Velocity Mode Valve Positioning Temperature Control

Many industrial processes are heated by means of gas or oil, which is burned in a combustion chamber somewhere within the system. There are two techniques used for dispensing the fuel. Valves are switched in a high fire / low fire method, or, motor actuated valve positioners rotate a variable flow valve to dispense the desired amount of fuel. In the case of a variable flow valve, there is typically a mechanical linkage between the valve and the motor actuator.

Traditionally, for those processes using motor actuated valves, this is done by actually controlling the instantaneous position of the valve in a closed loop fashion. Valve position is measured by means of a feedback slidewire and this signal is used as the process variable in a position control loop. The temperature controller sends a position setpoint signal to the position controller to achieve the desired results. Some temperature controllers have physical triac or relay outputs to switch voltage directly to the valve motor whereas others simply provide an analog signal such as 4-20mA to a valve positioner driver circuit.