Leif Zars

Rev. 01/03/2000


Let me begin by saying that in the swimming pool industry any potential suction hazard exists at the Main Drain Sump or Suction Outlet. And to be accurately quantified it should be measured there. However this is easier said than done.


Numerous tests have provided significant information regarding the suction values at the main drain sump under a variety of conditions. This information can be helpful in developing suction potential indicators that are more easily measured or calculated from conditions outside the main drain sump.

Typical of such tests were those conducted by the author and reported in papers entitled "SUCTION ENTRAPMENT TESTS", and "VACUUM ELIMINATION DEVICES". These tests utilized a Wika Tronic Pressure Transmitter Model WK33398, 0 to 30 "Hg vacuum located in the main drain sump, and a Monarch Paperless Data Recorder Model M120450 as distributed by Davis Instruments (1-800-368-2516). Memory Card Catalog No. M120476 will also be required for the transmit data from the recorder to a computer for plotting.

The plotted data seems to properly represent the intensity of the suction incident. The size of the curve extending below 4.5 "Hg (2.2 PSI), which is considered the threshold of tolerable suction by the body, graphically illustrates this hazard.

One such typical plotted curve is included herein, showing a calculated area of 11.53 "Hg-Sec.

Data from numerous such curves is contained in Table 1. In this table the time of excessive suction and the maximum vacuum of this suction is compared to the actual area generated by the respective suction curves. The above referenced curve is shown on line 26 in this table.

There appears to be reasonable correlation between the calculated areas (duration of time, times the maximum suction) and the measured areas of the dangerous portion of these curves plotted by the above referenced recording data devices.

Comparing the measured vs. the calculated amounts of dangerous suction, the quantity of 0.735 was determined to be the average ratio between them. This is obviously due to the shape of the generated curves - being broadest at the upper portions and reducing in width in their lower portions.

Consequently to correlate time duration and maximum intensity of measured main drain suction with the real area of such a curve we would multiply the maximum suction by 0.735, and then multiply this by the time duration in seconds.

Typically this might look like (again using the above referenced curve):

22.2 x 0.735 x 0.7 = 11.53* "Hg-Seconds

Converting this to Pounds per Square Inch rather that Inches of Mercury:

0.491 x 22.2 x 0.735 x 0.7 = 5.61 PSI - Seconds

Again, this time duration was measured from the time suction exceeded 4.5 "Hg until suction was broken and it again dropped below 4.5 "Hg in the main drain sump.

The above method would allow for the measurement of the time duration and only the maximum vacuum intensity in order to reasonably closely calculate the suction danger rather than having to plot a suction curve as attached.


In the author’s "VISUAL STUDY OF PROLAPSE" (prolapse is commonly referred to as evisceration) report it was observed that the maximum constant suction at the main drain sump should not exceed 5 "Hg (2.45 PSI) which is close to the currently used maximum of 2.2 PSI. Utilizing the 2.2 PSI to be on the conservative side, we calculated all of "danger suction values" as those in excess of 2.2 PSI or 4.5" Hg.

Secondly, in the Prolapse report it was established that the maximum allowable intermittent suction should not exceed 11 "Hg-Seconds, which converts to 5.40 PSI-Seconds.

This leads us to the following statement regarding momentum and force: ‘The product of net force and the time during which it acts is defined as Impulse typical units for this would be (#)(Sec.)". Setting the area of the suction venerability as a constant value of 1 square inch, the unit of IMPULSE would appear applicable as the uniform unit for momentary suction measurements.

* Rounded


Next we will attempt to relate various combinations of swimming pool suction piping with the Impulse factor generated by its sudden stoppage and release of suction by various suction relief devices.


Momentum = m (v) Where m = Weight/32
Units In #-Sec.

Impulse = (Net Force)(Time)

Impulse = Resistance To Change In
Momentum In #- Sec.

In the following Tables for PVC piping from 1 " through 4", first calculations are shown deriving the weight of the fluid in the various lengths of pipe.

Next the Momentum of the fluid flowing in the pipes at 6 ft/Sec and 8 Ft/Sec. is calculated using the above formula.

Then is shown the Impulse generated for each flow when it is stopped in various time periods ranging from 0.25 Seconds up through 1.0 Second.

Finally, those values falling at or below 5.40 Pound-Seconds (the safety limit) are outlined on these Tables within a border for ease in viewing.

It becomes obvious in viewing these Tables that a shorter duration of time is critical in order to obtain low Impulse values, and that suction stoppage values of 1.0 second or larger provide few opportunities for operations within these guidelines.

Further it will be noted that the larger pipe sizes naturally create more inertia of fluid flow and that consequently only the extremely short time periods of 0.25 Seconds for complete suction stoppage can provide Impulse values within the guidelines.

Pipe velocity flow rates shown are 6 Ft/Sec. for Public and Semi Public pools, and 8 Ft/Sec. for Residential pools. Naturally the higher velocity of 8 Ft/Sec. contributes to larger Impulse values but it should be remembered that the likelihood of 3" or 4" suction piping on a residential pool is very unlikely. These sizes are usually reserved for Public Pools where flow rates are limited to 6 Ft/Sec., which is helpful in keeping the Impulse numbers low.

It is also obvious that suction breaking devices should be kept close to the main drains served so as to reduce pipe lengths thus reducing Impulse values.


Now, how to rate suction limiting devices or systems?

It would appear that all such devices could and should be laboratory tested against a standard which utilizes an Impulse number (Net Force and the time during which it acts such as 5.40 Pound-Sec.) And that the results of such testing along with the maximum allowable pipe size would be clearly and permanently displayed on the device. This testing and rating must include a certain reasonable amount of suction piping to reflect normal field conditions, and this length and size should be shown as a portion of the rating. Allowable flow velocity should also be shown as 6 or 8 FPS.

Any additional piping between the device and the main drain sump (including and unreleased fluid path down stream from the device) must be taken into account in determining the eventual effectiveness and rating of the device.

The rating of the device as installed should be a combination of the device with normal piping (laboratory rating) plus any added Impulse factor from the above additional piping. The combination of these two figures must remain below the danger Impulse threshold of 5.40 Pound-Sec.

This could be verified in the field by indicating on the device not only the laboratory rating but also the amount to be added to the this rating for each additional 5 feet of piping or fraction thereof that is unrelieved. Reasonable field observation and measurement could verify the conditions. Again, this total must be kept below the stated maximum Impulse level of 5.40 # Sec.

A typical Laboratory rating may read: Device Rated As: 3.36 Pound-Sec. Impulse with 25’ of 2" Pipe. Add 0.34 Pound-Sec. for each 5’ or less of Additional pipe.

It might also read: Rating: 3.36 PS@ 25’/2"+0.34"/5’

It should be added that most suction breaking devices tested would serve very well as a means of avoiding suction entrapment by the timely relieving suction. The real question is regarding the ability of the device or system to provide protection from prolapse. Here, the quick timing of the release is mandatory.

Regarding the current availability of suction breaking devices (electrical, mechanical, electrical/mechanical, and vent systems) it should be noted that it is critical to insure reliability in its actuation - perhaps not over 1 failure in 50,000 by laboratory certification.

The consideration of using milli-seconds as a standard for suction elimination devices is another possibility. The primary difficulty viewed here is that each installation would have to be field measured for compliance. And then too, no consideration for the intensity of the suction is given when time alone is the standard.


It should be mentioned that there are several other ways to avoid suction entrapment or prolapse in swimming pools.

The first and most obvious is of course having a main drain that is too large to be covered by a body. Typical of this would be an 18" x 18" square main drain, which has a diagonal of roughly 24" open area. Also there is a design that is similar to a trench drain measuring 4" by 24" which again can not be covered due to its length. These drains would seem to require little extra in the way of safety measures outside of limiting the grate flow to 1 Ft/Sec and the related pipe flow to 6 Ft/Sec.

Next would be dual main drains on a single suction line. This arrangement on the surface appears satisfactory, however it has the drawback that if a child covers one drain, the other drain could be a waiting death trap. This is not too likely in a deep pool, but probable in a shallow play pool.

And finally, other drain arrangements or configurations are certain to appear in the foreseeable future to satisfactorily address this problem.


Finally a word about Hair entanglement and its place in this entrapment discussion. This can be just as deadly and has not been evaluated in my earlier tests. It is common sense to keep grate velocities at or below 1 Ft/Sec to prevent turbulence. But it would appear that a snap out grate, with say a 5 pound pull, would have a significant advantage. Yes, it would expose a main drain sump for the moment but with the proper protection as discussed above perhaps this would no longer be an issue.


In the Prolapse report the maximum safe figures of 2.45 PSI (5" Hg) for constant suction and 5.40 Pound-Sec. (11.0 "Hg-Sec) for intermittent suction were developed.

In the report on vacuum elimination Devices the Range in "Inch Seconds for various devices was:

RANGE in "Inch Seconds"
(Where the Upper Impulse Limit is 11.0)

Electrical Mechanical Atmospheric Vent. Dual Drains
10.28 to
13.53 to
13.73 to
1.52 to
0.14 to

Converting this to Pound-Sec. this would be:

RANGE in "Pound-Seconds"
(Where the Upper Impulse Limit is 11.0)

Electrical Mechanical Atmospheric Vent. Dual Drains
5.05 to
6.64 to
6.74 to
0.75 to
0.07 to

From the above it is clear that both the Dual main Drains and the Atmospheric Vent Device are well within the desired limits.

It is felt that other devices should be able to be plumbed, relocated or otherwise modified to accomplish the safe release from the exposure to prolapse.

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