In a split-range control loop, the controller output is split and sent to two or more control valves. The splitter defines how each valve is sequenced as the controller output changes from 0 to 100%. In most split-range applications, the controller adjusts the opening of one of the valves when its output is in the range of 0 to 50% and the other valve when its output is in the range of 50% to 100%.
PIC-01 controls the pressure of the separator for liquid-vapour hydrocarbons, by means of a split-range controller with the output signal divided and sent to two pressure control valves PV-A and PV-B. When the pressure increases, the fluid must be discharged to burn. When the pressure decreases, combustible gas is introduced to compensate for the pressure of the separator.
The fuel gas valve (PV-B) must close in response to the increased pressure in the separator, while the flared valve (PV-A) should open when the pressure rises beyond the set point.
- When the pressure increases beyond the set point in the 0-50% range of the controller output, PV-B will close from fully open to fully closed.
- When the pressure increases beyond the set point in the 50-100% range of the controller output, PV-A will open from completely close to fully open.
In this case, the service of both control valves is different, with respect to the use of fuel gas and burning for pressure control. Another use case of the split range control loop is when a control valve cannot be suitably designed to cover the full operating range of the controller. In that case, the valve with a smaller Cv operated between a range of 0-50% and the other operated for a range of 50-100%.
The ease with which a split-range controller can be replaced by two ordinary controllers depends on several factors. If the “division” is calculated in the DCS or the PLC so that each valve has its own output from the control system, then the addition of a new controller is simply a matter of software configuration.