Effects of a Surface Engineered Metallic Coating on Elastomeric Valve Stem Seal Leakage Page: 16
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that re-packing could reduce stem leakage despite existing clearance variations and stem
BP Research and Engineering tested a number of different graphite packing
products and standard asbestos compression packing (Harrison et al. 1995). For this
research, they used two control valves that had been removed from process service in
their testing. In addition, several different valve stems of varying degrees of wear were
used in the testing. This was to simulate the range of plant conditions that may be
encountered in retrofitting packing. The tests were conducted using methane as a test gas
and leak measurements were performed in accordance with EPA Test Method 21. The
control valves were initially cycled 100 times at a pressure of 7 bar. Pressure was then
increased to 20 bar and the valves were cycled an additional 100 times. Pressure was
then raised to the final test pressure of 40 bar and the valves were then cycled at a rate of
6 cycles a minute. The valves were cycled at this rate for a period of an hour followed by
a rest period of 15 minutes. The cycle and rest periods were repeated until conclusion of
the test. Testing was concluded when valve stem leakage exceeded 10,000 ppmv after
one re-torquing of the packing gland. BP found that graphite packing had much lower
leakage than the standard asbestos packing. The initial leakage for the asbestos packing
was well over 1000 ppmv, and the test concluded at 200 cycles. The graphite packing
initial leakage ranged from approximately 1 to 100 ppmv. The graphite packing tests
were concluded after 3000 to 9000 cycles. From this testing, BP Engineering and
Research determined that valves at BP Chemicals' facilities should be repacked using the
improved graphite packing because re-packing would significantly reduce emissions.
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Taylor, John Abner. Effects of a Surface Engineered Metallic Coating on Elastomeric Valve Stem Seal Leakage, thesis, December 2000; Denton, Texas. (digital.library.unt.edu/ark:/67531/metadc2690/m1/24/: accessed November 22, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .