Perth1,
There may be a basis for you to exclude the exit loss when piping is discharging to atmosphere if you are relieving a compressible fluid.
There used to be a very good website that covered compressible flow and had a good discussion about your question but I've had problems accessing it lately. Here's the actual link that discusses your question..
Just in case, here's cached version of it....
A similar discussion can be found in the "Guidelines for Pressure Relief and Effluent Handling Systems" which is an AICHE, CCPS publication. Here's an excerpt...
"The pressure change at the exit (change from station 2 to station 3) is not included in the calculations on this section for the following reasons:
- If the flow is choked at station 2, P3 cannot be computed from a value of
P2 (see para 2.10.2 for a discussion of the pressure discontinuity across a
choke point).
- 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 larger size (expansion), if the flow is not choked at station 2."
I would suggest you get the AICHE publication and review the complete section on the referenced subject.
Typically, I have always accounted for an exit loss in the outlet piping when discharging to atmosphere and it is only recently that I have found the above references. For new designs, I'll continue to leave the exit loss in the calculation just as an additional allowance for piping runs that may not be well defined. For checking existing systems, certainly no harm to include the exit loss if pressure drop is not a problem. If it is a problem, I may have to think more about that since old habits die hard.