'We knew the existing system quite well, but we had no possibility to be onboard before the docking to check what was working and could be used, what was not working, and also how to install the measuring cabinets and interface with the automation system onboard,' says Peter E. Köhler, Scanjet Macron managing director in Finland.
'Together with the chief engineer we found easy-to-access locations for the new cabinets. Supports for the cabinets had to be made and welded and new piping for supply air to the cabinets had to be pulled. We also reduced the number of original small measuring cabinets using our larger cabinets thus saving space and costs,' he adds.
'We used the existing cabling and upgraded the communication to modern digital Modbus. In order to get the tank levels to the automation system, we developed completely new communication units acting like the ship’s digital automation system sending queries to the measuring cabinets for tank levels,' expresses Köhler. The tank levels were then converted into analogue output signals using our converter cards and connected to the automation system.
International rules demand that all tanks influencing a vessel’s stability must be sounded for tank level. For practical reasons, remote sounding was developed many years ago on passenger ships because of the large number of tanks and compartments. Once a pressure transmitter inside a tank is damaged, this transmitter cannot be replaced unless the tank is empty and accessible.
For fuel tanks, side penetration is not accepted for passenger ships due to fire risks. So also for fuel tanks the pressure transmitter must be located inside the tank. These transmitters are often damaged by pressure shocks when filling the tanks, or by high temperature inside the tanks.
Electro-pneumatic systems do not have the problems pressure transmitters have, recounts Köhler. The sensors are located inside the measuring cabinets, and are easily accessible for checking the performance of the sensors and the system. Using the ship’s instrument air, the electro-pneumatic sensors measure the fluid pressure inside the tanks. This pressure is converted into tank levels considering the density of fluid inside the tank.
All measuring lines are automatically cleaned by the air pressure, making the system self-cleaning.
'Scanjet Macron’s tank level gauging system has been developed based on the extensive experience we have from various tank level gauging systems, and several hundreds of ships on which we have installed electro-pneumatic and other tank level gauging system,' says Köhler.
In the event of a sensor problem, performance can be checked locally in the cabinet. A damaged sensor can be replaced by the crew in less than five minutes without any calibration needed. All other tanks are measuring the tank levels when the damaged sensor is replaced so no need to shut down the system, remarks the Scanjet Macron chief.