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Swim jets, suction and cavitation
ABOVE: In a relatively short time cavitation will damage the volute and the impellor to a point where the pump becomes useless. Image: www. pumpsandsystems.com
By Cal Stanley
I
was recently contacted by a consultant in another state. He believed that a particular pool’s plumbing particularly the suction system - was incorrectly installed, potentially dangerous, and did not conform with the requirements of AS1926.3. As somewhat of an expert in pool hydraulics, this concerned me. As too many suction systems in pools have a potential to trap unwary users, particularly young ones, I took this seriously and asked that he send me all the information he had. Pool suction systems must conform with the requirements of the current edition of AS1926.3 – 2010. This Standard requires suction outlets connected to a common pipe to be not less than 600mm apart, centre to centre. One set was only 500mm apart. This maximum permitted flow rate is embossed on every outlet cover. The covers used in this pool all had a flow rate of 420 litres per minute (lpm). Exceeding the rated cover makes entrapment under water, particularly of young children, a real possibility. Not having full plumbing details I was unable to determine the actual flow through these pipes and thus through each cover fitting. However, I believe it would likely have exceeded the 420lpm limit by a considerable margin even with the pumps cavitating. The pool was fitted with three such systems each with two outlets. All were connected to 3hp pumps by 50mm pipes. Each pump was cavitating badly and very noisy. I understand that the swim jet flow at the outlets of each system was little stronger than that of a normal filtration return eyeball. The pool owner wasn’t happy!
26
SPLASH!
February/March 2022
“Good hydraulics mandates that the suction pipe to any pump be of a size sufficient to permit a constant flow, and sufficient to fill all wet parts of pump and return water to pool.”
Cavitation
A 3hp pump cannot draw sufficient flow through 50mm pipes to keep the pipes and the pump full of water: the pipe resistance is too much, so the pump cavitates, and in this case would have been actually pumping about the same flow as a 1.5hp pump. Good hydraulics mandates that the suction pipe to any pump be of a size sufficient to permit a constant flow, and sufficient to fill all wet parts of the pump and return water to pool. These pumps were never fully primed and trying to suck far more water than the pipes permitted. A centrifugal pump works by creating a lowpressure zone in its volute – which houses the spinning impellor. This low pressure is what causes the water to flow into a pump. When the inflow is restricted, pressure drops so far that the available molecules of water begin to implode. This implosion is called cavitation, and it not only creates considerable noise, in a relatively short time it will damage the volute and the impellor to a point where the pump becomes useless. This may be hard to believe, but go online and you can find examples where even solid metal impellers and volutes have been damaged beyond repair by cavitation. What hope for a plastic one? A pump that constantly cavitates is noisy and has a very short life span. The “swim jets” in question at this particular project were actually just normal eyeballs. They weren’t really swim jets and provided no resistance to swimmers. I understand that the pool installer is rectifying matters as I write. None of us are perfect and all capable of mistakes. I like those pool builders/ installers who owns their mistakes. Too many try to blame anyone but themselves. n For these articles Cal Stanley draws on his 30 years’ experience and success in pool construction, having run award-winning pool construction company Neptune Pools in Western Australia for three decades. He currently works as a pool consultant and delivers hydraulics courses for the swimming pool industry. Contact: neptunepools@westnet.com.au.