valve. In comparison, the pressure loss of the stop valve is larger than the pressure loss of the gate valve. This is determined by the internal structure of the valve. For different material flow rates and the effect of adjusting the flow rate, the effect of using different types of valves will be quite different. In the case of high-speed flow, it is generally not advisable to use a cut-off valve, because in the case of high-speed flow, the pressure loss of the shut-off valve is the largest, but the cut-off valve has a much better regulating effect on the flow than the gate valve and the ball valve. When selecting the type of valve, consider the material used to make the valve based on the temperature and the corrosion of the media. 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, which can prolong the service life of the valve without greatly increasing the cost. Simply pursuing high grade materials will only result in unnecessary waste of materials.

Packing Box Leakage Countermeasure

Because of the leakage of the seal of the valve packing box and the leakage of the seal of the valve body connection, it is the key to the leakage of the valve. Therefore, special attention is required.

The traditional soft packing seal is the axial pressure that depends on the packing gland, so that a certain radial contact stress is achieved between the valve stem and the packing and the packing and the stuffing box side wall to achieve sealing. Therefore, the axial force of the gland must be quite large, which results in increased friction torque between the packing and the valve stem, increased wear, and rapid wear of the soft seal packing. Therefore, the gland bolt must be tightened frequently or the packing must be replaced to ensure Better sealing effect.

The use of suitable packing seals and packing seals can increase the reliability of valve use and prolong service life. The use of a combination of flexible graphite ring fillers provides better sealing than a flexible graphite ring filler. Currently, there are many cases where a single flexible graphite ring packing is used in China; and in foreign countries, the use of a flexible graphite ring packing combination has become popular and has achieved good results.

Eliminates leaks in the body connection area

The sealing of the valve body connection is a static seal in terms of its sealing properties, which should meet the following requirements:

(1) Adapt to sudden changes in temperature and pressure;

(2) multiple removal without damaging the sealing element;

(3) Simple and compact structure with less metal consumption;

(4) Insensitive to vibration and shock loads;

(5) It can meet the use requirements of various working media.

The connection part of the valve body is usually sealed with a beryllium groove or a concave-convex flat gasket. In recent years, the “O”-shaped sealing ring seal has also been widely used.

The Zen-type flat gasket seal is a flat gasket installed in a closed groove, this structure on the sealing surface, can produce a high sealing specific pressure, usually far beyond 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 4.0 MPa. The disadvantage of this sealing structure is that when the valve is disassembled, the gasket is difficult to remove from the sealing groove, and if it is hard to remove, it will often damage the gasket.

Concave and flat gasket seal is to install the flat gasket on the sealing surface of the concave-convex flange. Compared with the gutter type flat gasket sealing structure, it has the following advantages:

(1) When the valve is disassembled, the gasket is easily removed; (2) Since the sealing groove is stepped, the processing performance is better.

Flat gasket material, according to process parameters and fluid properties, can choose aluminum, copper, 1Cr18Ni9Ti and rubber asbestos board. Fluoroplastics are also commonly used as gasket sealing materials, but due to their cold flow properties, improper design of the sealing structure will lead to undesirable consequences.

The "O"-shaped sealing ring has a simple structure and is easy to manufacture. As long as the sealing structure is designed properly, it can produce enough radial extrusion deformation after assembly, and it can be sealed without axial loading. Therefore, it is used as a flange. Connecting the seal reduces the structural size of the flange and thus reduces the weight of the valve.

For low-pressure, small-diameter valves, in order to reduce the size and weight of the valve body, the valve body is generally internally threaded, and the sealing element at the connection can also be a flat gasket or "O"-shaped sealing ring.

Valve stem leakage countermeasure

The valve stem is an important force component in the valve. The valve stem material must have sufficient strength and toughness, be able to resist corrosion of media, atmosphere and fillers, scratch resistance, and good process performance. In order to improve the corrosion resistance and wear resistance of the stem surface, the surface should generally be reinforced. At present, most of the materials used in the production of valve stems are martensitic stainless steels, but the crevice corrosion resistance of these stainless steels is weak. This is due to the effect of acidic substances, which causes localized corrosion of the passive film on the surface of the stem. Therefore, it is best to use austenitic-ferritic duplex stainless steel, which can enhance the ability of the valve stem to resist crevice corrosion, so that the leakage at the valve stem is effectively controlled.

The leakage of valves in the petrochemical industry is a problem that cannot be ignored. It will have a major impact on the economics, safety, and environment of production. The key to eliminate the valve leakage is to optimize the design of the valve structure, reasonably select the material of the valve according to different process conditions, improve the quality control level of the valve in the production process, and optimize the selection of the valve according to the process operating system. With the advancement of science and technology, especially the emergence of new materials, some valves with new structures have been put into use. They have good performance and long life and can meet the needs of production development.

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