How do you size a control valve?

Control valve sizing determines the flow coefficient Cv (or Kv in metric) needed to pass the required flow at the available pressure drop. For a liquid, Cv equals the flow in gpm times the square root of the specific gravity divided by the pressure drop in psi. For gas and steam the equation adds an expansion factor Y and a pressure-drop ratio x. You then check the valve operates between about 20 and 80 percent open and is free of choking, cavitation and excessive velocity.

What is the difference between Cv and Kv?

Cv and Kv are both flow coefficients. Cv is defined as the flow of water in US gallons per minute at 60 degrees Fahrenheit that passes through the valve at a pressure drop of 1 psi. Kv is the flow of water in cubic metres per hour at a pressure drop of 1 bar. They convert with Kv equals 0.865 times Cv, and Cv equals 1.156 times Kv.

What is choked flow in a control valve?

Choked flow is the condition where increasing the pressure drop across the valve no longer increases the flow. For liquids it is linked to the onset of cavitation or flashing and begins when the pressure drop reaches FL squared times the inlet pressure minus the factored vapour pressure. For gases it occurs when the pressure-drop ratio x reaches the factored xT, where the expansion factor Y falls to its minimum of 0.667.

What is cavitation and how is it different from flashing?

Cavitation occurs when the local pressure in the valve falls below the fluid vapour pressure and forms vapour bubbles that then collapse as the pressure recovers downstream, causing noise, vibration and trim damage. Flashing occurs when the downstream pressure stays below the vapour pressure so the bubbles do not collapse and a two-phase mixture continues, causing erosion. The key difference is whether the downstream pressure is above the vapour pressure (cavitation) or below it (flashing).

What is the pressure recovery factor FL?

FL is the liquid pressure recovery factor. It describes how much pressure the valve recovers downstream of the vena contracta. A high FL near 0.9, typical of globe valves, means little recovery and a higher choked pressure drop. A low FL, typical of butterfly and ball valves, means strong recovery and earlier cavitation and choking. FL is a property of the valve style and trim and should be taken from manufacturer data.

What valve opening percentage should I size for?

A good design has the valve operating between roughly 20 and 80 percent open at the normal flow, so there is turndown to reduce flow and margin to increase it. A valve running near fully open cannot pass more flow and gives poor control, while a valve barely open at normal flow is oversized and controls poorly at low openings. This tool reports the opening percentage on the selected inherent characteristic.

Which valve characteristic should I choose, linear or equal percentage?

Equal percentage is preferred when most of the system pressure drop is in the pipe and equipment rather than the valve, which is common in process service, because it compensates for the changing valve authority and gives near-linear installed behaviour. Linear is suited to systems where the valve takes most of the pressure drop, such as level and some flow control. Quick opening suits on-off and relief-style duty.

What is the expansion factor Y in gas valve sizing?

The expansion factor Y accounts for the change in gas density as it expands through the valve. It equals 1 minus x divided by three times the product of the specific heat ratio factor and xT, where x is the pressure-drop ratio. Y ranges from 1 at very low pressure drop to a minimum of 0.667 at choked flow, and it reduces the effective capacity compared with an incompressible calculation.

What is valve authority?

Valve authority is the ratio of the pressure drop across the fully open valve to the total pressure drop across the valve plus the rest of the controlled circuit. An authority of about 0.25 to 0.5 gives good control. Low authority distorts the installed characteristic and makes control coarse near the closed position.

Does this calculator handle steam?

Yes. Steam is sized with the compressible gas equations using the steam density at the inlet condition and the expansion factor. The tool estimates density from pressure and temperature and flags when the steam is at or below saturation, in which case you should confirm the density from steam tables and account for wetness.

What is the Reynolds number factor FR in valve sizing?

FR is the Reynolds number factor from IEC 60534-2-1. It corrects the required flow coefficient when flow through the valve is not fully turbulent, which happens with viscous liquids or very low flow. FR equals 1 in turbulent flow, above a valve Reynolds number of about 10000, and falls below 1 in the transitional and laminar regions, increasing the required Cv.

What is the piping geometry factor Fp?

Fp accounts for reducers, expanders and other fittings attached to the valve. When a smaller valve is installed in a larger line, the fittings add resistance and reduce the effective capacity, so Fp is less than 1 and the required Cv increases. Fp equals 1 when the valve is the same size as the line.

How is control valve noise calculated?

Aerodynamic noise from gas and steam valves is estimated with the IEC 60534-8-3 method, which converts the mechanical stream power at the vena contracta into acoustic power using an efficiency factor that depends on the flow regime, then subtracts the transmission loss through the pipe wall to give an external sound pressure level in dBA. Liquid hydrodynamic noise uses the IEC 60534-8-4 approach, where noise rises sharply once cavitation begins.

Is this control valve sizing calculator accurate and free?

The calculator is free and works offline. The sizing follows the IEC 60534-2-1 equations, and the gas constant was verified against the standard industry universal-gas constant. Results are preliminary screening grade because the valve recovery factors FL and xT are typical representative values, vapour pressures are reference values, and noise is a screening indicator rather than an IEC 60534-8 sound-level prediction. Confirm the final valve selection with the manufacturer and your project specification.

AIEnginear · Control Valve Sizing

Name: Control Valve Sizing Calculator and Engineering Suite
Author: AIEnginear

IEC 60534-2-1 · liquid / gas / steam · Cv, Kv, cavitation, noise, choked flow