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The influence of valve leakage and its solution

Loss Caused by valve leakage

Different media,Valve Leakage caused by economic loss and environmental pollution degree is different. If it is a gaseous medium, it is easy to spread in the air, and its pollution radius will be large. The liquid medium will produce pollution through the penetration of the ground, and may also flow into flood drainage channels through open or hidden ditches, polluting rivers, lakes and other environments. The incident that once occurred in the Pesticide Factory in India was leaked out of a 45T liquid poison tank with a 45T liquid highly toxic cylinder of methyl vinegar burst through the valve, and suddenly turned into thick smoke and drifted in the densely populated urban area, in just a few days, the poison gas took the lives of 2800 people, so that more than 520 thousand people were poisoned to varying degrees, of which more than 100 thousands people were permanently disabled. About 20% of the valves in the industrial installation have leakage problems, and the direct economic losses caused by them account for about 50% of the total loss in the petrochemical industry taking a set of 5000kt/a scale of the normal pressure reduction device as an example, in general, there are about 5000 set valves, and the economic losses caused by 20% of the valve leakage and the pollution caused by the environment can indeed not be underestimated, if from the perspective of the entire petrochemical industry, the situation is more serious.

Countermeasures-- Optimization Design Selection

Whether the degree of valve leakage can be minimized and the service life can be reached at the longest depends on whether the valve design selection is reasonable, the selection of sealing form, the quality of the valve product, whether the installation construction and production operation are in line with the specifications etc., and to a large extent, it depends on the optimization of the valve design selection.

The optimization of valve design and selection involves the selection of valve form, the design and manufacture of valve itself, the selection of valve materials and many other problems. Selection of valve type which should consider requirements of technological conditions and the design specifications or other aspects of the overall optimization.
The use of valves, Medium temperature, Pressure, Flow rate, pressure drop and corrosive medium directly affect the selection of valves, but also according to the temperature and corrosive medium, choose the material used in the manufacture of valves. According to the construction and actual operation experience, the selection of valve in addition to meet the relevant technological requirements, design specifications, but also fully consider various specific circumstances, so that it as far as possible with the operating conditions match, maximum to meet the requirements of use.

For liquids and gases, the effect of using gate or globe valves will be significantly different from ball valves. The air tightness of ball valves is better than gate&globe valves, and the speed of opening and closing is much faster, but the temperature range is much smaller than gate and globe valves. In general, most ball valves can not be used at high temperatures. Gate valve and globe valves are more stable at high temperatures. The pressure loss of ball valve is smaller than gate&globe valve which the pressure loss of globe valve is larger than that of gate valves, which is determined by the internal structure of the valve. For different material flow rate and the effect of regulating flow, the effect of using different types of valves will be very different. Globe valves are generally not suitable for high-speed flow, because the pressure loss of globe valve is the greatest in high- speed flow, but the flow regulation of glove valves is much better than gate valves and ball valves. In the choice of valve type also need according to the temperature and corrosion of the medium to consider the manufacture of valve materials used. The material used in the valve must meet the requirements of the working point, the reasonable selection of spool and valve body materials can prolong the service life of the valve without greatly increasing the cost. Pure pursuit of high grade materials will only lead to unnecessary waste of materials.

Packing Leakage Countermeasure

Due to the leakage of the sealing of the valve packing box and the sealing of the valve body, it is the key to the leakage of the valve, so special attention should be paid to it.

The soft packing seal is achieved by the axial pressure of the packing gland, which produces a radial contact stress between the stem and the packing as well as between the packing and packing box side wall. Therefore, the axial force of the gland must be quite large, which causes the friction torque between the packing and valve stem to increase,so it is necessary to tighten the gland bolt frequent or replace the packing in order to ensure a better sealing effect.

The use of suitable packing seal and packing seal combination can improve the reliability of the valve, prolong the service life, it the combination of flexible graphite ring packing is used, the sealing effect is better than that of only flexible graphite ring packing.  
Eliminate Leakage at body connection
Valve body connection part seal need meets the following requirements:  
1: Can adapt to the rapid change of temperature and pressure
2: Multiple disassembly without damage to sealing elements
3: Simple structure, compact, less metal consumption
4: Insensitive to vibration and impact load
5: Can meet the requirements of various working media

The connecting part of valve body, usually using birch groove type or concave-convex flat gasket seal, in recent years, O ring seal has also been widely used.


 O Shaped sealing ring, its simple structure, easy to manufacture, as long as the sealing structure design is reasonable, after assembly can produce enough redial extrusion deformation, can not be axial loading can achieve sealing, therefore, its flange connection seal, can reduce the size of the flange structure, so as to reduce the weight of the valve.

For low-pressure of small diameter valves, in order to reduce the size and weight of the valve body structure, the valve body is generally connected by internal thread, and the sealing element at the joint can also be used Flat gasket or O Ring.

Stem Leakage Countermeasures

Valve stem is an important force parts in the valve, so stem material must have enough strength and toughness, resistance to medium, atmosphere and packing corrosion, abrasion resistance, good process performance. In order to improve the corrosion resistance and wear resistance of the valve stem surface, the surface should be strengthened. At present, the manufacture of valve shtem material, most of the domestic use of martensitic stainless steel, but this material corrosion resistance is weak. That is due to the role of acid substances, local corrosion caused by the destruction of the passive film on the surface of the valve stem, so it is best to use austenitic ferrite duplex stainless steel can enhance the ability of the valve stem gap corrosion, so that the leakage at the valve stem is effectively controlled.

The leakage of valve in petrochemical industry is a big problem that can not ignored, it will bring great impact on the economy,safety and environment of production. The key to eliminate valve leakage lies in optimizing valve structure design, rationally selecting valve according to different technological conditions, improving valve quality control level in production process, and optimizing valve selection according to technological operating system. With the progress of science and technology, especially the emergence of new materials, some valves of new structure have been put into use, they have good performance, long life can meet the needs of production development. 

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