Butterfly control valves carry a typical FL (Liquid Pressure Recovery Factor) of 0.55 to 0.68 for a centric-disc build, with a global benchmark figure of 0.61 cited across sizing references [S1][S2].
FL and xT are the two dimensionless coefficients defined in IEC 60534-2-1 and ANSI/ISA-75.01.01 that govern choked liquid and gas flow through a control valve; they are not safety factors and they are not adjustable in the field [S2].
What FL and xT Actually Measure
FL is the ratio of total pressure drop (P1 minus P2) to the total pressure drop including the minimum vena-contracta pressure (P1 minus Pvc), so a high FL means little recovery and a low FL means aggressive recovery downstream of the seat [S2][S3].
xT is the normalised pressure-drop ratio (ΔP / P1) at which compressible flow chokes; once the operating drop x exceeds Fk times xT, the vena contracta hits Mach 1 and mass flow stops climbing with further downstream pressure reduction [S2].
For reference, the same dimensional set spans 0.20 (full-port ball) up to 0.98 (anti-cavitation globe cage), and a butterfly at FL = 0.90 would mean slow recovery, while 0.60 would mean aggressive recovery [S2].
Butterfly vs Globe vs Ball on a Criteria Grid
Using a three-axis comparison (FL band, cavitation resistance, gas-choking tolerance), the rotary and through-bore styles cluster at the high-recovery, low-FL end of the scale [S2][S1].
Standard butterfly (centric disc): FL 0.55-0.68, very low cavitation resistance, smooth-bore rotary path with no flow tortuosity [S2].
V-notch ball (segmented contour): FL 0.60-0.75, low cavitation resistance, slightly better rangeability than a centric butterfly at the cost of higher torque [S2].
Full-port ball (through-conduit): FL 0.20-0.50, extremely low cavitation resistance, lowest FL of any common control style and only chosen when pigging or full-bore flow is mandatory [S2].
Globe, standard contoured plug: FL 0.85-0.90, high cavitation resistance, the default for throttling clean liquids where choked-drop margin matters [S2].
Globe, multi-port anti-cavitation cage: FL 0.92-0.98, excellent cavitation resistance, the trim of choice for pump recirculation and high-head feedwater service [S2].
Butterfly Cavitation Threshold in Numbers
The critical cavitation index is σ_choked = 1 / FL^2, which for a centric butterfly at FL = 0.55 gives σ_choked of about 3.31, dropping to about 2.16 at FL = 0.68 [S2].
Translated to operating pressure drop: a butterfly with FL = 0.60 starts choking when the actual pressure drop reaches roughly 36% of effective inlet pressure (P1 minus Pv), while a globe at FL = 0.90 holds until 81% [S2].
That gap is the engineering reason why a butterfly is rarely picked for clean-water pump recirculation loops where ΔP routinely exceeds 50% of P1; the cavity collapses in the disc wake and shreds the seat within a season [S2][S4].
How xT Compounds the Problem in Compressible Service
On gas and steam duty, the relevant metric is xT, and a butterfly's smoother flow transition than a globe gives a smaller xT, which means the downstream pipe run before the next obstruction must be longer to recover pressure cleanly [S3].
For an LNG let-down or CO2 regulator station, a butterfly at low xT can hit sonic choking at modest ΔP/P1 ratios, capping Cv and forcing the spec writer to drop one line size or switch to a globe [S3].
Reading the same passage on cavitation prevention for a butterfly, the rule is simple: a high FL pushes ΔP_choked higher, while a low FL brings choking and cavitation on early, and the same author reminds that downstream reducer geometry can shift an installed butterfly toward XTP rather than the bare xT [S3][S6].
Where the FL = 0.61 Benchmark Comes From
The single-value 0.61 butterfly figure appears in published training content and matches the FL = 0.63 ball and FL = 0.83 globe references used to teach pressure-recovery math [S1].
It is a representative mid-band, not a guaranteed supplier number; Emerson's traditional sizing handbook (Fisher) explicitly notes that FLP, the installed recovery factor, must be derived from test when reducers or fittings sit on either flange, since attached fittings change the vena-contracta pressure field [S7].
For a centric wafer butterfly between flanges with no reducers, the published 0.61 is a defensible assumption; for the same valve installed between a 2:1 inlet reducer and a 1.5:1 outlet expander, expect FLP to drop further and the choked drop to fall with it [S7].
Selection Rules and Common Sizing Errors
Rule 1: spec a butterfly only when the service is clean, the liquid has a low vapour-pressure margin, and ΔP is below roughly 30% of P1; above that, size up two steps or switch to a globe cage trim [S2].
Rule 2: for cavitation-sensitive service, compute σ = (P1 - Pv) / (P1 - P2) and verify σ > 1 / FL^2 with FL pulled from the actual trim, not a generic catalogue [S2].
Rule 3: for gas and steam, compute x = (P1 - P2) / P1 and confirm x < Fk xT; if Fk xT is below 0.3, the valve is choking before it reaches design stroke, which is a sizing error, not a control problem [S2][S3].
Rule 4: never use FL or xT as a safety margin; treat them as measured trim properties, because treating them as fudge factors is the most common reason butterfly valves fail prematurely in chemical and pulp service [S3][S6].
Standards, Sizing Equations, and Trackable Next Steps
The two governing standards are IEC 60534-2-1 (industrial-process control valves, Part 2-1: flow capacity, sizing equations for incompressible and compressible fluids) and ANSI/ISA-75.01.01, which carry the same FL, xT, and Fk definitions in consistent form [S2].
For gas service, the Fk factor (specific-heat ratio correction) and the combined XTP for valves installed between fittings are still derived experimentally under IEC 60534-2-1 Annex D, not read off a generic curve, so any spec copy that quotes XTP without a test report should be treated as marketing copy [S2][S7].
To watch on the next 6 months: any vendor update that publishes FL and xT curves versus disc angle for high-performance double-offset butterflies, since current public data still centres on the centric FL = 0.55 to 0.68 band, and double-offset geometry typically moves the recovery point downward by 0.05 to 0.10 [S2][S5].
For readers who want a worked example of cavitation prediction and trim selection in adjacent rotating equipment, the recent Ball screw vs lead screw efficiency and backlash decision matrix walks through similar ΔP and torque trade-offs, and the PTFE vs Graphite Braided Packing spec map covers the gland side of the same valve once a butterfly is committed to throttling service.
Detailed specification references: butterfly valve, control valve, and construction machinery and equipment.