By Francis Lowe
The pump is the heart of your pool water system, circulating water so that it can be filtered, heated, cleaned and chemically treated. Without a pump the pool will quickly become useless.Therefore, when first installing or replacing your pump it is extremely important that it is the correct size for your pool. It is essential that the pump is neither too big nor too small, as there can be problems associated with both, such as insufficient filtration or the creation of too much pressure on the pipe work or filter installation. The aim is to install a pump which will filter your entire pool water volume every 6 to 8 hours, depending on the swimming load.
When you are simply replacing a worn out pump, many people will automatically put in a pump of the same size as they are taking out. For this to work it depends on the original being correct in the first place. Consequently, it is worth doing the correct sizing exercise regardless.
The factors that need to be considered can be summed up as “flow rate” and “head”.
The flow rate is the amount of water moved in a given time, usually expressed in gallons per minute. Before you can find the flow rate, you need to know the volume of your pool. All measurements should be taken in feet. To find the average depth of your pool, take an average of the deepest and shallowest points.
Pool Water Volumes.
Rectangular Pool : Length x Width x Average Depth x 6.25 = Gallons
Round Pool : Radius x Radius x Average Depth x 20 = Gallons
Shaped or Oval Pools : Avg Width x Avg Length x Avg Depth x 6.25 = Gallons
Once you know the volume, decide how quickly you want to turnover the pool water. Ideally this will be about every 6 hours, but can vary according to bather load. If we assume turnover required is six hours then the flow rate in hours is the volume divided by 6. This figure must then be divided by 60 to get gallons per minute.
Having worked out the flow rate that we need, we then have to look at the head loss, which is a measure of the resistance, for example friction in pipe work, that the water will encounter on its journey around the circulation system.
This is an extremely difficult thing to establish accurately. There are simply too many unknowns and too many variables. For example, to be 100% accurate you would need to know exactly how many feet of pipe you have, exactly how many joints, where they are, who manufactured them, what they are made of, etc,etc.
In practice it is only feasible to take a best guess. It is accepted in the industry that for most pools an average “head” is 10m or 33ft.This would be reasonably accurate for most pools where the pump is sited about 30 feet from the pool and is on the same level. Use this as a starting point and adjust upwards if your installation is further away, has lots of bends, or is at a lower level.
Once we have calculated the required flow rate, and estimated the head, then we need to look at the pump curve charts provided by the pump supplier.
On the vertical axis the scale is measured in “Feet Head” or “Metres Head”, whilst the horizontal scale is “Flow Rate”. Find the point at which your two readings meet, go to the next curve up, and this is then the correct size pump for your pool.
Francis Lowe is a leading expert in swimming pool maintenance with a huge wealth of experience developed from 40 years in the industry.
To see a full range of pool equipment, including swimming pool pumps and pool pumps please see the website of specialist supplier poolwarehouseuk.com
