Re: pressure test thoughts

it sure great having you around.Chris.
----- Original Message -----
From: "Christopher Wright" <chrisw@skypoint.com> To: <PipingDesign@yahoogroups.com> Sent: Wednesday, September 07, 2005 10:01 AM Subject: Re: [PipingDesign]pressure test thoughts

>
> On Sep 7, 2005, at 1:34 AM, Narendra Roy wrote:
>
> > Has any one viewed that the hydrotest is a good means
> > of stress relieving of the welds in piping and vessels
> > which are not subject to PWHT?
> The hydro-test doesn't stress-relieve welds. It can act as part of the
> shakedown process, but load cycling simply redistributes local residual
> stress, so that it can shake down to elastic response. PWHT actually
> makes metallurgical changes while stress relaxes due to creep.
>
> Several lifetimes ago I helped develop a hydro-test methodology that
> showed how vessels shake down. When you test a vessel you keep close
> tabs on the water you pump into it and if you plot the water influx
> against pressure you can see the onset of localized yielding. What
> you're seeing is the volume change (plus a contribution for water
> compressibility and sometimes relative movement between mating parts)
> The curve is linear at first, then it starts to bend over as local
> yielding begins, in welded joints or at stress concentrations. When you
> release the pressure and re-pressurize, the vessel response is linear,
> parallel to the first pressure excursion, but it doesn't start to go
> non-linear until you get to the maximum pressure of the previous cycle.
> It's real interesting to watch, and has some significant advantages
> over strain gages, although we used strain gages, too.
>
> Christopher Wright P.E. |"They couldn't hit an elephant at
> chrisw@skypoint.com | this distance" (last words of Gen.
> .......................................| John Sedgwick, Spotsylvania
> 1864)
> <a href="http://www.skypoint.com/~chrisw/">http://www.skypoint.com/~chrisw/</a>
>
>
>
>
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Received on Wed Sep 07 17:37:00 2005