Hi all,

I recently installed Ceasar II V5.1 and tried to open an older calculation where we have a pipeline that can operate under full vacuum. A pressure of -1 Bar was entered. When I do an error-check I get a message that pressures below 0 are not allowed. The help function however says it is.
Why can't I enter a -1 bar pressure anymore?

You should check you old calc ... Caesar does not do negative pressure. It's a beam element program and the failure mode under negative pressure is buckling collapse, whic is not compatible with a 'beam'. Use the search facility in this forum for other posts on this subject.

Cheers.

Not strictly correct.

Particular care should be taken with expansion joints...

from Pressing F1 in Caesar

Note: CAESAR II addresses negative pressures as follows: the absolute value of the longitudinal pressure stress (PD/4t) term is added to the appropriate code equations; pressure thrust forces applied to expansion joint ends will be compressive; and buckling is not addressed in CAESAR II.


Which begs the question, whats changed>?????????????????????????

Version 5.10 gives you an error if you try to run with negative pressures.

This change was made because (a) you shouldn't be doing this, and (b) many ignored all the warnings.

I'll get the "help text" changed.

fair enough, but (as i have just discovered - fancy that!) my old calc (by a contractor)won't run without changing it. something i don't want to spend time on.

Also, what was wrong with having the expansion joint go into compression? seem a logical thing to do, especially on large sizes.

I'm aware of the stress issues, but as a sense check for movement on flexibles, was it totally worthless??

Totally worthless - maybe not. However, we ran into this all the time and either:

a) Repeatedly spent an unwarranted amount of time on the phone explaining that the "beam element" and the "piping codes" don't address negative pressures.

b) Repeatedly had to explain to managers and supervisors why we let the software run with a negative pressure - the argument being we should have somehow prevented the "non-expert user" from doing this.

Hopefully this change solves both problems.

Accepted

But for the first time in Caesar's history, your latest issue is now not backwardly compatable.
Its a good job i still have 5.00 in my machine.
And what is Coade's recomendation WRT to negative pressure forces on the bellows?? (local forces to mimic external pressure force??)

Yes, of course. Add point forces on both ends of the joint to simulate compression of the expansion joint.

Achh du Liber the conundrum "Fix" something for people who shouldn't be using the software unsupervised....

A classic no win situation ;<)

Vacuum pressure design is checked outside of CII which is standard practice. Since we still need to generate support loads using CII for thermal, occ, etc... for vacuum systems with expansion joints, a prudent comprimise to consider is to allow negative pressures on EXPANSION JOINT ELEMENTS only.

Force cards (F1,F2,...) usually fall through the cracks when lightweights end up reruning calcs and the originating engineers documentation does not state anything about the forces on the title page, stress iso, report...

At least these critical forces would not be lost in the wrong hands.

Many stress guys have touble figuring out if the force is a load or reaction never mind a expansion joint pressure thrust direction or what a pressure thrust is.

Thanks for the explanation. I know CeasarII doesn't check the pipe for buckling but I have one question: Can you please re-implement the negative pressure feature?

Imho deleting it is not a real improvement. It will lead to errors when not all extra forces are added to compensators when running a vacuum calculation. Especially cause CeasarII calculates that force itself when you have an internal pressure.
Changing a good feature in a program cause the program is used by a lot of people that shouldn't use it is not the way to go imo. (Although I can understand that you're tired of all phonecalls about this).

Once we rely on machines to do the thinking for us we are lost; useless thoughtless relics destined to become slaves to the machine.

If we allow people to input a negative pressure then we are doing the same disservice to the engineer using the program. They will think they are getting external pressure effects on their vaccum piping when they are not. We will think about this some more. Perhaps we could ignore the negative pressure (make it zero), as long as it is not below -1 bar, on all elements except bellows elements where it could be applied as before. This would solve the backwards compatibility issue. But I don't think it will make it "dummy proof" even if we put up a warning explaining what we are doing (and oh, all the tech support calls we will get on this one!).