Swimming pool outlets – what could be simpler?

A group of us from four European countries assembled in Paris to consider revising the British and European Standard for swimming pool outlets. To put this into context, we were faced with a compilation of French and German proposed amendments to the existing BS EN 13451-3 Swimming Pool Equipment standard on pool inlets and outlets.

Also incorporated, were proposals from the Netherlands Blue Cap group (safety of pool outlets consumer group). There are 23 pages of comments! The UK have submitted no proposed amendments to the standard on the basis that with twenty years of experience and no reported accidents, we are more or less happy with it. There are certain elements that we would wish to see changed, like the removal of gravity flow outlet circulation but these have all been picked up for change by other countries in proposed amendments.

Following an earlier meeting in June when we achieved “not a lot”. We all knew that this meeting had to tackle the issue of dealing with the French, German and Netherlands amendments to the revision to the 13451 3 Standard. This was crunch time, make or break, which made it all the more remarkable that a room full of experts spent virtually two days talking about one issue! It is the paragraph 4.6.1 in the standard concerning pool outlets and, more particularly, pools where there is only one outlet.

I will try to explain but it is complicated, and I suspect bedevilled with vested interest! If I can explain it to you, I might understand it better myself!

What’s clear…

What’s clear? Nothing, except that one factor is certain. The flow through a pool outlet to avoid entrapment should be ≤0.5m/sec. Why this is, is not so clear but it is boldly stated in the European Standard. It is also equally clear in the American Virginia Baker law on pool outlets, although here it is in feet and inches, not metric, however it also has some scientific evidence to back this up. I have video evidence showing this reality in some testing by the ASTMS the American standard body used in law regulation.

I have also a peer reviewed paper published on entrapment by Human Kinetics which shows the same effect. Both these authoritative pieces of research show, that when the flow rate is low, then, if there are two pool outlets both with a flow rate of ≤0.5m/sec and one is blocked completely, a vacuum will not form and there will be no life-threatening entrapment or serious injury. (Out of interest, it seems injury from outlet suction is potentially as fatal as drowning from entrapment. In some deaths from this occurrence, it was not the entrapment and drowning causing death.

Rather, it was the impact on the internal organs which ruptures the aorta). In the ASTMS research a 15lb very buoyant rubber duck is placed over one branch of an outlet submerged in a tank of water. It completely covers the outlet, stopping the flow. The other outlet is open. The duck rises serenely to the surface. Unscathed! In the second Human Kinetics study a paunchy American researcher pulls his stomach onto an outlet. Temporarily held for an instant, his buoyancy, just like the duck, frees him from its grip. He is not held, or entrapped.

I presume this is because in both cases there are firstly two outlets, so the flow from the blocked outlet transfers to the free outlet. In the existing EN standard, we say that the flow should transfer 100 per cent. And secondly that the flow rate is low enough for this transfer to take effect. Certainly, in the ASTMS testing they show that when the flow rate is increased by using a lower dimension pipe size connected to the outlet the duck becomes trapped on the outlet.

This raises the issue of which is the most significant factor, the flow rate, or the 100 per cent transfer of the flow. Perhaps not significant until you apply the same rationale in the case of a pool with a single outlet.

Entrapment requirements in global safety standards

The entrapment requirements in global safety standards are clear for all pools built today. Two pool outlets and a flow through them of ≤0. 5m/sec is a necessary requirement. In Europe and America, we are at least together on this. (Despite Donald Trump confusing would with wouldn’t). But what then of existing pools that have only one pool outlet or if a pool installer/builder/architect/engineer wishes to only use one pool outlet. Can this be done safely with no fear of entrapment. Some very major grill manufactures say it can be done and its very safe!

There are a number of ways in which it is considered possible to do this.

Firstly, and the overriding consideration is that the water flow rate through the outlet is the same ≤0.5m/sec.


If the outlet is so large that one bather cannot block more than 50 per cent of it with their body,


If the type of outlet grill used is designed with some peripheral flow around its circumference and has a raised profile such that a bather’s body cannot block it easily. And even if it does, that the peripheral flow continues although the body blocks the raised area. This method is the one which the grill manufacturers favour.

Thirdly, if the outlet is just that big >1m2 it cannot conceivably be blocked by a bather. In America, they call this an unblockable pool outlet and say, that means they are safe.

If these single pool outlet solutions to prevent entrapment work, then in writing a standard you are supposed to be able to prove it, so it can be tested. In the case of a double drain pool outlet no test is necessary you can calculate the flow to achieve the ≤0.5m/sec and you can see how many outlets there are. In the case of a single outlet though, in all but the very big outlet, greater than 1m2 which you can measure, you need a test to be able to demonstrate that it works. This poses a whole series of additional challenges. To show no more than 50 per cent of the outlet is capable of being blocked, you can make a template or a dummy or a replica body, and see how much space it occupies on the outlet grill, but what size do you make it? Then, is it enough just to prove the 50 per cent size factor, or should you also have to demonstrate that if this were a person stuck on the outlet that they could apply an appropriate force to free themselves. Or, like the duck or the fat researchers, should they just rise up by gravity, and if so, what weight and buoyancy should apply to the dummy? Do you have to make a replica for children and adults, or is it sufficient to just use an adult dummy?

Similarly, with the pool outlet of the type with raised grill profile with peripheral flow, then is the size of the outlet grill significant, or can it be demonstrated that no matter how a body covers it the peripheral flow will always ensure that a vacuum cannot form and trap a bather. In which case how do we test this. How do we replicate human flesh, in order to prove that say, 95 per cent of the world population if they found themselves on a pool outlet grill would not become trapped by their flesh congealing around the grill and preventing the peripheral flow?

In both cases, the 50 per cent pool outlet and the peripheral flow outlet, this brings into the consideration the issues earlier. Is it the ≤ 0.5 m/sec flow rate that is significant? Is it the 100 per cent transfer of flow that is significant? Or is it simply that the flow can transfer?

In the case of the 50 per cent flow single pool outlet, the logic for the 50 per cent is that this makes the single outlet comparable to the double outlet system. You can block 50 per cent of the single grill, or one outlet and 100 per cent of the water flow will transfer and no entrapment happen.

But is this the case? On the start of the second day of our meeting I challenged the experts present to tell me what size a single outlet should be in a standard 25m pool. The one that we use as an example in Swimming Pool Water, with a turnover of three hours and a circulation rate through the pool of 172m3/hr. None of the experts came up with an answer, which I found concerning. Their explanation was that a single outlet of this size was something they would never contemplate. In Swimming Pool Water, we show that a 25m pool requires, two pool outlets of 0.160m2area. That is each outlet having a square grill measuring 0.41 metres square to ensure a flow rate through the grill of less than ≤ 0.5m/sec. This means that to replace the two grills with one single grill, it would need to be grill area of 0.320m2, or 0.82 metre square. That’s a big grill but no bigger than the one at Tunstall baths where I spent my formative years swimming and ending up working at. (never did me any harm, or anyone else for that matter)

This poses the question, using the criteria, we have deployed so far. If in a 25m pool, we require a pool outlet of 0.32m2 and this ensures that the flow rate through it is ≤0.5 m/sec. Then to ensure a safe double outlet system, to meet the current European and USA standard do we need, two pool outlets of this size? Meaning an additional outlet of 0.32m2? Or, as we state in Swimming Pool Water, will two pool outlets of 0.160m2 be sufficient? In which case we must be acknowledging that if one outlet is completely blocked it cannot transfer 100 per cent of the flow at ≤ 0.5 m/sec!

The flow rate through the remaining outlet must be at a considerably increased rate, nearly double. Using this argument for the 50 per cent outlet grill. If a bather blocks 50 per cent then is a 100 per cent flow transfer necessary? Be it that this must increase the flow rate to more than the 0.5m/sec rate. If so, how critical is both the 100 per cent transfer and the 0.5m/sec? Or, will any flow rate or transfer rate be sufficient just as long as a vacuum is not formed? So too with the peripheral flow pool outlet. If it cannot be blocked and the peripheral flow is maintained albeit that it is not a rate to provide 100 per cent transfer or a flow 0.5m/sec or below. Does it matter? Just as long as the flow continuing around the periphery prevents a vacuum?

To be honest after two days, or in my case since 2001 when we developed the first standard, the issue is not resolved. At the working group, we have as an interim agreed that double drain pool outlets are required in all new pools and that means ≤0.5m/sec flow rate and 100 per cent transfer flow rate. And we have in pools with only one outlet reached as far as only allowing outlet grills greater than 1m2. We have not ventured as far as allowing peripheral suction pool outlet grills or pool outlet grills that cannot be blocked by one bather for more than 50 per cent. We have no idea how to test these if we should include them, hence the impasse and hence yet another meeting will be required.

As I left the meeting room at the end of our second day to spend another night desperately trying to make myself understood in the bistros of Paris, I turned to our Chairman Samuel. Would you mind sending me a copy of your notes I asked. My wife will never believe that I went to Paris to spend two whole days discussing single pool outlets with no clear outcome!

Ralph Riley 20 August 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.)