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In the past, the main consideration for control valve packing selection is the operating temperature of packing. When the temperature is lower than 200℃, use PTFE packing; When the temperature is higher than 200℃, the packing is graphite. Now, when selecting the packing, not only to consider the working temperature, but also to consider the affection of process control by the friction of packing, such as hysteresis, sealing quality, service life, and so on.
The principle of choosing packing is to reduce the friction and prolong the service life as far as possible under the premise of ensuring the required level of leakage.
For the friction of a graphite packing, it is much greater than that of a PTFE packing, so PTFE packing should be used as long as the temperature permits.
Summarize the main points of packing structure design and application:
1. Different packing boxes and packing structures are suitable for different applications. The correction selection of the packing box and packing structure, can reduce the friction and prolong the service life under the required sealing performance.
2. In the packing structure, the packing layers and length are not the more than better usually packing length is 1.5 times the stem diameter.
3. It is very important to improve the surface finish and surface accuracy of the stem and packing box. For PTFE packing, the finish of stem and the packing box is not enough, and it will reduce the sealing performance and cause increasing leakage;
4. The design of the packing structure should be able to automatically adjust the compression force and compensate for the reduction of the compression force caused by wear, etc., so as to facilitate on-site maintenance and installation.
5. The improper installation of the control valve will cause uneven friction, for example, when the horizontal installation, the stem dead weight will increase pressure on the lower line of packing, increase friction; And for the upper line of the packing, it has poor sealing performance due to the pressing force reduction.
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