Short answer: one flow rate and one pressure drop are not enough for a reliable Cv/Kv calculation. For each of the minimum, normal and maximum operating cases you need the flow rate, the inlet and outlet pressure at that case, the temperature and the fluid properties at those conditions (for liquids: density, viscosity, vapor pressure and critical pressure; for gases: molecular weight, compressibility factor and specific heat ratio). On top of that, the supplier needs the line size and schedule, any reducers, the shutoff requirement, the allowable noise, the fail position, the air supply, the signal/positioner, materials and the hazardous area classification. The calculation is done with this data; the selection is then made by checking whether the calculated Kv range fits the controllable range of the candidate valve.
This applies to throttling control valves in liquid, gas or steam service where process data is known. On/off isolation valves, self-operated regulators and safety valves are selected with different data sets. Cavitation, flashing and choked flow are not covered in detail here; this article only identifies the data needed to check for them.
Why a single "normal" case is not enough
A control valve takes up part of the available system pressure difference. In a pumped line, as flow increases the pump discharge pressure falls and piping and equipment losses rise, so the ΔP left across the valve shrinks. At low flow the opposite happens: pump pressure rises, losses drop, and the valve operates nearly closed with a much larger ΔP. Holding ΔP constant and simply scaling flow therefore understates the capacity needed at the maximum case and overstates it at the minimum case.
The three cases answer different questions: the maximum case shows whether the valve has enough capacity, the normal case shows where the valve will spend most of its time, and the minimum case shows whether it can still control stably close to its seat. If there is an additional condition such as start-up, turndown operation or an alternative product, give it as a separate operating case.
Control valve RFQ data form: per operating case
The form below lists the items that should be filled in separately for each operating case in a request for quotation, and why each one matters. The standard layout of a control valve data sheet is defined in IEC 60534-7; this form is not a copy of that standard but a practical checklist focused on sizing inputs.
Table 1 – Data required for each operating case
Data | Min | Normal | Max | Why it is needed / effect if missing |
Flow rate (with units: m³/h, kg/h or Nm³/h, stating the reference condition) | Required | Required | Required | Primary Kv/Cv input. If the unit or reference condition is unclear, a gas sizing can be completely off. |
Inlet pressure P1 (state absolute or gauge) | Required | Required | Required | Needed for ΔP and, for gases, expansion. If assumed constant, capacity comes out wrong. |
Outlet pressure P2 | Required | Required | Required | Sets ΔP; for liquids, P2 relative to vapor pressure is needed for choked flow and flashing checks. |
Inlet temperature | Required | Required | Required | Density, viscosity and vapor pressure change with temperature; it also drives material and packing selection. |
Liquid: density or specific gravity | Required | Required | Required | Used directly in Kv; if missing, the supplier may assume water. |
Liquid: viscosity | Required | Required | Required | Viscous fluids may need a non-turbulent flow correction; the value at the lowest temperature matters. |
Liquid: vapor pressure and critical pressure | Required | Required | Required | Needed for choked flow, cavitation and flashing checks; without them these risks cannot be assessed. |
Gas/steam: molecular weight or density, compressibility factor (Z), specific heat ratio | Required | Required | Required | Gas expansion and the choked flow limit depend on these; for steam also state the degree of superheat. |
Phase (liquid, gas, two-phase, solids content) | Required | Required | Required | Two-phase flow or entrained solids cannot be sized correctly with standard liquid/gas equations; this changes trim and body selection. |
A practical note: pressures should be the values at the valve inlet and outlet flanges. If pump discharge or vessel pressures are passed on without deducting the intermediate line losses, ΔP will look larger than it is.
Single-value design, mechanical and control data
Table 2 – Case-independent data and the effect if it is missing
Data | What to provide | What happens if it is missing? |
Line size, schedule and reducers | Inlet/outlet pipe size and wall thickness; whether reducers are acceptable if the valve is smaller than the line | Reducers lower installed capacity; without the line size, suppliers propose different body sizes for the same Kv. |
Design pressure and temperature | Line design values, pressure class and end connection | Body pressure class and materials may be selected wrongly. |
Shutoff requirement | Seat leakage class (named by IEC 60534-4 or ANSI/FCI 70-2) and the maximum pressure difference at shutoff | The class alone is not enough; without shutoff ΔP the actuator thrust and seat design cannot be set. |
Allowable noise | Project limit and measurement condition (e.g. at a stated distance from the valve) | The need for low-noise trim in gas and steam service cannot be assessed. |
Fail position | On loss of air or signal: fail open, fail closed or fail in last position | Actuator type and spring action cannot be fixed; a choice that conflicts with the safety function may be made. |
Air supply | Minimum and maximum supply pressure, air quality | Actuator size and shutoff force cannot be verified. |
Signal and positioner | 4–20 mA, HART or fieldbus; diagnostics, position feedback, limit switches, solenoid | Accessory scope varies from bid to bid. |
Materials | Body, trim and gasket materials; corrosive, erosive or sour service details | Even with correct capacity, service life and suitability problems may arise. |
Hazardous area | Zone classification, gas group, temperature class; SIL target if applicable | Protection type of electrical accessories cannot be chosen; ATEX/IECEx equipment conformity and SIL are separate topics and must be requested separately. |
Control purpose | Controlled variable, loop type, required response; any characteristic preference | Inherent characteristic and stroking speed are chosen arbitrarily. |
Kv and Cv: definition, conversion and a teaching example
Kv is the flow of water in m³/h through the valve at a pressure drop of 1 bar; Cv is the flow of water in US gallons per minute at a pressure drop of 1 psi. Both express the same physical quantity in different units. Derived from the unit definitions (the SI values of 1 US gallon and 1 psi), Kv ≈ 0.865 Cv and Cv ≈ 1.156 Kv. Always check which coefficient a quotation states; mixing them up misjudges capacity by about 16%.
The example below is a teaching scenario built to show why data is requested per case; it is not a real project. Assumptions: liquid service, turbulent and non-choked flow, valve at line size (no reducer effect), no viscosity correction, specific gravity ρ/ρ0 = 0.85. Under these conditions the simplified expression Kv = Q·√((ρ/ρ0)/ΔP) applies (Q in m³/h, ΔP in bar). Because of the pump curve, inlet pressure falls as flow rises, while outlet pressure stays at 6.0 bar(g).
Table 3 – Teaching example: required Kv by operating case (ρ/ρ0 = 0.85)
Case | Q (m³/h) | P1 (bar g) | P2 (bar g) | ΔP (bar) | Required Kv | Required Cv | Kv if ΔP were held at 2.5 bar |
Min | 12 | 10.0 | 6.0 | 4.0 | 5.53 | 6.4 | 7.00 |
Normal | 40 | 8.5 | 6.0 | 2.5 | 23.32 | 27.0 | 23.32 |
Max | 55 | 7.6 | 6.0 | 1.6 | 40.09 | 46.3 | 32.07 |
The result makes two points. First, with ΔP held constant, the Kv required at the maximum case would be calculated 20% low and the valve could end up undersized. Second, the flow ratio is 55/12 ≈ 4.58, while the required Kv ratio is 40.09/5.53 ≈ 7.25; the control range the valve must cover is wider than the flow range.
This simple expression does not replace an IEC 60534-2-1 calculation or sizing software. The standard method includes terms such as the piping geometry factor, the liquid pressure recovery factor, the choked flow limit, a Reynolds-number correction and, for gases, the expansion factor; the standard itself states that its liquid equations are not intended for non-Newtonian fluids or slurries. The final Kv/Cv is calculated with the selected valve's manufacturer data and the applicable standard.
The selected valve: opening at normal flow and controllability at minimum flow
Take a hypothetical valve with a rated Kv of 63 in the example. The ratio of required Kv to rated Kv is 8.8% at minimum, 37% at normal and 64% at maximum. These ratios are not travel percentages: the travel that corresponds to a given Kv ratio depends on the valve's inherent characteristic (linear, equal percentage, etc.) and on the installed characteristic that results from ΔP changing with flow. Travel values must be calculated with the candidate valve's characteristic data from the manufacturer.
· Capacity margin at maximum: The valve should deliver the required Kv at the maximum case without running into full travel; how much margin to leave is set by the project specification and control strategy.
· Operating region at normal: Where the valve spends most of its time, the characteristic should be stable and the gain reasonable. Use the project specification and the manufacturer's recommendation rather than a universal opening range.
· Controllability at minimum: The Kv required at the minimum case should stay above the minimum usable flow coefficient stated by the manufacturer. Rangeability, as manufacturers state it, is the ratio of the maximum flow coefficient to the minimum usable flow coefficient and varies with valve design.
· If the range does not fit: Options include reduced trim or port, a different characteristic, a different body type, or two valves in parallel or split range.
Limits of this approach and unsuitable conditions
· If choked flow, cavitation or flashing is possible at the minimum or maximum case, the simple Kv expression is not valid; valve type and trim must be reviewed for these phenomena separately.
· Standard equations cannot be applied directly to two-phase flow, slurries, solids-laden or non-Newtonian fluids; a dedicated evaluation with the manufacturer is required.
· High-viscosity liquids or very small flows may need a laminar/transitional flow correction.
· For gas and steam, expansion and the choked flow limit become decisive as ΔP/P1 grows; the liquid expression cannot be used.
· For multi-stage trims and high-ΔP service, sizing follows the manufacturer's stage calculation.
· If the valve also performs a safety function (e.g. emergency shutoff), the control and safety functions must be defined separately and the SIL requirement stated on its own.
How missing data distorts bid comparison
When data is missing, each supplier fills the gap with its own assumption: one assumes water density, another holds ΔP constant, a third ignores the reducers. The same tag then receives different body sizes, trims and actuators, and the bids cannot be compared technically even if their prices sit side by side. The most common consequences are:
· Only the normal case given: insufficient capacity at maximum, or unstable operation very close to the seat at minimum.
· No vapor pressure or critical pressure: cavitation and flashing risk stays invisible at bid stage and surfaces at commissioning.
· No shutoff ΔP or fail position: the actuator is undersized, or the valve goes to an unexpected position on air failure.
· No hazardous area or signal data: positioners and accessories must be replaced later.
Related product and service pages
Related pages
· control valves: General overview of control valve types.
· globe control valve: Sliding-stem design often considered for high-ΔP, precise control service.
· segmented ball valves: Rotary control applications that need a wide control range.
· high-performance butterfly control valve: High-capacity control service in large line sizes.
· measurement and control engineering services: Review of process data, instrument selection and commissioning support.
Frequently asked questions
The supplier only gives Cv. How do I convert it to Kv?
Use Kv ≈ 0.865 × Cv; conversely Cv ≈ 1.156 × Kv. When comparing bids, convert all values to the same coefficient first.
What if the minimum flow is not known?
Review start-up, turndown and likely future operating conditions to set a realistic minimum. Without a minimum case, you cannot judge whether the valve can control close to its seat.
Is stating the leakage class enough to define shutoff?
No. Give the pressure difference at shutoff and the flow direction along with the class; actuator thrust and seat design are selected from this information.
If I have already calculated Kv, can I put only Kv on the data sheet?
No. The supplier also needs the raw process data to recalculate for its own valve per IEC 60534-2-1 and to perform cavitation, noise and shutoff checks.