Steam Orifice / PRV Capacity (Napier)
Choked steam through an orifice or PRV by Napier's formula.
Example
You enter
- Orifice / seat area (in²) 0.5
- Upstream absolute pressure (psia) 100
- Downstream absolute pressure (psia) 30
- Discharge coefficient Cd (~0.6 orifice, ~1 nozzle) 0.9
You get
- Steam capacity 2317.5
Checked against published worked examples from: Napier's formula / ASME/API 520.
Details, formula, and sources
ScopeFlow chokes when P2 < 0.58 x P1, and the capacity W = 51.5 x Cd x A x P1 (saturated, the API 520 constant) 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 or tablechoked = downstream_p_psia < 0.58 x upstream_p_psia; capacity W = 51.5 x discharge_coeff x orifice_area_in2 x upstream_p_psia [lb/hr] (saturated, choked).
Source and editionNapier's formula / ASME/API 520 / Grashof steam orifice / PRV capacity, by name.
Free accessNapier's formula is a classical, published steam-flow relation; ASME/API and the valve manufacturer govern the actual certification.
What governsEstimate. AHJ and licensed professional govern.
API field namesorifice_area_in2, upstream_p_psia, downstream_p_psia, discharge_coeff, steam_capacity_lb_hr
- Choked flow chokes at P2 < 0.58 x P1; capacity then depends only on the upstream pressureNapier / Grashof
- Saturated steam Napier is for saturated steam; superheat needs a Ksh factor; apply the discharge coefficientASME/API 520