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It is a usual practice to measure the cargo level aboard ship from time to time. The values obtained must be very accurate as inaccurate values may lead to environmental hazards or pollution. If the cargo overflows it will definitely cause pollution, so continuous monitoring of the cargo level is important.
There are different methods of gauging a cargo tank. Some of the devices used for the measurement of the cargo level on different tankers are:
- Float gauges
- Radar gauges
- Ultrasonic gauges
- UTI indicators
- Slip tubes
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1. Float gauges
Float gauges have been used widely on all gas carriers. Their construction is very simple, consisting of a float attached by a tape to an indicating device. The indicating device can be arranged so that the reading can be read out remotely or locally as desired. These are also fitted with isolation valves so that the float alone can be taken out and be serviced if required. A typical float level gauge used in a gas tanker is shown below:
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2. Radar gauges
This is another type of gauging equipment that operates on the principle of radar. This type of gauge is used on all modern tankers now-a-days for their accuracy and reliability. Radar gauges operate at very high frequencies (11 gigahertz). In case of gas carriers, the siting of the transmitter on the tank dome is very important for the most accurate operation.
Advantages of Radar gauges
- Since the antenna is the only moving part inside the tank, it is highly accurate and highly reliable.
- Radar waves are most suitable because they are not affected by the atmosphere above the cargo in the tank.
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3. Ultrasonic gauges
The working principle of the ultrasonic gauge is very similar to that of the echo sounders. Usually they will measure either the ullage (unfilled space) or they will measure the depth of the liquid by reflecting sound waves from the liquid surface. The receiver may be located either on the tank dome (in gas carriers) or at the bottom of the tank. This type of gauge is not reliable, to be specific, because there may be problems in obtaining satisfactory readings when loading by spray lines. Another disadvantage is that the reading may vary depending upon changes in temperature and pressure.
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4. UTI indicators
These are used widely on crude oil tankers. This type of gauge, as the name implies, measures the ullage, temperature, and interface. These are used during the final phases of loading and during the voyage. Sensors are provided at the bottom to indicate the water interface since most of the crude will contain water.
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5. Slip tubes
As per the Gas Codes, slip tubes are classified under restricted type of gauging device. These are used widely on gas tankers. They are grouped under restricted type of gauging since there is a possibility of some amount of cargo vapor or liquid being released to the atmosphere during the measurement of the level. Care should be taken while using these types of gauges so that it doesn’t cause any harm to the environment. Some terminals forbid the use of these types of gauges for some types of cargoes.
They have an orifice (with a diameter restricted to 1.5 mm, unless an excess overflow valve is fitted) at the upper end which allows the liquid or vapor to be released to the atmosphere. The reading can be observed from the markings on the tube when the liquid level has reached them. Initially the vapor will be vented and then the liquid level rises.