Task

The fluid level in several supply tanks is to be constantly maintained within a certain range. The tanks are fed from a feed tank that is continuously kept at a constant level.

Solution

The supply tanks each feature two level switches, one for minimum filling level and one for maximum filling level. The undershoot of the minimum filling level is signaled to the master control system via the AirLINE electrical/pneumatic automation system. The diaphragm valve for filling the tank then opens. The valve is closed again when the maximum filling level is reached (upper level switch). The filling level in the feed tank is maintained on a constant level by means of a local control loop. A continuously measuring ultrasonic level transmitter detects the filling level in the feed tank. The closed-loop filling level control function is performed by a diaphragm control valve with attached positioner. The positioner incorporates a process controller to whose actual value input the signal output of the level transmitter is connected. The set-point value of the control loop is preset via a 4 . 20 mA signal which is made available by the electrical/pneumatic automation system.