The response of flow rate through a control valve depends on the friction losses in the piping in which it is installed as well as the controller signal.  The installed characteristic (a graph of flow rate to controller signal) is usually nonlinear and different from the nominal (inherent) characteristic of the isolated valve.  Understanding the process piping influence on the valve is essential to choosing a right inherent characteristic for the valve to relatively linearize the flow rate response to the controller output, and/or gain scheduling the controller.