Posted by Ray Cronise on November 4, 2008 at 7:42am
This was a rather long response to my previous blog on replacing drain covers to member, Casey Treese in Las Vegas. If you haven't read part 1, you might want to do that first by clicking here.Casey's response was:Excellent article! I liked that you included field built sumps too. I have virtually no experience with these but it got me thinking about how I would go about hydraulic calcs and determining velocities for these custom sumps. Let's say the shapely design was the easy part and I'm good with the above diagrams but, after that, how would I approach the the system design as a whole? Does the numbers game change with a field built sump? Is it easy or hard to "lasso" the dangerous flows that lurk in the more volumonuous pools?The key to eliminating suction entrapment world wide is for us all to better understand what submerged suctions do and don't do. As we replace covers, there are several important issues we must keep in mind. The three most important are:1) flow rate (measured in GPM)2) flow rate (measured in GPM)3) flow rate (measured in GPM)As it turns out, we are rapidly moving beyond the entire velocity concept in safety for piping and drains. It served us well for years, but you will see that the only real number we need is the system flow rate which is measured in GPM, not FPS. Velocity was a good start, but it does not tell the whole story and often velocity alone can lead us down a wrong path. Historically we have placed far too much importance on TDH estimations. They are just that estimates. Velocity, estimates, and rules of thumb are very rapidly falling aside as we describe accurate performance criteria that can be measured and verified in the field.Let me give you an example. It was long held by many state codes that if we limited the velocity through a grate to 1.5 fps (velocity) that made the drain safe. The velocity does not directly effect the suction, so this requirement is related to hair entrapment/entanglement. While it was the a great start, we found there are unforeseen issues with this approach.When we started testing covers with a FULL HEAD of hair in the new ASME standard, rather than just a "pony tail" in the old ASME standard, we found something very surprising. The hair would begin to spread out, or mat, on the surface of cover resulting in the velocity of water through the remaining cover opening increasing. Where the velocity increases is the place the hair will ultimately become entangled or trapped in the cover. We found that while our 1.5 fps velocity was a great start, it was NOT as good as actually testing the cover. Ironically, the geometry for covers that have the highest flow rate rating and DO NOT trap hair often have velocities far in excess of 1.5 fps. In other words the safest drains we TEST for hair entanglement, out perform these existing "rules of thumb" we have worked with for years.We are very quickly moving into a new realm in swimming pool circulation system design safety. Verifying through direct measurement that your as-built swimming pool is safe is always better than any piles of plans or calculations that suggest it is safe. An accurate measurement in the field always trumps a general rule of thumb. Always. It also uncovers the situations when we make mistakes. Our in field measurement is the SYSTEM FLOW RATE in GPM. We cannot practically measure velocity directly, so why bother with them in the first place.The second, and most IMPORTANT step, is all professional pool builders should always attempt to eliminate the hazard completely in every pool. We will cover this more later, but suction ports provide little to no contribution to pool circulation. A vast majority of submerged suction ports (not all), dual outlet or not, can simply be eliminated. Remember, it is maintenance of the cover, systems not built to standard, or systems that were modified after the fact that caused most reported suction entrapment cases. If we ELIMINATE the hazard, not just MITIGATE it, we are moving beyond our habits of yesterday and into a realm of suction entrapment free swimming pools. As an industry, we've have taught circulation incorrectly for so long that there is a lot of confusion, but as I travel the country and meet more and more people that have given it a try, they are AMAZED to find how well drainless pools work.We continue to need submerged suction (drains) for high volume water features, in floor cleaning systems, and raised beam spas among other things, but drains do little for water circulation or distribution of sanitizer. And when we do use them, let's all commit to measure and verify they are working correctly and safely.On the other hand, If you believe your pool has a dead spot (diving hopper, steps, coves and swimouts) - place a floor return or directional return to push clean water into that spot. Consider reversing the flow through existing single main drain pools.In the end be sure you always try to first eliminate the hazard and when you can't eliminate then be sure to verify installed submerged outlets are safe.
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