Pumps on ships — the technology
Arguably pumps on ships represent some of the oldest technology to be found. Bilge pumps on ships were an essential item on old wooden sailing vessels because all wooden ships leak to some degree and seafarers have needed to remove the accumulated water on a regular basis.
Early bilge pumps on ships were of the suction type operated by levers and later by systems that included a metal flywheel to speed operation. Pumps on ships can be of many types including suction or vacuum pumps but also screw, gear, lobe and centrifugal.
In essence all pumps are similar in that there is an input and output and a means of motive power. Pumps that are part of machinery with a rotating element such as the main engine may be driven from it by gears but otherwise the motor can be an internal combustion engine, a hydraulic or electric motor.
Pumps on ships are rarely developed solely for use on board ships but are variants of pumps used in industrial processes and shore-based power production. Even the pumps used on oil and chemical tankers have applications in other industries
A look at the technology of pumps on ships
One of the simplest types of pumps but found in a variety of situations on board vessels including ballast, bilge, fire, general service, cooling and deepwell cargo pumps among other applications. Strictly speaking, centrifugal pumps add energy to an already moving fluid. They are therefore not self-priming so either require a feed to the inlet or as is common rely on a gravity feed with the inlet immersed.
The pump typically comprises a motor, a drive shaft connected to the impeller, a volute or housing, seals and bearings. Fluid is drawn into the suction eye of the rotating impeller when the pump is operating and forced by centrifugal pressure to the outlet via the volute. As the fluid leaves the impeller under pressure, a pressure drop is produced at the eye of the impeller creating suction and drawing the fluid into the pump.
Sealing is provided by a mechanical seal or by packed gland. For the former cooling water is supplied from the discharge side of the pump. For the latter cooling is provided by the allowance of slight leakage, lubrication is by a grease filled manual lubricator. Depending upon the length of the drive shaft, one or more bearings will be fitted. The efficiency of the pump is affected by both the shape of the housing and impeller designs.
Although the working of centrifugal pumps is basic, there is great variety in their design depending upon application and pumps for different purposes can look very different despite the operating principle being the same. The choice of materials for impeller, housing , shaft and bearings will depend upon the intended use of the pump.
Most other types of pump found on ships fall into the positive displacement category. These are generally self-priming and require a safety valve to limit maximum pressure and cannot be started against a closed discharge valve. Although they are self-priming, most makers recommend that where possible to reduce wear or the risk of seizure pumps should be flooded with liquid before starting.
It is considered that this family of pumps on ships are more suited to low to medium flow rates and can handle higher viscosity fluids than centrifugal pumps. Positive displacement pumps fall into two types – reciprocating or rotary. Reciprocating pumps are also referred to as piston pumps while the rotating category includes types such a gear, screw and lobe pumps.
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.
Types of pumps on ships
A piston pump can be single or double acting meaning it operates either on one or both directions of the piston travel in the cylinder. In a single acting pump there are an inlet and outlet valves at one end of the cylinder. With the outlet valve closed and the inlet valve open the travel of the piston away from the valves creates suction which draws the liquid into the cylinder. At the end of the piston travel the inlet valve closes and the discharge valve opens so that as the piston returns the liquid in the cylinder is forced into the discharge line. Piston pumps are frequently used for bilge pumping and tank stripping.
In a double acting piston pump, there are inlet and outlet valves at each end of the cylinder operating in opposite action. So that liquid is constantly drawn into the cylinder and discharged regardless of the direction of travel of the piston. A radial piston pump will have several pistons contained in a circular housing and actuated by a central eccentric drive cam. In an axial pump the cylinders are operated by a swash plate the angle of which can be varied to control flow.
In a gear pump the inlet and outlet sides are separated by a pair of meshing toothed wheels or gears housed within the pump casing, one of the gears is attached to a shaft driven by the pump motor and the other is driven by this drive gear. The wheels are a close fit within the casing.
When the pump is started, any air or gas is trapped between each pair of consecutive teeth and dragged along the casing from suction to discharge side till no more air is left on the suction side. Liquid is then drawn into suction line under atmospheric pressure, subsequently this liquid is trapped and moved around the casing into the discharge side and pumping of liquid will commence.
Gear pumps are motor driven through a chain or wheel drive. Control of flow rate is achieved by a bypass valve or controlling the speed of the motor. Gear pumps are commonly used for fuel and lube oil transfer. In order to meet high pressure requirements it is quite common for one or more booster pumps to be installed along the transfer line.
Lobe pumps are a variation of gear pumps in which the meshing toothed wheels are replaced by wheels with a small number of lobes – usually two three or four. The lobes interlock as they rotate moving fluid around the casing similar to gear pumps.
Screw pumps are one of the oldest means of moving fluids and liquids using the principle developed by Archimedes. In an Achimedean screw, a helicoid rotating inside a cylinder transports the fluid along the cylinder. Modern screw pumps employ the same basic concept but tend to use two or three screws instead of a single screw. They are mainly used on board ship to pump high pressure viscous fluids such as fuels and lubes and hydraulic fluids.
Because the fluid is moved axially along the pump, there is much less turbulence than in a centrifugal pump. This is desirable as it reduces foaming in the fluid.
Searching for savings
Because so many units are used on board ships, a pump manufacturer’s success in the shipping sector is closely tied to the rise and fall in vessel demand and also to the latest regulatory requirements in terms of machinery in which pumps are major components. Other important issues are energy, cost and space and the size of a piece of equipment’s footprint.
Newbuilding activity is currently well below the heights of the first decade and a half of the 21st century but is on a par with the historical average. Major issues in pump development in recent years have been driven by efficiency requirements as much as anything else although issues with low-sulphur fuel oils have also been a factor. Because all pumps except those driven directly by the main engine have a power demand, the cost of bunkers and the EEDI requirements have seen pump makers looking at ways to reduce running costs.
The combined power consumption of all the pumps and compressors onboard ships is a significant cost to owners and a promising area for savings. It is reckoned that the power consumption of pumps and fans could be reduced by as much as 60% compared to older products and cutting pump speed by 10% will lower power consumption by 27%.
Using frequency-controlled drives, adopting magnetic drive and the use of new lighter-weight and more resilient materials, while enhancing performance, are hardly the most headline-grabbing achievements but these are the areas where pump makers are competing in the marine market.
Few pumps last the lifetime of a ship and maintenance of pumps is a major cost element in terms of spare parts and time needed so replacement of older pumps can be a benefit both in terms of reliability and in many cases energy consumption. In a conventional pump leakage is a recurring problem as the pump ages and wears and seals are degraded. Leakage also means loss of pressure which can be a secondary problem depending upon the system the pump is part of.
**Magnetic drive pumps** are a comparative new development in shipping but over the last ten years or so, most makers have begun including them in their ranges. A magnetic or mag drive pump separates the motor from the pump unit as no connecting shaft is needed. Instead the pump components are housed in canister shell which contains a driven magnet component.
The torque of the pump motor is transferred without contact through the magnetic coupling to the pump screws. This is achieved by means of several magnets located on two rotors, one connected to the motor and one to the pump shaft, which rotate in sync when in operation. The pump shaft is hermetically sealed along with the inner rotor. The pump screws are driven contact-free through the magnets on the outer and inner rotor. Because the magnetic coupling is non-contact, there is no grinding action and no wear on the coupling. The magnetic coupling completely replaces the mechanical face seal and introduces a safety margin for start-up, which reduces the risk of damage from running dry.
The arrangement does not permit any leakage to enter the motor unit thus reducing pump failures. Also because there are no seals around the shaft, mag drive pumps are highly suited to aggressive fluids or those containing abrasives such as cat fines which are found in much of the HFO bunkers supplied today. On the downside, liquids containing iron particles or rust scale can be a problem as the particles accumulate around the magnets affecting pump performance. This type of pump is also more expensive so there is a payback period to take into account although some makers claim that along with operational benefits, mag drive pumps can deliver a massive energy saving – perhaps as much as 50%.
Regardless of pump type, materials research has shown improvement in performance and reliability while designs have reduced the number of moving parts and consequently reducing wear, particularly in applications where the abrasive action of products causes excessive wear. Ceramics and tungsten or silicon carbide have lowered the amount of wear in some pumps and other new materials, such as titanium alloys and advanced duplex or super-duplex steel are also finding their way into newer models.
Older ships have pumps that are mostly analogue but more recently computer-driven and electronic aspects of pumps and compressors have brought improvements in performance and condition monitoring. Supervisory control and data acquisition (SCADA) systems can provide distributed control of the pump or compressor’s processes and a variable-speed drive, through which flow may be adjusted, for example to save energy, has become an integral part of many systems. Since some pumps operate almost constantly when the ship is sailing, any improvements in running costs can be significant.