I am modelling a vacuum pipe system. The pressure in the pipe is 0.050 bara (Absolute Pressure).

While modelling in caesar i am writing -0.950 bar to the pressure box (Since caesar works with gauge pressure) and before doing stress anlysis caesar gives error like caesar cannot proceed with negative pressure.

How i will be able model vacum pipes with negative pressure?


I will not be able to answer your further questions since i will be out of office during the day.

Waiting for your advices..

Thanks a lot ..

The piping codes do not address negative pressures.

So we connot use Caesar II Program to solve vacuum pipe systems?

Can we assume positive pressure instead of negative pressure? CAn we model 0.95 bar internal pressure instead of 0.95 external pressure?

As far as i know the pressure effects only the stress in the pipe.(I mean hoop stress) so there is no effect in the restraint forces.In my design i only check Sustain -operating and sismic cases.

The "hoop stress" equation is rearranged such that you solve for "t", to determine the pipe thickness. After that, (for process and power piping), the hoop stress is not important - it doesn't factor into the "Code Stress".

Depending on which load case you're talking about, pressure is a component of the "Code Stress" in some form of "longitudinal pressure stress". Most Codes define this as PD/4t (although there are more exact forms of this equaiton). So you basically add PD/4t as a term in the stress equation. If you run with a negative pressure, the PD/4t term would reduce the "code stress", which is non-conservative.

In earlier versions of CAESAR II we did allow people to run with negative pressures, but when the longitudinal pressure stres was computed we used |P|D/4t, so the term was always positive.

In the last two versions of CAESAR II we changed things to just throw an error, because we got tired of explaining to people why the software wouldn't design stiffening rings or address buckling (you can't do that with simple 3D Beam Elements). In the next version (5.30) we'll probably allow negative pressures again, but there will be a warning regarding the limitations.

Richard,

Whether Bucking effect also will address in next rivison.5.30

No - never. Flexibility analysis uses simple "3D Beam" elements in a linear elastic analysis.