Suction inlet hydraulics...
often under estimated and always misunderstood.
Confusion
Many pool builders have confusion when designing suction inlets (aka: main drains, suction inlets, equalizer lines). Why? Because there are THREE criteria that must be met in their design.
Cover Flow Rate
First, the safety cover that protects the suction pipe from human contact has a flow rating. This flow rating is different depending where the cover is mounted - on the floor or side wall. The flow rating of the cover is expressed in Gallons Per Minute (GPM). The GPM rating is the maximum that is allowed to flow through the cover (total GPM flow rate of all of the pipes under the cover).
Sump Dimensions
A sump is a hollow pocket below the cover. Builders can use a pre-fabricated plastic bowl that is attached to the suction pipe. The plastic sump is then cast into the concrete wall.
Builders can also use a "site fabricated" sump. This is merely a divot carved into the wet concrete during construction. A site fabricated sump must be minimum of 1.5 times as deep as the suction inlet pipe is wide (e.g.: 2 inch diameter pipe requires a 3 inch deep sump, a 6 inch pipe requires a 9 inch deep sump...).
Line Velocity
This is where the confusion lies. In addition to the above two criteria, there is a "speed limit" for the water. The system cannot be designed so that the line velocity in the branch suction lines (pipe under the drain cover) exceeds 3 Feet per Second (FPS).
Though the main suction line (trunk line) is allowed to flow at 8 FPS, merely splitting the trunk line into 2 will not reduce the line velocity of the suction inlets. They would then only flow at 4 FPS.
To achieve a lower flow rate in the branch lines one needs to increase the pipe diameter, so that the velocity within the pipe decreases. Yes, this also means that the sump needs to be deepened due to the larger pipe.
It's the LAW
These standards were not just made up... they're a part of the International Residential Code (Appendix G) and published industry standards (ANSI/APSP 7-2006).
Contact the author, Paolo Benedetti of Aquatic Technology Pool and Spa at:
info@aquatictechnology.com or 408-776-8220.
Visit his website at: www.aquatictechnology.com.
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