In the past, the main consideration for the selection of control valve packing was the packing operating temperature. When the temperature was below 200 degrees, PTFE was selected, and when the temperature was above 200 degrees, graphite was selected. Now, when selecting packing, not only the operating temperature should be considered, but also the impact of packing friction on the control process, such as hysteresis, sealing quality and service life.

The principle of packing selection is to reduce friction and extend service life as much as possible under the premise of leakage level.

The friction of the system using graphite packing is much greater than that of the PTFE packing system. Therefore, as long as the temperature permits, the PTFE system should be used.

To summarize the key points of packing structure design and application:

1. Different packing boxes and packing structures are suitable for different applications. Reasonable selection of packing boxes and packing structures can reduce friction and extend service life under the required sealing conditions.

2. In the packing structure, the number of packing layers and length are not the more the better. Usually the packing length is 1.5 times the valve stem diameter.

3. It is very important to improve the processing finish and surface accuracy of the valve stem and stuffing box. For PTFE packing, the valve stem and stuffing box are not smooth enough, which will reduce the seal and increase the leakage.

4. The packing structure design should be able to automatically adjust the clamping force to compensate for the decrease in clamping force caused by wear, etc., and it should be conducive to on-site maintenance and installation.

5. Improper installation of the regulating valve will cause uneven friction. For example, when installed horizontally, the deadweight of the valve stem will cause pressure on the packing below the horizontal line, increasing friction, while the packing above the horizontal line will have poor sealing performance due to the reduced clamping force.