Viewpoint: options for pumping liquids

If you are pumping liquids from sumps or tanks situated below ground, there are three main options: submersible, self-priming and cantilever.

These three pump types come with a range of pros and cons, but you should always bear in mind the type of liquids that will be pumped.

If you are only going to pump storm water or sewage, then the selection process will be based on location, accessibility and budget.

However, if you are pumping process effluent which is corrosive, abrasive or a hot liquid, the choice of pumps available should be carefully considered.

Submersible pumps

Submersible pumps are hermetically sealed and are considered a ‘fit and forget’ solution. They are inherently self-priming and don’t suffer cavitation issues. Certain models also have the ability to pump solids.

If you are pumping process effluent which is corrosive, abrasive or a hot liquid, the choice of pumps available should be carefully considered

Small, lightweight submersible pumps can be cost effective when a cold clean liquid is pumped, but when flow rates and pump sizes increase, the ancillary equipment required can quickly add cost.

And while there are versions that will handle corrosive or abrasive liquids and solids, any leakage into the pump casing via gaskets or cable entry inevitably proves disastrous.

Self-priming

In comparison, self-priming pumps tend to be a more cost-effective solution. Maintenance is often easier as the installation is above ground. Life expectancy also tends to be longer than a comparable submersible pump.

To operate, a simple suction pipe is immersed into the liquid, and after an initial filling, the pump is automatically primed to a height of 7.5m — although the discharge must be free to vent, or an air release valve should be fitted to the discharge.

Open blade impellers allow the passage of solid particles and a check valve in the suction flange prevents a back syphon effect when the pump is stopped. This helps retain liquid in the pump casing ready for the next priming cycle.

ATEX-certified self-priming pumps are also available. However, selfpriming pumps are not suitable for pumping hot liquids on a suction lift.

Cantilever pumps

Cantilever pumps, meanwhile, don’t have any submerged bearings. So these types of pumps are capable of handling solid, abrasive or corrosive liquids without the need of external flushing.

The benefit of having no seals in contact with the liquid means temperatures can exceed 500°C and the pump can dry run without causing damage.

The cheapest pump to buy is not necessarily the one that will have the lowest life cost or even be suitable for your application

Cantilever pumps are designed to prime automatically when the liquid covers the impeller, so the pump can even run without level controls.

There is also a variety of material options. These include cast iron or AISI 316 through to Sanicro 28 and wearresisting steel.

Various impellers are also available. Vortex impellers, capable of passing solids up to 90% of the discharge port, can be fitted. There are also options to fit channel impellers and closed/ open impellers.

Meanwhile, robust construction, coupled with no mechanical wear, means a cantilever pump has virtually no maintenance cost — which is the payback for a higher purchase price.

To buy cheap?

Clearly, if you are faced with pumping process liquids that are high temperature, corrosive, abrasive or mixed effluents that contain solids, you need to consider the various options available to you. The cheapest pump to buy is not necessarily the one that will have the lowest life cost or even be suitable for your application.

• Peter Staddon is managing director of The Pump Company.