Reciprocating pumps are positive displacement pumps in which a piston or plunger reciprocates inside a cylinder to move liquid. When the motion is in one direction, suction of liquid takes place in the cylinder and as it moves in the other direction, the liquid is expelled out of the cylinder through the delivery valve.
They are more used for relatively small capacities at high pressures. On ships reciprocating pumps are still used where reliable high pressure delivery, self priming capability and accurate displacement are required.
Reciprocating pumps are used on ships where small quantities of liquid need to be delivered at high pressure. Their main applications include:
Their use depends on the ship’s design and the requirements of the particular system.
Reciprocating pumps can be found in several shipboard systems. Depending on the ship’s design, they may be used as:
Marine references also identify reciprocating pumps as stripping and bilge pump applications on ships.
Because they are positive displacement pumps, a relief valve is important to protect the pump if the discharge line becomes closed or restricted.
The pump can broadly be divided into the liquid end and the power end. The liquid end handles the pumped fluid, while the power end supplies the mechanical movement required for reciprocation.
Important components include:
Depending on the design, the power end may include:
The crank mechanism converts rotary motion from the driver into the reciprocating motion required by the piston.
from the suction side. The pump may then operate irregularly, lose discharge pressure or show abnormal running behaviour. Common causes include suction line restrictions, insufficient suction head, air entering the suction system, defective valves and loss of prime.
Typical symptoms include:
If a reciprocating pump loses suction, the following should be checked systematically:
Air entering the suction system or a partial loss of prime can significantly affect pump performance.
Problem | Possible Cause | Corrective Action |
Low discharge pressure | Worn valves or piston rings | Inspect and renew as required |
No discharge | Blocked suction line or defective valves | Clear the line and inspect valves |
Pump loses suction | Air leakage or insufficient suction head | Check suction piping and liquid level |
Excessive vibration | Pulsation, loose foundation or mechanical wear | Check supports, foundation and pulsation arrangement |
Pump becomes noisy | Cavitation, air in liquid or damaged valves | Check suction conditions and valves |
High oil temperature | Low oil level or bearing problems | Check lubrication system |
Excessive packing leakage | Worn or incorrectly adjusted packing | Adjust or replace packing |
Low capacity | Worn piston rings, valves or restricted suction | Inspect liquid end components |
Fluctuating discharge pressure | Valve problems or inadequate pulsation damping | Inspect valves and air vessel/damper |
Troubleshooting references identify suction restrictions, air leakage, worn valves, insufficient NPSH, damaged piston rings and relief valve problems among common causes of poor reciprocating pump performance.
Overhauling is carried out when inspection indicates excessive wear, leakage, reduced capacity, abnormal noise or other mechanical problems. The exact procedure depends on the pump manufacturer’s manual.
Before starting an overhaul:
During a complete overhaul, the pump’s parts are normally inspected systematically.
The following components should be checked:
Inspection standards specifically call for checking cylinder liners for wear, ovality and scratches and pistons/plungers for wear, ovality and scoring.
The power end should also be examined for:
After reassembly, correct clearances, valve seating, packing adjustment, lubrication and alignment should be verified according to the manufacturer’s specifications.
Reciprocating pumps play a key role in positive displacement machines on board ships for applications calling for reliable pumping at comparatively high pressures and with relatively low flow rates. Proper functioning requires that suction systems, valves, piston or plunger, packing and lubrication system be in good condition. Routine inspection, systematic troubleshooting and proper overhauling maintains pump performance and prevents unexpected failures.
Because it displaces a definite volume of liquid during each operating cycle. The amount delivered is primarily related to displacement and operating speed.
Common reasons include a blocked suction line, air leakage, insufficient suction head, low liquid level, loss of prime and defective suction valves.
A positive displacement pump must be protected against excessive pressure if the discharge path becomes closed or restricted. A relief valve provides this protection.
The cylinder or liner, piston/plunger, piston rings, suction and discharge valves, valve seats, packing, piston rod, bearings and other power end components are inspected.
Possible causes include suction restrictions, worn valves, damaged piston rings, air entering the system, insufficient suction pressure and internal leakage.
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