CASE STUDIES
How Sea-Water (Raw-Water) Cooling Pumps Work
What is a raw-water cooling pump?
A raw-water pump, often called a sea-water pump, draws in untreated outside water, from the sea, a river or a cooling supply, and circulates it to carry heat away from an engine or a piece of machinery. "Raw water" means the untreated water taken from outside, as opposed to the treated coolant that runs in a sealed internal circuit. The raw water absorbs heat through a heat exchanger and is then discharged. This keeps the equipment at a safe working temperature.
Raw water and the closed circuit
Many engines and machines use two water circuits working together:
- The closed coolant circuit, which is sealed and circulates treated coolant around the engine itself.
- The raw-water circuit, which pumps outside water through a heat exchanger to cool that coolant, then sends the raw water overboard or to drain.
| Circuit | What it holds | Its job |
|---|---|---|
| Closed coolant circuit | Treated coolant, sealed in | Circulates around the engine to pick up heat |
| Raw-water circuit | Untreated outside water | Pumps through a heat exchanger to carry that heat away, then discharges the water |
Why the pump has to prime itself
A centrifugal pump moves water with a spinning impeller. The impeller throws water outwards to the outlet, and that creates the suction which pulls more water in behind it.
This only works when the pump is already full of water. A plain centrifugal pump is not self-priming. Start one with air in the suction line and the impeller simply spins the air around, with no suction and no flow.
On an engine the pump usually sits above the water it has to draw, so the suction line holds air every time the engine has been stopped. A raw-water pump therefore uses a self-priming design:
- On start-up it primes. Water held in the pump body is set spinning and forms a moving ring inside the casing. That ring carries the air out of the suction line, a little on each turn, until water has been drawn up and the pump is full.
- Once full it pumps. The pump settles into normal centrifugal running for as long as the engine runs.
Because the pump is driven from the engine, it also has to prime at low engine speed, not only at full speed.
AVM supplies Gilkes engine cooling pumps, a British maker of self-priming centrifugal pumps used on marine vessels and gensets. Their impellers are bronze, which stands up to sea water far better than a plain steel part. Pumps are specified against the engine model and the engine builder's part number rather than chosen freely.
Looking after a raw-water pump
Raw water carries salt, silt and small debris, which is why a raw-water pump is maintained differently from the sealed side of the cooling system.
- Never run the pump dry. There is nothing to cool or lubricate the parts inside it. Check that the sea cock is open and the strainer is clear before starting.
- Keep the inlet strainer clean. A blocked strainer starves the pump and is the most common cause of poor cooling.
- Watch for leaks at the shaft seal. A drip that gets steadily worse means the seal is passing.
- Act on rising engine temperature. Losing raw-water flow shows up as a hot engine before it shows up at the pump.
Where raw-water pumps are used
Raw-water pumps are used wherever outside water is the most practical way to remove heat: marine engines and gensets, where sea water is abundant, along with shore-based and industrial plant drawing cooling water from a nearby supply. In a marine hub like Singapore, raw-water cooling is common across vessels and waterfront installations.
To see the sea-water pumps AVM supplies, visit the sea-water pump category.
Frequently asked questions
What does a raw-water pump do?
It draws in untreated outside water and circulates it through a heat exchanger to carry heat away from an engine and keep it at a safe temperature.
What is the difference between raw water and coolant?
Raw water is untreated water taken from outside, such as sea or river water. Coolant is the treated fluid in a sealed internal circuit. The raw water cools the coolant through a heat exchanger.
Are centrifugal pumps self-priming?
Not by default. An ordinary centrifugal pump has to be full of water before it will pump. A raw-water pump on an engine uses a self-priming design instead.
Why must a raw-water pump never run dry?
With no water passing through it, there is nothing to cool or lubricate the parts inside the pump, and damage follows quickly.
Why is the impeller made of bronze?
Because it runs in sea water. Bronze resists the corrosion that would attack a plain steel part.
How is a replacement raw-water pump specified?
By the engine it is fitted to. The engine model and the engine builder's part number identify the right pump.