On Nov 2, 2005, at 5:57 AM, John Luf wrote:
> As I understand the testing procedure is along this
> line. The pressure is raised slowly watching the
> volume being input.
Funny you should mention. I used to get involved in this kind of
testing in a previous life. We didn't do it for pipelines, just for
specialized pressure vessels. If it's done properly it's precise enough
to determine highly localized yielding or even redistribution of weld
residual stress, but if you get to the point where you can't increase
the pressure with additional water influx, you've already distorted the
vessel to the point where the deformation is great enough so it won't
pass the 'no visible permanent distortion' part of the Code hydro-test
requirements. This can happen well before burst if the vessel details
and material are ductile. You also need to vent all the air from the
system or the test won't be reliable, since the compression of air
bubbles diddles the results. The curve of pressure plotted against
influx will be a straight line, if the system is properly filled.
I mentioned a test I was running where the vessel failed from a
pre-existing crack in a weld. We were keeping track of the water influx
very carefully, and had taken serious steps to vent _all_ the air from
the vessel, We'd just finished reading some strain gages and recording
the water influx when the the vessel let go with a helluva bang. It had
split pretty much from end to end and the resulting jet of water had
removed much of a sheet metal building wall. No one was hurt, although
I'd just come back in off a platform on top of the vessel (an 8 foot
dia 25 foot long test tank) when it blew. If I'd been out there, I'd've
probably been tossed off by the rupture. It had held pressure for 3 or
4 minutes before burst without a drop in pressure or any indication of
yielding. It was a straight brittle fracture in a weld HAZ, which
should have been caught (mea culpa) by routine pre-test radiography. It
might even have been found visually. That's why I've been pounding the
table about inspection. We'd had a couple of years of good service out
of the vessel, and the test was supposed to be a routine maintenance
thing--no point doing any NDT becasue everything has been fine up to
now. Just like the damn stock market--everything is always fine until
it isn't fine any more.
Christopher Wright P.E. |"They couldn't hit an elephant at
<a href="/group/PipingDesign/post?postID=wmjfC_nJ5WPjCHCtQztUGBKXnHAnayNAtYhNlDJtcmICB9AkP0Bw7GXZridhfw85oTP-DjdQeJ7EhWgWtqU">chrisw@skypoint.com</a> | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania1864)
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