Packing material is 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 member. Normally such packing is formed of a resilient material 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 leakage of undesired fluids into the environment. Also, it is desired to use packing material formed of polytetrafluorethylene (PTFE) because of its low frictional impact on an operating member.
Generally, because of its low elasticity, it has been found necessary to continually load PTFE packing in the containment assembly in order to enable the packing to set in and maintain a reliable and tight fluid seal against the operating member and the housing. However, in attempts to use PTFE packing in a live loaded packing configuration at extremely elevated temperatures and pressure conditions, the PTFE packing tends to deform and cold flow and eventually extrude out of the sealing area. Substantial extrusion of the PTFE packing leads to fluid leakage and increased wear and possible damage to the operating member and packing.
For example, fluid control valves are operated in environments where the operating pressure is less than about 1000 psi (6895 kPa) and the temperature of the fluid is less than about 200.degree. F. (94.degree. C.). However, certain applications of fluid valves require use in much higher pressure and temperature operational environments such as greater than 1000 psi (6895 kPa) and around 450.degree. F. (232.degree. C.). As an example, in the control of boiler feed water in power generating plants there is a requirement for fluid valves to reliably operate in pressures around 7200 psi (49644 kPa) and in temperature ranges of around 450.degree. F. (232.degree. C.).
Normally, valves operating at such pressure and temperature extremes use graphite packing material. (See for example U.S. Pat. No. 4,364,542 suggesting the use of a graphite packing material at temperatures above 250.degree. F. (107.degree. C.). However, the use of graphite packing leads to increased valve stem friction as the valve stem strokes through the packing thereby severely limiting valve operation and leading to increased wear on the packing material.
While desired, prior attempts to utilize PTFE packing at elevated pressure and temperature ranges was normally not recommended nor found feasible, particularly under live loaded packing conditions as indicated previously. In such attempts, it was found that the PTFE packing material extruded due to the severe pressures and movement of the operating member such as a valve stem, which tended to remove the material from the packing in long strands of string-like extrusions. Eventually, the continued removal and extruding of material from the packing led to a loss of sealing ability for the packing and thereby requiring early replacement of the packing material and possibly the operating member itself.
In U.S. Pat. No. 4,886,241, assigned to the same assignee as herein, there is described a packing containment structure for use in fluid valves at high pressure and high temperature conditions around 7200 psi (49644 kPa) and 450.degree. F. (232.degree. C.). This patent describes the use of PTFE packing material with an anti-extrusion wiper ring on each side of the packing material. The wiper rings are formed of high temperature organic or inorganic fiber material with a nitrile elastomeric binder. Such a packing containment structure has been found suitable for use at the described high pressures and temperatures where the fluid is water or other non-reactive fluid. However, due to the fiber material used for the anti-extrusion wiper rings, this packing containment structure is not recommended for use where the fluid is chemically active and where extremely stringent fluid control requirements must be met.
Because of the highly inert properties of PTFE it is desired to use such material for the anti-extrusion wiper ring in the configuration of the packing containment described in the aforementioned U.S. Pat. No. 4,886,241. However, the known undesired deformation and cold flow properties of PTFE negatively impact on the use of this material at the extreme temperatures and pressure conditions presently under consideration. Furthermore, while the aforementioned U.S. Pat. No. 4,886,241 describes how to use desired PTFE packing material at these elevated temperature and pressure conditions, only a fiber material with a nitrile elastomeric binder, i.e., a non-PTFE material, was suggested for the anti-extrusion wiper rings.
Accordingly, it is desired to provide a packing containment for use with chemically reactive fluids in high pressure and high temperature conditions such as greater than 1000 psi (6895 kPa) and around 450.degree. F. (232.degree. C.) with PTFE packing material and which inhibits the extrusion of PTFE.