Cv and Kv are flow coefficients used in control-valve sizing. They use different customary unit conventions, so a conversion is useful when comparing a valve specified in US units with a metric valve document. Emerson states Kv = 0.865 × Cv. Reversing that relationship gives Cv = Kv ÷ 0.865, or approximately 1.1561 × Kv. This two-row lookup covers only the coefficient conversion; it is not a valve-sizing table.

Liquid Valve Flow Coefficients Cv and Kv

What the conversion does

The conversion translates the numerical coefficient between two conventions. It does not change the physical valve or make two valve models equivalent. A Cv value and a Kv value still need to be tied to the same valve travel, test basis, and manufacturer data. Use the relation in the direction shown: multiply Cv by 0.865 to obtain Kv, or divide Kv by 0.865 to obtain Cv. The reverse multiplier shown in the table is calculated from the stated Emerson factor and rounded for display.

What a coefficient does not tell you

A flow coefficient alone does not establish the operating flow. Liquid valve sizing also depends on the fluid’s specific gravity, upstream and downstream pressures, piping geometry, attached fittings, and valve recovery characteristics. Emerson’s handbook describes a maximum allowable pressure drop and the possibility of choked flow. For a liquid, vaporization can limit the flow as pressure falls through the valve. A simple Cv-to-Kv conversion does not evaluate cavitation, flashing, noise, erosion, or required controllability.

Use consistent pressure and fluid data

When calculating a required coefficient, make sure flow units, pressure units, and the source equation’s constants agree. The Emerson handbook notes that the pressure terms in its sizing equations are absolute. Mixing gauge pressure with an absolute-pressure equation can create a serious error. Record the fluid temperature, density or specific gravity, vapor pressure, and intended operating range. For a liquid near its boiling point or a fluid with unusual properties, the standard liquid equation may not be sufficient.

Account for connected piping

Reducers, elbows, tees, and other fittings immediately attached to a valve may alter the piping geometry factor. The handbook recommends determining the appropriate factor when fittings are present and indicates that experimental evaluation may be appropriate for the specified valve and installation. A catalog coefficient measured in a laboratory does not automatically represent a valve installed between reducers. Keep the valve, fitting arrangement, line size, and sizing method together in the engineering record.

When to seek a full sizing check

Use the conversion as a unit-reference step when reading a datasheet or comparing published values. Use the current valve-manufacturer sizing procedure for selection. Confirm that the chosen body and trim can handle the expected minimum, normal, and maximum flow; the pressure drop; the operating temperature; and the required rangeability. Where choked flow, cavitation, flashing, high noise, or hazardous fluid service is possible, request manufacturer engineering review. A converted coefficient is a convenient comparison value, not an approval for service.

For procurement, state the coefficient convention and reference conditions explicitly. If a supplier provides Kv while the project specification lists Cv, record the conversion and keep the original value beside it. This avoids a rounded number replacing the manufacturer’s published rating. For critical systems, check the applicable control-valve standard, the latest handbook edition, the actual valve travel, and the detailed calculation before releasing the selection.

Keep enough precision during the calculation to avoid compounding rounding. For example, the inverse factor is calculated from the handbook’s 0.865 relationship; a displayed value of 1.1561 is a readable approximation. Use the unrounded factor in a spreadsheet or calculation record when the conversion affects a specification. The selected manufacturer coefficient and the original unit convention should remain traceable to the valve datasheet.

References

• Emerson Control Valve Handbook, 6th edition
• Emerson Fisher Control Valve Sourcebook

This reference only converts Cv and Kv. Verify the current valve-sizing method, pressure basis, flow regime, fittings, fluid properties, and manufacturer limits for the actual service.