The impact of valve leakage and solutions

solutions

Optimized design selection

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 fluid, as compared to the use of cut-off valve and a gate valve or a ball valve, which effects vary significantly. 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.

Stuffing box leakage countermeasure

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.

Eliminate leakage from 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 ) Adapt to sudden changes in temperature and pressure;

( 2 ) Multiple disassembly without damaging the sealing element;

( 3 ) The structure is simple and compact, and the metal consumption is small;

( 4 ) not sensitive to vibration and shock loads;

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

The connection part of the valve body is usually sealed by a birch groove or a concave-convex flat gasket. In recent years, the "O" -shaped seal 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.

The “O” -shaped sealing ring has a simple structure and is easy to manufacture. As long as the sealing structure is designed reasonably, sufficient radial extrusion deformation can be generated after assembly, and the sealing can be achieved without axial loading, so 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.

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 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, the use of austenitic - ferritic duplex stainless steel can enhance the crevice corrosion resistance of the valve stem, so that the leakage at the valve stem can be effectively controlled.

The leakage of valves in petrochemical industry production cannot be ignored, and it will have a major impact on the economy, safety and environment of production. The key to eliminating valve leakage is to optimize the design of the valve structure, rationally 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 valve selection according to the process operating system. With the advancement of science and technology, especially the emergence of new materials, some new structural valves have been put into use, they have good performance and long life, which can meet the needs of production development.

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