New pump too big?

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JasonLion said:
Does the inherent pump efficiency number change at the edges of the head curve?
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Yes and it changes by a lot. The hydraulic power (output) at dead head is 0 but the shaft power and resultant motor power is still well above 0. On the left most side of the head curve, re-circulation losses dominate. The pump is actually still pumping water even though no water exists the pump. It simply recirculates from the exit of the impeller back to the pump inlet through the impeller ring. On the right most part of the head curve, the dominate loss is friction due to the high flow rates. These extra efficiency losses are not captured in the head curve because that is referenced to the pump inlet and outlet ports but they still affect the efficiency of the pump.


This doesn't seem to exactly agree with either of the stated positions.
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Why do you say that because to me it shows the same trend as the Intelliflo power curve.
 
Yes, in order to realize the efficiency gain with larger pipe, you have to reduce the pump run time. Otherwise, if you keep the run time the same, you end up using more power.

But if your next question is going to be, how much less should I run the pump, then that is more difficult to answer. Theoretically, a plumbing system with higher flow rate should need less run time to clean a pool than one with a slower flow rate. However, there have been studies that show a typical pool only needs 3-4 hours of run time no mater the size of the pump or the size of the plumbing. However, they do recommend using larger pipe for better flow rates but they don't correlate this with less run time. If the study is correct about run time, that would mean you are better off with small high head loss plumbing but that seems counter intuitive. However, a smaller pump will always be more efficient so that is always a good idea.

My main point is that things are not always black and white. It really is going to depend on how you end up running the pump and if you can actually lower run time. Chances are you could probably reduce pump run time with your current setup and save energy costs and you might find out that to keep the pool the same cleanliness the run times are about the same with the two sets of plumbing.

This is why I generally do not recommend changing pipe size for at least the underground pipe because that can be quite expensive and you may never recoup the costs. However, if you are changing pumps anyway, then up sizing the pad plumbing is not such a big deal and would probably have at least some benefit especially on the suction side of the plumbing.

For your situation, I would suggest that you leave things as they are because you will probably never recoup the cost of the changes. Second, I would suggest experimenting with run time and see how low you can go.

As an example, I have a 20k pool with a 1/2 HP pump and during the heat of the summer I run my pump only 4 hours per day (~2 hrs high speed and ~ 2 hrs on low speed). This is less than 1/2 turnover per day and it keeps the pool chlorinated properly and as clean as I need it.
 
mas985 said:
Why do you say that because to me it shows the same trend as the Intelliflo power curve.
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Back several posts ago the message was higher flow rates mean more power draw. In my data from the power calculator, that is true over most of the head curve, but doesn't appear to hold at the high flow end of the range. The power calculator shows less total energy usage, but more total flow at the highest flow rate. The data from the power calculator appears to invalidate your point, thus my comment about not supporting your position, and my question.

Your response clears things up. The inherent efficiency drops at the ends of the range. When I make that adjustment to the inputs for the power calculator, I get a data set that shows consistent numbers where higher flow rates mean more power draw across the entire range. The power draw at the low end of the range goes up, but not enough to be higher than it is at higher flow rates. At the high end of the range the power draw goes up significantly, enough to make the data consistent again.
 
Ok, I understand the confusion. I was originally speaking in general terms but in reality the power curve gets a little strange near the end of the head curve as can be seen by the Intelliflo power curve. This is due to the internal dynamics of the pump and the rapid change in efficiency. So this area is not easily predictable in terms of performance but from 0 GPM to the best efficiency point, the power curve is very close to linear and very predictable.
 
Thanks! I'm still trying to wrap my head around how less resistance to flow causes more power draw. I guess for me, I will only be happy when I see it with my own eyes by taking measurements. I appreciate everyone's time in trying to convince me and I can see I am probably wrong based on experience with pumps. At this point I'll never have confidence either way until I take my own measurements of the same system with an amp-meter while making changes to pipe restriction!
 
This is waaaay to complicated for me. I am going to go sit by the pool and have a beer!
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Yep. We have taken a simple fact and started to beat it to death. I am reminded a bit of the saying, " If it ain't broke, keep fixing it until it is"

For we folks who's heads hurt frequently during contemplation, it is sufficient to know that increasing psi in your system results in less energy consumption. FWIW, the difference in energy savings isn't really anything that has any demonstrative effect on your lifestyle so it certainly isn't worth worrying too much about it. I'm off to visit NOLA and have a beer with him. :mrgreen:
 
DanielP said:
Thanks! I'm still trying to wrap my head around how less resistance to flow causes more power draw.
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Again, you are only focusing on resistance and ignoring flow rate. It takes higher motor power to create higher flow rates and since less resistance means higher flow rates in a pump, the motor needs to generate more power. Remember water has a lot of mass and it takes energy to move it and even more energy to move it faster.

But another way to look at this that may make more sense to you is that it will take more motor power for higher resistance if the flow rate is kept the same. So if you were to compare the energy required to move water at say 50 GPM in both 1.25" and 1.5" plumbing, the 1.25" plumbing requires more motor energy. But the only way to do this with the same pump would be to increase the RPM which is only possible in a VS pump.


At this point I'll never have confidence either way until I take my own measurements of the same system with an amp-meter while making changes to pipe restriction!
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Well then you may have a problem because you can't measure real power draw on an AC induction motor with just a clamp on amp meter. The reason is because of something called power factor and a good explanation is here:

http://en.wikipedia.org/wiki/Power_factor

You also need to know the phase difference between the voltage and the current. The power factor will change along a pump's head curve and is smallest at the left most point on the head curve. As the operating point moves left from BEP, the power factor drops and the reactive power grows while the real power drops. This can mean that the measured amps remains somewhat constant while real power is falling.

The easiest way to measure power for a pump motor is to use the power companies' power meter on the side of the house. Take a power reading with the pump on vs off and make sure nothing else in the house is turning on or off between readings.

But how do you plan on adding resistance to the plumbing? The slope of the power curve is fairly flat, so you will need to change the resistance by a lot to get a measurable change in power.

Again, the CEC and APSP have plenty of measured data on their web sites that you can look over.
 

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After reading more and talking to pump people I am convinced you all are correct and I am wrong :)

I had already cut my old pipe to the skimmer so I will be changing it to 1.5". Would anyone like to take a stab at what my feet of head will be if I keep everything in my sig except for the new 1.5" skimmer pipes that will replace 3 feet of 1.25" of pipe and two 1.25" 90º elbows?
 
Finally! :hammer:

BTW, who are these so called pump people? We have quite a few on this site too. :cool:

The absolute change in head loss depends upon the current operating point which is dependent on all of the plumbing components. Your signature is a little ambiguous as to what is on each side of the plumbing (suction vs return) and there are a few missing pieces so I would need the following:

Suction side as you specified:
1 Skimmer
3' Pipe (compare 1.25" to 1.5") - Is this the total suction pipe?
2 90s (compare 1.25" to 1.5") - Is this all of the suction side fittings?
Any valves?

Return Side, please review and fill in missing pieces:
3' 1.5" Flex Hose
Backwash valve size? (1.5"/2")
Filter Size (inches or lbs)?
Return Pipe Length and Size?
Return fittings - Number and type?
# of return eyeballs? (1?)
Return eyeball diameter (inches)?
Any valves or anything else in the plumbing?

If you had a filter pressure measurement with the new pump, then I will only need the filter pressure and the height of the pressure gauge above water level.
 
BTW, who are these so called pump people? We have quite a few on this site too. :cool:
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That is a secret!

I don't need a comparison now, what I really want to know is if I will be operating in an efficient range of the pump or if I should get a different pump. I currently don't have a pump hooked up, I plan to do the plumbing today after a trip (or two or three) to the hardware store.

Let me get it all hooked up so I can be more precise.

Have a good weekend if your not on!
 
The PSI on the filter is 18 and the gauge is 1/2" above water level.


I had typed all the following up before I read your last statement but I'm including it in case it is relevant in the future:

Suction Side (all 1.5"):
1 Skimmer
13" - 1.5" pipe
2 pipe-to-thread adapters
1 shut off valve
2 90º elbows

Return Side:
26" - 1.5" flex hose (quality spa hose, no deep grooves)
1.5" Backwash valve
21" Cooper sand filter (2.2 sq ft)
1 - 1.5" pipe-to-pipe adapter
2 - 1.5" thread/pipe adapters
3 - 1.5" to 1.25" adapters
8' - 1.25" pipe
6 - 1.25" 90º elbows
4 - 1.25" thread/pipe adapters
1 - 1.25" shut off valve
1 - 1.25" union
1 - 1.5" tablet feeder
1 - 3/4" return eyeball
 
DanielP said:
Suction Side (all 1.5"):
1 Skimmer
13" - 1.5" pipe
2 pipe-to-thread adapters
1 shut off valve
2 90º elbows
Click to expand...

Only 13" of suction pipe?

Before you had said that you replace 3' of pipe. Usually for an AG pool the suction and return pipe is about the same length to get to/from the pool.
 
18 PSI is a bit higher than what I would expect given the description of your plumbing. I had expected around 16 PSI. Has the filter been back-washed recently or might it be a little dirty?

But assuming the pressure is correct, here is my estimate for the current operating point:

36 GPM
47' Head
932 Watts

The plumbing curve can be expressed as the following equation:

Plumbing Head (ft) = 0.0355 * GPM^2
 
Thanks!
18 is what it runs after my first backwash/rinse/backwash of the season. Last year with the old pump it usually ran around 10 psi after a backwash.

Do you think changing the return to 1.5" will be better or worse on efficiency? I already have everything but a $10 shut-off valve/union to change over. I know (now) it will use more watts with more flow but I plan to run the pump less to make up for it.

Also, since it would use more power with bigger pipe, should I partially close the return valve if I need to run it 24/7 to reduce flow/power draw?
 

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