Limiting Suction Outlet Plumbing Velocities

Standards Mandate the MAXIMUM SPEED LIMIT
 
Standards dictate that the MAXIMUM velocity limit in the plumbing at the suction outlets is 6 feet-per-second (ANSI/APSP-7).  I recommend that the maximum designed velocity on the suction side of the pump be limited to 4 feet-per-second.

Why lower that the maximum?  In suction lines speed is not a good thing.  In fact, excessive speed causes equipment wear, friction, lost energy and pump noise.

Since pump selection is the last task performed when designing a suction system, oftentimes there is not a pump curve that EXACTLY fits the system requirements.  By designing below the maximum allowed velocity, the designer has the ability to select the closest pump curve, even if it slightly exceeds the 4 FPS design - safely remaining below the maximum.

Isolation Valves 

When isolation valves are placed in suction lines between a pump and the suction outlets, any singular suction line supplying water to the pump MUST be capable of providing 100% of the systems’ designed flow rate at a maximum line velocity of 4.5 FPS.  

This is required, because if valves are closed and lines are improperly isolated, and if the pipes are not of sufficient diameter, the line velocity would quickly exceed the 6 FPS standard.

Energy Efficiency

Energy efficiency is greatly enhanced by lowering the line velocity.  Cavitation, operating noises & suction entrapment hazards are also reduced by lowering the line velocity.

Line velocities cannot be reduced much lower than 4 FPS, otherwise pipe scour would not occur.  Debris and sediment could eventually collect in the pipes.

Paolo Benedetti, SWD Aquatic Artist, Watershape Consultant, Expert Witness, International Construction Management 
Contact the author at: info@aquatictechnology.com or 408-776-8220 
"Creating water as art."™ 
Aquatic Technology Pool and Spa© 
©www.aquatictechnology.com All rights reserved.

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

Swimming Pool Construction Expert Witness, Consultant, Designer and Contractor Paolo (Paul) Benedetti discusses defective hydraulic design and plumbing deficiencies.

Swimming Pool Construction Expert Witness, Consultant, Designer and Contractor Paolo (Paul) Benedetti discusses defective hydraulic design and plumbing deficiencies.


Getting it from Here to There... Safely & Efficiently
Hydraulics is the science and physics involved with the properties and movement of liquids.  In swimming pools, this involves the movement of water through piping, pumps, equipment and fittings.  

Swimming pool are the most common form of construction that is almost entirely "design & build."  That is, the person who sells the product also designs and builds it.

And therein lies the problem.  A pool salesman has absolutely no business specifying pipe and pump sizes during a sales call.  Nor is it proper for the plumbing subcontractor to define the pipe sizes during installation.

Engineered Systems 
The hydraulics of a swimming pool must be properly designed in order to operate efficiently, quietly and safely.

The line velocities (the speed at which the water travels through the pipes) must be maintained within defined parameters.  Too slow and debris will settle in the pipes.  Too fast and the system develops excessive resistance (dynamic head), noise, water hammer, premature equipment failure, pump cavitation and an unsafe bather environment (excessive suction).


Whether the system is designed by a mechanical engineer or through the use of hydraulic design software (created by an engineer), strict adherence to the standards must be maintained.


The national standards quantify the absolute maximums for line velocities.  This does not mean that this is the goal... it is the speed limit, the maximum speed that the water can travel.


In conjunction with the line velocities, other factors such as gallons per minute (gpm), flow rates, turnover rates, equipment and fitting head pressures and the total dynamic head (tdh) of the complete system must be accounted for.


Restrictions to flow
Every length of pipe, pipe fitting, valve, filter, apparatus and trim fitting contributes to resistance in the system.  Smaller diameter pipes have a lower maximum flow rate than larger pipes.  The hydraulics of the system must be defined, before the exact pipe size can be specified.

The Correct Design Process 
Starting with a scaled site map (project layout), the plumbing is diagrammed and laid out.  The lengths of pipe and the pipe fittings are added up.  The required system flow rate is extracted from the turnover rate.  

A "guesstimate" is made from flow rate charts as to the appropriate pipe size.  The filter(s) is chosen based upon the filter's allowable flow rates.  The dynamic head (resistance to flow) of the other system components are also added to the system design calculation.


The system's TDH is then calculated from all of these variables.  Finally, looking at manufacturer pump curves, a pump size is chosen.  The TDH and required flow rate should fall within the center of the pump's operating curve.


Sometimes the TDH of the system may be to great.  This is easily adjusted by increasing the diameter of the system's plumbing.  This drops the restrictions and therefore the TDH.

Arbitrarily selecting pipe size and pump horsepower is asking for trouble.  Pumps that are forced to operate outside of their operating curves are less efficient, noisy (cavitation) and prone to premature failure.


Graeme Baker Safety Act
The GBSA was the governments response to horrific and tragic entrapment accidents.  These accidents were a direct result of poorly designed hydraulic systems.


Almost every system involved in suction entrapment accidents had excessive line velocities at the suction points (drains).  The easiest means to decrease the velocity at a drain, is to install multiple drains.  In addition to lowering the line velocities, the alternate suction points allow for the diversion of the suction in the event of obstruction of any single drain.  Properly spaced and placed, blockage of multiple drains is virtually impossible.


Drain Grate Flow Rates
Split drains have been the minimum standard for years.  For years prior to the GBSA, the IAPMO and NSPI (now APSP) Swimming Pool Plumbing Standard specified a minimum of 2 suction points for each pump.  But swimming pool contractors have ignored this published standard repeatedly.


How many single skimmer pools with a single main drain exist in the US?  When the skimmer becomes blocked, the main drain becomes the SOLE SUCTION POINT for the pump.  This is hardly 2 suction points!


The grates for main drains also have a maximum flow rate.  The flow rating is lowered when the grate is mounted in a horizontal position (pool wall or spa bench).  This means that 2 drains in the floor may suffice.  However, when one drain is located horizontally, the system may require 3 drains (2 on the horizontal & one on the floor).


This is a contractor induced system design flaw that is still occurring.


As a goal, the suction at the drain inlets should be a minimal as practical.  This is especially important for vertical inlets, as bathers with long hair may become entangled in grates with excessive flow rates.


The solution to these issues is a properly designed, engineered, specified and installed hydraulic system.  

As a result, the owner will have a safe bathing environment that will operate at peak efficiency and with minimal noise.