Steam PRV Orifice Area for a Required Capacity (Napier)
The orifice / seat area a required relief capacity needs.
Example
You enter
- Required relief capacity (lb/hr) 5000
- Upstream absolute pressure (psia) 100
- Discharge coefficient Cd (~0.6 orifice, ~1 nozzle) 0.9
You get
- Required orifice / seat area 1.079 in^2
- Choke threshold (0.58 x P1) 58
Checked against published worked examples from: PDHonline (Whitesides).
Details, formula, and sources
ScopeA = W / (51.5 x Cd x P1) from the choked Napier form (API 520 constant). A 5,000 lb/hr relief at 100 psia and Cd 0.9 wants about 1.08 in^2 - round up to a standard API 526 orifice letter. Assumes choked flow (P2 < 0.58 x P1); saturated steam. A sizing aid, not a relief-valve certification; ASME/API and the valve maker govern.
Formula or tablerequired_area_in2 = required_capacity_lb_hr / (51.5 x discharge_coeff x upstream_p_psia), the inverse of the choked Napier capacity W = 51.5 x Cd x A x P1; choke_threshold = 0.58 x upstream_p_psia.
Source and editionNapier's formula / ASME/API 520 choked steam capacity, solved for the orifice area, 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 namesrequired_capacity_lb_hr, upstream_p_psia, discharge_coeff, required_area_in2, choke_threshold_psia
- Choked flow assumes choked flow (P2 < 0.58 x P1); capacity depends only on the upstream pressureNapier / Grashof
- Round up to API letter round A up to a standard API 526 orifice letterAPI 526
- Saturated steam Napier is for saturated steam; superheat needs a Ksh factor; apply the discharge coefficientASME/API 520