FEATURE BY DAVID J. PETERSON, P.E., SWD POOL AND SPA HYDRAULICS GOING BEYOND CODE REQUIREMENTS 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 inside 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 The pool industry needs to look at the big picture, shifting from codelimit design to best practices design. Photos courtesy Watershape Consulting/Design by Skip Phillips, Questar Pools T he 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 22 ■ Pool & Spa Marketing ■ January 2010 PSM_JAN10.indd 22 12/30/09 8:38:09 AM