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Failed
Steel Nipple
A metallurgical failure analysis was requested of a
wrought steel, one inch diameter, six inch long,
nipple that failed on a gas production site. It was
reported that an operator had opened a one inch,
3,000 psig WOG (Water/Oil/Gas) in-line valve and as a result of that
action, the blowdown line connected to the valve was
subjected to an instantaneous internal gas pressure
of approximately 1,000 psig. Furthermore, the one
inch blowdown line contained a right angle (90
degree) elbow approximately four feet above the one
inch blowdown line shut off valve. On the high
pressure side of the shut off valve, a wrought
steel, threaded nipple six inches in length was
threaded into a thread-o-let which was in turn
welded into a 90 degree elbow which was flanged on
one end.
Photograph A: Close-up view of threaded end of fractured nipple.
Metallurgical testing included
stereomicroscopy,
scanning electron microscopy (SEM), transverse and
longitudinal metallographic examination, and energy
dispersive spectroscopy (EDS). The results of those
tests illustrated that the six inches long, one inch
diameter, threaded wrought steel nipple failed as a
result of overload bending forces. When the blowdown
line was opened, the 1,000 psig gas instantly
started expanding to atmospheric pressure. This gas
expansion was exacerbated by the abrupt, 90 degree
directional change at the elbow. The bending forces
experienced by the subject, six inch long, one inch
diameter, wrought steel nipple exceeded the bending
strength of the nipple at the first engaged thread
and the pipe failed in bending overload.
Photograph B: Close-up view of bent six inch failed nipple. Nipple is on high pressure side of in-line valve.
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 Dr.
R. Craig Jerner, Ph.D., PE specializes in accident
investigation and metallurgical failure analysis,
with over 30 years experience as a metallurgical
consultant and accident investigator. He has
testified as a metallurgical expert in over 250
depositions and more than 70 court appearances. If you or someone you know should need the services of Dr. Jerner and J.E.I. Metallurgical, please visit our web site at the buttons below, or e-mail Dr. Jerner --- r.c.jerner@metallurgist.com |
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