The challenge: build a fire-fighting ship to put out a blaze on an island in the middle of a lake.

Positive displacement (piston) pump

Pros

• Great efficiency
• Easily manoeuvred craft due to jets from hoses

Cons

• Engines must be accurately married
• Tight tolerances of engines means pump's life is limited

Centrifugal pump

Pros

• Simpler to get working
• Ready-made vessel and a means to power the pump

Cons

• Inefficient – for every gallon pumped, one-tenth goes back.
• Heavy – flotation and steering are critical

What are the chief requirements when putting out a fire?
A mechanically powered pump with high pressure rather than a high flow rate.

What is a pump?
A machine used to transport, raise or compress water, using tubes or other machinery. Water pumps have a large range of pressures – from a fraction of a pound to more than 10,000lb per square inch (psi). Originally used to lift water from wells and later from mines, by the 20th century there was a proliferation of pump types for all kinds of uses, from water supply and sewage pumping to pumping oil through pipelines and fire-fighting.

What is the best type of pump to use?
That depends. The two main types are positive displacement pumps and centrifugal (retro-dynamic) pumps. Both have advantages and disadvantages when it comes to being mounted on a craft and then being used to put out a fire.

What is a positive displacement pump and how does it work?
Piston pumps are the commonest examples of these and work by using suction created by a vacuum to draw water into a closed space. Operated mechanically or by hand, a handle is attached to a piston inside a pipe. Lifting the piston creates a partial vacuum in the pipe and water is drawn up, through the pipe and into a chamber in the pipe. A one-way valve closes after water is pumped into the chamber; this keeps the water from flowing back down. More and more pumps of the piston pull more water into the chamber, which then produces a jet of water out of the other end. To raise the pressure, the power or torque must be increased.
 
The Archimedes screw, invented over 2,000 years ago, is an early version of a positive displacement pump. When a corkscrew-shaped mechanism within a pipe is turned, it pulls water upward. Prior to the invention of engines, this type of pump was powered by humans and animals.
 
Other examples of positive displacement pumps are diaphragm pumps, peristaltic pumps and rag-and-chain pumps.

Positive displacement pumps are very efficient and deliver virtually all of the water that is pumped through them. But their engines need to be accurately married and the tight tolerances of the engine mean that the pump has a limited life.

What is a centrifugal pump and how does it work?
This uses 'rotating impellers' – motor-driven propellers – that create a flow of water as they turn. The blades are immersed in the water to be pumped. As the impeller rotates at up to 1,500 revolutions per minute (rpm), water enters the pump near the axis of the blades and is swept out toward their ends at high pressure. Water is continuously pumped without the use of valves.

Axial flow pumps are another type of centrifugal pump. In these, the flow goes in the same direction as the shaft. They have high flow rates and low pressures.

Radial flow pumps are another sub-group. They have curved veins welded to a flat, solid disc, and on top of this is placed another flat disc with a hole in the middle. Water coming through this hole hits the spinning veins, which forces the water perpendicular to the direction of the shaft. Radial flow pumps have low flows and high pressures.

Useful pump facts

• The higher the water has to be lifted, the greater the pressure that must be dealt with: for instance, 10m (33ft) of water generates 1 bar – 15 pounds per square inch (psi) – of pressure.
• A human being can produce up to 150 watts of power. Therefore three people could realistically power either a positive displacement or centrifugal pump.
• Equivalent lift (head) is 40% of the throw distance. So spraying water 10m is the equivalent of a 4m (13ft) head.
• To throw the water 20m (66ft), a pressure of 2 bars (30psi) is needed.

Find out more from our Scrapheap Challenge advisers about a possible solution for a centrifugal pump and a positive displacement pump.

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