Pool tank

Emptying and Refilling

The first thing to consider before going ahead with this task is whether it is really necessary to empty the pool at all. Many repairs to the pool lining and/or tiles etc. can be carried out by trained divers, without the need to empty the swimming pool at all. A lot of structural damage can be done to the construction if this task is not carried out correctly.

However, there are occasions where the pool water will need to be emptied. An example would be if any broken glass somehow found its way into the pool water. Because glass is completely invisible when submerged in water, the entire pool contents would need to be emptied and a thorough clean-up operation carried out to ensure that all traces of glass have been removed.

If you have assessed the requirement to empty and have decide to go ahead, here's what you should do:

  1. Carry out a suitable and sufficient risk assessment for this job before going ahead with anything. This risk assessment will need to be carried out by a person who is competent and understands all of the hazards and risks involved.
  2. Contact the local water supplier and the Environment Agency and inform them of what you intend to do. You may need their explicit permission before going ahead. Also, they will require you to remove all of the chlorine from the water prior to discharging and may also require you to discharge the water at a slower rate than you were originally intending.
  3. Before releasing any water, turn off the air and water heating system and let the temperature come down to as close to the ambient temperature as possible.
  4. Neutralise all of the chlorine in the water using sodium thiosulphate. Every 1 gram of free chlorine will need 5 grams of sodium thiosulphate to neutralise it. For example, if your pool volume is 450 cubic metres and your free chlorine reading is 2.0 mg/l, then there is 900 grams of free chlorine. Times this by 5 (4500g) and you have the amount of sodium thiosulphate you will need to add.
  5. Start discharging the water. This needs to be done slowly, at a rate of no more than 750mm per 24 hour period. So for a pool that's 2 metres deep, it's going to take the best part of 3 days to empty it.
  6. Before refilling, try to get the pool tiles to as close as possible to the incoming water temperature. This will obviously be more difficult to achieve in the winter months, so have a think about when would be the best time to schedule this work. Heat the water slowly at a rate of no more than 0.25 degrees Celsius per hour. So if the water is, say, 5 degrees Celsius, you may be looking at a four-day period in order to get it up to bathing temperature.

If a pool is emptied, then the bottom and sides should be scrubbed thoroughly with 10 mg/l chlorinated water before refilling. It should be flushed thoroughly to drain before refilling. Check the integrity of the structure while the pool is empty.

Pool tank profile

It is recommended that all pool profiles are based on a number of important safety principles:

  • abrupt changes in depth should be avoided in water less than 1.5 m in depth;
  • steep gradients should be avoided - a maximum gradient of 1 in 15 is recommended for water depths up to 1.5 m;
  • changes in depth should be clearly identified by the use of colour-contrasted materials or patterned finishes so as to indicate to bathers when they are proceeding to water of a different depth. Where colour is used, this should not reduce the visibility of a body lying on the pool bottom;
  • a minimum water depth of 1 m is recommended for larger pools used for training and/or competition. For small community pools without a separate learner pool, a depth of 900 mm is recommended because this is more appropriate to young children and for teaching purposes.

The introduction of a movable floor(s)/bulkhead(s) will affect the pool tank profile and will create a wider range of different profiles. Care should be taken to ensure no additional hazards are created. The overall profile should still meet the above principles and where this is impractical, or cannot be achieved, options for controlling any potential hazards need to be considered.

Pool tank edge

The pool tank edge should be colour-contrasted with the pool water so as to render it clearly visible to bathers in the water and on the pool surround. This is particularly important for deck-level pools where the pool edge may be partially submerged.

Fixed raised pool ends are recommended for main pools with deck-level edge channels, where a pool is used predominantly for training and/or racing. The raised ends help the swimmer to easily identify the end walls of the tank.

In a leisure pool where the pool tank bottom slopes gently from a beach area to deeper water, there is no need to highlight the water’s edge providing there are no ‘upstands’ or steps between the pool and its surrounds.

Pool tank detailing

It is recommended that the detailed design of the pool tank should ensure that:

  • The pool tank should have no sharp edges or projections that could cause injury to bathers, especially below the water level. Careful consideration will need to be given to the design of recesses, ledges, or rails so as to ensure that they are not a hazard;
  • Wave machine openings, sumps, or inlets and outlets of the pool water circulation system should have suitable protective covers or grilles. They should be designed to prevent limbs and fingers getting trapped. Undue suction should not be created, which could result in a body being held against a grille, and there should be no exposed sharp edges. This is particularly important in areas of moving water;
  • There should be at least two outlets per suction line at a sufficient distance apart to prevent a body being drawn or trapped by two suction line outlets. The amount of suction produced at any single outlet position should not be sufficient to result in a body being drawn towards it and held in position or entangle hair;
  • where handrails are provided, they should be recessed into the pool tank in such a way that it is not possible for limbs to become trapped between the grab-rail and the rear wall of the recess or the tank wall;
  • If a resting ledge is to be provided this should be recessed into the pool wall. If, for some reason, this is not possible, the ledge should be colour-contrasted and warning signs displayed to alert bathers, who are entering the water, to its presence.
  • If tiles are used for the pool tank lining, epoxy grouts should be used as these are resistant to grout attack.

Pool tank bottom

A slip-resistant and non-abrasive finish should be provided in the following areas:

  • on the end walls of the pool as a turning pad to aid tumble turns or for swimmers starting backstroke events;
  • in leisure pools on the beach area and other shallow water areas where bathers may become unbalanced when a wave machine or other feature is operating.

If racing lines are not to be included then a line running along the centre of the pool will assist bathers to determine sudden changes in water depth. The ability to see the bottom of the pool clearly is essential to effective lifeguarding.

Pool floor patterns which would make it more difficult to recognise a body at the bottom of the pool should not be used.

The pool bottom should be kept clear of contamination, algae, and general debris by daily sweeping, suction cleaning or other means. This is especially important with deck-level pools because up to 80% of the water flow can be leaving via the surface, meaning that there will not be much water flow at the bottom of the pool to help keep it clean and free of algae and other staining.

Movable floors and bulkheads

Movable floors are being used more extensively to change the water depth over part or all of the pool tank area in order to achieve greater programming flexibility. They allow more activities to be accommodated within a single pool area or improve activities that may be compromised by a fixed depth of water. There is evidence of greater through-put and reduced net operating cost where they are used, particularly for 50 m pools.

The use of this technique to create a ‘dry’ activities space is usually limited by the wet humid conditions within the pool hall. However, learner pool floors which can be raised to the level of the pool deck surround, are sometimes used as a holding area for competitors when an event is taking place in the main pool.

Movable floors can be adjusted from a depth of a few centimetres for carer and baby classes to a safe depth of 5 m for a person diving from a 10 m diving board.

Where a movable floor is provided as part of a learner pool, automatically folding steps can be integrated with the movable floor to allow mother and child, or those with ambulant disabilities, to access the pool with greater ease, regardless of its set depth.

The turnover period of pools with moveable floors should be appropriate to the pool at its shallowest point (ie potentially biggest bathing load).

There are two types of bulkheads: those which traverse laterally (and when not in use, are stationed at one end of the pool); and those which move vertically (and when in their lowered position, are housed in a recess in the pool floor).

Bulkheads can be used to:

  • Divide the water area so it can be used for different activities simultaneously. This is often desirable for safety reasons
  • Reduce the length of an existing pool to 25 m, the length recognised by the ASA for training and competition
  • Provide measurable distances where accuracy is important
  • Provide a safety barrier to the edge of a movable floor.

If possible, moveable floors and bulkheads should be brought to the surface and tilted so that both surfaces (including the underside) can be cleaned. This may be required every 6 months.

If moveable floors do not tilt for cleaning, they should have access hatches or manholes to allow access. Cleaning then requires specialist divers who should work to the Diving at Work Regulations 1997.

Inlets and Outlets

Inlets and outlets, grilles and covers should be designed in accordance with BS EN 13451-3. They should be inspected visually every day and once a month subject to closer examination for obstruction, impact damage and vandalism and to make sure that they are correctly in place. If they are damaged or missing, swimming should be suspended immediately.

Inlets: in water less than 800mm in depth and in sensitive areas (steps, teaching points, beside base inlets, etc.) the velocity of the water entering the pool should not exceed 0.5m/s. In other areas, the velocity of the water entering the pool should not exceed 2.0m/s.

Outlets can cause entrapment and therefore have the capacity for serious harm. PWTAG guidance is that all pools should be tested to show that outlets comply with BS EN 13451-3. New completed pools should have this certification when built. Where this is not the case, pool outlets should be tested by a competent authority to show that they comply.

Outlets should also be tested for hair entanglement.

Pool outlets should be designed and installed so as to reduce the potential for entrapment of the user. As a general requirement, water speed through the outlet grilles should be ≤0.5m/s.

Grilles in outlets and inlets should comply with the requirements of BS EN 13451-1 and have gaps no greater than 8mm to prevent entrapment hazards.

All wall and floor outlets should be fitted with a sump to a design that accords with BS EN 13451-3. Additionally at least one of the following two requirements should be met.

  • Multiple suction outlet systems should be designed in such a way that: there are at least two functioning suction outlets per suction line the distance between the nearest points of the perimeters of the devices is ≥2m if any one of the suction outlets becomes blocked, the flow through the remaining suction outlet/s shall accommodate 100% of the flow rate
  • it is not possible to isolate one of the outlet sump suction lines by means of a valve

In the case of suction outlet systems on existing pools with only one grille, the grille should be designed in such a way that it cannot be blocked:

  • one user cannot cover more than 50% of the opening
  • raised grilles can be domed opposite to the flow direction, with prevalent peripheral suction; the height of the dome shall be at least 10% of the main dimension single grilles should have a grille area of ≥1m2.

Balance Tanks

Balance tanks should be inspected at least once a year and cleaned as necessary. Debris should be removed and inner surfaces brushed and flushed down with 10mg/l chlorinated water, which can be returned to the circulation system via the filters.

Balance tanks should be regarded as confined spaces, and therefore the legal duties set out in the Confined Spaces Regulations 1997 may well apply.

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