EXPLORING THE EVOLUTION OF
Non-destructive examination (NDE) of valves by the manufacturer has been practiced almost since
the first valves were made. However,
for many years it was viewed primarily as just another manufacturing step.
A review of valve catalogs from the
postwar era during the mid 20th century shows remarkably little discussion of NDE, although photos and
descriptions of testing laboratories
and equipment are common. The
focus then was on testing samples of
castings from each heat level, and
once a product was determined to
“meet spec” that was about it, other
than tracking those heat numbers.
That situation has changed.
NDE has been around since the early days of the
process industries. More and more emphasis on
quality, however, has brought it front and center
both here and in other countries as new methods
are developed and ever-better designs and
products are challenged.
BY RON MERRICK
THE HISTORY OF
Ultrasonic examination of coating thickness
1924 radiography was first used to examine castings (by H. H. Lester for Boston
Until the end of the World War II era, some radiography was also performed
using radium as the source. This allowed greater penetration power compared to the
X-ray generators. Radium produces gamma rays, as do cobalt and iridium, which
were first used commercially in 1946 and eventually displaced radium because of
their increased strength. An irony of our business is that “X-ray” examinations are
now almost entirely done with gamma rays.
Other common NDE methods in use for a long time include dye penetrant tests,
which find surface flaws. This method has its origins in the “oil-and-whiting” test
used in the 19th century in which castings were wiped down using heavy oil, and
then painted with a suspension of chalk in alcohol. The suspension drew out some of
the oil that had penetrated into cracks and defects. Fluorescent dyes were first used
commercially in 1942 under the name Zyglo.