Instrumentation

Commonly used terms for Control Valve Sizing

Control Valve

A Control Valve is a final control element, which regulates the rate of fluid flow as the position of the valve plug, or disc is changed by the force from the actuator.

Flow Co-efficient of a Valve

Flow Co-efficient of a Valve is the flow capacity of a valve, commonly expressed by the Cv factor or Kv factor.

  • The Cv of a valve is defined as the quantity of water, at 60 °F, in US gal/min, that will flow through the valve at a specified travel with a pressure drop of 1 psi.
  • The Kv of a valve is defined as the quantity of water in m3/h, at a temperature between 5 °C and 40 °C that will flow through the valve at a specified travel with a pressure drop of 1 bar.
  • Kv = 0.856 Cv.

Cavitation

Occurs in liquid service when the pressure in the valve body falls below the vapour pressure of the liquid. The bubbles that are formed will implode immediately or shortly after leaving the valve, due to the downstream pressure of the control valve recovering to rise above the liquid vapour pressure.

Choked Flow

Occurs for an incompressible or compressible fluids when the fluid velocity at the vena contracta reaches sonic velocity.

Flashing

Occurs, for liquids only, when the pressure in the valve body falls below the liquid vapour pressure and when the bubbles thus formed remain as vapour in the fluid, owing to the fact that the downstream pressure of the control valve is at or below the liquid vapour pressure.

Control Valve Authority

Control Valve Authority is the ratio between the pressure drop across the control valve at a certain relative travel to the pressure drop across the control valve in its fully closed position.

Inherent equal percentage characteristic

Inherent equal percentage characteristic of a valve is a characteristic whereby equal increments of relative travel yield equal percentage changes of the relative flow coefficient.

Inherent linear characteristic

Inherent linear characteristic of a valve is a characteristic whereby equal increments of relative travel yield equal increments of relative flow coefficient.

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