Marine Hydraulic Systems

In my last article, we have seen the importance of hydraulics and its applications on board a ship. Let us discuss on the basic components and functioning of these hydraulic circuits.

A Basic Hydraulic Circuit:

Any hydraulic circuit is a closed system where the hydraulic fluid under pressure, after performing the work, returns back to the same system. Thus it is one of the main advantages of using hydraulic circuits. To obtain any hydraulic fluid under pressure, we require a pump unit with necessary pipelines and valves. It is highly essential to have an accumulator on the pump’s discharge side. The piping is usually of very small diameter which takes the fluid from the pump to the area where work is to be performed. The pump must have an oil reservoir where it takes suction.

The pump used is usually a positive displacement type, especially a gear pump. There is also a standby pump, which can be used in emergency or when one pump fails. The pump takes suction from the reservoir, which is filled with the hydraulic oil. The reservoir has a level gauge, from which the oil level can be witnessed. For safety reasons, the reservoir tank has in-built float type alarm, which signals when oil level is low. The second float trips the pump, below which the pump looses suction. The oil can be filled up in the reservoir, either manually or automatically. If the tank is topped up automatically, it is essential to have high level and low level alarm systems functioning. Both the pump units are mounted on top of the reservoir tank itself. The reservoir has a partition in between serving for two main reasons.

1. To avoid oil return mixing directly with clean oil in the reservoir and creating turbulence.
2. To avoid excessive oil shift on to one side, when ship rolls or pitches. The equalizing holes in the partition, helps to maintain same oil level on both sides of the reservoir.

The pictures for the above explanation have been attached below.

The pump discharges into the valve unit through relief valve. The relief valve, relieve excess oil pressure back into the reservoir tank, thus maintaining safety of the system. There are some systems which have two relief valve arrangements, one draining into the reservoir and other an external relief valve. The hydraulic oil under pressure is pumped through very small pipelines, to various valve units either located inside the engine room or in near the cargo control room. These valve units are usually electro-hydraulic type, where it is operated by solenoid activation and returned back to original position by spring force. These direction control valves are numerous in varieties matching individual requirements of the system.

Let us discuss the working of 3-way 4-port, spring-loaded, solenoid operated direction control valve. The valve has the above name because:

3-way: the hydraulic fluid can make three different paths through the valve.

4-port: every valve unit has four ports, two of which are hydraulic oil inlet and two are hydraulic oil outlets.

Solenoid-operated and spring-loaded: Each unit of the valve is activated by the signal from the solenoid and when the solenoid de-energizes, the spring force which is acting against the solenoid force, brings back the control valve in its initial position.

After the pressurized oil passes through the direction control valve, it reaches the hydraulic cylinder or hydraulic motor, where it uses its pressure energy and does work. The pressure is lost in performing the required work of either lifting, rotating etc. Thus the oil losses pressure and drains back into the reservoir. Any leakage in the pipelines or any other hydraulic machinery, the return oil to the reservoir volume is reduced and thus results in decrease in the reservoir tank level.

In my next article, we will discuss on the hydraulic starting arrangements for emergency generator and life boat/rescue boat.

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