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Are Main Drains Worthless Deathtraps?


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Let's look at the evidence scientifically.

It is my contention that the best use of suction is by the skimmers. I estimate that about 90 % of the debris that gets in the pool is buoyant. Buoyant being defined as stuff that floats for at least ten minutes. I estimate that about 5 % of debris that gets in the pool is neutrally buoyant. Neutrally buoyant being defined as stuff that floats for five minutes or less. And, the remainder of debris is non-buoyant. Non-buoyant being defined as stuff that does not float, but sinks readily.

Most of the debris that gets in the pool is blown in by the wind. Naturally, the lighter and less dense the debris is, the more it is blown by the wind. That is why most debris that gets in the pool floats.

If you have proper skimming action, the skimmers should catch about 90 to 95 % of all buoyant debris such as leaves, grass etc, perhaps 30 to 50 % of all neutrally buoyant debris and virtually none of the non-buoyant debris, such as sand etc. There should be at least one skimmer for every 200 square feet of pool

As you can see, there should be very little for the main drain to do in a properly designed pool with proper skimming action.

Now, let's look at the main drain. Of the non-buoyant debris, the main drain will catch virtually none, other than what happens to fall directly on it. Virtually all non-buoyant debris sinks rapidly and lands directly below the point where it hits the surface of the water.

Of the buoyant and neutrally buoyant debris, I estimate that the main drain will catch less than 5 to 10 percent, and the rest will come to rest on the floor. The floor can be brushed over and over to refloat the debris, but vacuuming is much faster and more effective.

Some people will claim that a main drain will help disperse chemicals that are added to the pool. If you wanted the chemicals to go into the filtration system, you could just dump them in the skimmer.

I think that as long as the chemicals are properly mixed and diluted before adding them to the water, there will be little benefit. Returns can be directed into the deep end with directional outlets to provide circulation.

It is my conclusion that in a properly designed pool, there is no need for a main drain; and that it is nothing more than a waste of perfectly good suction.

Now, on to safety.

A main drain can easily generate over 500 pounds of force. This force can hold down even the strongest person. The force can, and has, ripped the intestines out of people, leading to their deaths.

Main drains are a type of thing that is known as an "Attractive Nuisance". An "Attractive Nuisance" is something that is dangerous, but it is also interesting to people for some reason and it is something that is usually not immediately obvious to people, especially children, that it is dangerous. Kids love to play with main drain cover and find the suction interesting. They will often remove the covers if they can. The cover can become loose, lost or broken.

Once the cover is broken or removed, there are several ways a child can be injured. They can be held under water and drown, they can be disemboweled, they can get their hair, arms, legs, feet, fingers etc caught and be unable to get loose.

Many people now have automatic cleaners, such as the Polaris, that roam the entire floor, cleaning with suction generated by venturi action. Automatic cleaners are a good investment. An automatic cleaner totally eliminates any possible need or utility that a main drain might provide.

People without automatic cleaners should be brushing and vacuuming on a regular basis.

I recommend that people with young children avoid automatic suction cleaners, because children might play with the cleaner and become hurt by the suction.

There are only two possible uses that I can think of for a main drain. The first is if a person is continually getting large amounts of continuously neutrally buoyant particulates, such as dead algae. In that case, a main drain would provide some benefit. However, a competent pool owner should virtually never be in such a position.

The second is the main drain is a place to put a hydrostatic relief valve and the main drain could periodically be briefly opened to allow the enclosure to be cleared of debris.

Here are some good references:

(20/20: Pool Entrapment - Part 1)

(20/20: Pool Entrapment - Part 2)

(Swimming Pool Drain Safety)

http://www.youtube.com/watch?v=C54nZlnRtw0 (Swimming Pool Suction Entrapment)

(Is Your Pool's Drain Safe)

(Drain Testing using Dye For Effect on Pool Circulation)

http://www.fluent.com/news/pr/pr130.htm (Simulation of Water Circulation Improves Safety of Swimming Pools)

http://www.fluent.com/solutions/sports/CS1...ilogy-pools.pdf

In conclusion: It is my opinion that Main Drains are Worthless Deathtraps.

Edit: After reviewing the dye test video, I have serious doubts as to its authenticity. I do not believe that that drain is pulling 800 GPM.

800 gallons per minute is 107 cubic feet per minute = 1.78 cubic feet per second. There are 2.0625 cubic feet directly above the grate. The entire volume of water above the grate would be sucked in every 1.1587 seconds. There is no way that that is happening.

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The main purpose claimed historically of main drains is for circulation of water near the bottom of the pool. I don't believe it's ever really seriously been considered for removal of non-buoyant substances that fall from the surface. As you point out, that's primarily the responsibility of a pool cleaner which mostly roams along the pool bottom.

There have been quite a large number of reports (on multiple pool forums) of people with above-ground pools that got algae, either in the nascent cloudy stage or as a full green bloom, and even after shocking with chlorine the cloudiness was not able to be removed from the lower depths of the pool. Some solved this by use of a flocculant and vacuum-to-waste, but the main problem was poor circulation in the depths of the pool. Some of these pools were less expensive Intex pools which typically have weak pumps and small filters, but even those pool users who upgraded their pumps and filters still had difficulty clearing the lower depths of their pools. Pointing the return downward or diagonally down and to one side would usually help and the most common problems were with round pools of constant depth.

Now the question becomes whether this poor bottom circulation could be improved by pointing a return downward for a vertically circular circulation pattern as described in this blog post I had referred to earlier. I don't know the answer to that -- my hunch is that it would work IF there were proper placement of returns and skimmers in a non-traditional pattern. It seems that would work reasonably well in a square/rectangular pool. For a circular pools, perhaps a downward/diagonal flow is required. I think more studies (or flow models) need to be made for these other pool shapes/configurations.

As for entrapment, I thought that simply requiring two floor drains far enough apart (so that one person could not be on both simultaneously) pretty much solved that problem (and having a larger dome "2-level" area in a single drain is better than nothing). Also, if there is common suction from skimmers in addition to floor drains, then the amount of suction force is not as strong -- the greatest force (and problems) occurs when there is no other alternative suction flow path. Now commercial/public pools are another matter as they may be plumbed with separate pumps to just a floor drain and spas may only have one suction entry and significantly higher flow rates (for the jets). As far as I could tell, all of the links you gave with tragic examples all had one suction drain per pump.

The link to the dye test was interesting and completely inconsistent with dye tests I have made in my own pool in the past to see flows of circulation. If dye is added near the return flow in the deep end of my pool, it absolutely migrates down toward the floor drains (in addition to the simmer on the opposite side) and I could see that flow (mostly viewing from the shallow end where one can see the vertical profile). I don't know why the test in the video didn't show this except that the drain surface area was larger (though their flow rate was very high so should have compensated for that).

I'm not at all opposed to the idea of not having floor drains, but I just know that the very common round above-ground pools are pretty lousy in their circulation patterns near the bottom so something must be done to fix that. Of course, they typically don't have floor drains in the first place so the issue isn't one of entrapment but rather improving circulation.

Richard

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After reviewing the dye test video, I am not sure that they really were achieving close to 800 gallons per minute. The suction seemed too weak for 800 gpm. Perhaps they had some sort of error in their test or even faked it. Maybe it was 800 gpm, but I would want independent verification before believing it.

I have seen a 140-gpm flow through a 12 x 12 main drain pull a better vortex then what they showed.

However, I know from experience that a main drain really doesn't help keep a pool clean. I always leave the main drain off. You will get much better results using the suction for the skimmers.

Suction entrapment is based on several factors, such as pump size, plumbing size etc. Even with alternative flow paths, a running pump can generate several psi of suction at a main drain. For a 50-square inch available area main drain, every 1 psi of suction generates 50 pounds of force.

Another type of suction/mechanical entrapment is caused when a person with long hair gets near the drain and the drain sucks their hair in and the hair gets tangled and caught.

There can also be other mechanical entrapment where a person gets some part of their body trapped, such as hands, fingers etc.

As long as a person has adequate filtration and turnover, they should almost never be in a position where they need to clear their pool. If the pool can be cleared, it can be kept clear.

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I can't offer a link to a hydrodynamics study re: turnover, but I think main drains are essential to speedy and effective turnover and circulation (and temperature profile when using a heater), which I suspect could easily take 3-4x longer w/o main drains. Your views on cleaning are well taken, which is why I often close the ball valves controlling my mains between 1/3 - 1/2, to improve skimmer action (tons of cottonwoods in the ravine that girdles my subdivision).

Also, I believe the danger is virtually eliminated when using two drains spaced 6' apart, and a skimmer all the while.

I think your concerns are legitimate w/ older pools, especially commercial pools, but misplaced w/ newer residential installs. Just my opinion.

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I think main drains are essential to speedy and effective turnover and circulation (and temperature profile when using a heater), which I suspect could easily take 3-4x longer w/o main drains.

According to the study in this link (http://www.fluent.com/solutions/sports/CS103-trilogy-pools.pdf), properly designed pools without main drains are essentially equal to pools with main drains as far as circulation is concerned.

Pools without main drains would certainly not take "3-4x longer".

I think that if you have (8) properly placed returns, 3 to 4 skimmers, and sufficient flow rate in an 800 square foot, 30,000-gallon pool, your circulation to all areas of the pool would be very well distributed. I don't think that a main drain would make any significant contribution.

I would much rather use the available suction to add an additional skimmer than a main drain.

I think that properly designed, maintained and operated main drains can be reasonably safe. However, there are always going to be weaknesses in design, maintenance and operation. If even 5 % of main drains are not properly designed, maintained or operated, there will be a very significant risk to many people.

Covers routinely get removed or broken. The fact that the main drain is at the bottom of the pool significantly reduces the ability to inspect and maintain the cover. Health Department Inspectors are certainly not going to dive in the water to inspect the cover. The service person usually does not want to dive in the water, either.

I don't believe that the risk is worth whatever minor benefit a main drain might provide.

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I don't discount the potential problems, but I question the assertion that main drains categorically provide no benefit.

A particularly deep pool (> 8') would likely have improved circulation with a main drain. The cited study is by an outfit that builds 1-piece fiberglass pools, unlikely to be that deep.

Typical brushing patterns will tend to migrate sediment to the deepest part of the pool. A main drain help clean that out. A mechanical pool cleaner that traverses the bottom could have a similar effect; if you don't have one, you would probably benefit from a drain.

"Covers routinely get removed or broken" is an assertion worth testing; the few pools I have direct experience with do not have this problem.

--paulr

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I think that main drains are the kind of thing that just intuitively feels right, but when you study it scientifically, the proof is just not there to support the feeling.

I don't think that there would be a significant temperature differential from the bottom of the pool to the top in a properly designed and continuously circulating system.

Another reason to increase the flow rate of your skimmers is that if the flow rate of the skimmers is too low, the skimmer doors (weirs) will bang as swimmers splash and make waves. When the door bang, they will often break.

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High flow rate into a skimmer is worthwhile, however one could certainly have adequate skimmer flow without turning off the main drain. Thus the argument does not "scientifically" demonstrate that main drains should always be turned off. "I don't think there would be a significant temperature differential" is not a scientific argument, it is an intuitive argument; furthermore it is based on a "continuously circulating system" which is far from typical. The available scientific studies do not answer all the questions. This does not mean your intuition is wrong, but it means it is not proven in all cases.

I am all for data-driven conclusions, but the data must actually fully support the conclusion; the data presented so far does not do that.

--paulr

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After more consideration, my current opinion is that pools should be designed to operate without main drains.

However, they can have some utility and they do not have to be dangerous. Therefore, a main drain should be considered when designing a pool.

For normal operations, I would keep the main drain closed or partially open to allow a maximum of 10 % of the available suction to be pulled from the main drain. The main drain could be used more as needed or wanted.

There should always be at least two drains. The drains should have the latest approved safety covers. They should be inspected regularly to insure that the covers are intact and secure. The covers should be kept clean and free of debris.

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I suspect that main drains are a holdover from when swimming pools were designed much differently than today. Considering that the old skimmer troughs did not allow for rapid turnover, I think the main drains were utilized much more then, and simply didn't go away as designs changed. They can be utilized properlyand safely, and can contribute to good pool maintenance, but it would require installers to design differently.

I think using the phrase "worthless death traps" is the type of thinking that often generates a frantic media hype over a generally benign issue. If you focus on small portions of the data, you can find a way to label virtually anything "unsafe". If you look at the data on injuries and/or drownings over the past several of decades, bathtubs are a far greater risk than main drains. Lakes, rivers, ponds and streams should definitely be outlawed, and the oceans of the world should be drained and filled in.

Good judgement and proper supervision have no substitute. No mechanical device wll ever take the place of these. There were no more drownings or entrapment issues 40 years ago with single main drains in every pool, than there are now. Having said that, we should be looking for ways to design swimming pools to make them safer. If we actually had engineered plans based on physics, rather than just pretty drawings by "pool designers", this issue wouldn't exist. Since the world we live in is driven by profit, rather than by proper design & function, I don't see it happening, so we are forced to come up with mechanical devices to compensate.

Just my humble opinion...

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  • 3 weeks later...

A pool can be designed to work without a main drain. However, a well designed pool, designed to work without a main drain will still have one so the pool can be drained without dumping in a separate drainer pump. And if you live in a fire hazard area, you want that main drain hooked up (with valves) to your fire hose connection. Even if you don't want to stay and water down your roof, the firefighters can use it when they show up. Something to consider...

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