Pool outlets – the sequel

After my last monumental blog on the development of a European Standard for swimming pool outlets I realised I just couldn’t leave you hanging in the air with no definite outcome. As I write this, I have just left the follow up meeting at the AFNOR headquarters in Paris. It’s next to the Stade de France, where England have been entertained on many occasions by the French.

So to it was with this meeting, the French, and one Dutch man entertained my Anglo-Saxon views. The result was an outcome. It will not please everyone, makes little sense and of course will have to be ratified by the full committee of WG8. This was just a task group of WG8 established to deal with details that has proven to difficult for the main group to resolve.

What are the issues to resolve? How should a double outlet system, the safest system, recommended for all pools, be configured to provide a speed of water flow through it of less than 0.5m/sec. Ensuring that if one outlet should become blocked by some unfortunate bather then the flow of water, sucking the bather to it, should transfer 100% to the remaining free outlet.

Secondly, if the pool designer should choose to only use one pool outlet. Not two, as any sensible architect would. And if they did not choose to provide what the Americans call an unblockable outlet. Meaning it is bigger that one metre square. Then how big would it have to be to be safe? And how do you test it for safety?

First issue resolved

The first issue was resolved, not unfortunately by using the example in our book Swimming Pool Water. This calculates the size of outlet required to ensure a flow of less than 0.5 m/sec and then divides the flow between two outlets. The book example calculates this in a standard deck level 25m pool, as 2 outlets of 40cms square. Providing a combined flow rate of 0.5m/sec through them. The problem with this is to transfer the flow 100 per cent to the free outlet should the other become blocked means that the criteria for the flow rate of 0.5m/sec is at least doubled.

This begs the question as to whether a 100 per cent transfer of the flow could take place at all. It is certainly not complying with the standard. The answer we agreed was to calculate the necessary flow rate to achieve 0.5 m/sec and then to provide twin outlets of the same dimension. In effect, in the Swimming Pool Water example this means two outlets 80cms square with a normal flow rate through them of 0.25 m/sec. Then should a bather block one of them a 100 per cent flow transfer could take place through the other outlet and the flow rate through it probably stay in the region of 0.5 m/sec.

These are big outlets but the reason for this is that we in the UK require the flexibility, should we need it, of 100 per cent of the water flow through the bottom outlets. In reality most deck level pools in the UK have 20 per cent of the flow through the bottom outlet, when the pool is being used for swimming, with 80 per cent of the flow from the surface.

I cannot think of a good reason for requiring 100 per cent flow through the bottom outlet. The French base their calculations, for public and even domestic pools, on one third bottom outlet, two thirds surface. No matter what type of surface water draw off system, be it deck level, overflow or skimmers. The DIN used in Germany, Austria, Spain and many European countries has total surface water draw off, none from bottom outlets.

What about a single pool outlet?

On the second issue we decided that a single pool outlet should it become blocked by a bather must always be sufficiently large for at least 50 per cent of it to be free to allow water to pass. The flow rate criteria throughout the standard is still the same, no matter one, two or more outlets < 0.5m/sec in normal conditions. Then we said should the single outlet become blocked by a bather the flow rate could increase to 0.8 m/sec, but a vacuum could not form and suck a bathers body to it.

To work out what the implications of this proposal were on the size of the outlet we took anthropometric data published in a cross section of countries around the world. We averaged it out.

And after a silly debate about what bits of the body constitute a torso produced a template we were satisfied with as representing the largest torso of 95 per cent of the world population. It was 0.65.5m long by 0.36m wide. The implications of which are that for a single pool outlet, if you choose not to provide an unblockable size of 1m square then it must be 0.65.5m x 0.36m minimum and then be sufficiently large to allow an additional 50 per cent to remain free to facilitate a maximum flow rate when blocked of 0.8m/sec.

I will leave it to you to deduce the consequences. I did try to point them out but the other members were happy with this conclusions. I left the field of play, at AFNOR, in the vicinity of the Stade de France knowing that one day I must return.

Ralph Riley 12 September 2018

(Ralph Riley is the Vice Chairman of PWTAG, former Chief Exec of ISRM and provider and operator of many public swimming pools. He heads up the UK BSI representation on European and World sports standards. This blog represents Ralph’s views and not necessarily those of PWTAG.)