Control Valves calculators

Cv / Kv sizing and control valve calculations.

Control Valve Cv Sizing — Liquid

Required Cv from flow, ΔP and specific gravity.

Cv ↔ Kv Converter

Valve flow coefficients: US Cv, metric Kv and SI Av.

I/P Converter Scaling (4-20 mA ↔ 3-15 psi)

Current to pneumatic signal — mA, psi, kPa and bar in one place.

Control Valve Cv Sizing — Gas & Steam (IEC 60534)

Compressible sizing with expansion factor and choked-flow check.

Control Valve Characteristic & Installed Gain Calculator

Inherent vs installed curves from valve authority — linear, equal % or quick opening.

Pressure Relief Valve Sizing Calculator (API 520)

Required orifice area for gas, steam or liquid relief.

Control Valve Seat Leakage Class Calculator (FCI 70-2 / ANSI I-VI)

Allowable leakage rate for Class II through VI.

About control valves calculations

The control valve is where the control system finally touches the process, and sizing it correctly matters more than almost any other single decision in loop performance. The flow coefficient — Cv in US units, Kv in metric — expresses how much flow a valve passes at a reference pressure drop, and every sizing exercise reduces to computing the coefficient the process needs and selecting a valve that covers it with sensible travel margin.

Liquid and compressible service need genuinely different equations. Liquids are limited by cavitation and flashing when pressure inside the valve approaches vapour pressure. Gases and steam expand as they pass through, requiring an expansion factor, and choke at a limiting pressure ratio beyond which more pressure drop produces no more flow at all.

The most common and most expensive sizing error is oversizing. A valve doing all its work in the first 20% of travel hunts, wears its seat and controls poorly — and the extra capacity that felt like a safety margin becomes a maintenance problem.

How these tools fit together

  1. Establish the coincident conditions. Identify the flow and pressure drop that occur together. Maximum flow usually coincides with minimum available pressure drop, not maximum.
  2. Choose the right equation. Liquid service uses the basic square-root form; gas and steam need the expansion factor and a choked-flow check.
  3. Select with travel margin. Aim for maximum flow around 70-80% of travel and normal flow around 50-70%, while confirming minimum flow still needs more than about 10%.
  4. Complete the signal chain. The controller output reaches the actuator through an I/P converter, mapping 4-20 mA onto the 3-15 psi pneumatic standard.

Frequently asked questions

What is the difference between Cv and Kv?

They are the same property in different unit systems. Cv is US gallons per minute of water at 1 psi drop; Kv is cubic metres per hour at 1 bar drop. Cv equals 1.156 times Kv.

How much pressure drop should a control valve take?

Where the design does not fix it, a common allocation is 25 to 33 percent of the total dynamic system loss at maximum flow — enough authority to control without wasting excessive pumping energy.

All calculators are provided for reference and education. Verify independently before use in safety-critical work — see our disclaimer.