I'm concerned about my pipes being 1.25". I just purchased a Hayward 1 HP Power-flo LX (sp1580) but I haven't received it yet. I really don't know how much 'feet of head' I have, so I don't know if my 2.2 sqft sand filter will be OK with it. Will 1.25" pipe make my pump go out sooner or just use a lot more power?
New pump too big?
- Thread starter DanielP
- Start date
Welcome to tfp, DanielP 
That pump should be fine. With your flow throttled a little by the 1.25" pipe, the electrical current the pump requires also goes down. So the pump will run at lower KW but you will have to run longer to turnover the same amount of water.
Why was 1.25" pipe used?
That pump should be fine. With your flow throttled a little by the 1.25" pipe, the electrical current the pump requires also goes down. So the pump will run at lower KW but you will have to run longer to turnover the same amount of water.
Why was 1.25" pipe used?
Thanks, for the welcome 
Are you sure more resistance to flow will lower power draw?
I am not sure why they used 1.25", it seems a little strange.
Are you sure more resistance to flow will lower power draw?
I am not sure why they used 1.25", it seems a little strange.
Would you guys like to share where you are getting this calculation? I understand lower flow will draw less power if it's because of a different RPM motor or different impeller, but I don't understand how lower flow CAUSED by restriction, which makes the motor work harder, could possibly draw less power. I looked around and the only thing I could find was a calculator for estimating pump HP requirements. It said TDH increased with resistance to flow, and if I increased the TDH in the calculator it resulted in a higher HP requirement.
Do one of you have an clamp style amp-meter to test this with a fully-open and half-closed return system? If I had one, I would most certainly offer to do this test and show the results even if that means I am wrong.
Do one of you have an clamp style amp-meter to test this with a fully-open and half-closed return system? If I had one, I would most certainly offer to do this test and show the results even if that means I am wrong.
- May 3, 2007
- 16,886
- Pool Size
- 20000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
Basically it boils down to the shape of the pump's head curve. The energy delivered by a pump to the water is a product of pressure AND flow rate. So both determine power.
The shape of the head curve for a centrifugal pump is relatively flat at higher head loss and lower flow rates so the flow rate decreases much faster than the pressure increases, thereby decreasing the total power delivered to the impeller and consumed by the motor. This is why the energy factor of a pump as measured by gallons pumped/watt-hrs consumed decreases with increasing pressure and decreasing flow rate.
But if you still are not convinced, you can download pump measurements from the CEC web site and prove it to yourself. The tests performed on more restrictive plumbing curves show a decrease in power consumption but also a decrease in energy factor.
http://www.appliances.energy.ca.gov/AdvancedSearch.aspx
Also, this post shows a graph of the Intelliflo power draw with flow rate. You will note the drop in power vs an increase in head loss:
hydraulics-101-have-you-lost-your-head-t915.html#p6542
The shape of the head curve for a centrifugal pump is relatively flat at higher head loss and lower flow rates so the flow rate decreases much faster than the pressure increases, thereby decreasing the total power delivered to the impeller and consumed by the motor. This is why the energy factor of a pump as measured by gallons pumped/watt-hrs consumed decreases with increasing pressure and decreasing flow rate.
But if you still are not convinced, you can download pump measurements from the CEC web site and prove it to yourself. The tests performed on more restrictive plumbing curves show a decrease in power consumption but also a decrease in energy factor.
http://www.appliances.energy.ca.gov/AdvancedSearch.aspx
Also, this post shows a graph of the Intelliflo power draw with flow rate. You will note the drop in power vs an increase in head loss:
hydraulics-101-have-you-lost-your-head-t915.html#p6542
I believe if we saw a line representing resistance on a pump's head curve graph, we'd see the reason for the increase power draw at higher flow rates.
The head loss on the pump curve is the resistance to the flow ... or maybe I misunderstand you.
Look at mas985's first graph in this post: http://www.troublefreepool.com/hydraulics-101-have-you-lost-your-head-t915.html and you will see that the power goes down with reduced flow and the reduced flow is caused by increased head.DanielP said:
I agree this is kind of odd to me as well.
With a pump if you restrict the flow (which seems like it would make it work harder), head goes up, flow goes down, and power goes down.
If you put in a smaller impeller on the same motor (which seems like if would make the motor work less to maintain the same RPM), The flow goes down, and the head goes down {but you are on a different pump curve} and the power goes down
While difficult for me to grasp, I think the bottom line is that the power used is directly related to the water moved.
With a pump if you restrict the flow (which seems like it would make it work harder), head goes up, flow goes down, and power goes down.
If you put in a smaller impeller on the same motor (which seems like if would make the motor work less to maintain the same RPM), The flow goes down, and the head goes down {but you are on a different pump curve} and the power goes down
While difficult for me to grasp, I think the bottom line is that the power used is directly related to the water moved.
If you look here: engineeringtoolbox.com/pump-system-curves-d_635.html, you'll see that there is a "System curve" that represents "head loss". The head loss increases with flow rate and restrictions due to pipe size is a major factor in determining this loss. Greater friction through a pipe at high flow rates is a major contributor to the system curve going up. This resistance is the reason more power is used at higher flow rates.
You cannot say that because a pump curve shows higher power usage at higher flow rate that I will save power by restricting flow with smaller pipes. It is the pipe's friction or resistance to flow that cause the increased power demand.
You cannot say that because a pump curve shows higher power usage at higher flow rate that I will save power by restricting flow with smaller pipes. It is the pipe's friction or resistance to flow that cause the increased power demand.
linen said:
What we don't see is the system curve, nor does it show how a change in the pipe size effect the system curve which would affect the pump curve.
Let's try this once more. Pool pumps are constant speed, spinning at 3450 regardless of resistance. The lower the resistance, the more water they push....thus doing more work than if the resistance was higher. Less work = less electricity consumed.
This is not something we are making up nor do any of us have any interest in beating a dead horse.
This is not something we are making up nor do any of us have any interest in beating a dead horse.
Here is my take...The engineers tool box (love that site) curve you are showing is for a fixed plumbing setup...and of course as you increase the flow rate the head will go up. The only way a pool pump could follow this curve is if it was a variable speed pump that you increased to raise the flow rate (or a 2-speed that you went from low to high with) The curve that I pointed you too is assuming the head loss is from different plumbing setups with more or less restriction at one motor speed (typical of most pool pumps). So an example: looking at the intelflo's curve at 3450 rpm, to get a flow rate of 120 gpm your plumbing must have a head pressure of ~68 ft at that flow rate. If you added some more restriction into that plumbing (say a dirty filter), then your head would go up (the pressure your gauge is indicating would go up) and your flow rate would go down. At the same time your power consumption would also go down. Once you backwash the filter the plumbing head would go down (the pressure gauge reading would drop) and your flow rate would go back up. At the same time your power consumption would also go up.
Why does the system curve matter? From your link the operating point is always the intersection of the system curve and the performance curve correct? So the operating point will ALWAYS be along the pump performance curve. If we change the flow rate (i.e. by restricting the flow) we will move up and left on the performance curve to a new flow rate and head loss position. And the new system curve would pass through that point.
Dave's summary is better
Dave's summary is better
It will help a little to install a foot or two larger straight pipe going into the pump inlet side (whatever size the native pump fittings are), then step down to your existing pipe size, this has to do with pump cavitation, back pressure, etc. What you don't want is an elbow fitting right at the pump inlet side.
jblizzle said:Why does the system curve matter? From your link the operating point is always the intersection of the system curve and the performance curve correct? So the operating point will ALWAYS be along the pump performance curve. If we change the flow rate (i.e. by restricting the flow) we will move up and left on the performance curve to a new flow rate and head loss position. And the new system curve would pass through that point.
Dave's summary is better
You cannot have a pump curve without a system curve. The system curve determines what the head losses will be and by examining what effects the system curve, you can better understand why the power increases.
- May 3, 2007
- 16,886
- Pool Size
- 20000
- Surface
- Plaster
- Chlorine
- Salt Water Generator
- SWG Type
- Hayward Aqua Rite (T-15)
I would suggest you read the entire sticky. There is a head curve further down which shows multiple plumbing curves on top of the pump's head curve. The intersection of the system curve and the pump's head curve is defined as the operating point and there is only one operating point for a given pump, plumbing setup and pump speed.DanielP said:
But the whole point of a pump's head curve is it is independent of the plumbing system curve. It actually represents an infinite number of plumbing system curves. Points on the left side of the head curve represent higher head loss operating points while points on the right side of the curve represent lower head loss plumbing. The calculation of a specific plumbing curve can be quite involved but it can be done.
One other source I would suggest is reading through Joe Evan's (an expert in the water distribution industry) papers on his web site pumped101.com. He covers all these aspects of pumps.
[EDIT] I forgot to add this to address your original question:
The formulas below show the relationship between the hydraulic power and the electrical power. Basically the hydraulic power is a function of the head and flow rate at the operation point and the electrical power to the input of the motor is higher because of the motor and pump efficiency. So if you actually go along the pump's head curve, you will see the power peak near the pump's best efficiency point (right side of the head curve). This is where the product of head and GPM is the highest. However, the input to the motor remains fairly constant to the right of the best efficiency point because of the friction loses within the pump increase. This is why you see the Intelliflo power curve flatten on the right side of the chart.
Using the power calculator at http://www.engineeringtoolbox.com/pumps-power-d_505.html (default setting, Imperial units.) I entered GPH and head data from the published WhisperFlo WFE-12 head curve and got the following results:
This doesn't seem to exactly agree with either of the stated positions.
Does the inherent pump efficiency number change at the edges of the head curve?
Code:
GPM Head Power (shaft kW)
50 100' 1.57
80 92' 2.31
110 80' 2.77
140 60' 2.64Does the inherent pump efficiency number change at the edges of the head curve?
The data Jason just posted does seem to agree with Marks edited information above ... where Max power is near the "upper-right" "knee of the curve" at max efficiency.