Re: 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