Causes of Entrapment in Swimming Pools

You are an architect, engineer, contractor or builder of swimming pools. You have just started the construction of swimming pools or it has been your business for decades. We would like to share with you our knowledge about the technical safety of water drainage from pools. As a pool builder you can easily, and often without high costs, prevent suction entrapment of swimmers at the pool’s drainage intakes and the entanglement of long hair on drain covers.

To drain water from point A to point B.

We drain water from the pool for several reasons. For quality control, to purify polluted water in a filter and to supply water to a water slide or other water attraction. Water can be drained from the pool in different ways. By use of an overflow system, with suction intakes below the water surface, we may or may not include a movable bottom, and perhaps with a buffertank somewhere in between. We can drain water by free fall or we use one or multiple pumps in a single circulation system. But when is the safety of the swimmers in the pool in danger?

What makes water circulation systems in pools unsafe?

A high negative gauge pressure

There is a risk of entrapment present when a swimmer, that blocks the drain cover of a suction intake, is exposed to a negative gauge pressure (vacuum). How high this negative gauge pressure is depends on several factors: the number of suction intakes of the circulation system, the number of pumps and their position relative to the pool, the flow rate of water at various locations in the pipe system and the distances the water travels through pipes between the suction intakes and the pumps.

Water flowing through a pipe undergoes friction, because the water makes contact with the walls of the pipe. The faster the water flows through a pipe, the higher is the frictional resistance. This loss in energy is reflected by a pressure loss in the pipe. We use pumps to increase the pressure again so that the water still continues to flow at a certain flow rate. But what happens when the flow of water is fully or partly blocked by a swimmer?

In the worst case, water is drained by a single suction intake. A swimmer blocking such a suction intake, thereby stops the entire flow of water and is exposed to the mechanical strength of the pump. A life-threatening situation in which there is a high probability that the swimmer drowns or dies as a result of the severe injuries inflicted by the body when being exposed to high negative gauge pressures. For these reasons, many guidelines and standards describe that the application of at least two suction intakes in each individual circulation system is a necessity.

Analytical and experimental research of The Blue Cap Foundation, in collaboration with the University of Twente, demonstrates that a (partial) vacuum may also occur in circulation systems with multiple suction intakes. After blocking a suction intake, the flow rate through the other suction intakes increases. In these branched pipe sections more pressure loss will occur, allowing a negative gauge pressure to built up behind the blocked suction intake. How high this negative gauge pressure can become is highly dependent on the length of the branched pipe sections, the diameters of the pipes used and the flow rate of the water through these pipes. Simulations at our test pool show that the negative gauge pressure can range from several hundred to several thousand newtons, equal to a few tens up to a few hundred kilograms of weight. For children a load of a few hundred newtons can be enough to keep them trapped under water.

For this reason, the measurement of the negative pressure is a necessity. Only by measuring one will know for sure whether drainage intakes in existing pools are safe. Also for the construction of new pools the measurement of the negative gauge pressure is a useful tool to evaluate safety. More information about the measurement of the negative gauge pressure can be found here.

Too high water flow rates through drain covers

As explained above, high flow rates in the pipe can cause high pressures. A high flow rate through the drain cover of an intake can also incur unnecessary risks. Scientific research shows that there is a strong link between the entanglement of long hair on drain covers and high flow rates through these drain covers. European standards specify for this reason that the average flow rate through a drain cover should not exceed the 0.5 m/s.

Want to know more about what you can do as a pool builder to minimize these risks, please read further here.

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