Marine Pumps: Types, Working Principles, Applications and Maintenance Onboard Ships
Introduction
Marine pumps are among the most essential pieces of machinery onboard a ship. From circulating cooling water and transferring fuel oil to supplying lubricating oil and removing bilge water, pumps support several critical shipboard operations.
For marine engineers, understanding how pumps work, where different types are used, and how to identify common faults is important for maintaining machinery efficiency and ensuring safe vessel operations.
What Is a Pump?
A pump is a mechanical device that transfers liquid from one place to another by converting mechanical energy into hydraulic energy. Depending on its design, it may increase the liquid's pressure, maintain circulation, or move a specific volume of liquid.
Marine pumps are broadly divided into two categories:
Dynamic pumps: Transfer energy to the liquid through a rotating element. Centrifugal pumps are the most common example.
Positive displacement pumps: Move liquid by trapping and displacing a volume of it. Examples include gear, screw, and reciprocating pumps.
The choice of pump depends on factors such as the liquid being handled, required flow rate, discharge pressure, and operating conditions.
1. Centrifugal Pumps
Centrifugal pumps are widely used onboard ships for transferring and circulating liquids. They work using a rotating component called an impeller, which gives energy to the liquid and pushes it towards the pump casing.
The casing converts the liquid’s velocity into pressure, allowing it to flow through the discharge pipe. These pumps are commonly used in cooling water circulation, ballast systems, and general service applications.
How Does a Centrifugal Pump Work?
The pump uses a rotating impeller to transfer energy to the liquid. Liquid enters through the suction inlet and moves outward as the impeller rotates. The casing then helps convert the liquid's velocity into pressure before it leaves through the discharge outlet.
The main components include:
Impeller: Transfers energy to the liquid.
Casing: Collects the liquid and helps convert velocity into pressure.
Shaft: Transmits rotational power to the impeller.
Bearings: Support the rotating shaft.
Mechanical seal or gland packing: Helps prevent leakage along the shaft.
Suction and discharge connections: Allow liquid to enter and leave the pump.
Types of Centrifugal Pumps
Based on liquid flow, centrifugal pumps can be classified as:
Radial-flow pumps: Liquid moves mainly perpendicular to the shaft.
Axial-flow pumps: Liquid moves mainly parallel to the shaft.
Mixed-flow pumps: Liquid movement combines radial and axial components.
Pumps may also be classified as single-stage or multistage, depending on the number of impellers.
Applications Onboard Ships
Centrifugal pumps are used in several shipboard systems, including:
Seawater and freshwater cooling circulation
General water transfer
Firefighting systems, where the pump is designed for that service
Ballast and bilge services, depending on pump design
Other services requiring continuous liquid flow
What Is Priming?
Priming is the process of filling the pump casing and, where required, the suction line with liquid before starting the pump. Many conventional centrifugal pumps cannot pump effectively when air remains in the casing.
Failure to prime a pump correctly can result in little or no discharge and may cause damage if the pump runs dry.
2. Positive Displacement Pumps
Unlike centrifugal pumps, positive displacement pumps move liquid by trapping a volume and forcing it towards the discharge side. They are commonly used for applications requiring controlled delivery of liquids, including certain oils and fuels.
An important safety consideration is that a positive displacement pump can develop dangerously high pressure if its discharge is blocked. Suitable pressure-relief protection is therefore essential.
A. Gear Pumps
Gear pumps use rotating gears to transfer liquid through the pump casing. Liquid is carried in the spaces between the gear teeth and casing from the suction side to the discharge side.
Key features and applications:
Commonly used for fuel oil and lubricating oil transfer.
Suitable for many viscous liquids, depending on the design.
Deliver a positive-displacement flow.
Require appropriate protection against excessive discharge pressure.
Wear, excessive internal clearances, and damaged gears can reduce pump performance.
B. Screw Pumps
Screw pumps use rotating screws or spindles to move liquid along the pump casing. Depending on the design, they may use one or multiple screws to provide a smooth, continuous flow.
They are commonly used onboard ships for fuel oil, lubricating oil, and other suitable liquid-transfer duties.
Important components may include the screws or spindles, casing, shaft, bearings, and sealing arrangements.
Maintenance checks include:
Inspecting screws for wear or damage.
Checking bearings and shaft alignment.
Examining seals for leakage.
Monitoring unusual noise, vibration, and temperature.
Ensuring operating conditions remain within the manufacturer's limits.
C. Reciprocating Pumps
Reciprocating pumps operate through the back-and-forth movement of a piston or plunger. During the suction stroke, liquid enters the cylinder; during the delivery stroke, the liquid is forced towards the discharge line.
Depending on the design, reciprocating pumps may be single-acting or double-acting.
Their main components generally include the cylinder, piston or plunger, suction and delivery valves, and connecting mechanism.
An air vessel may be fitted to reduce fluctuations in flow. As with other positive displacement pumps, suitable overpressure protection is important.
D. Other Positive Displacement Pumps
Other designs include:
Rotary vane pumps: Use vanes in a rotor to form chambers that carry liquid through the pump.
Diaphragm pumps: Use a flexible diaphragm to displace liquid; the diaphragm can separate the liquid from the driving mechanism.
Lobe pumps: Use rotating lobes to transfer liquid through the casing.
Their suitability depends on the liquid properties, required performance, construction materials, and operating conditions.
3. Centrifugal Pumps vs Positive Displacement Pumps
Although both types move liquid, their operating principles and applications differ.
Working principle: Centrifugal pumps transfer energy through an impeller, while positive displacement pumps trap and displace liquid.
Flow characteristics: Centrifugal pump flow changes with system resistance and operating conditions. Positive displacement pumps deliver a volume per cycle or revolution, depending on their design.
Typical service: Centrifugal pumps are widely used for water circulation, while gear and screw pumps are commonly used for suitable oil-transfer duties.
Pressure protection: Positive displacement pumps need suitable overpressure protection because a blocked discharge can cause pressure to rise rapidly.
Priming: Many centrifugal pumps require priming; priming behaviour in positive displacement pumps depends on their design and operating conditions.
Selecting the correct pump helps maintain reliable operation and reduces the risk of equipment damage.
4. Cavitation in Marine Pumps
Cavitation is a condition in which vapour bubbles form in a liquid when local pressure falls sufficiently low. As these bubbles enter higher-pressure regions, they collapse and may damage internal pump surfaces.
Cavitation can reduce pump performance and cause excessive noise, vibration, and erosion.
Common Causes
Insufficient suction pressure.
Blocked suction strainers or filters.
Excessive resistance in the suction line.
High liquid temperature.
Inadequate Net Positive Suction Head (NPSH).
How Can Cavitation Be Prevented?
Keep suction lines and strainers clean.
Maintain adequate suction pressure.
Avoid unnecessary restrictions in the suction piping.
Operate the pump within its recommended limits.
Investigate unusual noise, vibration, or reduced discharge pressure promptly.
NPSH is an important factor in assessing whether sufficient pressure is available at the pump suction to avoid excessive cavitation. The available NPSH should meet the pump manufacturer's requirements.
5. Starting and Operating a Marine Pump
Correct starting procedures help prevent operational problems and protect pump components. The exact procedure varies according to the pump type and shipboard system, so the manufacturer's instructions and vessel procedures should always be followed.
Before Starting
Confirm that the correct pump has been selected for the required duty.
Check suction and discharge valve positions according to the operating procedure.
Ensure the pump is primed where required.
Check lubrication, seals, and visible leakage.
Verify that relevant protective devices are available and correctly set.
Confirm that the equipment is safe to operate.
During Operation
Monitor the pump regularly for changes in performance.
Check suction and discharge pressure.
Observe flow and motor current, where applicable.
Monitor bearing and casing temperatures.
Listen for unusual noise.
Check for excessive vibration and leakage.
Report abnormal conditions and record them as required.
6. Common Pump Problems and Troubleshooting
Pump problems may result from operating conditions, component wear, incorrect valve positions, or inadequate maintenance.
Pump fails to deliver liquid: Check priming, suction blockage, valve positions, and possible air ingress.
Low discharge pressure: Investigate suction conditions, internal wear, leakage, and operating conditions.
Excessive noise or vibration: Check for cavitation, bearing problems, misalignment, and loose components.
Leakage: Inspect seals, gland packing, joints, and connections.
Overheating: Check lubrication, bearing condition, alignment, and operating load.
Reduced performance: Inspect strainers, impellers or displacement elements, and internal clearances as appropriate.
Troubleshooting should begin with safe, basic checks. Any inspection or repair requiring dismantling must follow the vessel's isolation and safety procedures.
7. Marine Pump Maintenance
Preventive maintenance helps improve reliability, reduce unexpected breakdowns, and extend equipment life. Maintenance schedules should follow the manufacturer's recommendations and the vessel's planned maintenance system.
Important maintenance activities include:
Inspecting the pump for leakage, corrosion, and visible damage.
Checking bearing condition and lubrication.
Monitoring vibration and operating temperatures.
Inspecting seals and gland packing.
Checking coupling alignment where applicable.
Cleaning strainers and checking suction piping.
Inspecting impellers, gears, screws, valves, and other internal parts during overhauls.
Recording defects, repairs, and performance changes.
Before dismantling a pump, isolate the equipment, depressurise the system, and follow the vessel's lockout and safety procedures. Never rely solely on a closed valve to ensure safe isolation.
8. Why Marine Engineers Must Understand Pumps
Pumps support many essential shipboard operations, and even a minor fault can affect the performance of a larger system.
A marine engineer should understand:
The working principles of different pump types.
How to select a pump for a particular duty.
The importance of priming and suction conditions.
The causes and prevention of cavitation.
The role of relief valves in positive displacement pumps.
Routine inspection, troubleshooting, and maintenance procedures.
The importance of following manufacturer guidance and shipboard safety procedures.
Frequently Asked Questions
1. Which type of pump is commonly used for water circulation onboard ships?
Centrifugal pumps are widely used for water circulation because they can provide continuous flow.
2. Why do many centrifugal pumps require priming?
Priming removes air from the pump casing and fills it with liquid, allowing the pump to operate correctly.
3. Why is a relief valve important in a positive displacement pump?
It protects the pump and system from excessive pressure, particularly if the discharge line becomes blocked.
4. What is cavitation?
Cavitation occurs when vapour bubbles form due to low local pressure and collapse in higher-pressure regions, potentially damaging pump components.
5. Why is regular pump maintenance important onboard ships?
It helps identify wear and faults early, maintain performance, and reduce the risk of unexpected equipment failure.
Conclusion
Marine pumps play a vital role in keeping shipboard systems operational. Understanding centrifugal and positive displacement pumps, their working principles, applications, and common faults helps marine engineers operate equipment safely and maintain its reliability.
Regular inspection, correct operating practices, and timely maintenance are essential for dependable pump performance throughout a vessel's service life.
Always follow the pump manufacturer's instructions and the vessel's approved procedures for operation, maintenance, and safety.
