Consider Non-Recoverable Losses in Relief Valve Outlet Piping? 1

[Tonylmiller]

I understand that recoverable losses (such as due to velocity change in an enlarger) are NOT considered in pressure drop calculations for the inlet piping for a relief valve (API RP 520).

Does the same apply to the exit piping? Sometimes considerable enlargements in the outlet piping take place, and it could have a significant effect.

Thanks in advance!

Tony

 

[EGT01]

Tony,

Forgive me if I seem "picky" but I think it best to reference API RP-520 Part II inlet piping requirements as they are written:
"...the total non-recoverable pressure loss between the protected equipment and the pressure relief valve should not exceed 3 percent of the set pressure of the valve..."

As additional comment about the inlet piping requirements, I find it interesting what the API Technical Interpretations say about the inlet line non-recoverable losses:

http://committees.api.org/standards/tech/docs/520bti.xls

http://committees.api.org/standards/tech/reti.html

Now, for the exit piping, there isn't much discussion in RP-520 Part II about what is included in the pressure drop calculations and reference is made to API RP-521 for additional information on sizing of discharge piping systems for vapor or gas service.

From API RP-521:
"In any calculation method, the total frictional resistance should include the length of piping and the equivalent length of all fittings, valves, expansion or contraction losses, and any other flow resistances."

From another reference, "Guidelines for Pressure Relief and Effluent Handling Systems", AICHE, CCPS:
"The pressure change at the exit (change from station 2 to station 3) is not included in the calulations on this section for the following reasons:
1. If the flow is choked at station 2, P3 cannot be computed from a value of P2.
2. If the flow is not choked and the pipe discharges to a large reservoir (or to a same size section of piping), P2 = P3 and no computations across the exit are required (exit loss and velocity head recovery are equal).

In contrast to the above conditions, a pressure change at the exit must be determined if the pipe connects to a downstream piping run of a smaller size (reduction). The pressure change must also be determined if the pipe connects to a downstream piping run of a larger size (expansion), if the flow is not choked at station 2."

Of course its best to get the references and read through them yourself. Both references indicate that expansion and contraction losses should be accounted for in the outlet piping.

As far as not counting the exit loss mentioned in the AICHE guidelines item 2 above, I've only recently had the guidelines and have always interpreted API 521 to mean that an exit loss should be included as an expansion or other flow resistance. However, I don't think API ever specifically mentions "exit loss" and I've seen other references to support what the AICHE guidelines suggest.

http://www.optimal-systems.demon.co.uk/topic-pressure-at-pipe-end.htm

Of course "old habits die hard". For new designs, I don't see a big problem for leaving the exit loss in but when evaluating existing designs, you may want to apply more scrutiny to the evaluation, especially if the original designer did not include an exit loss.

 

[Tonylmiller]

Thank you very much.

Tony