Flow Characteristics
Equal Percentage Characteristic
The equal-percentage inherent characteristic produces a constant relative flow change for each increment of travel: every 1 % of additional travel multiplies the flow by the same factor. On a semi-log plot it is a straight line; arithmetically, Q = Qmin·Rh, with R the rangeability.
Its loop gain is low at small openings and high at large openings. That shape is exactly the compensation needed when the process gain rises with flow and the valve's pressure drop falls with flow — the standard situation in pumped liquid circuits. There the installed characteristic of an equal-percentage valve approximates linear, giving the controller a near-constant gain to tune against.
Typical uses: pressure and temperature loops on pumped systems, steam and heat-transfer duties with wide rangeability needs, and any application where the valve operates near closed for much of its life. Compare with linear and quick-opening, and see the full comparison article here.