Re: Vessels not failing in hydro test

Dear Friends,

Vessels do not fail during hydrotest due to the following factors:

1. The allowable maximum stress (fs) of the metal is

      factored to be lower than the yield stress of
      metal (fy).

   2. The designer should recommend the metal thickness

      readily available in the market, hence actual
      plate thickness is usually greater but not lesser

      than the design thickness.

   3. In my previous experience regarding material
      testing, the actual fy of a steel sample during
      actual testing is usually higher than what is
      specified in the product.  For example, a Grade
30
      metal (fy = 30,000psi) will reach up to Grade 35
      fy=35,000 psi) during testing.

Hope this answers the querry.

Joshua Rizaga
Engineering Manager
VWC, Philippines

• Christopher Wright <chrisw@skypoint.com> wrote:

>
> On Aug 29, 2005, at 3:35 AM, Tomy John wrote:
>
> > 1. Why vessels are not failing in hydro Test,
> where the pressure is
> > 1.5 times the design pressure.Becauce
> t=PD/(2*Str)(Hoop stress).
> The allowable stress at maximum allowable pressure
> is lesser of 2/3 the
> yield stress or the ultimate stress/3.5. (ASME Code
> Div 1). The proof
> factor isn't 1.5 any more but 1.3, so the hoop
> stress never gets to the
> yield value during hydro-test. There's also a 15%
> margin (roughly)
> between the actual yield strength and the
> statistical minimum which is
> specified for design. And it's a practical
> impossibility for vessels to
> be designed with the wall thickness precisely equal
> to the minimum
> required value for the few materials where the yield
> strength governs
> the design allowable.
> 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/
>
>

Send instant messages to your online friends http://uk.messenger.yahoo.com Received on Mon Aug 29 12:22:00 2005