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).
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).

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