Level monitoring and measuring systems are an important part of instrumentation and automation systems on board ships which can be used to determine how much liquid/ material is present within tanks, containers and equipment. Accurate level measuring is important to ensure the levels of fuel oil, lubricating oil, fresh water, ballast water, bilge water and cargo are monitored accurately.
Incorrect level measuring can cause issues such as overflow of tanks, damage to pumps, dry running and cargo loss. Different types of measuring systems are used depending on the type of liquid, tank layout and what level of accuracy is required. Methods of measuring liquid level include; float switches, capacitive switches, reed switches, air bubbler systems and radar level gauges.
A float switch is a simple method of detecting if a liquid has reached a certain level. They consist of a float element which moves up and down depending on the level of liquid.
Principle of Operation:
Applications on board ships:
Used on ships to;
A capacitive switch can detect if there is any liquid present by detecting any change in electrical capacitance. The switch usually consists of an electrode and electrical circuit to detect any change in dielectric properties of the liquid.
Principle of Operation:
Applications on board ships:
The sensor needs to be chosen and calibrated accordingly depending on the liquid being measured as different liquids have different dielectric properties.
A reed switch is an electrical switch which uses a magnetic field. The switch consists of two thin pieces of ferromagnetic metal contacts inside a small glass tube.
Principle of Operation:
As the float moves away the magnetic field becomes weaker and the contacts will go back to their original position.
Applications on board ships:
Used on ships to;
Advantages:
Fuel treatment is essential to protect the auxiliary engine and to ensure that combustion is efficient. Treatment is required because fuel may contain water, rust, sediments or other contaminants which may damage precision equipment.
The main goals of the fuel treatment process are to:
The settling tank and purifier have distinct but complementary roles. The settling tank allows some free water and more dense contamination to be removed while the centrifuge/purifier achieves better separation.
Fuel system problems can occur which cause the engine to start badly, run at inconsistent speed, lose power or fail altogether.
Common issues with the fuel oil system include:
These factors should be checked and addressed in routine maintenance of the system.
High/Low Overfill Alarms in Cargo Tanks are important safety devices which are used to prevent cargo overfilling beyond the allowed level during loading.
These are very important on oil tankers and other vessels which carry liquid cargo. If the tank is filled too much it can cause overfilling of cargo, pollution, damage to equipment and can be a serious hazard.
High Level Alarm
This can give an early warning when the cargo has reached a set level.
Once the liquid has reached the high level sensor, this will send an electrical signal to the cargo control room/alarm panel. The operator can then take action to reduce the flow of cargo.
Overfill Alarm
This is another safety alarm which is used to detect when the cargo has reached a higher level. This will activate once the liquid has reached a set level which is close to maximum allowed fill.
This will give another warning that the loading can be stopped.
Principle of Operation:
Importance:
If needed the high and overfill alarms should be independent and should be tested at set intervals and before any relevant cargo operations.
Air bubbler level gauging uses air pressure to measure liquid levels in tanks. This is often used in tanks where it would be inconvenient to fit moving equipment.
Principle of Operation:
A continuous supply of compressed air is passed through a pipe or dip tube going into the tank. The bottom of the tube is placed near the bottom of the tank.
As air is passed through, bubbles will form at the open end of the dip tube. The pressure will increase inside the tube until it equals the pressure from the liquid above.
This pressure is directly proportional to the height and density of the liquid.
The formula for this is;
P = ρgh
Where,
P = Pressure of liquid at dip tube opening
ρ = Density of liquid
g = Acceleration due to gravity
h = Height of liquid above the tube opening
The air pressure required to create continuous bubbling can be used to calculate the liquid level.
Applications on board ships:
Used on ships to;
Advantages:
Disadvantages:
Radar Level Gauging is a modern method of measuring liquid levels in tanks. This method is widely used on cargo tanks on board ships to give accurate readings without direct contact with the cargo.
Principle of Operation:
This works by using electromagnetic wave reflections.
The basic formula for this is;
D = (c x t)/2
Where,
D = Distance between radar antenna and liquid surface
c = Speed of electromagnetic waves
t = Time taken for signal to travel
The 2 is included because the signal needs to travel to the liquid and back.
The liquid level can be calculated by subtracting the distance from the reference height of the tank.
Types of Radar Level Gauging:
Applications on board ships:
Used on ships to;
Advantages:
Disadvantages:
Method | Working principle | Main application |
Float switch | Movement of a float with liquid level | High and low level alarms |
Capacitive switch | Change in electrical capacitance | Liquid presence and level detection |
Reed switch | Magnetic field operates electrical contacts | Tank level switching |
Air bubbler | Air pressure balances liquid pressure | Tank level measurement |
Radar gauging | Reflection of microwave signals | Continuous cargo tank measurement |
Level monitoring and measuring is an essential process to ensure safe tank and cargo operations. Float, capacitive and reed switches are all commonly used to detect liquid levels and give alarm signals. Air bubbler and radar systems can be used to measure liquid levels. Each system will have different principles of operation, advantages and disadvantages. Choosing the right system, regularly testing, and ensuring the equipment is properly maintained will help prevent tanks from overflowing, protect machinery and ensure safe operations on board ships.
To help the marine engineer and deck officers monitor the contents of tanks, prevent overfilling, protect pumps and ensure safe operations.
Level switches will give an ON/OFF signal when the liquid has reached a set level. Level transmitters will usually give a continuous signal.
They can give a warning during loading of cargo and help to prevent tanks from being filled beyond the allowed levels.
Measuring liquid levels by calculating the air pressure needed to create continuous bubbling from the dip tube.
It is able to continuously measure the level without direct contact (non contact).
Non contact radar uses microwave signals passing through the tank. Guided wave radar uses a probe/cable to pass the signal through the tank.
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