Low head, high flow inline horizontal propeller pumps for freshwater recirculating system

 In our latest case study, we are looking at a project we recently worked on for a UK engineering company.

 They contacted us to discuss a new freshwater recirculating test system they were designing and they specifically required low head, high flow inline pumps. They were having difficulties in finding pumps which would work with the small footprint available on site, and the low NPSH available.

What the client needed

The system needed pumps with a compact design to fit in the small footprint, but which also had the power to deliver high flows at low pressure. In addition, the pumps had to be able to be suitable to handle solids without damaging the pump.

The client couldn’t locate suitable pumps to perform the application, which is where North Ridge pumps came in. We listened to the client brief and found that a propeller pump was the best option for the low head, high flow challenge.

What is a propeller pump?

Propeller pumps are designed to pump low-viscosity fluids which might contain suspended particles, at high flows and very low heads.

They have an integrated motor submerged in the fluid which removes the need for cooled air, and also enables repeated starting and stopping of the pump without causing damage. In addition, propeller pumps are dry mounted and reversible in operation.

A propeller pump is actually a type of axial flow impeller pump. This type of pump design features a single or two-stage axial flow impeller which can produce high flows, with the maximum head limited to around 20m.

The axial flow impeller transports the fluid by using the pressure differences caused by the Bernoulli principle at the impeller vanes.

How birds and planes stay in the air

So what is the Bernoulli principle? It’s a seemingly counterintuitive theorem which states that as air or fluid moves around an object, it creates contradictory pressures on that object. In fact, faster air or fluid actually means less pressure while slower air or fluid produces more pressure.

In simple terms, this means that if you have a curved surface that makes air or fluid go faster over the top than the bottom, you produce a difference in pressure which creates lift. This is known as the Bernoulli Effect which you can see working every day with birds or aircraft in flight.

How does an axial flow impeller work?

The axial flow impeller is a common feature of a boat's propeller, a propeller-type mixer and a propeller pump. It can produce large flow at low pressures by using the the Bernoulli Effect, and by pumping the fluid in a direction parallel to the pump shaft. It works like this:

• Fluid enters the impeller via the suction flange and is pushed along the shaft by the      impeller blades 

• The pressure in the axial flow pump is created by the fluid flowing over the impeller blades 

• The fluid movement produced by the impeller blades creates high flow rates 

Where can they be used?

Axial flow impeller pumps or propeller pumps are ideal for applications where large amounts of water need to be moved quickly at low pressures, such as flood control, wastewater treatment or in this case, in a recirculation system with only a small amount of Net Positive Suction Head is available.

They can also be used in a ship’s anti-heeling system. This detects when the vessel is leaning too far to port or starboard, to the bow or to the stern. This can be caused by hard or fast turns, strong winds or uneven cargo loading.

The system detects the heeling angle of the ship and corrects it automatically using pumps. This ensures that the vessel remains level and stable, enabling loading and unloading to continue without any interruption, saving a considerable amount of time spent in the port and money.

What is Net Positive Suction Head (NPSH)?

Net Positive Suction Head or NPSH measures the pressure placed on a fluid on the suction side of a centrifugal type of pump, like a propeller pump.

The NPSH margin is critical but is often overlooked when specifying a pump. This is the difference between the NPSH available (NPSHa) at the pump’s inlet and the NPSH needed (NPSHr) by the pump to work without cavitation taking place. This happens when the pressure of a liquid goes below its vapour pressure. This can cause bubbles to form and collapse, permanently damaging the pump.

To prevent this, the NPSH margin must be positive to avoid cavitation. Pump designers and specifiers refer to the NPSH to make sure that pumps can operate effectively without suffering damage caused by cavitation, across all of the pump’s specified operating parameters.

So what’s the difference between a propeller pump and an impeller pump?

Basically, the main difference is their function: a propeller pump usually propels a vehicle through water while an axial flow impeller pump moves fluids. In fact, modern propeller and impeller pumps are both based on a design which uses Bernoulli's principle. 

But the two types of pumps differ in other ways. Propeller pumps can only generate linear thrust while impeller pumps can produce both axial and radial thrust. In terms of flow, this enters and leaves axially in a propeller pump but enters axially and leaves radially in an impeller pump.

• Axial thrust, also called axial load, works in parallel to the rotational axis. You’ll find axial loads in car axles, jet engines, wind turbines and industrial conveyor belts

• Radial thrust, or radial load, is perpendicular to the axis of rotation and is common in machine shafts, electric motors, car wheels and gearboxes

How did we solve our client’s problem?

At North Ridge Pumps, we are not limited in the type of pumps we can specify.

As the UK’s largest pump supplier with over a quarter of a century in the business, we are highly skilled at specifying pumps for a wide range of applications.

We use our specialist know-how to deliver the right specification for every client, not a standard solution straight off the shelf. Most important of all, we choose the best pump for every project brief offering the most cost-effective option.

For this commission, we specified horizontal inline propeller pumps fitted with SIEMENS 37 kW/400V-3Ph-50Hz/1450 RPM/IP56 motors. These had a capacity of 1200 m³/h at a head of 4m. The flanges were DN300 inlet/outlet.

If you have an application requiring the transfer, circulation or injection of fluids of varying viscosities and need guidance on the best pump to use, speak to North Ridge Pumps to see how we can help.

For more information on Low head, high flow inline horizontal propeller pumps for freshwater recirculating system talk to North Ridge Pumps Ltd