อัปเดต 2026-09-07

PSV Inlet/Outlet Line Pressure Drop

API RP 520 Part II §4/§7/§7.3

Standalone PSV installation-line calculator: derives inlet- and outlet-line ΔP from a {length, diameter, ΣK} description (or a direct ΔP entry) via an inverted Crane isothermal-flow solve, checks it against the API 520-II 3%/backpressure limits, and computes the open-discharge reaction force and elbow bending moment.

Open the calculator

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Line ΔP is obtained by bisecting the reused Crane Eq 1-7 isothermal-compressible-flow solver for the downstream pressure P2 that matches the target relieving mass flow (no new gas physics — ΣK fittings are folded in as an equivalent length Le = ΣK·D/f). Reaction force is an exact closed-form momentum + pressure-thrust balance; no iteration.

สมการ

Open-discharge reaction force, API 520-II §7.3

F = W ˙ \cdot V + (P_{2} - P_{a}) A_{2}

Strict SI (g_c = 1): Ẇ = relieving mass flow (kg/s), V = exit velocity (sonic/choked design case), P2 = exit static pressure, Pa = ambient, A2 = exit-plane area.

Elbow bending moment

M = F \cdot L_{a r m}

L_arm = perpendicular lever-arm distance from the elbow centerline to the reaction-thrust line of action.

Line ΔP acceptance ratios, API 520-II §4/§7

\frac{Δ P}{P _{se t}} \leq {0.03 L_{b p} inlet line (§4) outlet / built-up backpressure (§7)

Same acceptance ratios as the PSV Datasheet calculator; here ΔP itself is first derived from pipe geometry via the inverted Crane Eq 1-7 solve rather than entered directly.

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