Flow Rate
The measurement of gallons per minute (gpm) of water that is flowing through a circulation system at any given time.
Turnover Rate
The time (in hours) required for a recirculation system to move the number of gallons equal to the total volume of the pool, spa, or other aquatic venue.
Turnover and Flow Rates
To understand entrapment and ways in which it can be minimized, flow rates must be measured. Approved drain covers/SOFAs have specific flow rates that must never be exceeded.
A faster turnover rate results in a greater flow rate measured in gallons per minute (gpm). Other, non- circulation system drain covers/ SOFAs (e.g., for spa therapy jets) must also be properly sized and installed to handle the full flow rate of the attached pump or pumps.
Certified drain covers/SOFAs are rated in gallons per minute. The actual flow rate of a pumping system must be known before a compatible drain cover/ SOFA can be selected and installed.
Flow rate can be determined by:
• NSF/ANSI 50 certified flow meter that is properly installed and maintained.
• Total dynamic head measurement and pump curve.
Flow Meters
Flow meters must be installed correctly, and some require periodic calibration. There are different kinds of flow meters, and the type used greatly affects flow measurement accuracy. A common type is a rotameter that measures the volumetric flow rate of fluid in a closed tube. The inside of the tube has a variable area containing a float that moves up and down proportionally to changes in flow rate. While very accurate, rotameters must be installed vertically as gravity plays an important role in their function.
Where they are installed within a section of straight pipe is also critically important. Most flow meters, including rotameters, paddle wheels, magnetic, and ultrasonic meters, among others, convert the water velocity inside the pipe to gallons per minute. Pipe fittings, valves, and other equipment cause uneven velocity patterns within the pipe, before and especially after fittings and elbows. For this reason, most flow meters must be installed away from equipment that causes localized velocity disturbances. Typically, flow meter instructions specify a minimum 10 pipe diameters of straight pipe upstream and 5 pipe diameters downstream.
Flow meters that are not installed according to the manufacturer’s instructions should not be used for VGB Act compliance verification. All flow meter installations should be periodically validated using TDH measurement and the pump/impeller specific performance curve.
Total Dynamic Head
Total Dynamic Head (TDH) is a measure of a system’s resistance to flow. Each pool will have a unique flow rate based on the piping and fittings used. The same pump, on different pools, can produce significantly different flow rates. Operational changes to the same pool will also produce different flow rates. For example, cleaning a filter will lower the pool’s TDH, causing a higher flow rate. It is the resistance to flow in circulation systems that dictates the flow rate, not just the pump size.
Caution must be used when changing single-speed pumps to variable-speed pumps as a result of the U.S. Department of Energy’s 2021 Dedicated Purpose Pool Pump (DPPP) Law that prohibits the manufacturing of most singlespeed pumps approximately 1 to 5 horsepower. In almost all cases, the variable-speed pump will have the ability to move much more water than the single-speed pump it is replacing. While the goal of the new law is to save energy by operating pumps at lower speed (the minimum speed needed to achieve the proper turnover rate), they retain the potential to run at high-speed, increasing the risk of suction entrapment that may not have previously existed.
Historically, public health codes prescribed a minimum TDH that was used to size the circulation pump to assure it reliably exceeded the pool’s turnover flow rate when the filter was dirty. Today with the increased focus on building code compliance and energy efficiency, systems are being built with larger pipe, valves, and filters. This results in much lower TDH (less resistance) for the same size pool, resulting in much higher flow rates. For this reason, the flow rates associated with health code specified TDH values should never be used to size or validate VGB Act drain cover/SOFA compliance.
Before measuring TDH, the pool must be filled with water, configured to operate properly, and the filter media must be clean. For all other pumping systems, such as spa jet booster pumps, adjust the system to deliver the highest possible flow, i.e., pumps set to their high speed while all return-side control valves are set to the fully open position. If safe to do so, temporarily turn off the skimmers or gutter overflow system to measure the maximum potential flow through VGB Act regulated suction system.
TDH can be measured using the pressure and vacuum gauges on either side of the pump. The vacuum gauge is found on the suction side of the pump and measures vacuum in inches of mercury (in. Hg), provided the pump is above water level. For pumps below water level, a compound pressure gauge is often used because they are capable of reading above (psi) and below (in. Hg) atmospheric pressure. The pressure gauge is found on the discharge side of the pump and is measured in pounds per square inch (psi). All units of measurements are converted to feet of head, the units published by pump manufacturers. Pressure head is calculated by multiplying psi by 2.31; pressure and vacuum head is calculated by multiplying in. Hg by 1.13. These two values are then added together to give total feet of head.
For example: 21 psi X 2.31 = 48.5 feet of head (pressure side) 6 Hg X 1.13 = 6.8 feet of head (suction side) Total TDH = 48.5 + 6.8 = 55 feet of head The TDH value of 55 feet of head can now be used to determine flow rate. Feet of head can be converted to flow rate by going to the pump performance curve supplied by the manufacturer. As the resistance to flow increases, flow rate decreases.
Take your calculated TDH of 55 on the vertical scale of the graph and move horizontally across until it intersects with the pump curve. From there, follow vertically down to the horizontal axis to determine the flow rate.
In this example, the flow rate is 80 gpm (see illustration on page 15).
If the cover was rated at 100 gpm, then this flow rate of 80 gpm would be below the rating of the cover/SOFA and the pool would be compliant. If pressure was measured at 10 psi instead of 21 psi, this, for example, could be the difference between a clean and dirty filter. Now, when calculating TDH, using 10 psi in place of 21 psi, TDH has dropped to 30 feet of head. Using the same pump curve, the flow rate is now 110 gpm, and the SOFA would not be sufficiently rated. This example also serves to illustrate the difference between a health code specified TDH (55) and field verified TDH (30). Newer systems, designed for energy efficiency, will have an even lower TDH measurement.
If the TDH-based flow rate is less than the flow rating associated with the drain cover/SOFA – specific flow rating, then the pool would be in compliance.
However, if the TDH-based flow rate is higher, then this SOFA configuration would not be in compliance.
