Gear pump / gear wheel pump / positive displacement pump
- This is a rotary displacement or a rotary positive displacement pump.
- There are two gears, one is driven by an electric motor through a chain or wheel drive. The other gear is known as the idle gear.
- Initially when there is no liquid in the pump casing, there is air inside it.
- As the gears start to move, the air gets trapped between each pair of two consecutive teeth and is dragged along the casing to the discharge side until there is no air left.
- Liquid from the tank will thus rise up into the suction side and is discharged in a similar manner.
- For this reason the pump is a self-priming pump.
- These pumps give low discharge rate but a high discharge pressure.
- The flow rate is controlled by controlling the speed of the prime mover and also by the help of a by-pass valve attached to the suction and discharge lines.
This pump is used as lub-oil pump, boiler fuel oil pump, fuel oil transfer pump, main engine driven lub-oil pump.
- This type of a pump contains two shaft, each shaft has two screws, one right handed and other left handed.
- The screws are driven in phase by timing gear, unlike the gear pump in which one gear drives the other. This ensures that correct clearance is maintained between the screws. This avoids overheating and a possible seizure.
- When the screws rotate, their close relation to each other causes small pockets in the helices, these pockets move axially and have the same effect as a piston moving constantly in one direction.
- This type of a pump has suction sides on the outer ends from where the liquid is drawn into the screws and pumped inwards to discharge into the pump outlet.
- Even if the liquid level in the tank is below the pump level, the pump first draws in air or gases and creates a vacuum in the line, the liquid therefore rises up and reaches the pump.
- A relief valve is fitted to avoid build up of excessive pressure.
- Is self priming, thus is very useful for high vapour pressure liquids. (chemicals/liquified gases),
- Since these pumps can discharge both liquid and gases, they are very useful for tank draining and where the liquid flow is not constant, which may occur in lub oil systems of main engines, with the vessel rolling and pitching,
- They are suitable for operation at high rpm, and thus can be driven by electric motor,
- They are good for high viscosity fluids, pumps are quiet, smooth running and reliable.
Double acting reciprocating pump
- Such type of pumps have a bucket type piston inside a cylindrical liner, the bucket is lined with piston rings.
- The bucket is connected to the gear box by a piston rod, crosshead and a conrod assembly. This gear box is driven by an electric motor.
- The liner is connected to independent sets of spring type discharge and suction valves.
- The downward movement of the bucket creates a vacuum on the top of the bucket. Due to this, the suction valve of the top set opens and draws liquid into the liner.
- The upward movement of the bucket creates a suction below the bucket, the suction valve in the bottom set now opens and draws liquid into the liner
- The upward movement of the bucket also causes the liquid on the top of the bucket to be forced into the top set, thus causing the discharge valve to lift. As the discharge valve lifts, the liquid flows into the discharge line.
- The next downward movement similarly cause the liquid below the bucket to be forced into the bottom set, thus lifting the discharge valve. The liquid flows into the dischare line.
- The same downward movement cause the similar effect as explained in point 4.
- As this pump causes liquid to be discharged from both top and bottom, it is known as a double acting reciprocating pump.
- An air vessel is provided in the dischare line to reduce the pressure fluctuations caused by up and down movement of the bucket.
- A relief valve is also provided to avoid excessive pressure build up.
- As they can handle large volumes of air/gases, they can be used as stripping pumps for oil, chemical or gas tankers,
- As they are self priming and can handle high suction lifts, they can be used as priming pumps, engine room bilge pumps and cargo hold bilge pumps.
- Construction is complicated due to presence of suction valve, discharge valve, air vessel and relief valve. All positive displacement pumps need a relief valve to prevent excess built up of pressure if delivery lines are closed.
- This type of a pump contains a rotating impeller with a stationary casing.
- The impeller consists of curved vanes,
- There is an opening, eye at the centre, from where the liquid is drawn into the pump.
- The impeller is connected to an electric motor by means of a shaft.
- The impeller rotates at a very high speed.
- Fluid is thrown outward by the blades, due to centrifugal force.
- The high velocity fluid is collected in a specially shaped volute casing, where some of the inetic energy is converted into pressure energy.
- The fluid under pressure now leaves the impeller producing a drop in pressure behind it an the eye of the impeller,
- This causes the liquid from the suction line to flow into the pump, this fluid is also subsequently thrown out, the process continues.
- This is possible only when there is fluid in the pump initially,
- If there is no liquid initially, there will be air. As the air is very light density, for a centrifugal pump to throw out air, it will have to rotate at a very very high speed (turbo charger blower).
- For the above reason, a centrifugal pump is not a self priming pump. It has to initially primed with fluid.
Priming of centrifugal pump
- Use an air pump to initially drive out air from the suction branch and thus make the liquid rise into the eye of the impeller. Example emergency fire pump, bilge and ballast pump
- Priming can also be done by opening the sea chest valve, so that water which is at a higher level, flows inside the eye. Once liquid flows into the eye of the rotating impeller, suction can be changed over from the sea side to engine room bilges etc and pumping can now commence.
Difference between positive displacement pump and centrifugal pump
|Positive displacement||Centrifugal pump|
|1||Uses gears, screw for movement of air or fluids.||Uses an impeller moved by means of an eletric motor.|
|2||Are self priming pumps as they can move air also.||Are not self priming pumps, needs to be externally primed.|
|3||Can have very high discharge pressure.||Has comparitively low discharge pressures.|
|4||Have low dishcrage rates.||Can have very high discharge rates.|
|5||Should never be started with the dischage valve shut.||Can be started with the discharge valve shut.|
|6||Needs a relief valve to be fitted on the discharge side.||A relief valve need not be fitted.|
|7||These pumps can be used for effective stripping.||Cannot be used for stripping as they loose suction once the liquid level falls too low.|