Page 19 - Valve Selection Handbook

6 Valve Selection Handbook

Four broad classes of fluid tightness for valves can be distinguished:
nominal-leakage class, low-leakage class, steam class, and atom class.
The nominal- and low-leakage classes apply only to the seats of valves
that are not required to shut off tightly, as commonly in the case for the
control of flow rate. Steam-class fluid tightness is relevant to the seat,
stem, and body-joint seals of valves that are used for steam and most
other industrial applications. Atom-class fluid tightness applies to situa-
tions in which an extremely high degree of fluid tightness is required, as
in spacecraft and atomic power plant installations.
Lok 1 introduced the terms steam class and atom class for the fluid
tightness of gasketed seals, and proposed the following leakage criteria.
Steam Class:
Gas leakage rate 10 to 100 |ig/s per meter seal length.
Liquid leakage rate 0.1 to 1.0 jj,g/s per meter seal length.
Atom Class:
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3
Gas leakage rate 10~ to 10" fig/s per meter seal length.
In the United States, atom-class leakage is commonly referred to as
zero leakage. A technical report of the Jet Propulsion Laboratory, Califor-
nia Institute of Technology, defines zero leakage for spacecraft require-
2
ments. According to the report, zero leakage exists if surface tension
prevents the entry of liquid into leakage capillaries. Zero gas leakage as
such does not exist. Figure 2-1 shows an arbitrary curve constructed for
the use as a specification standard for zero gas leakage.
Proving Fluid Tightness

Most valves are intended for duties for which steam-class fluid tight-
ness is satisfactory. Tests for proving this degree of fluid tightness are
normally carried out with water, air, or inert gas. The tests are applied to
the valve body and the seat, and depending on the construction of the
valve, also to the stuffing-box back seat, but they frequently exclude the
stuffing box seal itself. When testing with water, the leakage rate is
metered in terms of either volume-per-time unit or liquid droplets per
time unit. Gas leakage may be metered by conducting the leakage gas
through either water or a bubble-forming liquid leak-detector agent, and
then counting the leakage gas bubbles per time unit. Using the bubble-
forming leakage-detector agent permits metering very low leakage rates,