Swimming Pool Main Drains Don't Move Pool Water

What Most People Don't Know About Swimming Pool Main Drains

Swimming pool main drains don't move water, pool water inlets do; swimming pool main drains can be used to receive water, but they don't actually move pool water. Let's take a look at the language on swimming pool circulation and swimming pool drains for most of the United States from 1928:


VII. Inlets and Outlets

A. All pools should be provided with an outlet at the deepest point of sufficient size to permit the pool to be completely drained in four hours or less. Outlet opening in the floor of the pool should be at least four times the area of the discharge pipe, to reduce suction currents. This opening must be covered with a proper grating.

B. In rectangular pools with deep water at or near one end, multiple outlets should be provided where the width of the pool is more than 20 feet. In such cases outlets should be spaced not more than 20 feet apart, nor more than 10 feet from side walls.

C. Proper pipe connections must be provided in recirculation pools to permit water being drained directly to the sewer, as well as to recirculation pumps. In making connections... prevent any possibility of sewage from the building or from outside backing up into the pool.

D. Inlets for fresh or re-purified water should be located to produce as far as possible a uniform circulation of water throughout the entire pool. In semi-artificial pools of irregular shape a careful study should be made of probable circulation currents and inlets located and spaced to provide as complete circulation as possible. All inlets should be located at the shallow water portion of the pool and not more than 1 foot below water line, except in case where reverse circulation is used as discussed in paragraph H.

E. Where the distance across the shallow portion of the pool is more than 20 feet, multiple inlets must be provided, so spaced that each inlet will serve a linear distance of not more than 20 feet. At spoon shaped rectangular pools where the outlets are located more than 5 feet from the end wall, inlets should be placed at both ends of the pool. At large pools with outlets near the center, inlets should be placed at the specified intervals entirely around the perimeter of the pool.

F. In small rectangular pools with only a single inlet and a single outlet, inlet and outlet should be located on a line drawn lengthwise through the center of the pool. Inlet orifices located at or below normal water level should be covered with a grating having openings of at least twice the orifice area.

G. Each inlet should be designed as an orifice and proportioned to supply the volume of water required at that particular point to obtain the best circulation. Inlet piping should be designed to provide at least twice the area of the inlet orifice. In large pools the inlet pipe system should be designed in sections with gates to permit regulation of the flow to different inlet orifices.

H. In a few cases pools have been designed for fresh water or re-purified water to enter at the deep point and overflow through outlets or skim gutters in the shallow portion. It is believed there may be some advantage in having flow through the pool in this direction, thus permitting floating matters and dirtier waters from the more crowded shallow area to be carried off more rapidly. The committee suggests that in designing piping systems for recirculation or flowing through pools, cross-connections be provided so that flow through the pool may be in the direction which experiments may prove most desirable. It is also suggested that the question of having skim gutters serve as overflows and outlets in recirculation or flowing through systems be studied more carefully, as it appears that such design may have certain material advantages.

The 1928 language was concise, accurate and does not conflict with the physical laws of fluids. That was a day when circulation systems were circulation systems and a drain was a drain!

What If We Still Built Swimming Pools Like They Did in 1928?

How did we get the 80/20, 60/40, 50/50 mandates? Where did we forget what was happening and why? Swimming pools were safer by the 1928 design standards than they are today. When you think of something new, like ANSI/APSP-7, and how many pages are included to deal with drains, then find out that we could eliminate all submerged suction like they did in 1928, you just shake your head.

Is a Swimming Pool Main Drain Necessary?

Consider an average size public pool 85 x 100 x 4.5 feet with 288,000 gallons. A six-hour turnover would require 800 GPM, which can be handled by a single 18 x 18 drain cover (tested to ASME A112 19.8 as safe, single suction; safety backup options allowed by the Virginia Graeme Baker Pool and Spa Safety Act include an automatic pump shut off, which has been tested to the ASME A112 19.17 SVRS standard).

Assuming all flow is going to the swimming pool main drain, what are the chances that debris on the floor will find it? Before you answer, consider that the drain has an area of 2.5 sq a. ft. and the pool is more than 8,500 sqft. A main drain is going to have little influence on dead spots some 100 or more feet away if it doesn't have a significant effect at 11 inches!

What We Got Right in 1928

What is also missing in 1928 is anything about "circular motion of the water." In fact, the writers went to great lengths to attempt to create "plug flow" where drains were used. They put limits on where the returns would be to encourage the water to flow toward the pool drain, moving dirty water on the way to the drain. It was a North American standard, so they did not consider Coriolis effect and toilets flushing in Australia either (if you think the rotation of the earth has an effect read this).

Not only is this 1928 language accurate and concise, it counters most, if not all, of the folklore that plagues the swimming pool industry today. Anecdotal evidence of cloudy pools associated with no drains, etc. don't really stand up when one actually goes out and measures what is going on; there are always other explanations.

The Bottom Line

The industry should not feel embarrassed by these blatant statements that counter what they were taught by codes and health departments. In the grand scheme of construction, knowing the details and physics behind circulation is a low priority. One can't really do much with it in the field, and industry standards writing committees are working to update the basic guidelines in the next generation standards.

What I do have a problem with, especially in light of someone's loved one dying, is the unwillingness to learn. All states should take note. This is not debatable with any scientific merit. There is not one person that is competent in fluid flow that would argue this point: Swimming pool main drains don't move water, pool water inlets do.

It's that simple. Drains can be used to receive water and should not be intended for anything else. Most of our standards would not let us build a pool following the water circulation language from 1928.

They were right then, we are wrong now.

Updated by Dr. John Mullen