One Hole or Two?

...the argument over safe suction sourcres

                This one’s always been a concern for pool designers, contractors, and operators.  And the health department guys around the country lose sleep over it, too. There remains significant disagreement on how safe one, two or more bottom drains or suction sources may be, how they should be designed and used, how far apart… just how they are to be tied in to the grand suction scheme of pool design. And now we have another source of argument and anxiety, that alternate source (the “equalizer fitting”) for skimmers.
Ok – Two main drains, properly covered, safely apart and with balanced plumbing are the norm now. Old public pools when rehabbed usually cannot be grandfathered with single drains except in pools over six feet in depth… and while there are many small variations, we all seem to agree on the double drains. Take a look at PrP #7 p.1, Pr #13 p.1, and PrP #24 p.4 for suction coverage. You’ll see in the second piece that Dr. Bill Rowley, one of our favorite pool engineers, has wrung the subject out well – using his own body as a testing device! Yep, it’s all been looked at, laid upon and laughed over. But there’s more, and it’s serious safety business.

Since we have resolved the main-drain thing, albeit to varying degrees throughout the various states and their various codes, we need now to look specifically at the most recent source of argument and opinion regarding skimmers.
First of all, we know there must be more than one source for water heading into a powerful (and dangerous) circulation pump in any pool or tub. For many years, however, one skimmer and one main drain has been the configuration usually considered safe – from hot tubs through wading pools to the three-meter diving well. More recently, two main drains separated by at least three feet (so one “body” couldn’t be sucked onto both) supplemented the skimmer or skimmers, even though the latter was for years considered adequate as a suction source if the bottom drain were blocked.  Multiple-skimmer requirements were added to most codes, but this was based on a pool’s surface area (often one for every 500 square feet) in consideration for adequate surface collection, not simply for suction safety alone.

As most of us know, many major institutional pools don’t have skimmers at all; they utilize full-perimeter gutters to provide surface collection. But there are thousands of public pools still using skimmers, along with a few hundred thousand back-yard pools and tubs. We can assume, therefore, that most of our PPOA members deal with skimmers in one way or another. Reviewing the skimmer concept for a minute, we know that an eight-inch opening in the tile line, fitted with a floating weir, does a pretty good job scrounging up surface particulate in a fan-shaped area maybe five feet out into the pool. The skimming water, rushing over the weir, falls through a leaf basket then into the plumbing which feeds the suction side of the circ pump.  But what if the water level drops below the opening of the skimmer (or the skimmers)? 
This is where the equalizer thing comes into play. (Where th’ heck did that name come from?) As shown in the sketch, this alternate suction water, laying idle for weeks or months, immediately becomes the back-up water source as the float valve (or other gadget) in the bottom of the skimmer – below the basket – keeps air from being ingested by shutting off the primary route and allowing, or directing, suction from the pool wall below the skimmer arrangement. This literal hole-in-the-wall has a fitting designed with safety and anti-hair-entanglement in mind.
Cool. But do we need them on all the skimmers in a pool or just a few?  How big should they be? And should there be, as is now being suggested or enforced, two per skimmer??
We, at PPOA, are thinking twice…  First of all, even the most substantial commercial skimmers don’t draw more than 35 gpm. That’s one of the reasons that so many are needed around the perimeter of a large public pool, especially when codes call for surface water to be equal or greater than bottom water exiting the pool. Even the plumbing from the skimmer is limiting. If the pipe from each skimmer – heading for a much larger perimeter suction line – were 2-inch PVC, the maximum suction capacity, at 5 feet-per-second, is 50 gpm. And designers usually go lower. If the plumbing is of just the 1.5-inch variety, that flow is about to exceed the pipe’s flow limit at less than 25 gpm. And we’ve even seen one-inch pipe used, nearly useless for this suction device. Remember, this is the pipe leading from the skimmer system to that serious main suction header encircling the pool – a serious suction deal!

That alternate suction source (yes, the equalizer), virtually always plumbed with the same size pipe, would be equally challenged to do the job if the normal skimmer flow were diverted. But why two? Wouldn’t it be better to have these two alternate sources – with better combined flow characteristics – even though they’d be feeding one crummy, flow-limiting pipe departing the skimmer?
The truth is, it’s probably not better.  If the suction line from the skimmer package to the header gathering suction flow is, well, whatever size, that four or six or ten feet of plumbing is the limiting factor in resistance to flow. Adding a larger few inches of “equalizer” pipe – or two, or a dozen pipes – which collectively exceed the capacity of the normally engaged skimmer line, simply will not improve the flow characteristics of that final feed line at all.

                Are we forgetting about safety here?  The second equalizer might appear to mitigate the suction of that starved skimmer if blocked by a swimmer’s arm, foot, or other body surface. With well over two million skimmers in use in the USA, the PPOA’s safety department has never heard of an “entrapment” on an equalizer fitting. The sources of water for the suction line would still include the main drains (bottom outlets) and all other skimmers around the pool. In experimentation we have found, even with an under-filled two-skimmer pool, the suction appeared minimal at the alternate source below one of the skimmers.

The bigger concern is in the case of the single-skimmer vessel – like a typical back-yard pool.  In a one-skimmer pool, the alternate source – in the event the water level gets too low for the skimmer throat to draw – is still only supplementing the main drain’s flow and whose suction should be safe – as it is always designed to handle 100% (or more) of the total flow. This single-skimmer arrangement is the pool configuration of greatest concern, however, and where a few health inspectors lay down the law.
Especially for rehabilitation projects, re-plasters and fiberglass jobs, we do find some plans in which inspectors are requiring the installation of a second equalizer for each skimmer.  If there are multiple skimmers on the pool, we do question the technical or safety-related appropriateness of this choice. There is no doubt that the requirement will add significantly to the cost of the project, and may even be the straw that broke the project’s budget. And we see it of virtually no benefit.

But shouldn’t we be required use two equalizers for the skimmer of a one-skimmer pool? We already know that, hydraulically, there’s no particular advantage. In new-pool design, however, it certainly might be reasonable from a safety standpoint. Safety concerns have been growing in pool designs for years. Nonetheless, no state code in the U.S. requires it yet; so grandfather acceptability should remain for now. Single equalizers certainly function for a few hundred thousand pools already out there, and will remain typical in the design for thousands more in the near future.
A suction-operated, safety shut-down switch, now required for many hotel spas and tubs, might just solve this paradox; it certainly would for rehabs of old pools where busting out the bond beam for a second equalizer line seems clearly excessive.

                Let PPOA know your thoughts on this subject, especially if you have heard of a dangerous situation or any degree of suction entrapment in your knowledge or memory. In your editor’s 40-year experience, this is the first time he has ever heard of equalizer concerns. Obviously, we at PPOA would like to stay on top of this one – as well as all other safety issues, of course – hence our considerable interest in this new view of the dangers involved in circulation suction sources.  

~  kw

© 2007 Professional Pool Operators of America