When it comes to valve packing, the type of graphite used significantly impacts both installation difficulty and sealing performance. Regular graphite, with its loose structure and lower strength, compresses easily, making installation relatively simple, regardless of the number of packing rings. However, high-purity graphite, crafted from larger flake materials, offers superior resilience and compressive strength, leading to more challenging installations. This difference raises important considerations about the optimal number of packing rings for achieving effective sealing, challenging traditional beliefs about how many rings are needed for optimal performance.
Lower-quality graphite, due to its loose composition, poor strength and weak resilience, deforms and expands easily when compressed, making installation relatively simple regardless of the number of packing rings used.
In contrast, high-quality graphite rings are made from larger flake graphite raw materials, giving them significantly better resilience and compressive strength. This leads to noticeable differences in installation difficulty when using a higher number of rings.
In practical applications, installing high-quality packing in valves can be challenging. A high torque is often required on the gland bolts to achieve effective sealing. This is because the inherent elasticity of graphite packing creates significant rebound force when multiple rings are stacked. The tightening force applied by the gland bolts must constantly counteract this rebound, making it difficult to transmit force to the bottom layers and allowing the graphite to expand laterally for optimal sealing. This results in reduced efficiency, increased costs, and compromised sealing performance.
Traditionally, it was widely believed that increasing the number of packing rings would enhance sealing performance. However, since the internal state of the packing cannot be directly observed, its effectiveness is often judged based on pressure testing results.
In cast valve mold designs, packing box depths were made excessively deep under the assumption that higher pressures required deeper stuffing boxes. As a result, some valves require 7–8 rings, while others may need over 10 rings to be fully packed.
However, in reality, the optimal sealing performance is achieved with just 5–6 rings. This ensures an ideal balance of downward force transmission and lateral stress distribution, leading to better sealing efficiency.