Which standard does FieldEngineersKit follow?

API RP 520 Part II §4.2 for open-discharge vapor/gas reaction forces, with ASME VIII-1 Appendix M guidance. Momentum matches the Part II US customary term when converted to SI.

Why is DLF often 2.0?

For sudden popping on open vent elbows, API 520 Part II allows a dynamic load factor up to 2.0 as a screening amplification of the steady thrust.

Can I use this for liquid relief?

No. Liquid, two-phase, and water-hammer cases need dedicated transient piping analysis — not this vapor/gas screen.

Safety Relief Valve (PSV) Reaction Force Calculator (API 520 Part II)

Engineering Reference & ASME Code Basis

1. Core Formula & Variable Definitions

API Recommended Practice 520 Part II (6th Ed., 2015) §4.2 Reaction Forces · ASME BPVC Section VIII Division 1 Appendix M (Para. M-14) · ASME B31.1 Appendix II (safety valve installation guidance) · ASME B36.10M outlet pipe inside diameter

v = √[ 2k/(k+1) · (R/M) · T ] · F_mom = (W/3600)·v

F_press = max(0, P_e − P_atm)·A_exit · F_total = (F_mom + F_press)·DLF

API RP 520 Part II §4.2 open-discharge vapor/gas screen · ASME VIII-1 Appendix M guidance · R = 8314 J/(kmol·K)

Default duty CO₂ · 25 000 kg/h · NPS 4F_total 5.98 kN (DLF 2.0)F_steady 2.99 kNDLF 1.0 – 2.0

Sonic exit velocity from stagnation temperature matches the API 520 Part II US customary momentum term when converted to SI. Exit static pressure uses continuity at T_e = T·2/(k+1). Outlet area from ASME B36.10 pipe ID. Base bending moment assumes a 1.5 m (4.92 ft) vent-elbow lever arm for screening only.

F_total (Peak dynamic reaction force) — Steady reaction scaled by DLF for sudden popping (N or lbf).
W (Nameplate relieving capacity) — Mass flow rate on the PSV nameplate (kg/h or lb/h).
v (Exit velocity) — Sonic exit velocity from isentropic critical flow at relieving temperature.
P_e (Exit static pressure) — Absolute static pressure at the sonic exit plane from mass continuity.
A_exit (Outlet pipe flow area) — π/4 · ID² from selected NPS and schedule (B36.10).
DLF (Dynamic load factor) — Popping amplification (typically 1.1–2.0 for open vent elbows).

2. Screening Assumptions

Vapor/gas open-discharge reaction screen with optional closed-header momentum-only mode. Not a substitute for transient piping analysis.

Default duty25 000 kg/h CO₂ · 180 °C · NPS 4 Sch 40 · DLF 2.0

Hero F_total = 5.98 kN; F_steady = 2.99 kN (momentum + exit pressure).

Outlet areaASME B36.10 ID for selected NPS / schedule

A_exit = π/4 · ID². Sch 10 / 40 / 80 supported in Phase-1.

Base moment arm1.5 m (4.92 ft) screening offset

M_bending = F_total · 1.5 m for open vent elbow. Adjust in detailed layout.

Out of scopeLiquid · two-phase · water hammer · full closed-header P₁/P₂ layout

Use Caesar II / equivalent dynamic piping analysis for those cases.

Quick Reference Lookup Table

Open-discharge screening — CO₂ at 180 °C, DLF 2.0, Sch 40 outlet
W (kg/h)	NPS	F_steady (kN)	F_total (kN)	v (m/s)
10 000	3	1.05	2.09	311
25 000	4	2.99	5.98	311
25 000	6	2.16	4.32	311
50 000	6	5.76	11.51	311
50 000	8	4.37	8.75	311
100 000	8	12.02	24.04	311

Forces scale with W; exit velocity depends on T, M, and k (not NPS). Larger NPS reduces exit pressure thrust. Confirm project fluid data and piping dynamics for design.

3. Applicability & Fluid Limits

API 520 Part II reaction-force screening applies to single-phase vapor/gas relieving through an open vent or simplified closed header.

Group / Regime	Range / Condition	Limit / Criterion	Engineering Notes
Vapor / gas (ideal-gas screen)	−50 to 600 °C input band	—	Uses MW and k from API 520–style presets (CO₂, air, steam, N₂, methane, light HC) or custom entries.
Open discharge vent elbow	Same	—	Includes exit pressure thrust above atmosphere and DLF up to 2.0 for popping.
Closed discharge header	Same	—	Momentum-only screen — layout-specific pressure terms are not modeled.
Liquid / two-phase / flashing	N/A	—	Out of scope — requires multiphase transient analysis.

Code Applicability & Safety Boundaries

Screening only — not a certified piping support design
Confirm nameplate capacity units (kg/h vs lb/h) before applying DLF

4. Step-by-Step Worked Example

Field VerificationShow

Worked example — CO₂ · 25 000 kg/h · NPS 4 Sch 40

Open-discharge PSV relieving carbon dioxide at 25 000 kg/h and 180 °C through an NPS 4 Schedule 40 vent (ID 102.26 mm), DLF = 2.0, P_atm = 1.013 barA.

W:25 000 kg/hT:180 °C (453.15 K)Gas:CO₂ · M = 44.01 · k = 1.30Outlet:NPS 4 Sch 40

Sonic exit velocity

v = √[2k/(k+1)·(R/M)·T]

Result:v ≈ 311 m/s

Momentum and pressure thrust

F_mom = (W/3600)·v; P_e from continuity; F_press = (P_e−P_atm)·A

Result:F_mom ≈ 2.16 kN; F_press ≈ 0.83 kN; F_steady ≈ 2.99 kN

Apply DLF

F_total = F_steady · 2.0

Result:F_total ≈ 5.98 kN

Conclusion: Peak screening reaction is F_total = 5.98 kN (F_steady = 2.99 kN). Use this thrust with the project support/layout model; liquid or two-phase relief is out of scope.

5. How to screen PSV open-discharge reaction force

1
Enter nameplate capacity and temperature
Use the PSV nameplate relieving rate W and relieving temperature in the active unit system.
2
Select gas and outlet pipe
Pick a preset gas (or custom MW/k) and the discharge NPS / schedule for A_exit.
3
Set DLF and discharge mode
Open discharge includes exit pressure thrust; closed header uses momentum only. DLF 2.0 is typical for open vent popping screens.
4
Read F_total and export
Hero shows F_total. Badges show F_steady and DLF. Export CSV/PDF or carry NPS/temperature to related pipe and nozzle tools.

6. Frequently Asked Questions & Technical References

Show

A field screen of the thrust on discharge piping when a safety valve pops — momentum of the vapor jet plus exit pressure above atmosphere, often multiplied by a dynamic load factor (DLF).