Most ship processes require objects or substances to be moved from one location to another or a force to be applied to hold, shape or compass a product. Some examples are movement of the ship through water, opening and shutting a water tight door, shutting of quick closing valve, discharge of ship's cargo, auto tensioning of ship moorings, heaving up of anchor, etc.
Such activities are performed by prime movers. Though most prime movers are diesel or electrical devices, however other means of providing prime movers are also there. Enclosed fluids can also be used to convey energy from one location to another and, consequently, to produce rotary or linear motion or apply a force. Fluid based systems using liquids as transmission media are called Hydraulic Systems.
Basic Equipment and Circuits
Although systems are designed to suit particular application, basic circuits and associated equipments can be identified as shown in the figure.
In the figure motor speed is controlled by adjusting valve 'A' to control "spill". This circuit could be of a windlass mechanism or of a deck crane mechanism. In case of circuit of windlass, the motor shaft will be doing the duty of heaving up the load of the anchor.
Five main categories of equipment to achieve this power transmission in a hydraulic system are:
- The hydraulic pump to convert mechanical into hydraulic energy.
- Valves to allow this hydraulic energy to be controlled.
- Hydraulic cylinders to convert the hydraulic energy into linear force and motion or
- Hydraulic motors to convert the hydraulic energy into continuous rotary motion
- Ancillary equipment including filters, heat exchangers, tanks, pipes, etc. to complete practical circuits.
Symbolic representation of the hydraulic circuit is shown below.
Advantages and Disadvantages of Hydraulic Systems
- A convenient method of transferring power over relatively long distances from, say, a central pump room to remote operating sites in the ship; where necessary, complete local control of operations can be achieved.
- Fully variable speed of both linear and rotary motion, with good "inching" capability and smooth take up of load; in all cases power is continuously transmitted whilst speed changes take place.
- High static forces or torques can be achieved and maintained indefinitely. Example is of lifting and keeping lifted large weights of cargo.
- Complete safety and reliability is assured under the most difficult environmental conditions; overload conditions are safeguarded by using a relief valve to limit maximum output torques or forces.
- Significant cost savings as alternative solutions for many requirements.
- It is a messy and therefore the areas around he system may not be always ship shape.
- Some high pressure hydraulic systems have caused serious accidents because of high pressure.
- Leak in hydraulic system can be the cause of major fires and or explosions.
"Basic Marine Engineering" by J.K. Dhar