By Dave Peterson
The following is the text, somewhat altered for this audience, of a letter I’ve sent a number of health departments in the course of working on projects for our clients.
There is no doubt that the recent wave of legislation, codes and standards regarding suction entrapment has caused confusion – not just in the pool and spa industry, but also among lawmakers, inspectors and contractors as well as pool and spa owners. These mandates, however well intended, have all too often been confusing or contradictory and frequently reflect neither common sense nor sound engineering.
Our firm and others have been retained by numerous owners, contractors and maintenance companies to guide them through these murky waters. Along the way, I’ve become particularly frustrated by the apparent lack of understanding of basic physics among the authorities who have jurisdiction. In pointing this out, I’m not picking on any particular individuals or groups, because I understand that it takes many people to write, administer and enforce regulations.
In fact, I have no one but myself to blame for not having gotten more involved earlier in the process.
Time and again since the rules went into effect, I’ve run into existing drains that are out of compliance – which is not at all surprising – for which the resulting fixes are so exorbitantly expensive that abandoning the pool is often the best solution. Even with new pool construction, I’ve encountered plan checkers who are stunned to see plans that do not comply with the new regulations – yet recognize that it’s not because designers are ignorant of code requirements, but rather that the codes themselves simply do not comport with sound engineering standards.
It is my contention that the very best way out of this mess – one that complies at least with the spirit of the law while eliminating suction entrapment risks – is to get rid of main drains wherever possible in both existing pools and in new construction. Non-compliant drains on existing pools could be abandoned, while new projects may be designed without suction outlets.
DRAINS AND SUCTION
To appreciate this argument fully, we must first consider the history of main drains and then the physical characteristics of water itself. On both levels, it is clear that main drains are genuinely obsolete. (And just because there are some existing pools where skimming is too inadequate to allow for the complete elimination of suction outlets, that’s no excuse for not applying modern design principles to new projects!)
If we go back in time about 90 years, many pools of that era used a fill-and-dump or fill-and-draw method of maintenance. At the time, water was not seen as the critical resource it is today, and effective pump and filtration technologies were still both being developed. (Pressure sand filters were not commercially available until the 1950s, for instance, while diatomaceous-earth filtration came to prominence in the ’60s and cartridge filters took hold in the ’80s and ’90s.)
The fill-and-dump process is an obvious solution to a lack of suitable water-maintenance technology – and is much easier to implement if the pool can be completely drained via a port set in the deepest part of the pool, more or less like a bathtub drain. This is why the term “main drain” was applied at the dawn of the modern pool industry and why it has stuck for so long, even though these outlets have for generations been connected to pumps for recirculation and now have nothing to do with draining pools.
So why are main drains obsolete?
In many projects, skimmer equalizers are used to protect pumps from low water levels and from the possibility of the skimmer line being blocked by leaves or other heavy debris. When installed, these suction devices should be done in split pairs as a safety measure.
In my book, skimmer equalizers are usually unnecessary. Water-level issues are better addressed through installation of an autofill device, while in the case of blockage the worst that can happen is damage to the pump – a far less serious consequence than an entrapment incident. Moreover, blockages aren’t much of an issue at all with indoor pools, although the requirements are the same.
I say get rid of them wherever possible and, more important, fix the code!-- D.P.
Discussions of that third point can get interesting. In fact, I’ve actually had health officials tell me that the pool pump is used to drain the pool – an incorrect statement that defies physics. While it is true that a pool pump can be used to drain a pool partially, it is only a matter of time before the pump loses prime and stops functioning.
After all, in most installations the equipment sits on a pad at deck level, which means the pump’s suction port is at least a foot above the nominal waterline. The friction loss on the suction side of the pump averages about five feet of head loss, which means the pump “feels” as if it’s lifting deck-level water from a depth of about six feet when set at deck level. A pump used in this sort of application is only designed to lift water seven to eight feet, so this leaves us with only about two feet of available drawdown in the pool.
A good pump might do a little better, but we are not likely going to find a typical pool pump capable of draining a pool that’s five feet deep or deeper. This isn’t a manufacturer’s dodge: This is physics and the limitations it puts on pumps subject to atmospheric pressure, pump design and tolerances – and we can’t change it.
This is why every maintenance person I know carries a submersible pump on his or her truck. If main drains truly worked as drains, why would this expense be necessary? It’s not just because submersibles give us the ability to run the drained water through a filter or direct the discharge to a specific location – the soil, perhaps? – instead of dumping it down the sewer or into a storm drain.
PHYSICS IN ACTION
Now let’s look at the second reason we should consider main drains as obsolete – that is, physics and specifically how it relates to maintenance.
For starters, it is generally clear that we really do not need drains or suction outlets for pool maintenance. Historically, the belief has been that floating debris is removed by a skimmer or gutter system and that sinking debris eventually moves toward and is pulled into an outlet or outlets set in deepest part of the pool. We now know, however, that debris is not sucked into these outlets: It must be pushed there, and “there” can be anywhere, including the skimmer alone if that’s what is desired.
Again, the laws of physics cannot be changed. Just as with the fundamental constraint that limits how high a pump can lift water to drain a pool, water itself has properties that cannot be overcome.
When physicists and engineers talk about forces that govern water’s behavior (or that of any other material), we often categorize them as compressive (pushing together), tensile (pulling apart) or shear (sliding with respect to each other). Liquid water has a unique ability to support compressive forces, but not tensile or shear forces.
Imagine two water molecules sitting on a table. If the molecules are touching each other and you push one into the other, you would expect both to move in the direction pushed. This is an example of compression. But try as you might to compress the two molecules into a smaller volume, water is essentially incompressible, so displacing the first molecule by one foot will displace the adjacent molecule by one foot.
In terms of pool operation, a return inlet jutting through the wall of the pool applies a compressive force to the molecules and they will move. (This is why the flow through a return inlet can be felt several feet away.) Displace the molecules at the pool wall by a few feet and the molecules in line will be displaced by an equal amount. Eventually, however, these molecules will slip out of line and the jet’s influence gets wider and weaker. Nonetheless, the compressive physics are undeniable.
Tensile force is different. Imagine the same two molecules on the table, but that this time you’re trying to pull one away from the other. The water is not “sticky,” so the molecule left behind just sits on the table by itself. It is not dragged along by the molecule you’ve moved, meaning that displacing one molecule has no effect on the adjacent molecule.
Back in the pool, when a suction outlet removes molecules at the outlet cover, it is not “pulling” in adjacent molecules: As each molecule is captured and removed by the pump, hydrostatic pressure on the remaining molecules forces them to fill the void. In other words, it is compressive force that squeezes molecules into the voids left by the departing molecules, not any tensile force. And the molecules could just as easily be debris or pathogens as water if they happen to be in the vicinity.
SUCTION AT WORK
Through the years, studies have shown how physics governs the flow of water in pools. At my company, we’ve checked into these phenomena on our own, dusting the floor and drains of a pool with silica sand and then turning on the pump.
Dealing with Reality
As mentioned at the start of the accompanying text, most of this narrative comes directly from letters I’ve written to various health departments to help clients solve problems related to compliance with provisions of the Virginia Graeme Baker Pool & Spa Safety Act. Many of these clients work with pools built in apartment complexes in the 1960s.
The stories are strikingly similar: At some point in the pool’s history, someone noticed an increase in water consumption and called in a leak-detection company that found the problem in the main drain. (Typically, pools of that era were built with copper pipes, and my suspicion is that, through the years, trichlor tablets had eaten holes in the copper.) To remedy the problem, someone simply plugged the drain, reinstalled its cover – and nobody was ever concerned about the non-functioning suction outlet.
Some of these pools had operated for 15 or 20 years or even longer with perfect water quality by the time the VGB Act prompted a new round of inspections. These pools all worked: Indeed, detailed chlorine residual tests conducted at multiple depths, locations and times have shown that the water circulates and mixes adequately. Furthermore, none of these pools had ever been cited for failing to meet health department standards despite inspections being done by various personnel and even by multiple jurisdictions.
As I see it, these watershapes offer compelling evidence that pools can operate without main drains or suction outlets while still containing healthy water. The problem is that the same health departments that had given passing marks to these pools in the past are now requiring owners to install main drains at the deepest parts of these pools for the ostensible purpose of draining and recirculation.
The copper pipes in these old pools still have holes in them, of course, so the only solution is to start all over with a new main-drain installation in a split-pair arrangement. This requires expensive demolition of the pool floor and some of the walls, plus extensive work on the decking along with trenching and plumbing tasks. In 2010 dollars, these costs are often greater than the original pool’s total price tag. And all of this is done to add an unnecessary suction outlet hazard to a perfectly functioning pool: After all, code requires it, and no government employee has the authority (or wants to take the risk) of making an exception for these pools.
What’s worse, I’ve been told by numerous builders that when they see these disabled drains, they simply cover them with a new, approved cover and add a safety vacuum release system (SVRS). Yes, just adding this device to a completely disabled line seems to be enough to enable an inspector to approve and reopen the pool as “VGB compliant” – despite the fact that the pool does not meet the antiquated health code requirement for a main drain!
I’ve always been honest about the projects I’m involved with, but I keep encountering builders and technicians who are willing to bend the truth – and health departments seem to be in denial about what’s happening. Let’s simply put an end to this game and revise the codes to allow pools to operate without main drains or suction outlets!
And let’s not forget shear forces. Imagine those two lonely molecules sitting on the table again: If you slide one along the other you are imparting a shear force, but the water molecules are not sticky so the second molecule does not move beyond maybe rotating a bit. This is not the same as, say, a pile of straw where sliding one piece out will likely drag others along with it: That simply doesn’t happen with water. In effect, suction outlets fail because of water’s inability to support shear and tensile forces.
To be sure, I’ve oversimplified a bit in laying out these examples by saying that water molecules aren’t sticky. Water molecules do bond together due to weak electrical attractions, but those forces – which we see as adhesion and cohesion when we overfill a glass of water – are negligibly weak compared to the compressive, tensile, and shear forces that influence water’s behavior in watershapes. In that context, in other words, it’s safe to say that water is not sticky.
So how do suction outlets remove debris? The answer is that they only do so if we push debris to them and that the best way to do so is with good circulation via return inlets. This also means that the suction outlets don’t need to be at the deepest point of the pool or even on its floor. In fact, we have built pools without any suction outlets, and their water is perfectly clean because the debris eventually finds itself at the weir of the skimmer.
To be sure, sand isn’t buoyant enough to get up to the skimmer, but this sort of debris does not seem to affect water clarity or chemistry the way lighter, more deleterious material might. We handle such debris with weekly vacuuming or deploy an automatic pool cleaner for a few hours a week.
BACK TO BASICS
To bring this discussion to a conclusion, it is my belief that outlets are not needed at the deepest part of a pool – and maybe not at all.
This is an important concept for many reasons. First, there are many pools – even heavily regulated commercial pools – that never had suction outlets yet perform well from maintenance and sanitation standpoints. Second, given all the attention on suction-outlet safety, the safest of all pools would be one in which the hazard is completely eliminated. Why, then, do some health officials insist on owners spending tens of thousands of dollars to retrofit existing pools with unnecessary, ineffective suction outlets? Is it only because an antiquated code requires them?
I believe it’s time for everyone, from lawmakers and local authorities to watershapers and pool owners, to recognize that we do not need main drains. In fact, we would all do well to eliminate them wherever possible and effectively say goodbye to the suction entrapment risks that come along with them.
Dave Peterson is president of Watershape Consulting of San Diego, Calif. He’s been part of the watershaping industry since 1994, starting his own firm in 2004 after stints with an aquatic-engineering firm and a manufacturer. A registered civil engineer, he now supports other watershape professionals worldwide with design, engineering and construction-management services and may be reached via his web site, www.watershapeconsulting.com.