In the past, the main consideration for packing selection of pneumatic control valve was the working temperature of packing. PTFE was selected when the temperature was lower than 200 ℃, and graphite was used when the temperature was higher than 200 ℃. Now, when selecting packing, not only the working temperature but also the influence of packing friction on the control process should be considered, such as return difference, sealing quality and service life.
The selection principle of packing for pneumatic control valve is to reduce friction and prolong service life as far as possible under the premise of ensuring the required leakage level. The friction of graphite packing system is much greater than that of PTFE packing system. Therefore, PTFE system should be used as long as the temperature is allowed. The key points of structural design and application of filler are summarized
1. Different stuffing boxes and packing structures are suitable for different applications. Reasonable selection of stuffing boxes and packing structures can reduce friction and prolong service life under the required sealing conditions.
In general, the longer the stem and packing, the better the length is.
3. It is very important to improve the machining finish and surface precision of valve stem and stuffing box. For PTFE packing, if the finish of valve stem and stuffing box is not enough, the sealing will drop and the leakage will increase.
4. The packing structure design should be able to automatically adjust the compression force, compensate for the decrease of the compression force due to wear and so on, which should be conducive to on-site maintenance and installation.
5. Improper installation of pneumatic control valve will cause uneven friction. For example, when the valve rod is installed horizontally, the self weight of the valve rod will cause pressure on the packing at the lower part of the horizontal line, which will increase the friction, while the packing on the upper part of the horizontal line will have poor sealing performance due to the reduction of the compression force.