How do swimming pool pumps work?

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swimming pool circulation pump with strainer basket and valves

The circulation pumps are the ‘heart’ of the circulation system. They are designed to continuously pump water around the system at a pre-determined rate called the flow rate. In larger installations, there are usually several pumps working at the same time, with additional pumps on standby. In smaller installations, there may only be one single pump.

How do swimming pool pumps work?

Swimming pool pumps work by having an impeller (which is similar to a propeller), which is housed within the pump casing and is connected to an electric motor, which rotates it at high speed. This causes water to be sucked into the pump on one side (the suction side) and forced out of the pump on the other side (the delivery side).

How do swimming pool pumps work

Single or Multiple Pumps?

It is usually advantageous to have more than one single circulation pump fitted to the system. By having two or more circulation pumps fitted, you have a ‘back-up’ as and when the circulation pumps require repair or replacement. If you only had one pump, you would have to put the pool out of use if it stopped working properly for any reason whilst the necessary repairs are carried out.

Another good reason for having multiple pumps is to save money on energy costs. Using an example of a standard 25-metre swimming pool with a volume of 420 cubic metres, if we needed to achieve a turnover time of 3 hours, the flow rate would need to be 140m3/hr. This is worked out as follows:

420m3 / 3 hours = 140m3/hr

If only one circulation pump was installed on the system, then that pump would need to be powerful enough to deliver the full flow rate. But if two smaller pumps were installed, and ran together at the same time in order to achieve the desired turnover time, there would be the option of turning one of those pumps off overnight when the pool is not in use.

This would slow down the turnover time, which would give much better filtration efficiency.  It would also allow significant cost savings over the course of a year.  The amount of money that can be saved can be calculated by referring to the electricity bill and looking at the amount charged per kilowatt-hour.

An alternative method for making cost savings in relation to circulation is the use of variable speed drives (VSD’s). These are devices that are interlinked to the power supply to the pump(s) that have the ability to alter the speed of the pumps (and therefore, reduce the costs) in response to changing conditions in the pool.

Strainer Basket

Before the water enters the impeller section of the pump, it passes through a pre-pump strainer basket. The pre-pump strainer basket is designed to trap larger items of physical pollution before it can get into the pump itself, where it would cause damage.

The strainer basket sits inside a vessel (known as the strainer pot) and can be removed for cleaning. They will need to be cleaned out regularly to prevent them from becoming completely blocked with debris. 

The pre-pump strainer is a separate component to the pump on large systems, but on smaller systems, it is usually an integral component of the pump.

Both types (separate and integrated) have lids which can be removed to take the strainer basket out for cleaning. The integrated type usually has a lid that is see-through and can be turned anti-clockwise on a thread. The strainer vessels that are separate to the pump usually have more robust lids that are bolted down onto the strainer vessel. The image below shows a strainer basket which has been removed from the strainer pot for cleaning.

Strainer basket from a swimming pool pump removed and sitting on top of the strainer pot

Care must be taken when replacing the lids after having had the strainer vessel open as it can often be the case that the rubber O-ring that forms an airtight seal between the lid and the strainer vessel can be misaligned when replacing the lids and therefore won’t form a good, airtight seal and will start to suck air into the circulation system when the circulation pumps are turned on.

Suction from pool

The suction generated by the impeller inside the pump is what causes the water to be sucked from the pool tank.

Within the pool tank, there are usually outlets at the bottom of the pool (these are sometimes called sumps or drains). You can see them in the image below; they are the square ‘grills’ at the bottom of the pool. Pool water is drawn through these drains and sucked through pipework into the swimming pool plant room.

how do swimming pool pumps work

Water is also leaving the pool tank via some sort of surface water draw-off system. There are three main types:

  • Overflow Channel
  • Skimmer Basket
  • Deck-Level

The image above shows a deck-level system where the water constantly overspills over the top and into a drainage channel that goes around the perimeter of the pool. It does this because the water is kept level with the deck of the pool (hence the name; deck-level). From there is drains down into a balance tank, which is an in-ground tank that accommodates the over-spilt water. From there, it is sucked through to the swimming pool plant room via the pumps.

On to Filtration

Once the water is drawn through the impeller, it is forced along the pipework on the other side of the pump by the centrifugal action. the water is thrown outwards by the vanes (think: blades) of the impeller. The water has nowhere to go since it is enclosed within the pump casing, except out of the pipe leading away from the pump. The velocity of the water is transferred to pressure and travels along the pipework on the other side of the pump. This is known as the delivery side, or the pressure side of the system.

The water is then delivered to the filtration system, where the pressure of the water within the pipework (generated by the impeller of the pump) forces the water downwards through a filter medium such as sand.

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