x_T is the valve’s terminal pressure-drop ratio factor for gas service. Choking starts near x = F_k·x_T.

F_L is the liquid pressure recovery factor. Cavitation screening uses ΔP_cav = F_L²(P1 − r_c·Pv).

Does this include pipe reducers (F_P)?

No. This screening assumes F_P = 1. Installed reducers need the full IEC 60534-2-1 piping geometry correction.

Control Valve Choked Flow & xT / FL Screening Calculator

Engineering Reference & ASME Code Basis

1. Core Formula & Variable Definitions

ANSI/ISA-75.01.01-2012 (IEC 60534-2-1 Mod) — Flow capacity equations, §5 choked flow · IEC 60534-2-1:2011 — Sizing equations for fluid flow under installed conditions · ISA-75.02.01-2008 — Capacity test procedures (x_T and F_L measurement)

x = (P₁ − P₂) / P₁

x_choked = F_k · x_T · F_k = k / 1.40

ΔP_cav = F_L² · (P₁ − r_c · P_v) · r_c = 0.96 − 0.28√(P_v/P_c)

ISA-75.01.01 / IEC 60534-2-1 §5 · F_P = 1 (no reducers)

Hero Regime · marginGas x vs x_chokedLiquid ΔP vs ΔP_cavF_P 1.0 assumed

Absolute pressures only. Gas choke when x ≥ F_k·x_T. Liquid cavitation when ΔP ≥ F_L²(P1 − r_c·Pv); flashing when P2 ≤ Pv. Manufacturer x_T / F_L preferred over typical trim presets.

x (Pressure differential ratio) — x = (P1 − P2) / P1 for gas / vapor service.
x_T (Pressure differential ratio factor) — Valve trim factor without attached fittings (typ. 0.20–0.80).
F_k (Specific-heat ratio factor) — F_k = k / 1.40.
F_L (Liquid pressure recovery factor) — Trim recovery factor for cavitation screening (typ. 0.50–0.95).
r_c (Critical pressure ratio factor) — r_c = 0.96 − 0.28√(Pv/Pc).
ΔP_max (Maximum effective differential) — Gas: x_choked·P1. Liquid: ΔP_cav.

2. Screening Rules Matching This Calculator

Checks gas choke (x vs F_k·x_T) and liquid cavitation / flashing (ΔP vs F_L² limit) using ISA-75.01 / IEC 60534-2-1 §5 forms. Absolute pressures only; F_P = 1.

Gas choke limitx_choked = F_k · x_T · F_k = k / 1.40

Choking when x = (P1 − P2)/P1 ≥ x_choked. Hero reports regime and margin ratio.

Liquid cavitationΔP_cav = F_L²(P1 − r_c·Pv)

r_c = 0.96 − 0.28√(Pv/Pc). Flashing when P2 ≤ Pv.

Pressure basisAbsolute P1, P2 only

Convert gauge to absolute before entry. Default gas duty uses bar abs.

Out of scope hereF_P reducers · two-phase · FF

Installed piping geometry F_P, two-phase choke, and FF flashing factors are not solved — use full IEC worksheet.

Quick Reference Lookup Table

Default gas duty (P1 = 10 bar abs · P2 = 4 · x_T = 0.70 · k = 1.40)
Quantity	Value	Unit	Notes
x	0.600	—	(10−4)/10
F_k	1.000	—	k/1.40
x_choked	0.700	—	F_k·x_T
Status	Non-choked	—	x < x_choked
ΔP_max	7.00	bar	If driven to choke

Hero reports regime and margin ratio. Absolute pressures required.

3. Regime & Trim Limits

Choke and cavitation limits depend on fluid regime and valve trim factors — not body metallurgy in this screening tool.

Group / Regime	Range / Condition	Limit / Criterion	Engineering Notes
Gas / vapor (x_T screening)	Per k at T	x_T typ. 0.20–0.80	Globe, butterfly, and ball presets in-app. Manufacturer x_T from ISA-75.02 test overrides presets.
Liquid (F_L · P_v · P_c)	Pv at flowing T	F_L typ. 0.50–0.95	High-recovery trims (high F_L) tolerate more ΔP before incipient cavitation.
Flashing service (P2 ≤ Pv)	N/A	Out of choked-gas model	Flashing is flagged separately from cavitation — needs hard-faced / anti-cav trim selection.
Installed reducers / expanders	N/A	F_P = 1 assumed	Upstream/downstream reducers change effective x_T and F_L — full IEC 60534-2-1 F_P correction required.

Code Applicability & Safety Boundaries

ISA-75.01 / IEC 60534-2-1 §5 screening — not a vendor sizing sheet substitute
Pair with valve-cv-sizing for capacity and control-valve-noise when ΔP is high

4. Step-by-Step Worked Example

Field VerificationShow

Worked example — air, x_T = 0.70

Globe valve on air, P1 = 10 bar abs, P2 = 4 bar abs, x_T = 0.70.

Fluid:Gas (k = 1.40)P1 / P2:10 / 4 bar absx_T:0.70

Differential ratio

x = (10 − 4) / 10

Result:0.600

Choke limit

x_choked = (1.40/1.40)·0.70

Result:0.700

Compare

0.600 < 0.700

Result:Non-choked · margin 0.86

Conclusion: Default duty is non-choked. Dropping P2 to 2 bar abs (x = 0.80) crosses the sonic limit.

5. How to screen control valve choked flow

1
Choose gas or liquid
Gas uses x_T / k; liquid uses F_L, P_v, and P_c.
2
Enter absolute P1 and P2
Convert gauge to absolute before entry.
3
Set trim factors
Use a typical trim preset or manufacturer x_T / F_L.
4
Read regime and ΔP_max
Hero shows choke / cavitation status and margin; rows show limits.

6. Frequently Asked Questions & Technical References

Show

A tool that checks whether valve ΔP reaches the ISA-75.01 / IEC 60534-2-1 gas choke limit (x_T) or liquid cavitation limit (F_L).