Proper Hydraulic Design - Swimming Pool Drains

Prefabricated Drain Channels and Covers
  
   A mental hurdle that is confusing to most swimming pool builders, designers, state and local health departments, building inspectors and even many drain manufacturers - is the compatibility of gallons per minute (GPM) through the cover and the feet per second (FPS) through an unblockable drain, suction sump or cover or the connected pipes.
   Drain sumps, channels and covers that comply with the national Virginia Graeme-Baker Safety Act (VGBSA) list a maximum GPM flow rating.  However, it does not mean that the pipe connections below that cover are permitted to flow at that rating.  

Pipes have a different standard
    
There are maximum line velocity standards that apply to the connected piping.  Simply stated, a 2" pipe can only flow 28 GPM at 1.5 FPS, or 43 GPM at 3 FPS where it connects to a drain sump.
    What the GPM drain cover rating allows are the grouping of different suction systems.  Multiple branch lines from different pump suctions may be grouped together under a cover, until their cumulative GPM flowrate meets the cover's rating.  This allows multiple system inlets to share a drain cover.
    Pre-fabricated drain channels pose another design obstacle.  They are classified under the VGBSA as "unblockable" drains due to their shape and size.  The VGBSA allows these drains to function as a single point suction, meaning that split drains are not required.  They can be ordered with various quantities of connection points for plumbing.
     Most of these channel drains only provide 2" connection ports that are limited (by law) to a mere 1.5 or 3 FPS (depending on the local standard).  That equates to ONLY 28 or 43 GPM per connection!  

Drain Designs  

1.   Single inlet unblockable channel drains are available with 196 GPM ratings.  
When installed in compliance with the FPS standard, it only has a maximum flow rating of 28 or 43 GPM. 
  2.   Dual inlet unblockable channel drains are available with 227 GPM ratings.  
However, when installed in compliance with the FPS standard, it only has a maximum GPM rating of 56 or 86 GPM.  
3.   Triple inlet unblockable channel drains are available with flow ratings of 320 GPM.   
Yet, when installed in compliance with the FPS standard they are limited to 84 or 129 GPM.
 
Show me the Math

When you are considering the purchase of a swimming pool, require that the hydraulic calculations (head loss and line velocities) and the plumbing schematics (line drawings) be included in the plans.  Require that the builder note in the plans or specifications, the pipe sizes and the maximum allowable line velocities of branch, trunk and return piping.    

This will ease in the verification of the system performance and compliance, by any project inspectors or consultants.  The next hurdle is getting them to actually build it that way!

The design of swimming pool circulation and piping systems are not rocket science.  But, it does require that one be familiar with the basics of hydraulic design, the idiosyncrasies of the codes, what component ratings really mean and the limitations of those components.

 
Paolo Benedetti - Aquatic Artist "Creating water as art."™ Aquatic Technology Pool and Spa ©www.aquatictechnology.com You may contact Paolo Benedetti at: info@aquatictechnology.com or at 408-776-8220