My feedback on main drains in an above ground pool...

[DennisP]

And I thoroughly disagree on your "conclusions"... Even with just over 4ft of water I can tell a difference in temp between the upper levels and the bottom without the main drains. With them on it is drastically different.

I would argue your assumption that since chemicals mix that temperature variances are also nil. That is far from the case.

Again, believe what you want. I have direct observation and quantifiable results to backup what I have posted. And unless you have the exact same pool setup, in the exact same environment, you have no point of reference other than flawed studies to fall back upon. I am an engineer, but far from blinded enough to think that those are the only answers and fully realize there are environmental factors that change everything compared to a set of "fixed examples". Your diatribe on pump dynamics show that you understand those aspects of engineering, so why be so obtuse on this???

[mas985]

Even with just over 4ft of water I can tell a difference in temp between the upper levels and the bottom without the main drains. With them on it is drastically different.

Within the first few minutes of the pump turning on, I would agree but I think you would have to agree that at some point even without a MD, the water temps would eventually equalize. It is illogical to think that the water temps would remain different at the top and bottom of the pool forever while the pump is running and constantly mixing the water. Even with a single return pointed horizontally, you would see with a dye test, return water mixing with the pool water at all levels of the pool. I really don't think it would take long at all for the temperatures to equalize.


But let me be clear, I am not disputing that MDs help to equalize the temperature difference. What I am saying is that there are other ways to accomplish same thing and they are equally as effective. As I mentioned before, I have one of my returns pointed down to my deep end and the deep end temperature at 9' equalizes very quickly, within minutes. So instead of a MD, you could have added a lower return and had the same results.

I would argue your assumption that since chemicals mix that temperature variances are also nil. That is far from the case.

Why would it not be the case? Both heat and chemicals move along with the water currents as they are both part of the water. Seems logical to me.

you have no point of reference other than flawed studies to fall back upon.

Actually I do, my own measurements and observations as well as a few posts from other POs that have pools without MDs and have no such problem with temperature difference. But why do you think these studies are flawed? They were performed by well respected universities and organizations. If you have any other contradictory scientific evidence then please share it. My point with the studies is that somebody actually went to the trouble of running experiments and simulations and documenting the results. This is what engineers and scientist do to confirm hypotheses.


I understand your reluctance to accept this evidence because you went to the expense and trouble of installing the MD and you really want it to have a benefit that you could not possibly have achieved in any other way but to me, the evidence is indisputable.

[Anonymous [GDPR] European in the UK]

And I thoroughly disagree on your "conclusions"... Even with just over 4ft of water I can tell a difference in temp between the upper levels and the bottom without the main drains. With them on it is drastically different.

I would argue your assumption that since chemicals mix that temperature variances are also nil. That is far from the case.

Again, believe what you want. I have direct observation and quantifiable results to backup what I have posted. And unless you have the exact same pool setup, in the exact same environment, you have no point of reference other than flawed studies to fall back upon. I am an engineer, but far from blinded enough to think that those are the only answers and fully realize there are environmental factors that change everything compared to a set of "fixed examples". Your diatribe on pump dynamics show that you understand those aspects of engineering, so why be so obtuse on this???

Oh, I don't follow your ideas Dennis and your Facts are not really facts they are as you see it but not facts.
Of course you get better flow with you M/D's as you don't have more than the 1 skimmer? how can you possible get the same flow with 1 skimmer compared to when the M/D's are involved?
As to the M/D's getting your pool up to temperature, it depends where your thermometer is. I have the pleasure of playing with pools set up like yours and also ones I have put a 3 port valve on so I can switch the M/D's from inlet to outlet.
Now you believe taking cold water from the bottom and feeding it into the top via returns is quickest. I don't, the water at the surface is warmer (fact) take the water from there and it takes less energy and therefore less time to heat that up than the colder water from the bottom that's a fact to. feed that into the bottom of the pool via the M/D's and the lower part of the pool gets warmer quicker as you said it causes things to mix quicker, yes it does whether it's used as an inlet or an outlet is mixes more that's a fact, you've agreed, I have tested it, it follows physics and you can't mess with physics.

The daft thing you said back a few posts about taking out your other return and wishing you hadn't put it? Go for it Mr engineer! then you'll loose performance as the friction and restriction you'll place on the system will create a great big headloss. You cannot compress a liquid so you can only get into the pump via skimmer and M/D's what the return line will allow to flow, the headloss and friction is just wasted energy.

I am not bemoaning M/D's, I was amazed that pools without them worked at first, the guy who taught me thought as you did. My pool doesn't have a M/D but I don't have a robot cleaner either as I don't need it a net and a pool Blaster Max is all I have and the Max is only for shear quickness as there is bound to be one leaf that misses the skimmers but my debris is taken out before it sinks in most cases but you are right I have to empty my skimmers more often.

Dennis where is the floor in the studies? they use the same software that keeps planes in the air and the maths behind it is very very accurate, the same is used in films to generate waves and fluid movements?
You need a study to prove it's floored, is there one?

[J_Palmer_Cass]

Regardless of the so called benefits of having a pair of master drains on the pool floor, they would be of little use to me. I do heat the pool with solar deck panels.

First, it is hard enough to properly place a pool liner exactly where it belongs when you only have to locate sidewall drains, skimmers and the return. Floor drains just makes the liner installation difficult.

Second, I don't want to have to deal with winterizing the piping that feeds the floor drains. Not quite sure how I could winterize the master drain plumbing with my pool setup either.

My pool liner fits like a glove, and I doubt that it would fit that well if I had used floor mounted master drains.

[mas985]

Yes the bullet does but it also has a way higher velocity, it's path decays so quickly as the water put's up an opposing force (Newton's 3rd law of motion).

But you are neglecting the movement of the water itself. When the pump first turns on, there is a lot of resistance but as time goes on and the pool currents are setup, the resistive force is reduced and the directional flow moves further and further away from the return reaching farther areas of the pool.

Also, I didn't mean to imply that the currents are only in one straight direction. I was mainly referring the fact that water does not drop like the bullet does. I understand that the water travels more or less in a cone and the cone angle is dependent upon the orifice size and shape.

except the single lines for the returns is way too much

You mentioned that you try to keep the port size the same between the suction and return sides but the above quote would indicate that you do not bring those same ports sizes (i.e diameter) all the way to the pump for at least the return side. If that is the case, the return side would definitely have more head loss due to less parallel pipe paths than the suction side. So I have to ask, isn't that defeating the purpose of the whole "port size" objective? Pipe, fittings and the exit orifice all add head loss to varying degrees so I don't see any benefit having the port sizes equal other than they have slightly less head loss than a smaller orifice eyeball. Is that the main objective?

Also, I am curious about your pump. Can you give more details about it (make/model)?

[Anonymous [GDPR] European in the UK]

Hi Mark, It was really for illustrative purposes more than what takes place over time, I admit to still learning and it's observational data, happy to have input from yourself, it's marvelous as you are one of the few to have such an understanding, so here goes.
The force of the bullet, or water dissipates, as we know energy can only be transferred so it begins to move more of the water, and more and more. Does the smaller faster orifice impart more or less energy than the slower moving but larger orifice? I think the restriction in the smaller fitting is robbing some of the energy. The fact that the water is nearer the surface means the air being less dense than water will allow as you describe allow a passage of water to be directed where you want, it's that rippling path on the surface. Does the larger less restrictive orifice moving more water, do the same, yes but a little slower, this I have observed to be actually more accurate at finding the path than the faster smaller orifice as that creates a more turbulent street so diverges more into the surrounding pool.

Most pool builders are obsessed with the circular path (Dennis demonstrated this several times) that swirling brings debris out of suspension into the dead spot in the middle of the pool where it sinks to the bottom. The same ideas of skimmer at one end and returns at the other is equally obsessed over. The directed return jet has control over leaves for a short distance only (10-12 feet) after which the flow decays and the energy previously put into the pool making it swirl takes over from the now diminished jet and the leaf can miss the skimmer, it then circulates once more and if it misses again arrives near the middle, partially water logged and beginning to sink. The pool robot salesman rubs their hands with glee as another willing customer arrives $$$.

Some pool builders do it slightly differently, returns one side and skimmers the other, the shortest distance, so much more obvious as the return jet, fast or slow will still have control over the leaf at that distance so it's easier to direct the leaf towards a skimmer (should almost always be more than one) add then that the return placed near the skimmer, facing downwards to action a horizontal rolling action creating lift and more debris is going into the skimmer without a main drain in sight.

To the other point:
If they ran say two single 1.5" lines but had a 2" to the pump from the skimmer that would be over the top unless as you said the return path was longer and more torturous, that's why we try to calculate the headloss in any pipe run. Depending on the path, number of fittings etc I would most likely run the same size pipe and separate the flows via Y's not T's to the returns but obviously each pool has it's own specific solution. As you said earlier the smaller orifice only adds 0.5 PSI, as my system runs between 0.25 and 0.75 PSI I really couldn't add 0.5 just to squirt water a little faster. LOL. I calculate the area of the pipe work and make the returns taper smoothly to the orifice dimensions that add up to total of the supplied pipe, I may vary that if I do want a directional faster flow and may also balance that out with the others.

I try to arrange the pad very accurately, pump inflow below the water line, return path above the water line but flowing down hill using the water in the pool as the minimum restriction possible, yes really obsessive, I want to try and run a system as a siphon one day. For now though, more like the HVAC just the minimal push from the pump but achieving reasonable flow. If I had a normal pool pump on there I would imagine it could cavitate especially with the pump going 0 to full RPM in under a second, I am surprised more pumps in the USA don't suffer cavitation because they are so powerful and the headloss in some cases is so high the water must be boiling inside the volute?

I the EU we have several pool companies that market monoblock filtration units in their pools, these pools have far the worst flow of any pool I work on, especially at the far end as they have a single return mounted very near the skimmer so water takes the short route. They do eventually (well sometimes) manage a bulk water movement but it's really poor and that is utilising a pump twice the power of a conventional pool (most USA pools are already using twice the power LOL) than we would in Europe but you do love add ons but even Sunny Optimism has a large pump for just a water fall but I digress. These pools still work when everything is within range so from that i deduced that we can really reduce the flow tremendously and likewise the power and still have a good pool. When the monoblock units have an algae issue then you really see how bad they are it can take more than a week to get them clear (bad course bag or pleated filters placed on the pump intakes rather than the outflow) I experimented on my own pool and even running the low power I do it still out performs the monoblock filtered pools but i have a lot more in reserve so if I need to get the water moving I can up the flow.

For commercial reasons I don't want to name the pumps I use but it's a lot more than just the pump, the whole circuit is improved where possible. I used to make many of the parts myself but as I realised that was more the icing on the cake I now adapt other parts to suit.

[mas985]

Does the smaller faster orifice impart more or less energy than the slower moving but larger orifice? I think the restriction in the smaller fitting is robbing some of the energy.

Yes, the total energy is less for a smaller orifice but the energy per unit area is higher directly in front so it is more focused and tends to push the water farther than the larger orifice. Same reason smaller spa jets feel more powerful.

As you said earlier the smaller orifice only adds 0.5 PSI, as my system runs between 0.25 and 0.75 PSI I really couldn't add 0.5 just to squirt water a little faster.

Not quite. The 0.5 PSI is at 70 GPM (full speed). Pressure loss is proportional to GPM^2. Given your RPM and wattage, I suspect your flow rates are much much lower. For 10 GPM the PSI loss is only 0.01 PSI.

I am surprised more pumps in the USA don't suffer cavitation because they are so powerful and the headloss in some cases is so high the water must be boiling inside the volute?

First, cavitation only occurs in the impeller inlet. As water travels outward in the impeller pressure rises and the vapor bubbles burst. So all the damage occurs in the impeller and fairly close to the inlet. The volute has the highest pressure in the system so water could not boil there. But the reason most pools do not have caviation problems is because PBs tend to install plumbing systems that have much higher head loss in the return plumbing than in the suction plumbing and this keeps the pump from cavitating.

[Anonymous [GDPR] European in the UK]

Yes, the total energy is less for a smaller orifice but the energy per unit area is higher directly in front so it is more focused and tends to push the water farther than the larger orifice. Same reason smaller spa jets feel more powerful.

Up close this is true, how about at a distance? The less overall power dissipates easier, the path decays. Because it's closer to the surface or pointing to the surface would help. It's like laminar flow jets all the rage but they are better than a conventional nozzle as that spray breaks up into the air because it's too turbulent to stay together. Just my observation.

Not quite. The 0.5 PSI is at 70 GPM (full speed). Pressure loss is proportional to GPM^2. Given your RPM and wattage, I suspect your flow rates are much much lower. For 10 GPM the PSI loss is only 0.01 PSI.

Yes true, my flow rate being around 31 US GPM would still be around 0.2 PSI (lazy guess, off to work) although my taper machined eyeballs probably less than the blunt end with smaller hole common to pool manufacturers.

First, cavitation only occurs in the impeller inlet. As water travels outward in the impeller pressure rises and the vapor bubbles burst. So all the damage occurs in the impeller and fairly close to the inlet. The volute has the highest pressure in the system so water could not boil there. But the reason most pools do not have caviation problems is because PBs tend to install plumbing systems that have much higher head loss in the return plumbing than in the suction plumbing and this keeps the pump from cavitating.

Yes, I wrote that badly, I meant the boiling temperature can be felt on the volute it's cavitation on the back of the impeller blades I was meaning, you put that so much better than me. I had to watch a YouTube video to re think what I had written. It was interesting to see the cavitation start so far ahead of the pump caused by the pressure drop in the valve the throttled down. https://www.youtube.com/watch?v=eMDAw0TXvUo
Because I have the pumps below the water line I don't need a self priming pump so the impeller design is different to a conventional pool pump even when the owners have the pump below water level they still use the "pool pump" cos that's what the label says......

Even running flat out at 2850 RPM I do not get cavitation, maybe because the pump increases rpm slowly it doesn't trigger the sudden pressure drop but also of course the dynamic head increases in proportion to the amount of water being moved
The massive savings in electricity, and less cleaning work with improved water clarity speak for themselves, Ben set out the Best guess idea and changed a lot of things, this is the first forum I have gone anywhere near explaining what I have done on pools but there would be a lot of very happy pool owners if they had the same system, running pools on a light bulbs worth of electricity! It's good for the co2 footprint as well, saving around 2 tons of co2 per season compared to a 3/4 HP motor and 1.5 HP probably double that.