Basic Packing Selection Considerations – The general rule for packing material selection that is suggested by most valve manufacturers is as follows:
• If the packing temperature is below 450 degrees F, select PTFE packing.
• If the packing temperature is above 450 degrees F, or if minimum leakage after a fire is required, select graphite packing.
Section 4.1.5 of SAES-J-700 is consistent with the above guideline except that it limits PTFE to a maximum temperature of 400 degrees F.
In addition to the above guidelines, packing selection criteria may include the following:
• Line pressure.
• Packing material compatibility with the process fluid.
• Compliance with NACE guidelines, if pertinent.
• Leakage requirements (environmental concerns, plant policies, etc.).
• Maintenance schedules and costs.
• Friction and valve performance (good process control).
Low Leakage Packing And Nonenvironmental Applications – Even when the 500 ppm emission requirement does not apply to a specific application, low-leakage packings may be considered for purposes of conserving the process fluid, controlling the fugitive emission (unwanted leakage) of toxic or polluting fluids, or minimizing packing maintenance and extending packing life.
Externally Loaded Packing And NACE Guidelines – Because of the material hardness limits that are imposed by the NACE guidelines, conventional spring-loaded packing arrangements are not generally available for sour applications; however, the Belleville springs that are included in externally loaded packing arrangements are typically made of an Inconel alloy that does conform to the NACE material guidelines. As a result, externally loaded packing arrangements expand the packing options that are available for sour applications.
Packing Specifications – Charts that illustrate the pressure and temperature limits
of the packing arrangements that have been discussed in this section are included in Fisher Specification Bulletin 59.1:062. Pressure and temperature limits are listed for two types of applications: (1) for environmental applications (when the 500 ppm standard must be met), and (2) for nonenvironmental applications.