startup modeling 1

[B.L.Smith]

Dear friends,

How can I model startup in CAESAR II?

 

[C2it]

Your question is rather vague.

What do you want to include in your analysis other than the many combinations of pressures temperatures and other loads that Caesar II already offers ?

 

[B.L.Smith]

I have 30" GRP pipe with 5 branches and 30bars pressure that is connected to a centrifugal pump in one side and a vessel in other side. I wanna analyze startup of this system with including pressure. What parameters should I put in harmonic analysis input like starting and ending frequency, increment, start node and stop node to startup analysis?

 

[C2it]

I have never needed to do such an analysis on a centrifugal pump system. In any case, the frequencies that might be there are going to constantly change during start-up and thus have no lasting harmonic effect. What do you expect to achieve from this ?

 

[B.L.Smith]

I want achieve force and stress at pump nozzle and branches during startup. I think I have to use water hammer/slug flow(Spectrum) analysis for these purposes but in CAESAR II manual are mention that for startup modeling harmonic analysis should(?)done(page 182/386).

 

[B.L.Smith]

Dear C2it,
Do you know how I can model spartup in CAESAR II?

 

[C2it]

I am not sure why you would expect slug flow in a pump system. Would that not suggest some sort of cavitation and risk of rotor damage ?

However, if you are thinking about water hammer / slug flow, you need to find reliable load / time data for your piping system. Assuming that data exists, then decide whether it is in any way cyclic or resonant. That will guide you towards response spectrum or time history analysis methods.

 

[B.L.Smith]

From Peng Book, We know if a load is applied statically(Like steady state flow)displacement response of a system is Yst and if it is applied suddenly displacement is 2Yst. Thus I think in startup of a piping system with a pump we should consider this concept. But I don't know how I can do this.
Thanks for your replies

 

[C2it]

Do you think that, if this supposed force were either significant or a problem, there would be evidence of it, maybe as a failure or unexpected displacement ?

Unless there is a true slug of liquid involved, all momentum effects of 'starting to flow' are self cancelling and should result in no external loads, assuming that there are no unrestrained expansion joints employed.

In my view, initiation of flow due to pump operation simply is not an instantateous load, rather it's a ramp up, so the (2x DAF) does not apply.

 

[B.L.Smith]

Dear C2it,

Thank you very much. We were working on a plant that a elbow failed during startup. Before your recent comment, I was thinking that force of startup(2*DLF)caused this failure.
Thanks again.

Sincerely,