Control Valve Flow Characteristic

Trim design will affect how the valve capacity changes as the valve moves through its complete travel. Because of the variation in trim design, many valves are not linear in nature. THE RELATIONSHIP BETWEEN VALVE CAPACITY AND VALVE TRAVEL IS KNOWN AS THE FLOW CHARACTERISTIC OF THE VALVE. Valve trims are specially designed, or characterized, in order to meet the large variety of control application needs. This is necessary because most control loops have some inherent nonlinearities, which you can compensate for when selecting control valve trim.

The percent of full flow through the valve is plotted against valve stem position. The curves shown are typical of those available from valve manufacturers. These curves are based on CONSTANT PRESSURE DROP across the valve and are called INHERENT FLOW CHARACTERISTICS.

The quick-opening characteristic provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control. So it is limited to on-off service, such as sequential operation in either batch or semi-continuous processes.

The majority of control applications are valves with linear, equal-percentage, or modified-flow characteristics.

Linear - flow capacity increases linearly with valve travel. Equal percentage - flow capacity increases exponentially with valve trim travel; equal increments of valve travel produce equal percentage changes in the existing Cv. A modified parabolic characteristic is approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.

When valves are installed with a pump, pipes, fittings, and other process equipment, the pressure drop across the valve will vary as the plug moves through its travel. When the actual flow in a system is plotted against valve opening, the curve is called the INSTALLED FLOW CHARACTERISTIC.                                                                  Inherrent Flow Characteristics For Common Valve Trim Designs