- In PipingDesign@yahoogroups.com, "master_halda"
<master_halda@y...> wrote:
> In my understanding (correct me if i'm wrong), hydrotest is meant for
> strength test of the conctructed pipeline where the pressure test is
> somewhat around 1.5 of the design pressure.
>
> While pneumatic test is usually done as a leak test on the pipeline
> system. Pneumatic test is always associated to the check on whether
> connection of flanges/weld are not leaking. Which usually is lower
> than that of the strength test.
The hydrotest isn't a strength test--the proof factor is too low. Think
about it--the maximum allowable stress is about 1/4 the UTS and
provides for a margin against burst of about 3 even in the rare case
where the entire system is designed so that it operates at the Code
maximum allowable stress. An overpressure of 1.5 times the allowable
pressure doesn't come anywhere near a measure of the strength of the
vessel.
In fact you wouldn't want to risk damaging a vessel or distorting it in
a shop test. Besides the most common failure mechanism (besides abuse)
is either local thinning due to corrosion or fatigue. Neither of these
mechanisms would be addressed in any such test.
The hydrotest originally was set to check for leaky riveted joints in
the days before welded vessels. The usage carried over to welded
vessels to determine weld soundness as well as check for the presence
of pinholes. The proof test set out in Section VIII to verify specific
details not covered by code rules is a strength test, since it measures
the bursting strength.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/~chrisw/
Received on Sun Dec 18 14:13:00 2005