Effects of valve leakage and countermeasures

                  Column:Technical article Time:2018-06-30
                  Valve industry in China are rising steadily, corresponding to the traditional markets of North America and the eu as a valve product, although still occupy a larger market share of global valve market...

                  Valve industry in China are rising steadily, corresponding to the traditional markets of North America and the eu as a valve product, although still occupy a larger market share of global valve market, but in terms of growth will be slow. According to the analysis report, the export growth rate of China's valve leading products will be about 25% on average in the next 3 years. Enterprises should seize this historical opportunity to realize leapfrog development and boost China's valve manufacturing industry from a manufacturing power to a manufacturing power!


                  Loss due to valve leakage


                  The economic loss caused by valve leakage in petrochemical industry and the degree of environmental pollution are different depending on the characteristics of media. If it is a gas medium, it will spread easily in the air, and its pollution radius will be larger. The liquid media will pollute the ground through infiltration, and may also contaminate rivers, lakes and other environments by flowing into human drainage channels through open or dark channels. On December 3, 1984 in the morning, India Bhopal pesticide plant one containing 45 t poisonous liquid cylinder atmosphere of vinegar acid armour poisonous liquid in the tank leak to break through the valve and tilting moment between into thick smoke waft in Bhopal, densely populated urban areas, in just a few days, gas killed 2800 people, more than 520000 people by different degree of mischief, and there are more than 100000 people lifelong disability. The former Soviet union's chernobyl nuclear power plant because of a valve and caused an unprecedented disaster, not only caused serious casualties, caused by the direct and indirect economic loss and environmental damage is difficult to measure. Statistics show that about 20% of the valve leakage problems in industrial devices, the direct economic losses caused by account for about 5% of the total loss of 0 in the petrochemical industry, with a 5000 kt/a scale often pressure-relief devices, for example, usually around 5000 valves, by 20% of the valve leakage caused by the economic loss and pollution to the environment to be reckoned with, if from the point of the petrochemical industry, the situation is more serious.


                  countermeasures


                  Optimize design selection


                  Can minimize the degree of valve leakage, achieve the longest service life and depends on the valve design and type selection is reasonable, the selection of sealing, the valve product quality, installation, construction and operation whether accord with standard, etc., and is largely depends on the valve on the selection of design optimization.


                  The optimization of valve design and selection involves the selection of valve form, design and manufacture of valve itself and the selection of valve materials. When selecting valve form, it should be considered from the aspects of process condition and design specification. The purpose of the valve, medium temperature, pressure, flow rate, pressure drop and the corrosive medium, etc., are directly affect the valve selection, but also according to the medium temperature and corrosion resistance, used in manufacturing valve materials. , according to the construction and practical operation experience, the selection of valves in addition to meet the technical requirements, design specification, also should fully consider various specific circumstances, to make it match the operating conditions as far as possible, maximum limit meets the requirement.


                  For liquid, gas, or two-phase fluids, the use of gate or globe valves is significantly different from the use of ball valves. The air tightness of ball valves is better than that of gate and globe valves, and the opening and closing speed is much faster, but the temperature range is much smaller than that of gate and globe valves. In general, most ball valves cannot be used at high temperatures. The performance of gate and globe valves is stable at high temperature. The pressure loss of ball valves is smaller than that of gate and globe valves, and the pressure loss of globe valves is greater than that of gate valves, which is determined by the internal structure of the valve. For different material flow rate and adjust the flow rate of the effect, using different types of valves will be very different. Should not be commonly use under the condition of high speed flow cut-off valve, because in the case of high-speed flow cut-off valve of the pressure loss is the largest, but stop valve on the discharge regulation was much better than gate valves and ball valves. When choosing the valve type, the material used in making the valve should also be considered according to the temperature and corrosion of the medium. The materials used in the valve must meet the requirements of the operating conditions of the valve. According to the technical requirements of the working point, reasonable selection of the material of the valve core and valve body can extend the service life of the valve without greatly increasing the cost. Pure pursuit of high quality materials will only lead to material waste.


                  Stuffing box leakage countermeasures


                  Because the leakage of valve stuffing box seal and the leakage of valve body connection seal are the key to valve leakage, special attention should be paid to it.


                  The traditional soft packing seal, is to rely on the packing gland axial pressure, make in the stem and packing, and packing and stuffing box wall, produce certain radial contact stress and sealed. Gland of axial force, therefore, must be quite big, it is caused by packing and increase the friction torque between the valve stem, wear, soft packing wear fast, thus must tighten gland bolts or change filler, often to keep up the good sealing effect.


                  The proper packing seal and packing seal combination can improve the reliability and prolong the service life of the valve. For example, the combination of flexible graphite ring filler is better than the sealing effect of only using flexible graphite ring filler. At present, single flexible graphite ring filler is widely used in China. In foreign countries, the use of flexible graphite ring packing combination has become popular and achieved good results.


                  Eliminate leakage of body connections


                  The valve body connection part shall be sealed, which is a static seal in terms of its sealing nature. It shall meet the following requirements:


                  (1) able to adapt to sharp changes in temperature and pressure;


                  (2) multiple disassembly without damaging the sealing element;


                  (3) simple structure, compact structure and low metal consumption;


                  (4) not sensitive to vibration and impact load;


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


                  The connection part of valve body is usually sealed with birch groove or concave and convex flat gasket. In recent years, "O" seal ring has been widely used.


                  Zen groove flat gasket seal, it is flat gasket is installed in the sealing groove, this kind of structure on the seal, can produce high sealing pressure, often far exceed the gasket material yield limit, so as to ensure the reliable sealing. It is suitable for medium and high pressure valves with pressure greater than or equal to 4.ompa. The disadvantage of this sealing structure is that when the valve is removed, it is difficult to remove the gasket from the sealing groove.


                  Concavo-convex flat gasket seal is to install the flat gasket on the sealing surface of the concavo-convex flange. Compared with the sealing structure of camphor groove flat gasket, it has the following advantages:


                  (1) when removing the valve, the gasket is easy to take out; (2) because the sealing groove is in a step shape, the processing performance is good.


                  According to the process parameters and fluid properties, aluminum, copper, 1Cr18Ni9Ti and rubber asbestos board can be used as material for flat gaskets. Fluorocarbon is also a common gasket sealing material, but due to its cold flow, if the sealing structure is not designed properly, it will lead to adverse consequences.


                  "O" ring, its simple structure, easy fabrication, as long as the seal structure design is reasonable, can produce enough after assembly of radial extrusion deformation, axial load can reach can not seal, therefore, it is used as sealing flange connections, can decrease the size of the structure of the flange, so as to reduce the weight of the valve.


                  For low pressure small size valves, the body is usually connected with internal threads to reduce the size and weight of the body structure, and the sealing elements at the joints can also be used with flat gaskets or "O" seal rings.


                  Stem leakage countermeasures


                  The valve stem is an important part of the valve under stress. The material of the valve stem must have enough strength and toughness to resist the corrosion of medium, atmosphere and packing, abrasion resistance and good technical performance. In order to improve the corrosion and wear resistance of stem surface, the surface should be strengthened. At present, the material of valve stem is made of, most of the domestic use of martensitic stainless steel, but this kind of stainless steel crevice corrosion resistant ability is weak, this is due to the effect of acid, the passive film on the surface of the valve stem damage of local corrosion, so it's best to use austenitic, ferritic duplex stainless steel, can enhance the ability of stem crevice corrosion resistance, thus making the valve stem leakage get effective control.


                  In petrochemical industry, valve leakage is a problem that cannot be ignored. It has great influence on the economy, safety and environment of production. Eliminate the valve leakage is the key to optimize the design of the valve structure, according to the different process conditions reasonably choose the material valve, valve in the level of quality control in the production process, and the optimization of valve type selection based on process operating system. With the development of science and technology, especially the appearance of new materials, some valves of new structure have been put into use.

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