Steam Orifice / PRV Capacity (Napier)
Choked steam through an orifice or PRV by Napier's formula: flow chokes when P2 < 0.58 x P1, and the capacity W = 51.43 x Cd x A x P1 (saturated) then depends only on the upstream pressure. A liquid Cv (square-root in pressure drop) is wrong for choked steam, which is linear in P1; superheat needs a Ksh factor. A sizing aid, not a relief-valve certification.
Formula and source
choked = downstream_p_psia < 0.58 x upstream_p_psia; capacity W = 51.43 x discharge_coeff x orifice_area_in2 x upstream_p_psia [lb/hr] (saturated, choked).
Napier's formula / ASME/API 520 / Grashof steam orifice / PRV capacity, by name.
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