5 replies to this topic

[Lucas]

Question here is: 0.83 mach speed in a relief line... Is it really a problem?

In a recent project, in order to not have to shut-down production / cut and weld flare lines / hydrotest flare header again, we're considering to use an existent tie-in of reduced internal diameter (6"). In one of our relief study cases, relief from a PSV (R 8" 300# x 10" 150#) sized for blocked discharge is passing through this tie-in with 0.83 mach.

So it's exceeding, for example, Norsok's design criteria of 0.5 or up to 0.7 mach for Relief Lines. It's also exceeding, but not so much, ro.v2 < 200.000 criteria.

Is it really a problem? Lenght of this reduced diameter tie-in is not more than 5 meters / 15 feet.

Thanks,

Lucas
3 years exp - Oil & Gas Production Surface Installations

[rxnarang]

Lucas,

The limits on velocity are normally imposed to limit vibrations and noise. There is no thermodynamic reason for the pipe not to evacuate fluids at 0.83 mach, provided the mechanical design in sound.

Of course, the client may not like the idea and impose his desires, but that is a separate matter.

So, for a short piece of pipe I see no trouble to going to 0.83 mach. Just support the pipe for sonic flow, which should be mechanically and conservatively safe.

Regards
Rajiv

[mishra.anand72@gmail.com]

Yes it is really a problem, if you cosider the scenario of poping up of PSV frequently.

[Zauberberg]

I agree with Rajiv. I have also seen (evaluated) existing flare systems and calculated that existing velocities in certain number of lines are exceeding 0.8Mach, which is considered as practical maximum in relief lines feeding the common header.

Especially if line is very short, I think you should not have any further concerns. 0.83 is quite OK number.

Regards,

[Lucas]

Rajiv / Zauberberg

Thanks for your opinions. If you don't mind, can you explain what do you mean by "support the pipe for sonic flow"? Thanks again,


"mishra.anand72"

That's Ok, for many reasons we don't expect this PSV opening frequently.


Lucas

[rxnarang]

Lucas,

I am not knowledegable on the exact criteria to use for support, but I do know that my piping guys are. If I write this requirement on a P&ID, the piping stress guys do the analysis.

I am aware of a number for frequency of the pipe structure which is used by some designers to analyse a pipe, in absence of any firm information. But I am not going to stick my neck out on this one! Best ask a stress guy.

Regards
Rajiv