Pumps
Most pool pumps are end-suction centrifugal style and perform according to the Affinity Laws:
- If the pump speed doubles, the flow rate doubles.
- If the pump speed doubles, the head quadruples.
- If the pump speed doubles, the power is multiplied by eight.
The benefit of using two-speed or variable-flow pumps is that lower speeds save significant amounts of energy.
Other considerations to reduce TDH include minimizing the length of pipe and number of fittings. Using long radius (sweep) fittings instead of sharp fittings also helps. New energy codes in California are actually mandating the use of long-radius fittings for energy consumption. This is expected to reduce TDH by 10 to 20 per cent.
Gravity systems
Gravity systems are channels or half-full pipes, positioned with a specific slope to transfer water using gravity instead of pumps. They are common for gutter pools and the like, where the water overflowing the gutter’s weir is collected and transferred to a surge basin, via gravity.
Manning’s equation is used to calculate flow through gravity systems based on the geometric configuration, the roughness of the channel or pipe and the slope. Unlike pressure systems, there is no minor or component head loss—by its very nature, the head loss is the change in water-level elevations between the two ends of the pipe or channel.
Like pressure systems, the pipe or channel size is critical and dependent on the flow rate. Additional design criteria include the slope of the pipe, indicated as a decimal (0.0208) or percentage (2.08 per cent), or elevation drop per unit length, such as 20 mm per meter (0.25 inches per foot.)
Table 2: Maximum flow rates for ASTM D1785-06 Sch 40 PVC at varying slopes |
||||
|---|---|---|---|---|
| Nominal | Slope = 0.5% | Slope = 1% | Slope = 2% | Slope = 4% |
| Size (OD) | Flow Rate | Flow Rate | Flow Rate | Flow Rate |
| 50 mm (1-1/2″) | 11 lpm (2.8 gpm) | 16 lpm (4.1 gpm) | 22 lpm (5.9 gpm) | 31 lpm (8.2 gpm) |
| 63 mm (2″) | 20 lpm (5.2 gpm) | 28 lpm (7.5 gpm) | 40 lpm (10.6 gpm) | 57 lpm (15.0 gpm) |
| 75 mm (2-1/2″) | 32 lpm (8.5 gpm) | 46 lpm (12.0 gpm) | 64 lpm (17.0 gpm) | 91 lpm (24.1 gpm) |
| 90 mm (3″) | 57 lpm (15.2 gpm) | 81 lpm (21.5 gpm) | 115 lpm (30.4 gpm) | 163 lpm (42.9 gpm) |
| 110 mm (4″) | 110 lpm (29.1 gpm) | 156 lpm (41.2 gpm) | 220 lpm (58.2 gpm) | 312 lpm (82.3 gpm) |
| 160 mm (6″) | 307 lpm (81.0 gpm) | 434 lpm (114.6 gpm) | 613 lpm (162.0 gpm) | 867 lpm (229.1 gpm) |
| 225 mm (8″) | 598 lpm (157.9 gpm) | 845 lpm (223.3 gpm) | 1,196 lpm (315.8 gpm) | 1,691 lpm (446.7 gpm) |
| 280 mm (10″) | 1,097 lpm (289.7 gpm) | 1,551 lpm (409.7 gpm) | 2,193 lpm (579.4 gpm) | 3,102 lpm (819.4 gpm) |
| 315 mm (12″) | 1,749 lpm (462.1 gpm) | 2,474 lpm (653.6 gpm) | 3,499 lpm (924.3 gpm) | 4,948 lpm (1307.1 gpm) |
