Swimming Pool Designer Expert Witness VGB Unblockable Drains

There has been a lot of confusion about VGB compliant BLOCKABLE & UNBLOCKABLE DRAIN COVERS.  

While the following national standards pertain to public/commercial pools, we advocate the same standards for residential vessels.  We do not discern a difference between the requirement for a safe hydraulic standards of private VS public/commercial vessels.  We feel that private pools should be just as safe as commercial pools when it comes to suction entrapment and safety.  After all, there are many more private pools than public pools on this planet!
 
While the MAXIMUM INDUSTRY STANDARDS allow 6 FPS, we advocate and promote a MAXIMUM SUCTION LINE VELOCITY OF 4 FPS IN THE TRUNK LINE and 2 FPS MAXIMUM THROUGH A BRANCH SUCTION LINE OR A DRAIN INLET (1 FPS when 1.5 FPS is the maximum standard).


First, let's start with an excerpt from a channel drain cover installation manaul:
 
WATER VELOCITY AND FLOW RATES -
The maximum water velocity through drain covers is limited by some local regulations, for example some state health departments limit the velocity through public pool drain covers to 1½ feet per second. This velocity limit is lower than the flow rating provided by the ANSI/APSP-16 2011 certification, therefore the local limit applies and must be followed. Never exceed the flow rating listed on the cover even if the local code does not provide a velocity limit.

PIPING – GENERAL
For new installations, the piping between drains must be sized to limit the velocity to six feet per second. This limit applies to the
branch piping and all fittings between multiple outlets and the tee leading back to the pump. If code requires a lower water velocity,
comply with the code. See the chart below for information on pipe size flow ratings at six feet per second.

HYDRAULIC SYSTEM SPECIFICATIONS -

ACCEPTABLE PIPE SIZE FOR MAXIMUM RECOMMENDED SYSTEM FLOW RATE PER APSP-7 (6 feet/second in the branch line):
Pipe Size                    1½"  2"   2½"   3"   4"
Flow Rate in GPM    45    80   110   160  230 (rounded down)
This is where the confusion lies - blockable drains & un-blockable drains and the relationship of line velocities.

First, Blockable drains:
 
Though a drain cover may be rated for 10,000 GPM, it is REALLY limited by the MAXIMUM LINE VELOCITY of the attached pipes.  On dual drain systems, in the event one drain is blocked, the REMAINING lines must be capable of sustaining 100% of the flow BELOW the 6 FPS maximum.  This means, that during normal operation the flow rate through a "blockable drain" must be 3 FPS or less (when a drain becomes blocked the line velocity through the remaining drain will double). 

BLOCKABLE DRAIN EXAMPLES:

1)  A pump is connected to a 4" trunk line, which is split into (2) 3" branch lines. The 4" line flows 234 GPM @ 6 FPS, while the 3" branch line flows 68 GPM @ 3 FPS. 68 GPM X 2 = 136 GPM.  The maximum flow rate of that system is 136 GPM.  When one drain is blocked the maximum line velocity of the remaining 3" branch line is limited to 136 GPM - therefore that is the MAXIMUM to be drawn through the 4" trunk line.

2)  A pump is connected to a 4" trunk line, which is split into (2) 4" branch lines. The 4" line flows 234 GPM @ 6 FPS, while the 4" branch lines flows 117 GPM @ 3 FPS.  The maximum flow rate of that system is 234 GPM.  When one drain is blocked the maximum line velocity of the remaining 4" branch line is limited to 234 GPM @ 6 FPS - therefore that is the MAXIMUM that may also be drawn through the 4" trunk line as well.

To select a compliant cover, one would merely select a drain cover that is equal to or above the above GPM flow rate at 3 FPS.  If the local standard was 1.5 FPS through the cover, they you'd look for a cover with 2X the GPM flow rating of the above system maximums.
 
THEREFORE, for blockable drains, the maximum line velocity at the pipes connected directly below the cover are limited to 3 FPS maximum (or lower).  In communities with 1.5 FPS standards, the plumbing would have to be split again or upsized to get to 1.5 FPS.

Now, onto UNBLOCKABLE DRAINS:

They are rated using TWO CRITERIA: GPM @ FPS.
Most states limit the FPS flowrate through an UN-BLOCKABLE COVER TO 3 FPS. 
Some states (Texas for example) have lowered the FPS through un-blockable covers to 1.5 FPS.
The rating for an un-blockable cover will look like this: 300 GPM at 3 FPS.
So, if you draw 600 GPM through the cover, the line velocity would be 6 FPS - exceeding the rating of the cover.  150 GPM through the cover, would bring the line velocity down to 1.5 FPS (to comply with states that have de-rated such covers).
 
UN-BLOCKABLE DRAIN EXAMPLES:

1) An un-blockable channel drain is rated at 300 GPM @ 3 FPS.  It is connected to a 4" trunk line, that is split into (2) 2.5" branch lines.  The 2.5" branch lines are connect to 2 inlets on the bottom of the channel drain.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 300 GPM @ 3 FPS
The line velocity of the attached 2.5" pipes are the limiting factors @ 88 GPM each X 2 = 176 GPM.
The maximum flowrate of this system is 176 GPM.

2) An un-blockable channel drain is rated at 300 GPM @ 3 FPS.  It is connected to a 4" trunk line & manifold, that is split into (3) 2.5" branch lines.  The 2.5" branch lines connect to 3 inlets on the bottom of the channel drain.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 300 GPM @ 3 FPS
Though the line velocity of the (3) attached 2.5" pipes would allow 264 GPM (e.g. 88 GPM each X 3 = 264 GPM), that would exceed the maximum line velocity of the 4" trunk line at 234 GPM. 
The maximum flowrate of this system is 234 GPM @ 6 FPS.

3) An un-blockable channel drain is rated at 300 GPM @ 3 FPS.  It is connected to a 4" trunk line & manifold, that is split into (2) 2.5" branch lines.  The 2.5" branch lines connect to 2 inlets on the bottom of the channel drain.  However, the local authority's standard specifies 1.5 FPS for un-blockable drains.
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: 150 GPM @ > 1.5 FPS < (a slower standard).
Though the line velocity of the (2) attached 2.5" pipes would allow 176 GPM (e.g. 88 GPM each X 2 = 176 GPM), that would exceed the maximum rating of the un-blockable cover at 1.5 FPS of 150 GPM. 
The maximum flowrate of this system is 150 GPM. 
Sidenote: the trunk line could not be downsized to 3", because 150 GPM would exceed 6 FPS in a 3" pipe.

4) An un-blockable channel drain is rated at 316 GPM @ 3.9 FPS when floor mounted.  It is connected to a 4" trunk line & manifold, that is split into (3) 2.5" branch lines.  The 2.5" branch lines connect to 3 inlets on the bottom of the channel drain.  However, the local authority's standard specifies 3 FPS for un-blockable drains (therefore, we must solve for the GPM rating at 3 FPS).
Consider the various maximum limits:
4" pipe: 234 GPM @ 6 FPS
2.5" pipe: 88 GPM @ 6 FPS
Un-blockable Drain Limit: XXX GPM @ > 3 FPS
316 GPM ÷ 3.9 FPS = 81 GPM/1 FPS
81 GPM X 3 GPM (our standard in this case) = 243 GPM @ 3 FPS
Though the line velocity of the (3) attached 2.5" pipes would allow 264 GPM (e.g. 88 GPM each X 3 = 264 GPM),
and the limit through the drain cover would allow 243 GPM @ 3 FPS,
BOTH OF THOSE FLOWRATES would exceed the maximum line velocity of the 4" trunk line at 234 GPM. 
Therefore, the maximum flowrate of this system is 234 GPM @ 6 FPS.
 
Note the differences in line velocities of the piping attached to bloackable VS un-blockable drains:

Blockable Covers: 
The maximum line velocity at the pipe connected below the cover is limited to 3 FPS.
Un-blockable Covers: 
The maximum line velocity at the pipe connected below the cover is limited to 6 FPS.

While these are the MAXIMUM INDUSTRY STANDARDS, we advocate and promote a MAXIMUM LINE VELOCITY OF 4 FPS IN THE TRUNK LINE and 2 FPS MAXIMUM THROUGH A BRANCH LINE OR A DRAIN INLET (1 FPS when 1.5 FPS is the maximum).
 
Hopefully, through these examples, you have come to realize that JUST BECAUSE A COVER HAS A MANUFACTURER'S "FLOW RATING," it is the ATTACHED PIPING SYSTEM THAT DICTATES the MAXIMUM GPM through a plumbing system.
 
Paolo Benedetti 
Aquatic Artist, Watershape Consultant, Expert Witness 
"Creating water as art."™ 
Aquatic Technology Pool and Spa 
©www.aquatictechnology.com

Swimming Pool Hydraulic Design Expert Witness

Swimming Pool Hydraulic Design & Plumbing

Aquatic Technology Pool & Spa expert witness Paolo Benedetti discusses proper hydraulic design and the benefits for the end user.

Safety

The paramount issue in proper hydraulic design of a swimming pool plumbing system is safety.  The system must be designed to maintain low line velocities at the suction inlets.  Lower line velocities result in a lessened chance of an entrapment or entanglement accident.  The suction at the drain is so low that any vacuum force can easily be overcome.

Efficiency and Energy Savings 

In order to lower the velocity of the water traveling through a piping system, the size of the pipes must be increased.  Lowering the restrictions that the pump must overcome, allows the pump to work more efficiently.  That efficiency results in a direct energy savings, as smaller pumps can be used to move more water.  Sounds simple, doesn't it?  But most of the swimming pool industry "doesn't get it."  Larger pipes, smaller pumps!  NOT larger pumps, smaller pipes!

By increasing the size of the pipes, the requisite amount of water can be circulated with a smaller pump, resulting in a saving of electricity (and money).  Additionally, the pumps operate much quieter and cooler.  Builders will charge more for the larger plumbing.  They are actually doing the owner a favor, as the owner will quickly recover that added cost and the compounded returns on that investment, over the life of the project.  And the savings will be compound even further as electrical costs continue to rise.

Hydraulically Sound Principals

The process of designing a hydraulic system is not difficult.  It does however, require that one possess the basic knowledge of fluid mechanics and some simple line velocity/head loss charts.

1.  Determine the required flow in GPM (to meet turnover rate or flow demands of jets, water features, etc.).
2.  Layout the system design, starting with an estimate of the pipe size required.  Select a pipe size on the suction side that will not exceed 6 FPS (nationwide standard/law).  4 FPS is ideal, as it will provide a margin of error and still scour collected debris from the piping.
3.  Calculate the feet of pipe & number of fittings - use the head loss charts to convert & calculate the head loss of the suction piping.
4.  Determine the equipment to be installed at the equipment pad (filter, heater, chemical controls, valves, etc.).  From the various manufacturer's information sheets, add all of these component head losses to the total from #3.
5.  Do the same for the return plumbing, as you did in #3.  Aim for no more than 8 FPS (nationwide standard/law).  6 FPS is ideal on the return side.  The piping size may be slightly smaller to create this slightly higher line velocity, but not always.  Add the return plumbing head loss to the total from #4.
6.  This grand total is the total head loss (restrictions) in the system.  This is called total dynamic head (TDH).  TDH really includes more than just the system loss, but we don't have time to discuss the other variables (e.g. static head, suction lift - positive or negative, etc.).
7.  NOW, the pump can be sized.  Utilizing pump flow curves, seek the desired flow in GPM at the calculated TDH (from #6).  The two values should meet in the middle of the pump's curve.  If they are at either extreme of the pump curve, either up or down size the pump.  Choosing a pump in the middle of the curve will allow for operational variations in the system, such as a dirty filter.
8.  National safety standards require that the line velocity at the suction inlets (e.g. drains) must be much lower than the line velocity in the main suction pipe (trunk line).  The national standard is 3 FPS, though some states and municipalities have a more restrictive 1.5 FPS standard.  
     Dropping the line velocity at the suction inlets is child's play.  The trunk line is merely divided into multiple balanced branch lines, until the applicable FPS standard is achieved.  This can be achieved in a number of manners.  
    The first method, requires that the trunk line be divided as many times as it takes to get below the required FPS standard.  The branch lines diameter is kept the same as the trunk line, so that the division applies evenly to all suction inlets.
    The second method, is a little more complicated to plan, but easier to install.  The diameter of the branch lines are increased larger than the main trunk line.  Utilizing the line velocity charts, one selects the larger diameter pipe required to drop the velocity to below the required standard.  Since a minimum of 2 suction inlets are required for each pump, this could greatly reduce the number of inlets required over method #1.

Prefabricated Drain Channels and Covers
    
   The next hurdle is confusing to most swimming pool builders, designers, state and local health departments, building inspectors and even many drain manufacturers - the compatibility of GPM and FPS through an unblockable drain, suction sump or cover.  Though drain sumps, channels and covers in compliance with the national Virginia Graeme-Baker Safety Act (VGBSA) list a maximum GPM flow rating, it does not mean that the pipe connections below that cover is permitted to flow at that rating.  
   The maximum line velocity standards still 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 drain cover rating does allow are the grouping of different suction systems.  Multiple branch lines from different pumping systems 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!  
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.

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 component applications.

Paolo Benedetti
Aquatic Artist, Construction & Design Consultant, Expert Witness
"Creating water as art."™
Aquatic Technology Pool and Spa
©www.aquatictechnology.com