# Thread: Calculating flow rate from Watts & RPM on var. speed pump

1. ## Calculating flow rate from Watts & RPM on var. speed pump

I'm just curious if it is possible to come up with a calculation for GPM and/or pressure given RPM and watts used, as reported by an Intelliflo VS pump. Obviously pressure and head come into play, but the Intelliflo VF pumps seem to be able to do this. Do the VF pumps have some additional sensors to measure flow rate and/or pressure to report these values, or is there some calculation that can be done given a series of RPM and watts used values? I've seen the charts published by Pentair for dynamic head vs. flow at different RPM values, but it would seem that GPM could be determined simply by knowing the power usage at a given rotational velocity. Am I right or wrong about this?

BTW, please forgive me as I know similar questions have been addressed in the past. However, I just cannot seem to find the answer despite many posts related to this.

Thanks!

2. ## Re: Calculating flow rate from Watts & RPM on variable speed

You are right that is theoretically possible to do that although there are some complications in practice. Each pump, VS or otherwise can be characterized by three parameters (Watts, GPM and RPM) which are unique at each operating point on any plumbing system. So theoretically, if you know two, you can determine the third.

The VF pump does not have any more sensors than any of the other VS pumps. It determines flow rate from Watts and RPM both of which are known, watts from a measurement and RPM from the drive setting. So the software simply looks up the appropriate GPM value that corresponds to the Watts and RPM measurements. Then through a hunting loop, they can zero in on a particular GPM setting by adjusting the RPM.

But the problem is that the slope of watts/GPM for a given RPM is quite shallow so when one inverts the equation to determine GPM, if there is a small error in watts, there can be a huge error in GPM. I believe Pentair calibrates the drive in such a way to maintain this accuracy but unfortunately the calibration tables are not available to the public.

Pentair publishes energy data in graph form (i.e. back of the manual) and there is also the California Energy Commission's limited measurements (2 RPMs and 3 plumbing curves), which by the way do not seem to match the published data in all cases so there is variance in the data as well which complicates things.

However, I do have a power equation that I use for the Intelliflo and works well when you know GPM and RPM and are looking for watts. But like I said the error tends to grow when inverting the equation to determine GPM from watts. The problem gets even worse at lower RPM.

So I have come up with several different methods to estimate pump flow rates from RPM and not directly from watts and seem to be more accurate.

The first method is the least accurate but only requires a somewhat vague knowledge of your plumbing setup. I have developed a set of plumbing curves with descriptions and you just need to match the description of your plumbing to that of the standards. This method is described in the Hydraulics sticky below. For each plumbing type there is a constant that when divided into RPM, will yield GPM.

The second method uses a description of your suction side plumbing in conjunction with your filter pressure and pump speed. This is a bit more accurate because it uses the filter pressure for the return side head loss. Only the suction side head loss is estimated from the plumbing description. To estimate flow rate for this, I would need just the filter pressure, gauge height above water level, type of backwash valve and a detailed description of the suction side plumbing (valves, distances, pipe sizes, # runs, etc).

The third method which is most accurate, requires a measurement of both the filter pressure and suction at the pump. Head is then calculated directly and flow rates determined from the head curves. Of course the accuracy is still limited to the accuracy of the gauges and to the head curve.

Also, it is important to do the calculation at high speeds and then use the pump affinity equations for the lower speeds because it is very difficult to make measurements of any sort at low speed.

It turns out that using the Watts method was no better in accuracy than the first method which is why I now offer three alternatives. In fact, I am skeptical that the VF is actually setting the flow rate to any sort of accuracy although Pentair claims it is within a few percent.

But I always have to ask, why do you really need to know your flow rate? Usually a rough estimate is all that is needed.

Here is a table I put together that shows the relationship between Watts, GPM and RPM for both the Intelliflo and EcoStar pumps. However, keep in mind that these values are extrapolated and interpolated from the 6 CEC measurement points so there is likely to be some error:

Watts, GPM & RPM Table

3. ## Re: Calculating flow rate from Watts & RPM on var. speed pum

Mark - Wow, thanks so much for the elaborate description. I figured there would be more to it than just a simple calculation. For my purposes, I would simply like a way to get a rough estimate of the flow rate when viewing the RPM and wattage parameters reported by my Intelliflo VS. I've been using an Autelis remote pool control unit that reports the pump parameters and I'm wondering if I might get Autelis to apply some calculations in their software to come up with approximate flow (and possibly pressure) results with a given RPM and wattage reported by the Intelliflo VS. I guess this could be done using your lookup table, so thanks for that. I was sort of hoping that this could be calculated from a simple calculation, but it would appear to not be so simple.

I too have noticed that the curves provided by Pentair for dynamic head vs. flow are quite flat. I'm surprised that they get such good (apparent) results with their methodology. Has anyone ever hooked up a real flow meter and tested the accuracy of an Intelliflo VF under normal and high pressure conditions? Wouldn't a DE filter needing a backwash throw these calculations off?

Cheers....

Craig

4. ## Re: Calculating flow rate from Watts & RPM on var. speed pum

The calculations include plumbing changes because both GPM and watts will change when the plumbing changes. Another way to look at the table is that along an RPM row, the head loss is changing so the various GPM/wattage values are for different plumbing set ups. However, the columns do not represent the same plumbing setup. A single plumbing setup cuts diagonally across the table.

If you want a simple formula that has limited accuracy, you can use the following for the Intelliflo:

Watts = 12.5 * GPM * (RPM/3450)^2 + 1371 * (RPM/3450)^3 + 82

This is a fit to the 6 points of the CEC data and has has fairly good accuracy (<5%). But again, I would suggest calibrating at high speeds and just use affinity equations for the lower speeds because the formulas error grows with reduced RPM. I put a calculator tab in the spreadsheet to show how I would do the calculation. You can either download the spreadsheet as excel or make a copy to your google docs.

5. ## Re: Calculating flow rate from Watts & RPM on var. speed pum

Mark, what do you mean by "affinity equations" and how would you implement them for the lower speeds? In viewing your spreadsheet, it was apparent to me that moving from right to left along a particular row (rpm value) results in increased resistance to flow, and thus (I assume) dynamic head. I can also see that for a given plumbing setup, the values are read off the diagonal. However, in a sense the "plumbing" does change as the filter starts to build up debris, leaves build up in the skimmer basket, or valves are turned to different positions. So, being able to calculate flow rate becomes a bit of a challenge. This is important though, as changes in flow could indicate that the filter needs to be changed, or that something is clogged. I probably should have gotten an Intelliflo VF for this, but it would still be nice to figure this stuff out using my pump.

BTW, is it correct that for a given speed, the energy used by the pump decreases as the resistance to flow (head) increases? Isn't this different than if a motor was directly connected to a load through its shaft? There is a direct relationship between flow rate and wattage for a given head value, but this relationship between head and power appears to be inverse for a given rpm value.

Thanks again for your help. This is really informative.

6. ## Re: Calculating flow rate from Watts & RPM on var. speed pum

From the Hydraulics 101 sticky:
Pump Affinity Equations

Pump affinity equations define how a pump's characteristics will change with speed assuming that the efficiency of the pump does not change. However, since a pump's efficiency does change with RPM, these equations can only be used as gross approximations. So if the flow rate, head or hydraulic HP is known for any one speed A, it can be calculated for another speed B using the following formulas.

GPM B = GPM A * (RPM B / RPM A)

Head B = Head A * (RPM B / RPM A) ^ 2

Hydraulic HP B = Hydraulic HP A * (RPM B / RPM A) ^ 3

BTW, is it correct that for a given speed, the energy used by the pump decreases as the resistance to flow (head) increases?
Yes that is correct.

Isn't this different than if a motor was directly connected to a load through its shaft?
The motor IS directly connected to the load which is the impeller.

One thing to keep in mind is that the load on an impeller is related to product of pressure and flow rate. If you plot hydraulic HP (i.e. load) of the impeller, you will see that it peaks on the right side of the head curve. This is defined as the best efficiency point (BEP). It is also the point where the motor draws the most power. As the operating point moves away from the BEP in either direction, the hydraulic HP drops. Towards the left, brake HP (BHP) and energy use drops but towards the right it remains constant because the BHP remains constant (i.e. lower hydraulic efficiency).

BEP is somewhat of a misnomer. It is the best POWER efficiency point but not the best energy factor (gallons/watt-hr). That would be the furthest to the right you can get without cavitating the pump.

7. ## Re: Calculating flow rate from Watts & RPM on var. speed pum

Hello Mr. Mark
Thank you for your table but unfortunately it didn't work with me can you try to upload it again

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