As pool industry professionals, we are being directed by the Virginia Graeme Baker Pool and Spa Safety Act to install anti-entrapment drain covers and safety vacuum release systems (SVRS) on many public pools. But we must understand the products we are installing.
We cannot expect every anti-entrapment-type suction outlet cover
to protect every sump, cut-off pipe or equalizer line equally. In
the same manner, we cannot expect every SVRS to protect various
flooded suction/suction lift scenarios in the same fashion.
It is very important that we, as industry professionals,
understand the term “differential hold-down force.”
Without it, systems can be retrofitted incorrectly and remain
subject to entrapment hazards.
What is hold-down force?
Hold-down force is vacuum pressure differential created by an
increase in resistance on the suction side of the pump. This could
happen if a cover were missing off a dual drain system and a sump
were to become blocked. There is an allowable 15 pounds maximum on
a standard 8-inch diameter sump when one of the two sumps is
blocked. Anything more can pose a danger.
Discussing the danger
We must understand that even dual drains can create a hazard if not
properly outfitted with certified anti-entrapment covers.
If one cover becomes detached or broken, and the sump underneath
becomes occluded by a bather, the remaining covered sump would then
be operating at full flow. The cover’s small open area could
create a significant pressure drop, resulting in a hazardous
dynamic hold-down force on the uncovered sump.
For example, consider a dual drain system with 8-inch sumps and
a sealing area of 40 square inches each. If one cover is missing
and a bather blocks its flow, the other drain will take the
pump’s entire flow. If the cover produces a pressure drop of
2 psi when operated at a full flow of 100 gpm, the resultant
dynamic hold-down force on the open sump would be in excess of 80
pounds. This could be enough force to entrap a younger bather.
This dynamic hold-down force is even more alarming with dual
12-by-12-inch sumps. An old cover with a larger flow rate could be
replaced with a newly certified cover that has a much smaller open
surface area. There again, in a dual drain scenario, if one cover
becomes broken or missing, the differential hold-down force could
A pressure differential can create problems in single-drain
systems as well. The pressure drop across a replacement cover with
a lower open surface area could cause the system vacuum levels to
increase, causing an existing installed SVRS, gravity feed
collector tank, or vent system not to operate as intended.
Sizing up solutions
For 8-inch dual drain suction outlets, pools should be outfitted
with larger diameter connector pipes (2.5- to 3 inches) and
properly sized suction plumbing and pumps. Installation of 90- and
45-degree elbows in the connector piping increases resistance, and
therefore hold-down force, when one sump is blocked. The use of 90-
and 45-degree elbow type fittings should be minimized, and avoided
If you are retrofitting new covers onto a drain, it may be
difficult to determine the exact cover to achieve 15 lbs. or less
of differential hold-down force. A tech may not even know what
configuration is underneath the drains.
However, techs should know that the larger the pump is, the more
open surface area is needed by the drain covers for a lower vacuum
If considering pipe size and open area on drain covers was
common practice, then our pools and spas would not harbor deadly
dynamic hold-down forces. As an industry, we must begin to
understand all the factors critical to suction entrapment
avoidance. Building pools and spas suction-safe is a simple,
cost-effective construction technique.
New technologies are becoming available every day. Your
opportunity, along with the potential for financial gain, makes a
proactive approach to suction entrapment avoidance a win-win