I have a few queries about relief valve loads (assuming discharge to atmosphere).

The 'thrust at vent pipe exit' which Caesar can calculate is taken from API520, correct?

This load acts in the opposite direction of the vent piping. What about the vertical force which should be acting at the relief valve? I can understand that the horizontal forces will be balanced for steady state flow, but the vertical force from the momentum change should still act upwards on the valve shouldn't it?

API520 seems to suggest there is single force on the vent pipe only when the PRV is connected to a vessel (in which case my 'missing' vertical force would be balanced by the tension in the pipe and vessel). For my situation however, the PRV is branching off a steam line, so this does not seem valid (as the upwards force could lift the pipe).

Also, should a PRV 'pop' case be run? That is, the moment when the PRV first opens and there is only a vertical force and a horizontal force acting at the valve.

I hope I'm making sense. I'd appreciate if someone could shed some light.

Peter

The loads provided by CAESAR II in the Relief Load Synthesis module are computed according to the paper "Steam Flow Through Safety Valve Vent Pipes", by Brandmaier and Knebel, ASME 1975.

CAESAR II isn't using anything from API-520.

I'm having trouble picturing how a relief valve going off is supposed to cause a pipe to lift up, assuming a typical open discharge up to the atmosphere.

Richard - thanks, my mistake.

Edward - sorry, perhaps I shouldn't have said 'lift' as it won't. I was more just trying to draw attention to the vertical force on the valve.

Perhaps I am misinterpreting the problem, but looking at the change in momentum of the fluid as it passes through the relief valve (a 90 degree change in direction)there should be a force acting with a vertical component upwards, and horizontal component in the opposite direction to the fluid direction as it leaves the valve.

I picture this as the same as slug flow creating forces at the elbows in a system (from the fluid change in momentum).

Dear PeteN,

Mr. Klein is right in having trouble in picturing the scenario; will we be safe with these young, Nextgen creative engineers applying their cheap ignorance instead of acquiring knowledge by hard way ! In this era of knowledge explosion, learn first, before exposing yourself.

Jim

Dear Jim (et al.),

I would very much appreciate it if contributors to this forum would refrain from making negative comments such as yours. This is supposed to be a forum for users to exchange useful technical information concerning pipe stress analysis in general, and CAESAR in particular; and to learn about these subjects from that exchange. Your comments do nothing to forward these goals. Please, if you cannot provide something useful, refrain from makiung any comment. Thanks.

Bruce

PeteN,

I think I know understand what you're talking about. I can't say I've ever heard that being a concern. In the case you describe, the forces should pretty well be balanced against the bottom of the header that you RV is coming off of.

From your description, I picture a steam header running horizontal with a vertical branch running from the top of the header to the bottom of the RV. It sounds like your expecting a net upward force along the length of that vertical segment.

Where can I find the paper „Steam Flow Through Safety Valve Vent Pipes"?

Regards!

ghx and anyone else concerned,

Send me an e-mail at techsupport@coade.com with your fax number and reference the technical paper and I can fax it to you.


Loren Brown
COADE Technical Support

Hello all,

I still think that B31.1 Nonmandatory Appendix II, "Rules for the design of Safety Valve Installations" is quite a good reference for all of what is being discussed here (complete with samples).

Loren, you are a saint.

Regards, John.

Another useful reference is "Analysis of Power Plant Safety and Relief Valve Vent Stacks", G.S. Liao, ASME Transactions, Oct. 1975. This paper and the other cited are available from the Linda Hall Library document services for a nominal fee.

Regard, Ken