During the early months of 1999 several vacuum elimination devices were tested to determine their effectivenell as based on their ability to eliminate excessive vacuum from a typical swimming pool main drain.

To quantify the time/intensity of suction, the unit "inch seconds" was used.  This is the amount of suction exceeding 4.48"Hg (the previously published number that was felt to be safe) over how many seconds in the main drain sump.  This is determined by the resultant area of the curve generated by the suction in excess of 4.48"Hg.

Basically five types of devices were tested:

Atmospheric Vent
Dual Main Drains

Typical plotted time suction curves on each of these devices follows in each corresponding section for each in this report.


MECHANICAL ELECTRICAL:  This device is extremely sophisticated and can be set up to monitor and react to a variety of conditions.  It will automatically reset its threshold depending upon the conditions encoutered.  It functions to perform two independent tasks as a means of interrupting excessive vacuum.   Simultaneously it will remove power from the pool and open a vent in the pump suction line.

Tests with a typical unit of this design indicated it ability to eliminate vacuum draw in the main drain sump was from 10.28-inch seconds to 25.48-inch seconds of vacuum (or 660 to 1800 milliseconds.)

ELECTRICAL:  This device senses excessive vacuum and in turn removes the power from the swimming pool pump.  Et can be set to react to various levels of suction signals.

Tests with two units of this design indicated their ability to eliiminate vacuum draw in the main drain sump was from 13.53-inch seconds to 41.06-inch seconds of vacuum (or 780 to 3600 milliseconds.)

MECHANICAL:  This device is dependent upon the mechanics of excessive vacuum triggering an atmospheric vent in the main drain line.  It can be set to react to various suction inputs.

Tests with a typical unit of this design indicated its ability to elminate vacuum draw in the main drain sump was from 13.73-inch seconds to 16.94-inch seconds of vacuum (or 765 to 900 milliseconds.)

ATMOSPHERIC VENT:  This device consits of properly designed and placed piping as part of the swimming pool main drain system.  Ther are no mechanical or electrical components.

Tests with six typical units of this design inicated their ability to eleminate vacuum draw in the main drain sump was for 1.52-inch seconds to 4.23-inch seconds of vacuum (or 330 to 443 milliseconds.)

DUAL MAIN DRAINS:  This is the currently specified method of prevention for suction entrapment or prolapse.  It consists of two equal main drains seperated by a distance of 3 feet.  The drains are plumbed hydraulically equal so that either one can handle the entire designed flow rate without exceeding maximums.

Tests with a typical set up of this design inicated its ability to eliminate vacuum draw in the restricted drain sump was from 0.14-inch seconds to 0.17-inch seconds of vacuum (or 105 to 113 milliseconds.)


These various devices seem to have their specific characteristics, which are generally repeatable in tests.  A summary of the general suction eliminating ability as measured in "inch seconds" and milliseconds of vacuum exceeding the "safe level" in the main drain sump of each category tested is given below.

RANGE in "Inch Seconds"

                                Mechanical/         Electrical         Mechanical         Atmospheric Vent.         Dual Drains
                                10.28 to                     13.53 to           13.73 to                         1.52 to                         0.14 to
                                 25.48                          41.06                 16.94                               4.23                             0.17

RANGE in Milliseconds

                                Mechanical/         Electrical         Mechanical         Atmospheric Vent.         Dual Drains
                                660 to                        780 to               765 to                         330 to                            105 to
                                1800                           3600                   900                               443                                 113


All things considered, it appears that it would be beneficial to keep the time/vacuum component in the main drain sump to its practical minimum.  Looking at the above table it is obvious that the dual maind drain sump arrangement produces the least component for suction entrapment or prolapse.  The atmospheric vent on the other hand seems to hold great promise.  It is not dependent on mechanical or electrical components.  It is less expensive to install than dual maind drains.   And for retrofit is immensely less expensive and less difficult to install.

An on site design of a main drain vent system can be made to work extremely effectively, however, there is no guarantee that it's functional dimensions can and will be maintained in the field.  Without strict adherence to its dimensional design the unit may not allow sufficient flowage, or may allow excessive main drain suction.

To work around this shorcoming a vent system has been designed and incorporated into a manufactured unit, "Vacuum Eliminator," that has the characteristics required built into it for all time.  Such units can easily be available in all current PVC pipe sizes.  Each is designed to appropriatley limit the suction as above, and yet allow for full flow design of the related main drain piping.   Also, this type of unit automatically resets itself to exactly the same limits time after time without any variation or attention.

The mechanical and electrical units generally have fail safe features built into them, however, there is still a mechanical or electrical component to be concerned about.

And finally, although the Dual Main Drains seem to provide the maximum degree of safety they haveone serious drawback that the vent system does not have.   That is that if a child or "float" covers one of the maind drains, the other main drain can be a waiting death trap.

With a single vent system the entire suction capability of any pump (1 HP or 20 HP) n any single main drain (of any size) is limited to the losw or suction provided by only 42 inches of gravity head - or well under the critical limit regardless of pipe sizes.

Related Reports:

"VISUAL STUDY OF PROLAPSE" by Leif Zars Dated 6/15/99
"VACUUM ELIMINATOR" by Leif Zars Dated 6/22/99
"SUCTION ENTRAPMENT TESTS" by Leif Zars Dated 2/6/98

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High Vacuum
Mechanical Vent Valve Opens and Electrical Motor Cuts Off

Seven Tests:    10.95 Inch Seconds Device "A"
                           11.53 Inch Seconds Device "A"
                           14.06 Inch Seconds Device "A"
                           12.72 Inch Seconds Device "A"  Average 12.35 Inch Seconds
                           12.07 Inch Seconds Device "A"
                           11.47 Inch Seconds Device "A"
                           13.67 Inch Seconds Device "A"

Five Tests:        13.30 Inch Seconds Device "B"
                           25.48 Inch Seconds Device "B"
                           10.28 Inch Seconds Device "B"  Average 14.06 Inch Seconds
                           10.60 Inch Seconds Device "B"
                           10.63 Inch Seconds Device "B"

Devices "A" and "B" quite similar

Gallons Per Minute Flow: 60 to 64
Pipe Size: 2" PVC
Main Drain Sump: 8" Round Hayward

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High Vacuum
Electrical Cut Off Switch

Three Tests:     41.06 Inch Seconds Device "A"
                           23.53 Inch Seconds Device "B"  Average 26.04 Inch Seconds
                           13.53 Inch Seconds Device "B"

Gallons Per Minute Flow: 60 to 64
Pipe Size:  2" PVC
Main Drain Sump:  8" Round Hayward - No Grate

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High Vacuum
Mechanical Vent Valve Opens

Seven Tests:     15.72 Inch Seconds
                            15.44 Inch Seconds
                            15.35 Inch Seconds
                            16.94 Inch Seconds      Average 15.47 Inch Seconds
                            15.99 Inch Seconds
                            13.73 Inch Seconds
                            15.16 Inch Seconds

Gallons Per Minute Flow: 60 to 64
Pipe Size:  2" PVC
Main Drain Sump:  8" Round Hayward - No Grate

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High Vacuum
Draws Water Below Water Level in Vent Line
    Breaks Suction

Eight Tests:     3.92 Inch Seconds Device "A"
                          3.75 Inch Seconds Device "A!'
                          3.65 Inch Seconds Device "A"
                          1.85 Inch Seconds Device "B"    Average 2.48 Inch Seconds
                          1.89 Inch Seconds Device "B"
                          1.73 Inch Seconds Device "B"
                          1.58 Inch Seconds Device "B"
                          1.52 Inch Seconds Device "B"

Eight Tests:     2.08 Inch Seconds Device "C"
                          2.19 Inch Seconds Device "C"
                          2.38 Inch Seconds Device "C"
                          1.87 Inch Seconds Device "D"    Average 2.66 Inch Seconds
                          2.17 Inch Seconds Device "D"
                          2.29 Inch Seconds Device "D"
                          4.10 Inch Seconds Device "E"
                          4.23 Inch Seconds Device "E"

Device "A" consisted of a 2" Vent Line Located in Main Drain Suction Line.
Device "B" consisted of a dual 2" vent line assembly in Main Drain Suction Line.
Device "C" consisted of a single 2" vent line assembly in Main Drain Suction Line.
Device "D" consisted of a single 1-1/2" vent line assembly in Main Drain Suction Line.
Device "E" consisted of a single 4" vent line assembly in Main Drain Suction Line.

Gallons Per Minute Flow: 60 to 64 Except
    Device "E" which was 286 GPM in 4" line with a 12" by 12" Main Drain Sump.
Pipe Size: 2" PVC Except Device "E" which was 4"
Main Drain Sump: 8" Round Hayward - No Grate Except
    Device "E" which was a 12" by 12" Sump with Grate.

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High Vacuum
Forces Suction to Opposite of Dual Main Drains

Two Tests:     0.14 Inch Seconds
                        0.17 Inch Seconds     Average 0.155

Gallons Per Minute Flow: 118
Pipe Size: 2" PVC
Dual Main Drain Sumps: 8" Round Hayward - No Grates

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Equipment Used
Suction Tests

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12" X 12" FOAM BLOCK