How to Choose and Maintain Marine Pumps

A ship’s pumping system includes piping that, if stretched out, would be many miles long, which includes both suction piping and discharge piping, as well as an assortment of marine pumps. Additionally, these systems often have backup marine pumps that interconnect with the piping, fittings, and valves. Ship pumping systems provide positive pressure – referred to as having “higher head” in pumping vernacular – that aids in the discharge of the fluid being pumped.

Marine pumps provide the energy to move this fluid at higher heads to overcome systemic pressure losses, usually resulting from friction within the piping. The assorted marine pumps on a ship perform a variety of vital functions that include lubricating equipment, heating, cooling, and cleaning, while others ensure the ship’s critical machinery is properly lubricated. Understanding how marine pumps fit into a ship’s pumping system is helpful knowledge for anyone tasked with sourcing a pump for a ship or other marine-related purposes.

Marine Pumps for Ships

Marine pumps are also commonly called navy pumps or ship pumps. For all ocean-going vessels, having the right marine pumps ensures their safe and efficient operation while minimizing hazards to the crew. They are used for essential applications on ships and sea-based oil and gas drilling platforms to manage water, waste, fuel, and ballast fluids. Marine pumps transfer these liquids between various tanks or compartments, a key factor in ensuring a ship remains stable and safe.

The type of sea-going vessels – whether for container ships, fishing vessels, navy ships, oil or gas tankers, passenger ships, research vessels, tugboats, or yachts – will determine the kind of needed marine pumps. Commonly used marine pumps on ships include vane pumps, screw pumps, reciprocating pumps, gear pumps, and centrifugal pumps. All these assorted marine pumps serve very distinct purposes on a vessel. For example, tugboats often use positive displacement marine pumps for firefighting, while they might utilize centrifugal marine pumps for ballast operations.   

Types of Marine Pumps

Selecting marine pumps depends on the application and the fluid to be circulated throughout a ship’s pumping system. Compressibility, density, surface tension, viscosity, and other qualities of the liquid being pumped. Other factors like the temperature throughout the system, pressure of the fluid, head to which fluid needs to be pumped, and flowrate also need to be contemplated. Marine pumps used onboard large vessels fall into two broad categories and are classified as positive placement pumps or dynamic pressure pumps.

Positive Displacement Marine Pumps

Positive displacement pumps self-prime, so they’re typically used for priming applications. Consisting of at least one chamber, though often comprised of multiple chambers depending on how they’re built, they alternate between empty and full. Usually used for applications with smaller or mid level discharge rates, positive displacement pumps often feature in pumping highly viscous liquids. They’re frequently used to facilitate higher pressures within the pumping system.

Types of positive displacement pumps used for marine applications include:

• Gear pumps: These marine pumps use gears that mesh with each other to transport fluid via displacement. Used commonly for hydraulic fluid, gear pumps can transfer highly viscous liquids.
• Piston pumps: A type of reciprocating pump, these marine pumps are reliable, provide decent suction capacity, and can even produce higher discharge pressure. However, they’re larger and more complex than other types of marine pumps. Used on modern tankers for pumping liquid cargo, piston pumps are also used for fuel, oil, and to circulate other fluids.
• Ram-type pumps: Like the gear pumps used commonly for hydraulic fluid, ram-type pumping systems use both electric motors as well as hydraulics to transport hydraulic fluid.
• Plunger Pumps: A type of reciprocating pump used to filter seawater through reverse osmosis, it’s similar to piston-type marine pumps, though it utilizes electricity while most piston pumps are powered manually.  
• Screw pumps: Using at least one screw, though sometimes several screws, these marine pumps can move fluids and solids along the axis of the screw or screws, which moves material down the screw’s spindle. Marine pumps with single screws tend to be used to transport such materials as bilge, contaminated seawater, sewage, and sludge. Twin screw pumps are sometimes used to replace gear pumps for the transfer of marine fuels, while those with three screws primarily are used primarily as an oil pump to lubricate the main engine.  
• Vane pump: Featuring an internal cylinder rotating around the motor’s shaft, these marine pumps house a sequence of vanes housed within various slots on the cylinder. Used primarily for transferring fuel, these vanes slide into and out of these openings to force pressurized fluid through the pump. As a positive displacement pump, it maintains pressure in spite of fluctuations in flowrate.

Dynamic Pressure Marine Pumps

Also known as roto-dynamic pumps, these marine pumps use lateral forces in their pumping action, which accelerates fluids typically via a rotating impeller. Certain systems with a dynamic pressure pump also use a positive displacement pump for priming. Normally used for applications that can handle a high or moderate rate of discharge, these pumps have a differential range from middling to low and are used to transport less viscous fluids. This category of marine pumps differentiates themselves from one another mainly due to their construction and their unique properties.

Types of dynamic pressure pumps used for marine applications include:

• Axial flow pumps: These marine pumps use a screw propeller to increase pressure, which causes liquid to accelerate axially within the pump’s blades. This rotational movement causes the fluid being pumped to move through the outlet guide vanes. They are used to circulate seawater, as well as for trimming and heeling applications.
• Centrifugal pumps: Perhaps one of the most versatile types of marine pumps, centrifugal pumps are used as ballast pumps, boiler feed pumps, condensate pumps, firefighting pumps, and many other marine applications. Utilizing an impeller, centrifugal pumps move fluid by transferring rotational energy derived from the motion of the impellers, with the rotating movement providing centrifugal force to move the liquid through their vane tips.
• Centrifugal-axial pumps: Also known as mixed flow pumps, these marine pumps are similar to axial flow pumps used in marine propulsion systems. Fluid moving through the impeller of such pumps is pushed by the impeller blades away from the pump’s shaft, suctioning it through at a greater than 90-degree angle.
• Submersible pumps: Often used to transport sewage from the ship, these marine pumps feature large channels so they won’t clog. As they remain submerged, these pumps feature a motor within a waterproof housing that drives an impeller to spin, which creates a low-pressure zone that draws the fluid through rapidly. Submersible pumps are also used as fire, emergency, bilge, and other such applications.

Selecting the Best Marine Pumps

There are a number of factors that should be considered when shopping for marine pumps. As marine conditions are often harsh, mechanical failures occur more than in many less severe environments. This is why many ships often have redundancies built into their pumping systems. Redundant systems also allow backup marine pumps to operate as the crew performs maintenance and repairs. Additionally, purchasing more reliable marine pumps at higher prices usually tends to be more cost-effective, as it means less maintenance and resulting downtime.

Critical systems especially rely on reliable marine pumps. Bilge pumps, for instance, prevent flooding by removing water from the ship, and backup marine pumps help mitigate pump system failures. Marine pumps need to be operationally sound in emergencies like flooding or fire. Whether considering primary or backup marine pumps, it’s vital to consider a number of basic factors.

When selecting marine pumps, it’s important to consider: 

• Compatibility: How a pump is powered will determine whether it’s the right one for the job. For example, electric marine pumps require a dependable electricity supply, whereas hydraulic pumps work via hydraulic fluid under pressure. The right pump depends on the types of power sources available on the ship.
• Flowrates: This refers to how much liquid passes through a ship’s pumping system over a specific time period. Flowrates of marine pumps are a good indicator of where they’re best used.
• Pump types: Marine pumps differ according to the application for which they’re used, along with their capabilities and features. For instance, positive displacement pumps offer a constant flowrate, so tend to be used for fuel transfers. Applications with higher flowrates and middling pressures commonly use centrifugal pumps.
• Upkeep: Marine pumps are often needed when ships are far away from port, so greater reliability should be a factor. Additionally, the easier marine pumps are to maintain, the better, especially for seagoing vessels.

Selecting the most appropriate pump for the job should consider all these factors.

Tips for Maintaining Marine Pumps 

Proper installation and maintenance of marine pumps is essential. Ships need a reliable pumping system to perform optimally. Keeping them running is the crew's job, and the following basic tips will improve any marine pump’s longevity.

Three basic tips for maintaining marine pumps are: 

• Regular inspections and proper lubrication: Routinely inspecting marine pumps and ensuring they have the proper amount of lubrication on pump parts to reduce friction will help lengthen the pump’s lifespan. Inspections should look for signs of damage or premature wear, especially of critical components, to ensure no leaks develop.  
• Following maintenance schedules: Manufacturer recommendations for scheduled maintenance should be adhered to, with specified maintenance done at the required intervals. This prevents potential issues and prolongs the life of marine pumps.
• Cleaning and filter maintenance: Keeping debris from getting into the pumping system requires regular cleaning to ensure a marine pump’s efficient operation. This should include filters, which should be monitored and cleaned regularly, along with replacement when recommended by manufacturer guidelines. Smooth pump operation relies on preventing clogs and maintaining optimal flowrates.

Following standard protocol for the maintenance of marine pumps will ensure they have long lifespans, which in turn will result in smoother operation of both the pumping system and the ship.

To learn more about marine pumps and their applications, contact the pump experts at Hayes Pump.