Even if a pool’s suction outlet cover is properly certified, it’s still crucial to ensure that it operates within parameters that preclude entrapment. Understanding concepts such as flow rate, open area and velocity will make this process much easier.
Velocity and flow rate
When designing a pool or spa, or replacing a pump, it’s important to determine what the project’s required turnover rate should be. The formula for turnover is as follows:
water volume (gallons)
divided by required turnover rate (hours)
divided by 60
equals gallons per minute (GPM)
The required turnover rate for a pool or spa is different from state to state — and in some cases municipality to municipality. Be sure to confirm the requirements for the pool or spa’s specific location.
100 = X
g= 36,000 gallon pool
h = 6-hour turnover rate
m = 60 minutes
x = Gallons Per Minute
Total dynamic head
The next step is to determine the system’s total dynamic head (TDH). For a specific measurement, the Hazen-Williams equation is ideal — it requires measuring the length of pipe and each fitting in the plumbing from suction to discharge; it also includes friction from filtration and heating elements in the system. Most states and health departments require a minimum of 60 feet of TDH.
Knowing the system’s GPM and TDH makes it possible to determine which pump is needed for the project by using a pump curve graph supplied by pump manufacturers. After selecting the pump that bests suits the project, the next step is to determine that pump’s maximum potential flow in GPM — in other words, the flow when all returns are open and the filter (if installed) is clean.
Next, apply the potential peak GPM of the circulation pump into the suction outlet cover hydraulics equation:
Plug in the 1.0 to 1.5 FPS velocity and apply the peak flow rate (GPM) of the pump, and the answer for Y (sq. in.) is the minimum open area required for the suction outlet cover.
Equation to solve for minimum open area:
(X*0.321)/(Z)= Y 2
Y = 21.4 sq. in.
X= 100 GPM (Flow rate)
Z = 1.5 FPS (Velocity)
Y = Minimum Open Area
Even so, it’s still wise to employ a suction outlet cover that has a larger open area than this minimum. For example, if the flow rate of an existing circulation pump is 100 GPM, the minimum open area required at a maximum cover velocity of 1.5 FPS is 21.4 sq. in., and the pool has an 8-inch round sump with a 12-sq.-in. open area cover, two options exist. The first is to replace the existing cover with an approved cover having an open area in excess of 21.4 sq. in.; the second is to install a smaller pump that still meets the required turnover rate, but reduces the existing cover velocity to less than 1.5 FPS.
As a general rule, the lower the flow rate, the less cover open area required.
The thought process of many service technicians servicing multiple drains runs along the lines, “This system’s turnover rate requires (for example) 100 GPM, and at a cover velocity of 1.5 FPS, I need 21.4 sq. in. open area for the suction outlet cover to be safe. Since this project is a dual drain application, I only need to have an open area of 10.7 sq. in. for each sump.”
However, if undersized drain covers are used for both drains, and if one cover becomes blocked, then the remaining drain will have all 100 GPM passing through it at an unsafe cover velocity. Thus, when calculating the needed open area for a dual drain system, it is best to consider the project as if it has only one operating suction outlet.
Utilizing this concept, if one outlet becomes blocked, the remaining outlet will still be capable of handling 100 percent of the pump flow at a safe cover velocity.
As a rule of thumb, when calculating the open area needed for two or more 8-inch drain situations, it’s better to consider only half of the sum total of the open areas of the outlets.
How the minimum open area would be reconsidered for each drain count:
2 drains = 21.4 sq. in. each
3 drains = 14.3 sq. in. each
4 drains = 10.7 sq. in. each
100 GPM (peak) • 1.5 FPS
21.4 sq. in. minimum open area
When selecting a certified suction outlet cover, it’s important to look for certain traits.
- High profile (domed design): This feature does not allow any area of the body to create a seal over the suction outlet.
- Durable material: Use products that are resistant to chemical degradation and ultraviolet (UV) light. The drain cover should not discolor or weaken enough to break prematurely. PVC is one of the suggested materials due to its proven resistance to the pool/spa environment.
- Large open area: Excessive cover velocity is the main cause of hair entanglement and mechanical entrapment. At cover velocities less than 1.5 FPS, the suction outlet is less likely to draw in hair and clothing. Cover open area of 20 square inches or more is recommended for pumps of 2 horsepower or less. Larger connecting pipe sizes are required to ensure safe hydraulics. Sump-to-sump connector pipe size and size of suction piping to the pump can relate directly to safer hydraulics. A larger pipe diameter results in a lower velocity for a given flow rate.
- Secure mount: Old-style drain covers mounted with only two screws are more susceptible to unauthorized removal. Covers with four points of contact provide a sturdier mount and make the cover more difficult to pry loose. Only use S/S 316 grade fasteners, or those supplied by the manufacturer.
Do the right thing
In the end, drain safety comes down to the builder or service tech, and what procedures are taken to best meet the safety requirements of clients and the bathing public. For every drain problem, both inexpensive and overpriced solutions are available. It’s up to the builder or service tech to perform the due diligence to determine which product best fits the needs of the job at hand. When it comes to safety, price should not be the deciding factor — bather safety should be.