Pump types
1) Displacement
The displacement pumping action is achieved by the reduction or increase in the volume of a space causing the liquid (or gas) to be physically moved. The method employed is either a piston in a cylinder using a reciprocating motion, or a rotating unit using vanes, gears or screws. The pump is double-acting, that is liquid is admitted to either side of the piston where it is alternately drawn in and discharged.
As the piston moves upwards, suction takes place below the piston and liquid is drawn in, the valve arrangement ensuring that the discharge valve cannot open on the suction stroke. Above the piston, the liquid is discharged and the suction valve remains closed. As the piston travels down, the operations of suction and discharge occur now on opposite sides.
An air vessel is usually fitted in the discharge pipework to dampen out the pressure variations during discharge. As the discharge pressure rises the air is compressed in the vessel, and as the pressure falls the air expands. The peak pressure energy is thus 'stored' in the air and returned to the system when the pressure falls. Air vessels are not fitted on reciprocating boiler feed pumps since they may introduce air into the de-aerated feedwater.
A relief valve is always fitted between the pump suction and discharge chambers to protect the pump should it be operated with a valve closed in the discharge line. Reciprocating displacement pumps are self-priming, will accept high suction lifts, produce the discharge pressure required by the system and can handle large amounts of vapour or entrained gases.
They are, however, complicated in construction with several moving parts requiring attention and maintenance. When starting the pump the suction and discharge valves must be opened. No valves in the discharge line must be closed, otherwise either the relief valve will lift or damage may occur to the pump when it is started. The pump is self-priming, but where possible to reduce wear or the risk of seizure it should be flooded with liquid before starting.
An electrically driven pump needs only to be switched on when it will run erratically for a short period until liquid is drawn into the pump. A steam-driven pump will require the usual draining and warming-through procedure before steam is gradually admitted. Most of the moving parts in the pump will require examination during overhaul. The pump piston, rings and cylinder liner must also be thoroughly checked. Ridges will eventually develop at the limits of the piston ring travel and these must be removed.
The suction and discharge valves must be refaced or ground in as required. Two different rotary displacement pumps. The action in each case results in the trapping of a quantity of liquid (or air) in a volume or space which becomes smaller at the discharge or outlet side. It should be noted that the liquid does not pass between the screw or gear teeth as they mesh but travels between the casing and the teeth.
The starting procedure is similar to that for the reciprocating displacement pump. Again a relief valve will be fitted between the suction and discharge chambers. The particular maintenance problem with this type of pump is the shaft sealing where the gland and packing arrangement must be appropriate for the material pumped.
Read also ➤Pump Definition
The rotating vane type will suffer wear at a rate depending upon the liquid pumped and its freedom from abrasive or corrosive substances. The screw pump must be correctly timed and if stripped for inspection care should be taken to assemble the screws correctly.
2) Axial-flow pump
An axial-flow pump uses a screw propeller to axially accelerate the liquid. The outlet passages and guide vanes are arranged to convert the velocity increase of the liquid into pressure. The pump casing is split either horizontally or vertically to provide access to the propeller. A mechanical seal prevents leakage where the shaft leaves the casing.
A thrust bearing of the tilting pad type is fitted on the drive shaft. The prime mover may be an electric motor or a steam turbine. The axial flow pump is used where large quantities of water at a low head are required, for example in a condenser circulating. The efficiency is equivalent to a low lift centrifugal pump and the higher speeds possible enable a smaller driving motor to be used.
The axial-flow pump is also suitable for supplementary use in a condenser scoop circulating system since the pump will offer little resistance to flow when idling. With scoop circulation the normal movement of the ship will draw in water; the pump would be in use only when the ship was moving slowly or stopped.
3) Centrifugal pump
In a centrifugal pump, liquid enters the centre or eye of the impeller and flows radially out between the vanes, its velocity being increased by the impeller rotation. A diffuser or volute is then used to convert most of the kinetic energy in the liquid into pressure. A vertical, single stage, single entry.
The main frame and casing, together with a motor support bracket, house the pumping element assembly. The pumping element is made up of a top cover, a pump shaft, an impeller, a bearing bush and a sealing arrangement around the shaft. The sealing arrangement may be a packed gland or a mechanical seal and the bearing lubrication system will vary according to the type of seal.
Replaceable wear rings are fitted to the impeller and the casing. The motor support bracket has two large apertures to provide access to the pumping element, and a coupling spacer is fitted between the motor and pump shaft to enable the removal of the pumping element without disturbing the motor.
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