The displacing pumping action is achieved by the reduction or increase in 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, screws or screws.
A reciprocating positive displacement pump is shown diagrammatically in the figure above to demonstrate the operating principle. The pump is double acting, that is liquid is admitted to either side of the piston where it is alternatively 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 during suction stroke. Above the piston, 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.
Why an Air Vessel is Fitted?
As indicated in the figure, positive displacement pumps are usually fitted with an air vessel. An air vessel usually fitted in the discharge pipe work 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 pressure falls. Air vessels are not fitted on the reciprocating boiler feed pumps since they may introduce air into the de-aerated water.
When starting the pump, the suction and discharge valves must be opened. It is important that no valves in the discharge line are closed, otherwise either the relief valve will lift or damage may occur to the pump when it is started. Positive displacement pumps are 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 only need 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 the steam is gradually admitted.
Use of Relief Valve
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.
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.
“Introduction to Marine Engineering” by T.A. Taylor