Pentair performance curve for dual speed, low head limit

[JoyfulNoise]

It's a well known phenomenon in fluid mechanics that the flow into a drain is not uniformly distributed across the drain area/opening. Most of the flow field is concentrated at the edges and the flow gradient drops off pretty quickly.

The same thing happens with large area electrical contacts and thru vias on a printed circuit board. Most of the current flow is at the edges of the conductor.

I believe it's all called the "shower drain" effect. I'll see if I can find some old research papers I had on it...


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[mas985]

I assumed is was 24x24 because it was twice the size of the ruler.

But area is actually much larger than that (2x) because of the depth of water.

 

[JamesW]

I will make a new thread for this.

Drain Testing using Dye For Effect

 

[rephil]

They aren't suggesting an equal run time in the study. It's an AVERAGE minimum suggested based on their findings across many pools. It's not a magic number either.

I don't know how this is hard to grasp. A pump moves water. That's ALL it does. Pump run time = water movement = turnover.

A = B = C. so A = C.

 

[mas985]

They aren't suggesting an equal run time in the study. It's an AVERAGE minimum suggested based on their findings across many pools. It's not a magic number either.

They are saying that most pools reached the same level of water quality within a 2 hour period. That is not really an average. The average would probably have been less that 2 hours.

Had the testing occurred at more frequent intervals, they may have been able to correlate that with the number of turnovers. But they didn't. So we have no idea if there is a corollary to turnover or not. That would be pure speculation. Since the circulation patterns are different in each pool due to the placement of the skimmers, MDs and returns, even if two pools had exactly the same turnover rates, that would not mean they have the same time to the same water quality. The only thing we know for sure is that most pools achieved the same water quality in 2 hours and none required more than 4 hours. That is why in at least this case, time is more relevant than turnovers.

I don't know how this is hard to grasp. A pump moves water. That's ALL it does. Pump run time = water movement = turnover.

A = B = C. so A = C.

Well, they are not equal right. There is a relationship between them and they are proportional and yes pump run time results is a specific turnover rate but only for a specific pool setup.


But again, you cannot use a target turnovers to determine run time because there is no way to determine what the "required" number of turnovers that a specific pool requires without doing some testing. For this reason, I go back to my first comment, turnovers are irrelevant when determining pump run time. They just don't matter. If you just run your pool for 2 hours and assuming you have enough FC for the CYA level, then that is all the circulation that most pools would ever need. That is what the study claims and that is all I am claiming.

 

[diggles]

Sorry rookie here. Whats head mean10 feet, 20 feet. Is that the length the water travels from pump to pool?

 

[mas985]

There is a more detailed explanation in the Hydraulics 101 link in my sig but basically, head is measured in feet because it represents the effective height that a pump can lift a liquid. It also refers to the dynamic head (i.e. friction loss) within the plumbing system. With pump curves, head is used instead of PSI because head is independent of the density of the liquid that the pump is pumping (e.g. industrial pumps) where as PSI is dependent on the density of the liquid. So there only needs to be one head curve for a pump that is used in multiple applications.

 

[rephil]

Imagine a bell curve. The middle is average (also median). The 25% on either side is "most pools", making up a total of 50%. So yes, when they say 2 hours they are effectively talking about an average.

At this point i feel like i'm being trolled, but i'll say it again. When a pump runs, it moves water at a certain velocity (GPM). GPM x time = turnover. So pump run time always, always, translates into some turnover amount.

Maybe this will help. If someone says it takes 2 hours to get from LA to San Diego, that is an average assuming usual traffic and going the speed limit. The conditions and driver are going to affect the time, obviously. But most people say 2 hours because it's easier to comprehend than saying "i'll be there in 120 miles".

But, when someone says it takes 2 hours to get to San Diego that doesn't tell the whole story. You won't stop driving after 2 hours if you're only halfway there due to traffic. This is why pump run time doesn't tell the whole story, it only represents an average based on "normal" conditions.

A system that only moves 1 GPM on a 50k pool is not going to take 2 hours to reach desired quality. Just like a car driving 1 MPH won't make it to San Diego in 2 hours.

 

[mas985]

Imagine a bell curve. The middle is average (also median). The 25% on either side is "most pools", making up a total of 50%. So yes, when they say 2 hours they are effectively talking about an average.

Why would you consider only the middle 50% of a probability density curve and ignore the lower tail which contains samples that are less than the 2 hour mark and should be included in the sample space of "most" pools less than 2 hours? If you include the lower 25% which you should, then you really are saying 75% of the pools are less than 2 hours which it could be. So I don't think you are using a probability density curve correctly. Here is how I would use a distribution curve:

Assuming one has the data to create this curve, the bottom axis of that curve, which BTW may not be normally distributed (aka bell), is run time required (0 to some undefined maximum time of the worst pool). Imagine the vertical axis as a cumulative distribution going from 0% to 100%. The 2 hour mark is somewhere on the x-axis. The paper indicated that "most" pools require less than 2 hours so therefore, the 2 hour mark is somewhere ABOVE the 50% CDF point and likely to be well above the 50% point otherwise they would have used different terminology rather than "most". So again, this does not reflect an average. As average run time would be the sum of the run times divided by 119. I don't believe they did this or they would have called it an average. What they likely did is run each pool for 2 hours and then counted which ones were satisfactory and which were not. At best, "most" is a majority, something greater than 50%. So at least 54 of the 119 pools required less than 2 hours but as I said before, it is likely far greater than 54 of the 119.

When a pump runs, it moves water at a certain velocity (GPM). GPM x time = turnover. So pump run time always, always, translates into some turnover amount.

That is the equation for gallons pumped in a period of time. The turnover equation is: Turnovers = (GPM * Run Time (min)) / Pool volume (g).

A system that only moves 1 GPM on a 50k pool is not going to take 2 hours to reach desired quality. Just like a car driving 1 MPH won't make it to San Diego in 2 hours.

Of course not but that was not the scope of the study. The study looked at set of 119 typical pools within FL. I would expect very few of these pools are that far outside the norm. Even a VS pump can't go much below about 15 GPM.


But just ignore the study if you feel compelled and find your own ideal run time. Just don't blindly assume you need 1, 2 or 3 turnovers because some web site on the internet says that's what you need. Based upon my experience as well as many others on this forum, it is likely to be much less than that.

 

[rephil]

The paper indicated that "most" pools require less than 2 hours

Actually it says most pools require 4 hours or less, not 2, see page 12. In fact, the first conclusion is that "a few hours" is recommended (page 11). They suggest 2 hours as the minimum, see page 13.

The study looked at set of 119 typical pools within FL. I would expect very few of these pools are that far outside the norm. Even a VS pump can't go much below about 15 GPM.

Correct. It means the turnover amount within those 2 hours would be somewhat similar across the pools in their study.

But just ignore the study

Clearly I'm not, as pointed out above. I am only stating that pump run time means absolutely nothing without the context of the system and pool that pump is attached to. You're only focused on the time quoted by the study, which I've said is an abstraction of turnover and is meant to simplify the results. The study goes much deeper than run time. So when you tell people "only run your pump for 2 hours" it doesn't do the study justice.

The turnover equation is:

Thanks for clearing that up, but I think the point was made and still stands. Run time is turnover.

 

[mas985]

I think you are missing a very key quote. See page 4 bullet #2.

In most pools, increasing the pump running time above two hours results in imperceptible improvement of water quality.

They are also saying that the pools were not the same and had "wide range of physical characteristics". Which means the turnovers were not the same (see below).

Actually it says most pools require 4 hours or less, not 2, see page 12.

Not quite. It says most pools can be maintained in "Sparkling Condition" which is different than the above quote which references water quality. This one is more about aesthetics while the first quote is about sanitation. They are different and require different run times but aesthetics is optional while sanitation is mandatory. But if you read on in that same paragraph it does mention that running 1 turnover is only necessary when removing dead algae. So they are basically saying the same thing we are that turnovers do not matter in day to day operation. They also give a range of a turnover from (6 to 12 hours) for study pools which also confirms that the pools in the study do not have the same turnovers.

They suggest 2 hours as the minimum, see page 13.

No they said the minimum does not exceed 2 hours which is different. There are some pools that can get away with less than 2 hours. This is consistent with the page 4 bullet above. Plus, we have already proven that with forum members (and myself).

So when you tell people "only run your pump for 2 hours" it doesn't do the study justice.

Actually, as I mentioned before, in pool school we do say to start at the 4 hours mark for full speed pumps and 8 hours for low speed to be conservative. What I usually tell people is most pools can get away with less than that and 2 hours per day is not an unreasonable target. Not just because of the study but because many of forum members have reported that they too can run for very short periods of time without ill effects. But the study only focuses on run time and as I mentioned above, it does say that a turnover is not necessary.


With a lot more measurements and time, could they have shown a correlation of water quality to turnover (or fraction there of) to water quality? Perhaps but as I mentioned before, that may not exist because the number of returns, skimmers, pool shape and how the water circulates in the pool, along with pump flow rate, determines how long a pump must run to properly circulate the water. This goes beyond the turnover calculation because two pools with the same turnover may not achieve the same water quality because they have different circulation patterns. So again, I still believe turnover is a very poor indicator for run time.

 

[rephil]

Not quite. It says most pools can be maintained in "Sparkling Condition" which is different than the above quote which references water quality. This one is more about aesthetics while the first quote is about sanitation. They are different and require different run times but aesthetics is optional while sanitation is mandatory.

I think a sparkling pool is what everyone on this forum is going for.

 

[mas985]

Well at least in my case, I can get both in less than two hours and probably the same for many others as well.