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Control Valves

Chapter 10.16 - Control Valve Features: Causes of Packing Failures

Causes of Packing Failures

There are several common factors that may lead to premature packing failure. Some of these are as follows.

Unacceptable leakage through the packing may be attributable to:

  • Loss of axial loading caused by packing wear
  • Damage to stem or packing box finish
  • Accuracy of stem guide installation
  • Improper maintenance
  • Excessive packing box temperatures
  • Vibration of the valve assembly
  • Material incompatibilities
  • Cold flow of the packing material
  • Different expansion rates of the packing and metal packing box components
  • Thermal cycling

Thermal cycling is a significant problem because the expansion rate of Teflon is ten times that of steel. The expansion and contraction of the packing in operation causes loss of axial loading.

Valve manufacturers normally ship all valves with packing installed. However, if the valve is not used as soon as it is delivered and is stored in a humid environment, it may be prone to pitting of its 316 stainless steel stem without drying or removal of its packing. In addition, stems manufactured of high nickel alloys are also susceptible to pitting under similar conditions. Therefore, valve packing should be removed if the valve is to be stored for an extended period prior to installation and use.

 
Attention to details of installation, finish and alignment are critical and can make order of magnitude increases in service life possible.

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