A Case for the Gravity Drainage Option

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By: Alex Fletcher

Maindrain2 300X225One of life’s greatest experiences is a long cool dip in the pool. Children choose swimming over any other recreational activity and it is one of the healthiest forms of aerobic exercise. i It is an activity that many families enjoy in the comfort of their own backyard.

Now imagine one of life’s worst experiences, having to bury your own child and worse yet, being there as they drown, helpless to save them. Try as you might, their bodies held on the bottom by an unseen force.

Unfortunately one of life’s greatest experiences can lead to this horrible scenario. According to the Consumer Product Safety Commission, between 1999 and 2010, twelve people drowned as a result of suction entrapment. Of those twelve, eleven were children ages six to seventeen. ii

Suction entrapment is an unseen force created by the suction of a pump connected directly to the suction outlet in a pool. Generally speaking it occurs when a person comes in contact with an improperly protected suction outlet and part of their body, jewelry, clothing or hair is entrapped, entangled or held down by the force of the pump. iii The amount of this force can be staggering; with as much as eighteen hundred pounds of hold down force, literally vaporizing the water in the pipe and sometimes disemboweling small children who have sat down on the suction outlet in a wading pool.

In a 2002 suction entrapment incident, a seven year old girl named Virginia Graeme Baker was trapped underwater by a direct suction main drain. Despite the desperate attempts by her mother to free her she would not budge. Two men finally pulled Virginia out, using enough force to shatter the 8” grate that had suctioned her to the bottom of the pool. Though she was quickly transported to the hospital it was too late to save the little girl’s life.

The Virginia Graeme Baker story has a twist though. Virginia was the granddaughter of Secretary of State James Baker III and as a result powerful forces were set in motion and on December 19, 2007 the Virginia Graeme Baker Pool and Spa Safety Act became Law. The Virginia Graeme Baker Act requires that after December 19, 2008 every swimming pool or spa drain cover manufactured, distributed or entered into commerce in the United States conform to the entrapment protection standards of the ASME/ANSI A112.19.8 performance standard or any successor standard regulating such swimming pool or drain cover. These drain covers are specially designed to prevent suction entrapment and were required on all public pools by that date.

Maindrain3But because suction entrapment often occurs when there is a missing or broken grate the law requires pools with a single main drain other than an unblockable drain to have a secondary layer of protection. The second layer can be one of the following five options:

I.            Safety Vacuum Release System – A safety vacuum release system which ceases operation of the pump, reverses the circulation flow or otherwise provides a vacuum release at a suction outlet when a blockage is detected, that has been tested by an independent third party and found to conform to ASME/ANSI standard A112.19.17 or ASTM standard F2287.

II.            Suction Limiting Vent System – A suction limiting vent system with a tamper resistant opening.

III.            Gravity Drainage System – A gravity drainage system that utilizes a collector tank.

IV.            Automatic Pump Shut-off System – An Automatic Pump Shut-off System.

V.            Drain Disablement – A device or system that disables the drain.

VI.            Other Systems – Any other system determined by the Commission to be equally as effective as, or better than, the systems described in subclauses (I) through (V) of this clause at preventing or eliminating the risk of injury or death associated with pool drainage systems. iv

Only two of these options remove Direct Suction from the swimming pool or spa, Gravity Drainage Systems and Drain Disablement. Many Health Departments still require main drains for proper filtration and chemical dispersion so if you have a main drain in the pool there is only one, Gravity Drainage.

Recognizing the danger posed by direct suction outlets in swimming pools and spas, the Florida State Department of Health has required gravity drainage on pools since 1977 and all spas since 1993. Of the approximately 31,000 pools in Florida on gravity flow there have been no entrapment injuries or deaths. Of the approximately 6,000 pools and spas on direct suction there have been 6 deaths. Statistically speaking the State of Florida has saved approximately 31 lives by requiring gravity drainage on its pools and spas. v The CPSC also has no record of an entrapment incident on a gravity drainage system. Florida is the only state to have such a requirement.

Maindrain4 300X225Citing their perfect safety record the Florida Department of Health sought to require all pools and spas that were still on direct suction be retrofitted to gravity flow. “Florida regulations have required gravity drainage for public pools since 1977 and for public spas since 1993 to prevent entrapment” according to Patti Anderson, Bureau Chief for Water Programs with the Florida Department of Health (DOH). “Gravity Drainage is the failsafe anti-entrapment system because there is no direct suction entrapment possibility, and it requires no maintenance or testing to ensure proper function of the anti-entrapment feature.” vi The DOH gave a graduated timeline over 3 years to have the work completed targeting the most dangerous pools first, wading pools, then spas and then pools. Most of these older pools were in some sort of disrepair and many were retrofitted and brought up to code in other areas as well.

Unfortunately as the deadline approached, in a move designed to circumvent the Health Department an amendment was added to a very popular construction bill to allow the other four options. The Department of Health, the United Pool and Spa Association and the Florida Swimming Pool Association opposed the legislation but were unsuccessful and the amendment became law.

When presented with these options none of the 18 engineers at the DOH would sign off on a method for the Department to approve plans for the installation of these other options. This left the Department with no alternative but to place all liability for any incidents involving the other options with the owner. Responsibility for proper installation, testing and operation lie squarely with the owner of the property. Many owners, being properly informed of the performance of the 5 options, the liability incurred and the ongoing maintenance and cost of testing, have chosen the Gravity Drainage Option. Many, unfortunately, lured by the promise of a savings of money have not.

Now a Gravity Drainage Standard is being written by the ASTM. Many on the original committee were SVRS manufacturers. The preliminary requirements coming from this committee would render many gravity systems deemed unsafe. With a perfect safety record how could this be possible? The UPSA and FSPA have since had members join the committee and have enlisted the University of North Florida to do comparison testing between Gravity Drainage and Direct Suction. As you will see in their report and in subsequent testing, the damage done by direct suction in an entrapment incident happens in the blink of an eye and the forces applied are staggering. When you couple this with the need for ongoing maintenance and the ease with which the other options can be disabled, there is only one option that makes sense when a main drain is present in a pool — a Gravity Drainage System.

A Gravity Drainage System works on the principle that a properly sized and located collector tank installed between the pool or spa and the pump can limit the amount of suction created when the main drain is blocked. The suction line to the pump is connected to the collector tank, not the main drain. Water flows from the main drain in the pool to the collector tank by gravity, and from the collector tank to the pump by pump-caused suction. With this recirculation system, the main drain is not a source of pump-caused suction.

In Florida the basic design requirements for a gravity system are as follows…

The main drain grate must meet ASME/ANSI A112.19.8-2007 for drain covers/grates.

The main drain outlet shall be connected to a collector tank.

The main drain sump and grate must be sized such that the maximum velocity of the water flow through the grate does not exceed 1.5 feet per second.

The piping connecting the main drain outlet to the collector tank must be sized so that the maximum velocity of the water flow from the main drain to the collector tank does not exceed 3 feet per second.

The capacity of the collector tank shall be at least one minute of the recirculated flow unless justified by an engineer. Vacuum filter tanks are considered collector tanks.

Collector tanks must have a minimum 2.25 square feet of water surface area. 

Maindrain5 300X224Because of Florida’s perfect record these design requirements were followed and a test tank was built by Vak Pak, Inc. in Jacksonville, Fla. The 1000 gallon test tank is designed to simulate the hydraulics of both a 100 gallon per minute pool and a 60 gallon a minute spa with a 120 gallon a minute therapy pump. They are set up to run both on direct suction and gravity flow. The University of North Florida then built an apparatus which would measure the hold down force created when the main drain was covered.

The results were dramatic to say the least. The direct suction system tests resulted in 1505 lbs. of hold down force normalized to 6.8 lbs. per square inch almost immediately after starting the pumps. Conversely the gravity flow system only resulted in 183 lbs. of hold down force normalized to approximately .8 lbs. per square inch and took approximately a minute to accomplish. The normalized psi is due to the 16”x16” open area under the device. vii

While this is certainly a striking difference it still did not definitively explain why no one has ever been entrapped on a gravity flow system. The apparatus built by the University of North Florida only lifted straight up and is not a good representation of how a body would act in the water. A body actually could not lift straight up but would either lift up from the front like a push up or roll from one side or the other in an effort to free itself. It was hypothesized that because there has never been an entrapment incident on a gravity system for two reasons:

There was no immediate force to entrap a person and a seal had to be created and even the smallest break in the seal relieved any suction force. Contrast this to direct suction where even when the main drain wasn’t sealed there was an incredible amount of force.

Because of the small amount of hold down force per square inch, even at maximum drawdown a body could easily break the seal.

Maindrain6 300X224Members of the United Pool and Spa Association decided to take the test a step further and designed a blocking element that could be lifted from one corner to test this theory. The 16”x16”x1/2” 11 lb. rubber mat was attached by the corner to a digital scale on a chain hoist and tested on both the gravity flow and direct suction main drains with an old flat style grate in place. This test yielded 7 lbs of force to lift the mat off the gravity flow system by the corner and 17 lbs of force from the direct suction system. Because most entrapment situations occur when there is a missing or broken grate the grates were removed and the tests run again. The gravity flow test resulted in the same 7 lbs of force. The test on the direct suction system was unsuccessful though because when the mat was placed over the drain without a grate the entire mat was sucked down into the drain. Even though the drain was not sealed the mat could not be removed until the pumps were shut off.

Again, there was a dramatic difference but not enough to conclusively say that you cannot be trapped on a gravity drainage main drain. There was only one thing left to do…conduct a live test with a human being on a gravity drainage main drain.

The first test was done on a wade pool with a 12”x12” main drain and a flow of 50 gallons per minute. It was run with the VGB approved grate in place and the subject was unable to seal the drain. Next the grate was removed and the tests run again. The subject, a 6’ 240 lbs. man was unable to seal the 12”x12” frame. A gasket was then made reducing the open area to 10”x10” and the test run again. This time the drain was sealed and the collector tank began to draw down. Afterwards, the subject reported that there was a definite pull at first from the kinetic energy of the water in motion but once that stopped it leveled off and did not increase appreciably as the collector tank drew down. Once the collector tank had drawn down as far as it could the subject was notified and easily removed himself from the drain. The test was run a number of times with the subject on his stomach and on his back with similar results each time. At no point could the subject not easily remove himself from the main drain.

Maindrain7 300X168A test was also run using the mat that was used in the test tank. It was placed over the main drain and once the collector tank had reached maximum draw down the subject tried to remove it by hand from an eyebolt in the middle. He could not remove it consistent with the 183 lbs of force required to lift it straight up as documented by UNF. He then grabbed it by the corner with just his thumb and forefinger and easily removed it also consistent with the tests run in the tank.

Next the same tests were run in a pool in 2’8” of water. This was a 75 gallon per minute system again with a 12”x12” main drain. The first difference became apparent when the subject could not hold himself on the main drain due to his buoyancy. A 23 lb. weight belt was employed and the subject was able to seal the drain. However, as soon as he took the belt off his back his buoyancy lifted him off the drain as there is virtually no force to hold you there in the beginning. The belt was used again and not removed and the collector tank lowered to the maximum draw down. The results were identical to that of the wade pool. At no point was the subject unable to remove himself from the main drain. Whether on his stomach or his back, whether he rolled off or did a push up, each time he easily removed himself even with the weight belt still on his back.

In a subsequent test in 4’6” of water the subject could not seal the 12”x12” main drain frame even with the aid of the gasket and the weight belt.

Because of the hydraulic requirements of 1.5’ per second through the main drain grate in Florida that necessitate a 12”x12” main drain frame and grate even a man of above average size could not seal the main drain with the grate missing.

There is no immediate force drawing you to the drain and a seal must be created even with the 10”x10” “gasket” in place.

Buoyancy is a large factor not included in any testing done up until the time. The subjects buoyancy allowed him to float above the main drain in the wade pool with no effects even in only 11” of water and the subject was unable to even create a seal in 2’ 8” of water without the aid of a weight belt. In 4’6” of water the subject could not create a seal even with the aid of the gasket and a weight belt.

While the overall number of the removal force seems large at 183 lbs it is only because of the large area of the drain and actually only results in approximately .8 lbs per square inch.

The seal is easily broken and as soon as it is the force disappears entirely and there is nothing to hold the subject in place.

Because a collector tank requires no power and has no moving parts there is no maintenance or ongoing testing required to ensure it is operating properly. Also, because of its design it would be very difficult to disable. This is not the case with the other options. SVRS’s and Automatic Pump Shut off Systems require power and ongoing testing to ensure they are operating properly. Suction Limiting Vent Systems can easily be disabled. Even Drain Disablement adversely affects the pool as it alters the proper dispersion of chemicals and creates “dead spots” in the pool. Clearly, the only sure-fire failsafe option to prevent suction entrapment when a main drain is present is a Gravity Drainage System.

If you are still not convinced compare the experience of our test subject with the following account taken from the website of the Law Firm handling estate of John Van-Joy Jr. who drowned in a hot tub at the Sandals’ Royal Bahamian Resort.

“While enjoying a once in a lifetime trip with his fiancée, Nicole Cleveland, John Van-Hoy Jr., drowned when he became entrapped in a spa at the Sandals’ Royal Bahamian Resort in Nassau, Bahamas. On December 28, 2010, at the end of the day, John was in approximately 3.5 feet of water in a small circular hot tub / spa at the resort, while Nicole relaxed nearby. John submerged his body under the warm waters, but never came up. As it turned out, his body had been “sucked” onto the single suction outlet drain at the bottom of the hot tub.”

“Although he was 33 years old, a former all-state baseball player, in excellent shape, and could bench press over 300 pounds, John could not free himself. Nicole, realizing something was horribly wrong when John did not re-surface, tried unsuccessfully to free John from the suction forces generated by the spa’s pump and water circulation systems. In response to Nicole’s screams for help, five more heroic guests jumped into the hot tub but collectively also could not free John from the suction outlet drain at the bottom of the spa. Complicating matters, experts who have visited the resort concluded, amongst other things, that there was no nearby emergency shut-off switch to turn off the spa’s suction pump in time to save John’s life. Ultimately, Nicole watched her fiancée die before her eyes. John left behind two boys, ages 15 and 5, his fiancée, two sisters, a brother and his parents.”

By Keith Brias and Richard Husk on May 24, 2011 viii

If a former athlete, aided by five guests could not get free from a direct suction main drain, what chance would a child have? It is not necessary to have direct suction in a swimming pool or spa. There is a vastly better option…Gravity Drainage.



The testing performed in this report should not be attempted.

The participants in this test are certified divers and industry professionals.

Attempting these tests on a direct suction main drain sump may be harmful or even fatal.


i Business Lexington, “Kentucky Needs to Update its Pool Code” by Tracynda Davis

 ii Consumer Product Safety Commission  1999-2010 Reported Circulation/Suction Entrapments Associated with Pools, Spas and Whirlpool Bathtubs, 2011 Report

iii Pool and Spa News, “The Five Types of Entrapment” by Rebecca Robledo 11/26/2010

iv “Virginia Graeme Baker Pool and Spa Safety Act”

v Robert Pryor, Florida State Department of Health

vi Florida Community Association Journal, “State Law to Join Federal Law” by Kathy Danforth July 2011

vii “Characterization of Swimming Pool and Spa Main Drain Technologies” James H. Fletcher, PHD Principal Investigator, University of North Florida 8/17/ 2012

viii http://news.braislaw.com/20