Control Valve Characteristic Curves: Linear, Equal Percentage, and Quick Opening

The flow characteristic of a control valve describes the relationship between valve stem position (expressed as percent of total travel) and the flow through the valve (expressed as percent of maximum flow) under constant pressure drop conditions. This inherent characteristic is a property of the valve's internal geometry, specifically the shape of the trim and the flow path. Selecting the correct flow characteristic for a control valve application is essential for achieving good process control performance across the full operating range. The three standard characteristics are linear, equal percentage, and quick opening.

Linear Flow Characteristic

A linear flow characteristic means that equal increments of stem travel produce equal increments of flow coefficient (Cv) change. If a linear valve moves from 50 percent open to 60 percent open (10 percent travel increment), its Cv increases by exactly the same amount as when it moves from 20 to 30 percent open. The inherent gain (change in flow per change in travel) is constant across the full valve range. Linear valves are appropriate for applications where the system pressure drop across the valve is approximately constant across the operating range, which is the case in pressure control loops and some flow control loops with low-pressure-drop piping systems. Linear trim is also preferred for liquid level control in constant head systems.

• Linear: equal flow change per equal travel increment, constant inherent gain

• Equal percentage: equal percentage flow change per equal travel increment, low gain at low flow

• Quick opening: maximum flow change near closed position, used for on-off or mixing

• Modified parabolic: compromise between linear and equal percentage characteristics

• Selection rule: match characteristic to system to achieve installed linear gain across operating range

Equal Percentage Characteristic

An equal percentage characteristic means that equal increments of stem travel produce equal percentage changes in Cv. If an equal percentage valve moves from 50 to 60 percent open and flow increases by 10 percent, then moving it from 60 to 70 percent open will also increase flow by approximately 10 percent of the new value (not the same absolute amount). This means the valve has very low gain (small flow change per unit travel) near the closed position and high gain near full open. The equal percentage characteristic compensates for the natural tendency of installed valves in real systems to have falling gain with increasing opening due to decreasing pressure drop across the valve as flow increases. Equal percentage trim is the most common selection for flow control in process industries because it produces a more linear installed characteristic in typical applications.

Quick Opening Characteristic and Installed Gain

Quick opening trim provides large flow increases for small amounts of stem travel near the closed position, with progressively smaller flow increments as the valve approaches full open. This characteristic is seldom used for modulating control because the high gain near closed makes precise flow control difficult in the lower travel range. Quick opening valves are used where it is important to establish maximum flow quickly, such as safety valve bypass applications, dump valve service, or on-off mixing applications where the valve need only be fully open or fully closed. The installed characteristic, which is the actual flow-vs-position relationship in a real piping system as opposed to the inherent characteristic measured under constant pressure drop, differs from the inherent characteristic and must be considered when selecting valve characteristics for control performance.