When the graphite packing is tightened and the packing is loaded, each packing ring must be installed in place in sequence until the complete set of packing is completed. This ensures an even distribution of axial stress and reduces friction and leakage rates, reducing or even preventing the need to replace packing or re-tighten glands in the future, thereby extending the service life of the packing set.
Then the load on the packing ring close to the functional part of the pressure surface is large, but the load on the bottom packing ring close to the process medium is so small that it can be ignored. Because it is the packing ring that reduces the axial load, the bottom packing ring is less effective at preventing media leakage, whether it is cut wound or molded packing. Another condition is that the load applied to the packing ring is excessive.
The friction force when the valve stem rotates is too large, which can damage the packing group. It can be seen from some data that if the installation is correct and standard, the stress relaxation level of the packing group after 100 mechanical cycles is not more than 20%, but if the installation is not correct, the stress will even be reduced by about 60%, so that the sealing Performance is no longer solid. The tetrafluoro-graphite packing is sealed by packing in the stuffing box, and pressing the graphite packing on the surface of the shaft through the packing gland.
Since the surface of the shaft is always rough, it can only partially fit with the graphite packing, and the parts are not in contact, forming countless labyrinths. When the pressurized medium passes through the surface of the shaft, the medium is throttled many times, and the seal is achieved by virtue of this "labyrinth effect". The fit and friction between the graphite packing and the shaft surface is also similar to that of a sliding bearing. There should be enough liquid to keep the seal smooth to ensure a certain life of the seal, the so-called "bearing effect". It can be seen that a good packing seal, That is, the combination of labyrinth effect and bearing effect.