3.3 Valve Characteristic

The relationship between the valve stem position and the flow is defined as the valve characteristic.  This relationship with constant (design value) pressure drop is termed the inherent characteristic, and the relationship in a specific process in which the pressure drop may vary with flow is termed the installed characteristic.  Two related units are used for the characteristic; one is flow in gallons of water per minute per stem percent that is used for sizing control valves.  The other is percent maximum flow per stem percent which is used to plot typical valve characteristics, for example, Marlin’s Figure 16.6 (2000).

  The flow through a restriction can often be represented by

relationship for flow through a restiction

(8)

with, in this expression, the characteristic expressed as a percentage of the maximum flow, and F_max is the maximum flow rate with a pressure drop of 1 psi.

A wide range of functional relationships for the C_v can be implemented through the detailed design of the shapes of the plug and seat.  Some typical characteristics are shown in Figure 8.  The valve characteristic relationship is usually selected to provide a nearly linear relationship between the stem position, which should be the controller output sent to the valve, and the controlled variable.  If this goal is achieved, constant controller tuning will be appropriate for the entire range of controller output (and flow rate).  To achieve this goal for a process with a constant process gain , a linear characteristic is appropriate, and when the process gain changes with
flow rate, the valve characteristic should be the inverse of the process non-linearity.

Figure 8.  Some typical inherent valve characteristics.

Also, note that linearizing the control loop is not always the most important goal.  For example, a valve that must increase the flow from zero rapidly to protect equipment should have a quick opening characteristic, even if this contributes to a nonlinear feedback loop.