Marine gauging systems are essential marine equipment on tanker content to check the contents inside the tank. This article explains various gauging system used currently across different kinds of ship tankers.
Marine gauging systems are essential marine equipment on ships and off-shore industries to accurately measure the amount of tank contents. Its successful use in inventory management in the marine sector prevents product lose, optimize product movement and protection of the marine environment.
Every modern chemical tanker must have an accurate and reliable tank gauging system. The system must be capable of measuring the tank content at any level of filling and must be independent of the high level and overflow control system. The Code specifies three different types of gauging systems dependent upon the flammability and toxicity of the cargo; open gauging, restricted gauging and closed gauging. Many chemical cargoes may not be gauged by manual dipping because to do so requires an opening to the atmosphere during operation. The use of completely closed gauging systems is necessary so that no vapour is emitted. Examples of closed systems are float gauges or radar systems. Indirect measuring methods such as flow metering may also be used. Cargo ships can be gauged through openings in the ullage space. This is called open gauging.
Float Gauges for Tankers
Tankers have closed gauges. They consist of a float which rises vertically on the liquid. It is attached to tape to an indicating device for local reading, with provision for a drive mechanism for remote reading. Certain important points to check while checking the gauges,
- Floats should be secured when at sea while measuring the tank contents.
- Remote and local readings should be compared frequently to determine discrepancies;
- Readings may need to be corrected to allow for tape and tank expansion or contraction, and ship trim and heel.
- Tapes should be checked regularly for free vertical movement of the float, and if damaged, should be replaced. Particular care is necessary with the rewind mechanisms which are carefully balanced: if obstructed, the gauge readings will be inaccurate.
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Different kind of Gauging systems
- Electro-pneumatic system- In this system, the air is rushed into the tank that contains liquid and the pressure is measured for the liquid to force out of the pipe.
- The hydrostatic pressure system- The level pressure transmitters are performing tank gauging by hydrostatic pressure that is applied on the transmitter. The pressure is converted to an electrical signal which is acquired by a slight change in sensor diaphragm that is changed into electrical resistance. The transmitters are based on rugged and reliable capacitive ceramic sensors qualified for measuring small filling heights with accuracy.
- Radar Tank gauging system- On cargo tankers, radar transmitters are installed on deck level. The time taken for the radar wave to travel between two transmitters placed on cargo is fed to a computer which checks the content of the tank. The radar gauge is installed outside the tank, making maintenance and repair a simple task for marine engineers. The electronic box and temperature and vapour pressure sensors can easily be removed, tested or replaced by simply closing the ball valve.
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