Circulation, Flow Rate and Turnover
Circulation
Swimming pool water is circulated around a system so that it can be filtered, heated and chemically treated before being returned to the pool.
Most circulation pumps work via centrifugal action. An outer casing encloses a rotating impeller that draws water in via vacuum suction. On the other side, the water is under pressure and gets forced along the pipework. The pumps must be primed (i.e. flooded with water) at all times for this to happen.
Flow rate
The flow rate is the speed at which water is flowing through the system. Usually expressed as cubic metres per hour and monitored by a component fitted to the system pipework (usually between the pumps and filters) called a flow rate meter.
If you cannot locate a flow rate meter on your system, it may be that it wasn’t fitted as part of the original installation (unfortunately quite common). You’ll need to get one fitted as soon as possible to calculate the turnover time (discussed below).
“It’s got the flow rate stamped onto the pump, so we don’t need a flow rate meter. Do we?”
Yes!
The flow rate given on the pump is not the same as the flow rate of the system as a whole.
See the image. The pumps data plate gives a flow rate of 1375 cubic metres per hour, but that’s the maximum capacity that was tested during production, against no resistance.
Turnover time
This is the time it takes to circulate the pool volume around the treatment system. It is calculated by dividing the pool volume by the flow rate.
If the turnover time is too long, pollution levels will start to build up in the pool. If the turnover time is too short, the water will travel too fast through the system. This reduces the effectiveness of filtration (slower filtration is usually better than faster filtration). Recommended turnover times are given below.
- Diving Pools 4 – 8 Hours
- Domestic Pools 4 – 8 Hours
- 50m Olympic Pools 3 – 4 Hours
- 25m General Use Pools 2.5 – 3 Hours
- Leisure Pools Over 1.5m Deep 2 – 2.5 Hours
- Leisure Pools 1 – 1.5m Deep 1 – 2 Hours
- Leisure Pools 0.5 – 1m Deep 0.5 – 1.25 Hours
- Leisure Pools Less Than 0.5m Deep 10 – 45 mins
- Hydrotherapy Pools 0.5 – 1.5 Hours
- Teaching Pools 0.5 – 1.5 Hours
- Waterside Splash Pools 0.5 – 1 Hour
- Interactive Water Features 20 mins
- Domestic Spas 15 mins
- Commercial Spas 6 mins
- Leisure Water Bubble Pools 5 – 20 mins
If you have a flow rate meter fitted to your system, work out your pools turnover time and make a note of it here. See how it compares to the recommended turnover times given above.
This information is important to include in the Normal Operating Procedures (NOP), a document that sets out how the pool should operate under normal day-to-day conditions.
Common Circulation Problems
Loss of prime
This is a situation where the pump and pipework, which under normal conditions is full with water, is empty.
The impeller will be rotating, but will not be able to create the vacuum necessary to create the suction required to pull the water through the pipework.
The slides go through how to re-prime the system.
Air ingress
If air is drawn into the system it can create problems with visibility in the pool due to the tiny air bubbles that become entrained in the water returning to the pool from the treatment system.
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.
Blockages
If problems are encountered with pressure loss, there might be a blockage. One of the most common places where blockages are encountered is in the skimmer baskets, especially with outdoor pools due to leaves etc. There might also be a build up of material in the pre-pump strainer basket if it hasn’t been maintained and cleaned out regularly.
Poor Design
Unfortunately, the design and construction of swimming pools and associated treatment systems is not tightly regulated and there are many examples of systems that are poorly designed. Examples of problems include:
- Incorrectly sized pipework
- Incorrect pipework material
- A ‘mish-mash’ of different pipework materials
- Too many right angled bends
- No valves fitted where there should be valves
- No pressure gauges fitted to filters
- No air bleed system fitted to filters
- No flow rate meter fitted to system
It’s not just contractors who are at fault – clients also need to take responsibility for not properly specifying a contract for the work, or selecting a contractor based solely on price, with inadequate consideration given to competence and quality.
