Control Valve Flow Characteristic

Update time: 22-05-04 Views: 1106

The flow characteristic is one of the important technical parameters of the control valve, and the deviation of the flow characteristic directly affects the stability of the automatic control system.

Flow Characteristic:Relationship between flow through the valve and percent rated travel as the latter is varied from 0 to 100 percent. This term should always be designated as either inherent flow characteristic or installed flow characteristic.

Inherent Characteristic: The relationship between the flow coefficient and the closure member (disk) travel as it is moved from the closed position to rated travel with constant pressure drop across the valve. Typically these characteristics are plotted on a curve where the horizontal axis is labeled in percent travel and the vertical axis is labeled as percent flow (or Cv) (figure 1-2). Because valve flow is a function of both the valve travel and the pressure drop across the valve, conducting flow characteristic tests at a constant pressure drop provides a systematic way of comparing one valve characteristic design to another. Typical valve characteristics conducted in this manner are named Linear, Equal-Percentage, and Quick Opening (figure 1-2).

Installed Characteristic: The relationship between the flow rate and the closure member (disk) travel as it is moved from the closed position to rated travel as the pressure drop across the valve is influenced by the varying process conditions.

Ideal flow characteristic: The inherent flow characteristic is also called the ideal flow characteristic. The ideal flow characteristic is a characteristic that assumes that the pressure difference between inlet and outlet sides does not change. The ideal flow characteristics include equal percentage, linear and quick opening.

Equal Percentage Characteristic: An inherent flow characteristic that, for equal increments of rated travel, will ideally give equal percentage changes of the flow coefficient (Cv) (figure 1-2).

Linear Characteristic: An inherent flow characteristic that can be represented by a straight line on a rectangular plot of flow coefficient (Cv) versus rated travel. Therefore equal increments of travel provide equal increments of flow coefficient, Cv (figure1-2).

Quick Opening Characteristic: An inherent flow characteristic in which a maximum flow coefficient is achieved with minimal closure member travel (figure 1-2).