Challenging tank level retrofit completed

The work was part of a comprehensive refit covering more than 780 items, including renovation and modernisation of two cabin decks, the buffet restaurant, sauna, and pool area on the 8,378gt vessel. For the tank gauging system, new instrumentation cabinets had to be connected to the old piping and tested for tightness. The tests showed that the piping was in relatively good condition and could be used for the Scanjet system. Some pipes were clogged, but the purging feature proved successful, even on some of the tanks which had not been in operation for several years.

International rules demand that all tanks influencing a vessel’s stability must be sounded for tank level. Because of the large numbers of tanks and compartments on passenger vessels, remote sounding systems have been developed, which can be either of pressure transmitter type, with transmitters inside the tanks, or an electro-pneumatic system where the sensors are in cabinets located in a safe area. If a pressure transmitter is damaged, this unit can only be replaced when the tank is empty and accessible, and this can lead to absence of soundings or false readings, putting the tank out of commission. Transmitters, particularly in fuel tanks, can often be damaged by pressure shocks when filling the tanks, or by high temperatures.

According to Scanjet Macron, which is part of the Swedish Scanjet group, electro-pneumatic systems do not have the same problems. The sensors are located inside the measuring cabinets, and are easily accessible for checking the performance of the sensors and the system. The sensors are connected to the tanks by small-diameter copper or stainless steel pipes. Using the ship’s instrument air, electro-pneumatic sensors measure the fluid pressure inside the tanks, and applying a correction for fluid density, the tank levels can be determined.

The measuring lines are automatically cleaned by the air pressure, making the system self-cleaning with negligible risk of flooding or clogging. The system can also be employed to detect water ingress in large compartments and voids and for establishing accurate flooding or filling rates.

The 184-passenger Hanseatic is classed as the “world’s only five-star expedition ship”, according to Berlitz Cruise Guide 2011.

“We knew the existing system [on Hanseatic] 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. “Time available for the design and installation of the new system was very short. 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.”

“The old system communication was very special – the signal was digital with own communication protocol and converted into analogue for interfacing with the automation system,” says Köhler. “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. These tank levels were then converted into analogue output signals using our converter cards and connected to the automation system. After some tuning our system worked perfectly. The tank levels and the draft measuring are now very accurate.”

“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,” adds Köhler. “Our sensors have no drift and 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 – no need to shut down the system. “Because all cabinets are on same communication line, we can call up any cabinet and tank from any cabinet, and check the performance of the sensors. There is no need to run from one cabinet to the other to check the performance. The system is also self-controlling. A sensor watchdog in every cabinet checks the sensor performance. In addition, alarms are shown for sensor failure, low supply air pressure, loss of power, and communication failure.”