Valves today face more challenging conditions
from a wider range of applications. As a result,
users are asking for more and better testing.
BY GREG JOHNSON
Two commonly used low temperature tests today measure the degrees for liquid nitrogen and liquid natural gas.
Filling a valve up with water, adding pressure and looking for leaks might
work for some valve specifications, but
many of today’s demanding valve
requirements call for much more stringent testing and evaluation. Special
service applications such as hazardous
fluids, nuclear power plants, high-pressure pipelines and more dictate a
much broader testing and inspection
regimen than traditional simple tests.
Many users are requesting valve
manufacturers prove their products will
operate satisfactorily at the higher and
lower temperatures and more extreme
pressures that their valves are advertised to reach. These may be the lowest
cryogenic temperatures or elevated
temperatures close to 1000° F (538° C).
Such tests call for specialized equipment
and test procedures.
The most common of these more
extreme tests is cryogenic testing. Such
testing is generally performed at temperatures ranging between - 50° F (- 46°
C) and -320° F (-196° C)—most often at
-320° F (-196° C), which is the temperature of liquid nitrogen (LN). Standard
practice is for the valve to be immersed
in the LN up to the packing gland area,
if the valve is equipped that way. The
packing must be kept out of the LN or it
could freeze the packing, seizing the
stem and causing the valve to lock up
and fail to operate. Because polymer
seals do not function well at cryogenic
temperatures, valve end connections
must be the type that makes a solid
mechanical connection. These include
threaded, flanged or caps welded onto
buttweld-end ends. Socketweld-end and
buttweld-end valves without welded-on
caps are very difficult to test at the lowest cryogenic temperatures.
One of the most popular low temperature services today is liquid natural gas
(LNG). Valves for LNG are sometimes
tested at -320° F (-196° C), but a more
accurate test is performed at the actual
LNG temperature of -260° F (-162° C).
Cryogenic testing is costly and hazardous and should only be performed by
experienced, trained personnel. The test
procedures for cryogenics are available
from several standards-making-organiza-