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Pool and spa hydraulics

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The pool industry needs to look at the big picture, shifting from code-limit design to best practices design.

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).

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