Thread: VS pump .. calc minimum RPM using filter psi?

1. VS pump .. calc minimum RPM using filter psi?

My goal is to run my pool at the lowest RPM possible using a pentair VS pump.

I have tried to figure-out the various equations and performance curves, but it feels like there are too many unknowns.

So, I went a different route. Using Pentair's web site for a 12k pool:
@1200 rpm, it cites 3.1 turnovers in 18 hours => a turnover every 5.8 hours ... ~35gal/min
@2400 rpm, it cites 6.2 turnovers in 18 hours => a turnover every 2.9 hours ... ~70gal/min

I want 3 turns per day, knowing I will run at 2400 for 3 hrs for the cleaner = so that is ~1 turn over
the other 2 turns can be achieved at 1200 for 12 hours.

I expect this to be a super best-case scenario ... I know I have to run at higher RPMs ... but how much?

I have 2 exit pipes of 2" from the equipment: one to the pool returns and the other to a set of bubblers on a tanning ledge.
At 1200, the bubbler spike is about 3" above the pool surface and the bubblers are 6" below the surface.
So, I know there is flow. Yet, the pressure reading in the DE filter barely rises.

My DE filter loss curve shows 35gal/min equates to 1psi of loss.

Should I run various RPMs, record the filter pressure, use the filter's loss-to-GPM graph, and derive the RPM-to-GPM for my system?

2. Re: VS pump .. calc minimum RPM using filter psi?

I can do a quick estimate for you with the following information:

Suction Pipe Diameter (in)
Suction Parallel Runs #
Suction Run Length (ft)
Filter Gauge Height Above Water Level (ft)
Filter Pressure (PSI) @ > 2000 RPM

3. Re: VS pump .. calc minimum RPM using filter psi?

Suction Pipe Diameter = 2in
Suction Parallel Runs = 3

I assume this is the pipe distance to the skimmers and drains?
Suction Run Length = 36', 81', 61'

Filter Gauge Height Above Water Level = 6ft 8in

Filter Pressure (PSI) @ > 2000 RPM
It seems to be fairly linear: PSI = 0.118*RPM - 21PSI
... tested between 2000 and 3110 RPM

Thanks for the help.
The equations used would be great too.

4. Re: VS pump .. calc minimum RPM using filter psi?

PSI is proportional to head loss and according to the pump affinity equations, head loss is proportional to the square of GPM and RPM so PSI should not be linear with RPM but RPM^2 instead.

It seems to be fairly linear: PSI = 0.118*RPM - 21PSI
... tested between 2000 and 3110 RPM
So at 3110 RPM, the pressure is 344 PSI?

How about just telling me what PSI you actually read at 3110 RPM.

5. Re: VS pump .. calc minimum RPM using filter psi?

missed a zero ... PSI = 0.0118*RPM - 21PSI

RPM PSI
2000 3
2200 4.8
2350 6.5
2600 9
2800 12
3110 16

Honestly, my ability to read PSI on the gauge is at best +/-0.25.

6. Re: VS pump .. calc minimum RPM using filter psi?

PSI gauge has about a 1 PSI accuracy as well plus the error tends to grow as the pressure drops. Anyway, fitting your data to the pump affinity equation, the PSI best fit line is:

PSI = 27.5 *(RPM/3450)^2 - 6.3

But that would imply that the filter gauge is actually 14.5' (6.3 * 2.31) above the water level. With gauge to water distance of 6.7', the Y-Intercept for the PSI equation should be -6.7/2.31 or -2.9. It seems that the filter gauge is reading about 3.4 PSI too low. 16 PSI is a little low for 2" plumbing even when compensating for the elevation change. I would have expected that the filter pressure would read a little over 19 PSI @ 3110 RPM for typical 2" plumbing. BTW, are all three suction ports open? And does each suction run go all the way from the pool to the pad?

If you know for sure that the gauge height is 6.7', and assuming the filter gauge is off by 3.4 PSI, then the corrected flow rate would be:

Flow Rate (GPM) ~ RPM / 35

Which BTW, is very typical of 2" plumbing with multiple suction runs so I think that the filter gauge is probably reading too low.

7. Re: VS pump .. calc minimum RPM using filter psi?

Very odd.
Is it common to replace a pressure gauge due to accuracy ... or is it really just needed for noticing a dirty filter?

I have stone 4' high above my slab. The filter pressure gauge is 3" above the stone line on the side of the house. Near the pool a retaining wall top is at the slab level and 2' vertically to the deck. The water line is 1/2 a 6" tile and coping it 2" above that.
4' + 3" + 2' + 3" + 2" = 6' 8"

All three suction lines terminate at the pad. They are 100% open.

I did not mention that I have a heat pump pressure-side on the pad after the filter.
It is not running, but I would expect there is some head loss as the water swirls around its 2" titanium coils.

LOL .. re: GPM ~= RPM/35
GPM = 2400/35 = 68.6

I guess with my common piping and distances, I fit the dummy profile for the pentair website, per the start of the thread.
@2400RPM = 70gpm

What if the pressure gauge is right? That would mean I am losing 7.8' of head in the pump, filter, or valves?
I would expect that kind of loss to make noise, but everything is quiet.

Tomorrow, I'll do the same test but close-off 2 of the three inputs to compare them to each other.

Thanks again for the help.

8. Re: VS pump .. calc minimum RPM using filter psi?

Originally Posted by dlandrum
What if the pressure gauge is right? That would mean I am losing 7.8' of head in the pump, filter, or valves?
Actually, just the opposite. If your filter gauge was correct, then there would appear to be some "missing" head loss. However, I went back and looked at your setup and I neglected to take into account that you had a DE filter and I would assume that is has a backwash valve. Depending on the model of backwash valve, the pressure loss can be quite significant and since some of this loss is between the pump and filter, the extra loss would not be reflected in the filter gauge. So the filter pressure could be correct, at least for the higher RPM values, and it would still result in the same flow rate estimate. BTW, what is the model of backwash valve that you have?

Also, if you are looking for a more accurate estimate, you can either install a flow meter or take a suction measurement and pressure measure at the pump drain plugs.

9. Re: VS pump .. calc minimum RPM using filter psi?

Good catch!

It is a pentair value.
manual: http://www.pentairpool.com/pdfs/Sta-Rit ... alveOM.pdf

pg 7 shows the head loss

It matches pretty close to what you were seeing in the data.

10. Re: VS pump .. calc minimum RPM using filter psi?

I am assuming you have the 2" version. Also, do you happen to have any check valves on the suction side of the plumbing? If so, what kind is it (flapper or spring loaded plunger)? Check valves don't follow the affinity equations because they are spring loaded so that can make the filter pressure flow rate even lower at lower RPM.

11. Re: VS pump .. calc minimum RPM using filter psi?

No check valves, just a diverter valve on each suction input.

12. Re: VS pump .. calc minimum RPM using filter psi?

I will be replacing my single speed 2 hp Whisperflo with an Intelliflo VS in the next couple of days. I know that there will be some trial and error figuring out the speed and time but in general how should I calculate the run time needed in relation to the speed. My Whisperflo needs to run 6 to 8 hours per day depending on the season. I will normally run the new pump at the lowest speed that will keep my chlorine generator going and a little spill on my water fall. But how do I calculate the my first trial time? While I am testing, is it as simple as old speed divided by new speed times the old time? In other words old speed 3450 divided new speed say 1200 times the old time 7 hours equals my first test time of about 21 hours? Or am I ay off?

13. Re: VS pump .. calc minimum RPM using filter psi?

I found the Pentair calculator to work pretty well.

http://pentairpool.com/pool-owner/resou ... /index.htm

Adjust the Variable Speed Pump RPM and Run Time; then look at the turnovers per day.

14. Re: VS pump .. calc minimum RPM using filter psi?

Keep in mind that the Pentair calculator does not take the plumbing size and restriction into account. That is a simple calculator which uses the relationship of GPM=RPM/35 and is only representative of the CEC Curve-C, 2" plumbing with multiple suction lines. If you want something a little better you can use the following estimates:

Variable Speed Pump Flow Rate Approximations

The following formulas use the affinity equations to yield a very rough estimate of flow rate.

1.5" Return Line with 1 x 1.5" Suction Line:
Intelliflo Flow Rate (GPM) = RPM / 47
EcoStar Flow Rate (GPM) = RPM / 48
Plumbing Head (ft)= 0.0167 * GPM^2 - CEC Curve A

1.5" Return Line with 2 x 1.5" Suction Lines or 1 x 2" Suction Line
Intelliflo Flow Rate (GPM) = RPM / 44
EcoStar Flow Rate (GPM) = RPM / 45
Plumbing Head (ft)= 0.0140 * GPM^2

2" Return Line with 1 x 2" Suction Line
Intelliflo Flow Rate (GPM) = RPM / 37
EcoStar Flow Rate (GPM) = RPM / 38
Plumbing Head (ft)= 0.0093 * GPM^2

2" Return Line with 2 x 2" Suction Lines or 1 x 2.5" Suction Line
Intelliflo Flow Rate (GPM) = RPM / 35
EcoStar Flow Rate (GPM) = RPM / 36
Plumbing Head (ft)= 0.0083 * GPM^2 - CEC Curve C

2.5" Return Line with 1 x 2.5" Suction Line
Intelliflo Flow Rate (GPM) = RPM / 34
EcoStar Flow Rate (GPM) = RPM / 35
Plumbing Head (ft)= 0.0074 * GPM^2

2.5" Return Line with 2 x 2.5" Suction Lines
Intelliflo Flow Rate (GPM) = RPM / 33
EcoStar Flow Rate (GPM) = RPM / 34
Plumbing Head (ft)= 0.0069 * GPM^

However, with a 2 HP pump and 7 hours of run time, you were probably getting over three turnovers per day which is more that you really needed for just filtering. For circulation, you can get away with just daytime running. So my first question would be why you think you needed 7 hours with the 2 HP pump?

15. Re: VS pump .. calc minimum RPM using filter psi?

When I 1st moved in, multiple pool people told me that 8 hours in the summer was about right. I have also found that it is about right to keep the pool clean. The Pentair calculator indicates that I would be getting about 1.6 turnovers per day @ 8 hours. I am not an expert but I have been told that 1.5 is better than 1.
I have 2" suction line(s) but I don't know how many. In either case, the numbers are close and would indeed give me more than 3 turnovers per day. However, none of the scenarios quoted come close to anything in the calculator.
So, two questions: Is more than 1 turnover per day helpful? Which is more accurate, the calculator or the quoted numbers above?
15K gallons, chlorine generator, waterfall spillover from seldom used spa.

16. Re: VS pump .. calc minimum RPM using filter psi?

Originally Posted by glennroe
Which is more accurate, the calculator or the quoted numbers above?
Actually, they are both the same for the CEC Curve-C: GPM = RPM / 35

I was just giving you alternatives for other plumbing configurations.

Next, there is really no set rule as to the number of turnovers per day only guidelines and lots of opinions. What you actually "need" can depend on several factors:

Chloriniation - This is the most important thing but with a SWCG, there is a minimum run time which will add enough chlorine even at a 100% setting to meet your target FC residual for the given CYA level. However, for most SWCGs this is a fairly easy thing to accomplish in a small amount of run time.

Circulation - This is important to maintain a FC residual in every part of the pool so algae cannot grow. However, less circulation is needed with a higher CYA residual (but more FC is needed) and also if you use a pool cover. Both things help reduce the amount of chlorine lost to UV extinction.

Filtering/Skimming - Lastly and IMHO less important than the first two but takes longer to accomplish, filtering and skimming helps remove debris from the water. This is mostly cosmetic and everyone seems to have a different opinion on what is acceptable. But running a pump on low speed filters the water slower and the skimming usually does not work as well.

For your situation, I would start off with a single turnover but at two speeds:

2 hrs @ 2000 RPM & 57 GPM
5 hrs @ 1000 RPM & 29 GPM

This way, you have exactly the same run time so you don't have to change the SWCG, you are turning over the water once per day and you have a period of high speed filtering to help with skimming and a cleaner if you have one. If you find that you don't need as much high speed run time, then you could go with this schedule:

1 hrs @ 2000 RPM & 57 GPM
7 hrs @ 1000 RPM & 29 GPM

Or even

0 hrs @ 2000 RPM & 57 GPM
9 hrs @ 1000 RPM & 29 GPM

But you will need to adjust the SWCG for lower FC chlorine production. So some trial and error may be required to "optimize" your run time.

[EDIT] - Corrected run time on low speed to maintain 1 turnover per day.

17. Re: VS pump .. calc minimum RPM using filter psi?

Great info, thanks.

18. Re: VS pump .. calc minimum RPM using filter psi?

In the last two schedules I posted, I just realized that I did not maintain 1 turnover per day for the low speed run time. I have since fixed those run times.

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