Effects of a Surface Engineered Metallic Coating on Elastomeric Valve Stem Seal Leakage Page: 15
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and Ledauphin 1997). It is generally accepted that ppmv leak measurements using
helium need to be double to approximate the leakage using methane (Lowe 1995). Leefe
and Davies performed a study of the correlation of valve stem leakage between methane,
helium, and steam. Their study showed that while there was considerable scatter on a
point by point basis, there was a reasonable correlation between methane and helium if
the results are averaged over a number of tests (Leefe and Davies 1995).
Lowe conducted laboratory testing of standard compression asbestos and special
graphite packing materials (Lowe 1995). He first tested the packing in a test assembly
that consisted of a stem and seal assembly mounted below a control valve operator.
Helium was used as the test medium. He found that leakage from the asbestos packing
could not be reduced below 10,000 ppmv despite repeated tightening of the packing. The
graphite packing had a much better performance. Initial leakage was less than 50 ppmv.
After 100 cycles, leakage had reached 200 ppmv. The testing was concluded after 4000
cycles when the leakage had reached 10,000 ppmv. Lowe also tested a number of
various size stock gate and globe valves. Several manufacturers provided these valves.
Helium was again used as the test medium. Each of these valves was tested at 7, 20, and
40 bar. At each pressure plateau, each valve was manually operated full open to closed.
Lowe found that most of these valves had leakage that was significant (well above 1000
ppmv). The highest measurement was 80,000 ppmv. Note that the leakage using
methane would have been even higher. Each valve was then repacked with the special
graphite packing. Leakage was dramatically reduced. Most of the re-packed valves had
no detectable leakage. The highest detectable leakage was 150 ppmv. Lowe concluded15
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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. (https://digital.library.unt.edu/ark:/67531/metadc2690/m1/23/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .