helpful. But what is often specified is a
look inside the walls of the valve, or as
this look is sometimes called, a volumetric examination. The best way to accomplish this for castings is by radiography.
Radiographic examinations (RT) can provide a useful look inside the pressure-con-taining wall of the valve and help to
determine the overall soundness of the
casting, as well as determine if potentially hazardous defects are present.
A layman reviewing radiographs of a
valve casting would be hard pressed to
determine whether or not the valve metal
is of good quality and fit for service. The
subjective nature of comparing reference
films and their “shades of gray” mean
that even some experienced radiographers have a hard time evaluating what
they are seeing. Radiographers that shoot
valve castings every day and diligently
scan their reference radiographs like the
latest issue of Sports Illustrated have a better chance of providing correct and
repeatable interpretations.
Casting defects are divided into various types and classes. These include
shrinkage, porosity, gas and inclusions.
The categorization of these defects is
generally viewed as a good way to judge
the overall soundness of the casting and
the workmanship of the foundry.
Defects such as hot tears, which are
areas of shrinkage open to the surface,
Fire testing involves pressuring the valve with water and then focusing several jets of flame onto
the valve.
or cracks are viewed as performance-affecting defects. Because of this, none
of these defects are allowed in most
casting evaluation standards. These
types of defects can reduce the effective wall thickness of a casting, causing
its pressure-retaining capability to be
compromised. Another issue with
cracks and hot tears is that they are
“stress-raisers,” which means they can
act to initiate further cracking of the
metal to the point that catastrophic
failure could occur.
Despite its subjectivity and limita-
tions, radiography is still the de facto
method of valve casting evaluation,
especially for critical refinery and power
plant applications.
NUCLEAR TESTING
The most critical valve applications
today are in the nuclear industry. Valves
for use in nuclear power plants, including nuclear-powered navy ships, are subjected to very rigorous examination
under a wide range of tests that would
make for a separate article. One of the
special tests required for on-shore
nuclear power plant service, for example, is seismic testing, which simulates
the stresses caused by earthquake activity. The joke in the valve industry used
to be that the stack of paperwork certifying a nuclear valve was often bigger
than the valve itself though today those
test reports and traceability documents
are stored on digital media.
CONCLUSION
With the widespread concern on safety
these days, both valve manufacturers and
end users have responded by creating
appropriate tests for valve integrity, especially for critical flow control applications. Many of these extreme tests were
randomly called out 50 years ago; but
today, they are commonplace. And, as
even more critical valve applications are
developed, undoubtedly, there will be
newer and tougher extreme valve tests to
ensure product integrity for those yet-to-be designed products. VM
Cryogenic testing is generally performed at temperatures ranging between - 50° F (- 46° C)
and -320° F (-196° C).
GREG JOHNSON is a contributing editor to Valve
Magazine and president of United Valve
( www.unitedvalve.com), Houston, TX. Reach
him at greg1950@unitedvalve.com.
