Effects of a Surface Engineered Metallic Coating on Elastomeric Valve Stem Seal Leakage Page: 14
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leaks 2000). This method involves the use of a portable leak detector. The type of
detector is not specified, however, the detector must be able to respond to the compound
measured. Measurements using this test method are required to be in parts per million by
volume. The instrument is also required to have a probe that sniffs the surface at
potential leak locations. EPA Test Method 21 provides the following general directions
for performing a survey of an individual source:
Place the probe inlet at the surface of the component interface where leakage
could occur. Move the probe along the interface periphery while observing the
instrument readout. If an increased meter reading is observed, slowly sample the
interface where leakage is indicated until the maximum meter reading is obtained.
Leave the probe inlet at this maximum reading location for approximately two
times the instrument response time. If the maximum observed meter reading is
greater than the leak definition in the applicable regulation, record and report the
results as specified in the regulation reporting requirements.
a. Valves - Leaks usually occur at the seal between the stem and the housing. Place
the probe at the interface where the stem exits the packing and sample the stem
circumference and the flange periphery. Survey valves of multipart assemblies
where a leak could occur.
Most of the recent research found that is related to valve stem seals is based, in
part, on EPA Test Method 21. Laboratory testing usually involves testing a valve, or a
valve stem and seal assembly at operating conditions. Instrumentation typically measures
leakage in parts per million by volume (ppmv) or can be correlated to an equivalent
reading in ppmv.
One of the major differences between laboratory testing and EPA test method is
the test gas used. The regulatory definition of a leak for many compounds is referenced
to instrumentation calibrated for methane. Due to safety considerations, most laboratory
and manufacturer tests use an inert gas. This is typically helium (Garrigues, Birembaut,
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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/m1/22/: accessed November 25, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; .