As well as leaving the pool via the surface water draw off system, water is also leaving via the outlets (drains). In a traditional swimming pool, these outlets are usually located on the floor of the deep end and are covered with a square grill. The following pictures show the outlets from above and below the pool tank.
The outlets of a swimming/spa pool can be extremely hazardous. The hazards are groups under the headings of entrapment hazards and entanglement hazards.
The outlets of a swimming/spa pool are connected to a powerful circulation pumping system. If the outlets are blocked, whatever is causing the blockage will be exposed to the suction force of the circulation pumping system. The circulation pumps will be sucking on whatever is causing the blockage, causing a vacuum. At this point, it will be very difficult to remove the blockage without turning off the circulation pump(s). If the blockage is a person, then tragic consequences can occur, including drowning, disembowelment and transanal evisceration, which is where internal organs are forcefully drawn out through the anus.
There are various ways that suction entrapment can be avoided:
- Emergency cut-off devices that automatically turn off the suction pumps when an increase in suction force is detected
- Multiple outlets being fitted so that even if one of the outlets gets covered, the remaining outlets take the increased water flow and prevent a vacuum being created at the blocked outlet. The distance between outlets should be a minimum of 2m.
- Outlets being designed so that it is impossible to cover them and form a seal. This can be achieved via having the grill surface area of sufficient size (the outlet should have a surface area greater than 1m2).
- It can also be achieved by the use of outlets that are designed to prevent a seal being formed around them when they are covered. These are called anti-vortex drain covers. Some examples below.
- Installing a break tank between the pool tank and the circulation system. The break tank is gravity fed, so there is no risk of being exposed to the suction of the circulation pump(s).
- Ensuring that the water velocity through each outlet is 0.5m/s or less.
- All outlets should be fitted to a sump where the outlet pipe is located a distance 1.5 x the pipe diameter from the grid.
- To prevent finger and toe entrapment the gap in the grille covering the outlet shall be a maximum of 8mm.
- Ensuring that all outlet fittings and fixtures comply with BS EN 13451–1 and 3.
There are also fittings that can be placed over a drain cover that is not an anti-vortex type that will go some way to gaining a similar effect.
Drain covers must be maintained. In order to prevent children getting fingers/toes trapped in the grill, the apertures should be no wider than 8mm.
Once the above design-based precautions have been considered, other precautions can be implemented such as providing training for all relevant staff regarding the dangers of suction entrapment
Entanglement hazards are slightly different to entrapment hazards. They involve hair being drawn into the outlet and then entwining on the other side of the outlet due to the circular motion of the water is it goes through the outlet. It may be impossible to free the hair from the outlet, even if the circulation pumps are turned off. People using spa pools are at an increased risk of suction entanglement due to the fact that they will be in close proximity to an outlet no matter where they are situated within the spa pool. For this reason, people using spa pools should be advised to tie long hair back and refrain from submerging their head under the water. Staff responsible for supervising the spa pool should be trained sufficiently so that they are aware of the hazards associated with outlets.