Diesel level system in Transnet tugboats

When leading shipbuilding company SA Shipyards manufactured

tugboats for Transnet, they needed a technical solution for diesel tank monitoring.

Instrotech – a Keller distributor for Southern Africa – specified an EX-rated submersible transmitter called the Keller 36 XWei. This is an intrinsically safe pressure transmitter for the conversion of pressure into an electrical signal, for use in hazardous environments.

Pressure transmitters are sensors with additional electronics that compensate for deviations of the pressure transducer and output the measurement results as standardised signals.

Featuring a microcontroller-based electronic evaluation to ensure maximum accuracy, the Keller 36 XWei is approved for use in high explosive gas and dust atmospheres – groups I (mining industry) and II (industrial applications) where there is a high risk of explosion.

The Keller 36 XWei has a floating piezoresistive sensor element that is free of outside influence from mechanical and thermal forces at the pressure connection. The A/D converter in the signal processor operates with a resolution of 16 bits (0.002% FS), using the signals from the pressure sensor and the integrated temperature sensor to calculate accurate compensated measurement values in just a few milliseconds.

With an atmospheric pressure vent in the cable, the Keller 36 XWei was placed in the centre of the tank for the uniformity of diesel head pressure and mounted 50 mm above the bottom of the tank to prevent any build-up.

Instrotech safely installed and monitored the diesel pressure head with a 4 mA to 20 mA output. The 5 m high port and starboard tanks were not uniform in shape. Therefore, Instrotech used sounding charts provided by SA Shipyards and the specific gravity of diesel to calculate the desired pressure at 16 critical points. They were also able to determine the volumetric capacity of diesel versus pressure head of diesel in the tanks.

Instrotech’s locally made programmable digital indicators are customised with a 16-point lineariser option, allowing the operator to key in the actual versus desired values, and thus displaying the quantity of diesel, in litres, within the tanks.

Keller’s 36 XWei EX-rated submersible pressure transmitter

Close-up of the volumetric indicators for the port and starboard fuel tanks

The 33X series of pressure transmitters have digital compensation with a mathematical model to achieve exceptional accuracy of 0.05% FS – in the temperature range 10˚C to 40˚C. Additional measurements and selections can optionally be used to achieve a precision (and also, if calibrated in an accredited test laboratory, an accuracy) of 0.01% FS. In addition to the digital RS485 interface, the X-line transmitters have a rangeable, analogue current or voltage output.