Packing materials are widely used to prevent fluid leakage around an operating member in a housing with fluid, such as a rotary shaft or a sliding stem in fluid control valves or in a reciprocating pump shaft. Normally such packing is formed of a resilient member and is placed under a static load by being bolted into position within a packing box around the operating member. In other instances the packing is subjected to spring loading in what is known as a live loaded packing configuration. Live loaded packing is particularly useful in attempting to prevent undesired leakage of fluids into the environment. Also, at operating temperature conditions above approximately 450.degree. F. (232.degree. C.) (i.e., operating temperature in the packing area) it is desired to use commonly available graphite packing rather than packing material formed of polytetrafluorethylene (PTFE) because PTFE packing tends to extrude at such elevated temperatures which could lead to packing failure and fluid leakage. At temperatures below 450.degree. F. (232.degree. C.) PTFE packing is desired because it has substantially lower friction than graphite packing. Excessive friction contributes to poor control of fluid processes.
As an example, certain applications of a fluid control valve require not only that the valve meet stringent leakage requirements and have low friction, but that it also meet a stringent packing-performance fire test to prevent catastrophic packing failure. Packing for fluid-control valves used in petroleum refineries, and in other chemical processing applications, is desired to have low friction and nearly zero leakage (i.e., less than 500 ppm as per EPA Method 21) of fluid around the top of the valve under normal operating conditions. In addition, it is desired that the packing not fail when exposed to high temperature thereby permitting excessive leakage of flammable fluids during a fire.
In such applications, graphite packing is well suited for preventing excessive leakage during a fire, however, the high packing stress required to provide nearly zero leakage during normal operating conditions results in high friction from the graphite packing, leading to less precise process control when compared to PTFE packing. Attempts to decrease the loading on the graphite packing provides a significant reduction in friction but leads to excessive leakage (greater than 500 ppm). To reduce the leakage and friction, it is desired to use PTFE packing material which can provide a tighter, longer lasting seal than graphite material. However, the use of PTFE packing at elevated packing temperatures is normally not recommended, and particularly where the valve must meet packing-performance fire tests, as in refinery applications. The potential extrusion, decomposition, and vaporization of PTFE packing under high temperature fire conditions would lead to catastrophic packing failure and undesired hazardous fluid leakage.
Thus, it is desired to provide packing for control valves which exhibit low friction, fire safety, nearly zero leakage, long service life, and reasonable cost concurrently.