Pool and spa hydraulics
by Sally Bouorm | January 1, 2010 3:12 pm
[1]By David J. Peterson, P.E., SWD
The pool and spa industry has been mired in entrapment codes, misinformation, media hype and politics for years. Everyone understands suction entrapments can be lethal, even horrific, but the industry needs to look beyond the suction outlets and assess the hydraulics of the complete system.
Right now, builders throughout North America are repeating the same mistakes that created the suction entrapment hazards in the first place. There is little guidance on best practices; the legally defensible standards are not understood, or even owned, by the very builders who are entrusted with the construction of safe and efficient hydraulic systems. It is time for the pool industry to make a paradigm shift from code-limit design to best practices design.
An example of code-limit design is turnover whereby the designer establishes the filtration rate based solely on the requirement that the turnover is done in, at most, ‘X’ minutes. However, a best-practice approach to turnover would use additional guidelines to justify a performance-based filtration rate.
Of course, safety is paramount—it is always a builder’s first consideration. Fortunately, it’s not hard to design for safety, leaving more time to focus on other important considerations, such as energy efficiency, acoustics, extended system life cycle, elimination of cost estimate and construction errors and ensuring functional systems.
Physics
Let’s start with the water molecule: H2O. This is the building block of life and the reason this industry exists. When uncountable numbers of these molecules are combined, we have an essentially incompressible fluid called water.
Water, like all molecules, has mass; in other words, a measure of an amount of material (atoms). The gravitational pull of the earth gives weight to that mass. We usually refer to the weight as a ‘specific weight,’ e.g. 9.8 kN/m3 (62.4 lbs/ft3).
If you are at a certain depth of water you get pressure, which is the specific weight multiplied by the depth to the point of interest. For example, at a depth of 3 m (9.8 ft), the pressure is 3 m x 9.8 kN/m3 (9.8 ft x 62.4 lbs/ft3) = 29.4 kN/m2 (614 lbs/ft2). This is equivalent to 29.4 kPa (4.26 psi), or simply ‘3 m (9.8 ft) of head.’ Pressure is measured in pipes using pressure gages.
Air is also a fluid, although it is compressible. When you are sitting at the beach, you feel 101 kPa (14.696 psi), due to atmospheric pressure caused by the weight of all the molecules stacked above you.
When water moves through pipes, fittings and equipment, it travels with a certain speed, or velocity. Like cars driving through a tunnel, there are certain legal speed limits. In fact, the absolute maximum speed of water in polyvinyl chloride (PVC) pipe is 8.8 km/h (5.5 mph), usually stated as 2.4 m/sec (8 ft/sec). We’ll come back to this speed limit later, because it is not the value we should be using for design and construction purposes.
As water molecules drag along the insides of the pipe, they create friction, which transfers and wastes pressure energy into kinetic energy (turbulence), heat energy, acoustic energy, and internal energy (molecular interaction). Friction loss is also known as ‘major loss’ to distinguish it from ‘minor losses,’ which are the pressure losses in fittings and simple devices due to the turbulence of the flowing water.
It is common in the pool industry for the minor losses to represent half the total pressure loss in the system—they are hardly ‘minor.’ Think of the fittings as equivalent lengths of pipe for computation of total dynamic head (TDH). For example, a 51-mm (2-in.) elbow might be equivalent to 1.8 m (6 ft) of pipe. All the equivalent pipe lengths are added to the actual pipe length; then, head loss tables are used to determine the pressure drop.
Additional pressure is also lost due to components such as filters, heaters, salt chlorinators, etc. Head loss information is typically provided by the manufacturer for certain flow rates. For example, a filter might have 1.2 m (4 ft) of head loss running at 227 lpm (60 gpm).
Energy
The lost pressure energy is easily measured by reading pressure gauges at various points in the plumbing system. The gages will show the pressure drops throughout the system, except across the pump where it increases. In fact, the increase in pressure across the pump exactly matches the total decrease throughout the rest of the circulating system. The sum of all pressure losses, plus the difference in elevation between the surface water level of the suction ports and the surface water level of the discharge ports, is commonly called TDH.
TDH is important because it is a direct measurement of the system’s efficiency and is used to select the pump. A properly sized pool filtration system will have a TDH of less than 12 m (40 ft) of head. Poorly designed systems may be twice that number.
TDH is directly related to the approximate square of the velocity—in other words, if the velocity is doubled, the TDH will almost quadruple. Thinking about it the other way, to cut TDH in half, velocity need only be reduced by about 25 per cent.
Velocity
How are velocities reduced to get the TDH to a reasonable level? First, reduce flow rates by moving only the minimal amount of water necessary. If the volumetric flow rate requirement is 114 lpm (30 gpm), do not pump 151 lpm (40 gpm). Second, properly size the pipes to handle the flow rate. Do not necessarily oversize—this might imply a careless and wasteful use of PVC. Sound engineering should be used to select the proper diameter.
To keep things simple, follow these guidelines:
- Minimum scouring velocity = 0.15 m/sec (0.5 ft/sec)
- Maximum suction-side velocity = 1.4 m/sec (4.5 ft/sec)
- Maximum discharge-side velocity = 2 m/sec (6.5 ft/sec)
| Table 1: Maximum flow rates for ASTM D1785-06 Sch 40 PVC | ||
| Nominal | Maximum suction flow at 1.4 m/sec | Maximum discharge flow at 2.0 m/sec |
| Size (OD) | (4.5 ft/sec) | (6.5 ft/sec) |
| 25 mm (3/4″) | 26 lpm (7 gpm) | 42 lpm (11 gpm) |
| 32 mm (1″) | 45 lpm (12 gpm) | 68 lpm (18 gpm) |
| 40 mm (1-1/4″) | 79 lpm (21 gpm) | 114 lpm (30 gpm) |
| 50 mm (1-1/2″) | 110 lpm (29 gpm) | 155 lpm (41 gpm) |
| 63 mm (2″) | 178 lpm (47 gpm) | 257 lpm (68 gpm) |
| 75 mm (2-1/2″) | 254 lpm (67 gpm) | 367 lpm (97 gpm) |
| 90 mm (3″) | 394 lpm (104 gpm) | 568 lpm (150 gpm) |
| 110 mm (4″) | 678 lpm (179 gpm) | 977 lpm (258 gpm) |
| 160 mm (6″) | 1,533 lpm (405 gpm) | 2,214 lpm (585 gpm) |
| 225 mm (8″) | 2,657 lpm (702 gpm) | 3,838 lpm (1,014 gpm) |
| 280 mm (10″) | 4,187 lpm (1,106 gpm) | 6,049 lpm (1,598 gpm) |
| 315 mm (12″) | 5,943 lpm (1,570 gpm) | 8,585 lpm (2,268 gpm) |
For example, if a 75,708-l (20,000-gal) pool requires a 360-minute turnover, the flow rate is 210 lpm (56 gpm). On the suction side of the pump, you will need a 75-mm (2.5-in.) outside diameter line; on the discharge side of the pump, you will need a 63-mm (2-in.) outside diameter Sch 40 PVC line.
Keep in mind this is a quick and easy way to initially size up all the lines. Long runs or numerous fittings may require the plumbing to be up-sized, thus reducing velocity and head loss even further.
It is also important to note there are only discrete sizes of pipe and fittings. This means that if, for example, the flow rate is 182 lpm (48 gpm), the suction-side diameter selected from the previous table is 75 mm (2.5 in.) This will have an actual velocity of only 1 m/sec (3.2 ft/sec). In other words, the 1.4 and 2 m/sec (4.5 and 6.5 ft/sec) suction and discharge velocity limits in this table occur at specific flow rates. However, most line size selections made with this table result in even lower velocities by design.
These criteria are what I would term a best-practices approach to line size selection. In fact, one of the benefits of our 1.4 and 2 m/sec (4.5 and 6.5 ft/sec) design limits is that if something changes, and the flow rate exceeds the design flow rate, there is a bit of headroom before the velocity exceeds the code limit.
[2]So, actual velocities may be less than these recommended limits, but line sizes might need to be increased anyway—how is that determination made? The answer is in the TDH analysis, which is too complicated to explain in this article. If you want to learn it, a detailed electronic worksheet—where pipe diameters, lengths, flow rates, fittings and equipment are all tallied and the TDH is calculated—is available. It’s all part of the Genesis 3 Advanced Fluid Engineering Design School package, which also includes software training.
How do we apply these criteria to size split-suction outlets? The easiest way is to size the primary suction line for the 1.4 m/sec (4.5 ft/sec) maximum, as selected in the table above, then maintain the same size pipe for all split outlets. This way, if one outlet is blocked, the other(s) can handle 100 per cent of the flow without a significant head loss in the branch piping. (This results in a dangerous vacuum force on the blocked cover.)
If all the branch plumbing on split-suction outlets is the same size as the primary suction line and sized for the 1.4 m/sec (4.5 ft/sec) maximum, during normal operation the flow rate and velocity will be halved to 0.7 m/sec (2.25 ft/sec), until one cover is blocked and the other handles 100 per cent of the flow at 1.4 m/sec (4.5 ft/sec) maximum. In the example of a triple-split set of suction outlets, each is expected to operate at a maximum of 0.47 m/sec (1.5 ft/sec), one-third of the 1.4 m/sec (4.5 ft/sec) design, until one cover is blocked and the other two roughly split the total flow for a velocity of 0.7 m/sec (2.25 ft/sec) in each line. When designing spas, a two-pump system, at minimum, is always used. Aerated venturi jets are never used with the filtration system. Spa jets are always on their own dedicated pump(s). The filtration system’s split suction outlets are usually located on the floor, separated by 0.9 m (3 ft) clear. The jet pump suction outlets are located in the footwell at the face of the bench, separated by 0.9 m (3 ft) or on different planes.
The next question becomes how to handle skimmer equalizer lines. Installion of skimmer equalizers is rarely recommended; existing ones should be plugged wherever possible. Skimmer equalizers are merely bandages for another problem: inadequate autofill mechanisms. Builders began installing equalizers when the water level dropped below the skimmer throat, causing the filter pump to run dry and be damaged. Now, we have reliable autofills; the risk of any suction entrapment hazard is much worse than a pump seal failure, or even an entire pump replacement. Builders are actively splitting these skimmer equalizers to comply with the code requirements—but elimination trumps mitigation every time.
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) |
Weirs
There are many types of weirs or spillway edges. Structurally, there is a wall that dams up water until the water rises above the wall and overflows to the other side. Rectangular weirs have level edges and some form of sidewalls that are generally used to control the width of the spillway. However, natural streams have water flowing over uneven boulders, and perimeter overflow spas or pools don’t have sidewalls. V-notch weirs have no bottom edge and instead rely on only two angled sidewalls to limit and control flow.
[3]For dam walls in spas or vanishing edges, we are also faced with a multitude of wall cap options, such as level surfaces, submerged surfaces that cut back into the pool side and surfaces that cut away from the pool. The wall cap configuration does not affect the flow much, but it has downstream performance implications and esthetic considerations, generally decided by the site and water shape design.
When referring to downstream performance in the context of weirs, the effect of the nappe, or face of the spillway, is being described. With a lot of flow, water can break away from the outside of the dam wall, creating a clear-sheet effect. At low flow rates, the water is likely to adhere to the wall. If the wall cap is cut away from the pool, the slope of the cap naturally accelerates the water in a direction that jumps off the wall.
There are lots of details and considerations for weirs and vanishing edge wall design. More information can be learned from the Genesis 3 Edge Program presented by Randy Beard and Skip Phillips. To determine how much water is flowing over the weir, measure the depth of water over the edge and refer to Table 3.
Table 3: Rectangular weir performance (Francis formula) |
|
|---|---|
| Weir Head | Flow rate per unit of weir width |
| 1.6 mm (1/16”) | 7 lpm/m (0.56 gpm/ft) |
| 3.2 mm (1/8”) | 20 lpm/m (1.61 gpm/ft) |
| 4.8 mm (3/16”) | 36 lpm/m (2.91 gpm/ft) |
| 6.4 mm (1/4”) | 56 lpm/m (4.48 gpm/ft) |
| 7.9 mm (5/16”) | 78 lpm/m (6.26 gpm/ft) |
| 9.5 mm (3/8”) | 102 lpm/m (8.19 gpm/ft) |
| 11 mm (7/16”) | 128 lpm/m (10.3 gpm/ft) |
| 13 mm (1/2”) | 157 lpm/m (12.6 gpm/ft) |
| 14 mm (9/16”) | 187 lpm/m (15.0 gpm/ft) |
| 16 mm (5/8”) | 219 lpm/m (17.6 gpm/ft) |
| 18 mm (11/16”) | 252 lpm/m (20.3 gpm/ft) |
| 19 mm (3/4”) | 287 lpm/m (23.1 gpm/ft) |
| 21 mm (13/16”) | 323 lpm/m (26.0 gpm/ft) |
| 22 mm (7/8”) | 361 lpm/m (29.0 gpm/ft) |
| 24 mm (15/16”) | 400 lpm/m (32.2 gpm/ft) |
| 25 mm (1”) | 440 lpm/m (35.4 gpm/ft) |
| 32 mm (1-1/4”) | 612 lpm/m (49.3 gpm/ft) |
| 38 mm (1-1/2”) | 801 lpm/m (64.5 gpm/ft) |
| 45 mm (1-3/4”) | 1,005 lpm/m (80.9 gpm/ft) |
| 51 mm (2”) | 1,222 lpm/m (98.4 gpm/ft) |
Information has been published stating a certain flow rate should be used for vanishing-edge design. Unfortunately, without qualifying all the key variables, a single recommendation is misleading and wrong. It is preferable to break things down into the influent and effluent systems.
The influent system would include the returning water that is spilling over the edge back to the pool. Obviously, there are many variables to size that system, including weir length, desired downstream effect (if any), pool usage, prevailing wind direction and strength, and even the edge detail. Esthetically, the design is typically configured for a low flow rate requirement of 7 lpm per linear meter (0.5 gpm/ft) of edge. Hydraulically, the system is typically designed to run at 25 to 37 lpm per linear meter (2 to 3 gpm/ft) of edge. This means more than 7 lpm per linear meter (0.5 gpm/ft) may be used if necessary, though this can be slowed down as low as possible to achieve the effect at low operational expense, and with less evaporation and potential splash out.
Finally, the effluent system (gravity lines to a surge basin, if present) is designed to handle twice the flow rate of the edge system—in this case, 25 to 75 lpm per linear meter (4 to 6 gpm/ft) of edge. This way, there is a 2:1 safety factor to ensure the gravity system will not be exceeded by normal operation of the edge system. This also provides extra capacity for surges caused by swimmer displacement and wave action.
All of this, of course, may vary significantly depending on the project. A competition pool probably needs a more robust system to keep the pool’s gutter flowing with all the wave action. Every project is different and has its own set of variables that need to be considered.
In conclusion
There is much more to learn than can ever be summarized in a magazine article. To truly stay on top of the game, the Genesis 3 Advanced Fluid Engineering Design School is a valuable program. With a CD of more than 20 custom watershape-specific worksheets, plus 20 hours of detailed instruction, it is unlike anything ever offered in the industry. The schedule is available at www.genesis3.com. The class is limited to only 40 students, so sign up quickly (a laptop is required).
| AVOIDING COMMON PLUMBING MISTAKES |
|---|
| One of the most common errors made among pool builders is undersized plumbing. A contractor once told me he sized the pool plumbing lines based on the horsepower of the pump. A 1.5-hp pump required 50-mm (1.5-in) plumbing; a 2-hp pump required a 63-mm (2-in.) plumbing; etc. Using our best practices approach, a 0.5-hp pool pump capable of flowing 340 lpm (90 gpm) at 9 m (30 ft) of head, would require 90-mm (3-in.) plumbing on the suction side of the pump. Historically, builders have also believed plumbing did not need to be any larger than the ports on the pumps. Some 0.5-hp pool pumps have 63-mm (2-in.) ports. For our 340-lpm (90-gpm) example, this would result in a line velocity of 2.6 m/sec (8.6 ft/sec)—exceeding the limits of several ANSI, NSPI, APSP and ASME standards. One might think 9 m (30 ft) of head is unrealistically low for this example. Keep in mind, however, that an unfiltered waterfall type feature is unlikely to have more than 9 m (30 ft) of total dynamic head if the plumbing is reasonably sized. The most important thing to note is that there is no relationship between pump horsepower and the size of the plumbing—the flow rate must be determined before sizing the pipe. |
David J. Peterson, P.E., SWD, is founder of Watershape Consulting Inc., a planning, design and engineering firm providing owners, architects, contractors and the legal profession with services relating to residential and commercial pools, spas and water features. Peterson is a registered civil engineer in multiple states, a member of the Society of Watershape Designers (SWD) and a Genesis 3 Platinum Member. He is also a member of the American Society of Civil Engineers, National Society of Professional Engineers, Construction Specifications Institute and American Concrete Institute.
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