Does increasing pipe size increase power requirements? 5

[danp129]

In regards to pool pumps, I was told that if I increased the pipe size from 1.25" to 1.5", my 1hp pump would draw more power because of the increased rate of flow. They backed up that claim with "head curves" from pool pump manufactures showing an increase in HP required for low-head, high-flow. It was my general understanding that reducing pipe friction would reduce power requirements, not increase them. My thoughts are that the reason for higher power draw at the higher flow rate on the mfg. curve chart is because there is higher friction. However, if the flow rate increases just because I reduced resistance to flow why would it draw more power when the RPM stays the same?

I am sorry if this is appears to be too basic of a question but it seems to go against logic. If they are correct, could somebody help a non-hydraulic engineer understand this?

For a background of what I've been told but haven't been convinced by, you can read this thread:

http://www.troublefreepool.com/new-pump-too-big-t57734.html

Note the graph below DOES NOT represent my single-speed 3450 RPM 1HP pool pump but is something I have been referred to, to visualize the increase in power draw.

Thank in advance for the engineering lesson!

 

[danp129]

Let's say you have an impeller that is 4" diameter and the vanes start at 1" from center and extend to the very outer edge. What happens to the pump performance curve if you take .25" of the vane off the outside and put it on the inside, thus keeping the vane the same length? Will peak HP requirement raise, drop, or stay the same?

 

[danp129]

cvg, mine is single-speed: voltage constant => motor stays same speed => pipe size increases reducing resistance => flow increases from less resistance => surprisingly more amps. That is what doesn't make sense to me, in every field less friction/resistance reduces power requirements not increase it. The only thing that explains why less resistance can cause more power draw to me is my own theory that the inner impeller, which requires less torque, contributes less as the flow rate increase.

The inner impeller always contributes less because its path of travel is much shorter, but how does that lack of contribution change at different rates of flow?

 

[LittleInch]

Danp129.

Stop thinking about the impellor, you won't get anywhere that way. I'll try one more analogy.

Your original pipe is like carrying one bucket of water up a 1: 4 slope. Your new pipe is like carrying two buckets of water up a slightly more gentle slope say 1:6. Which one do you think will take more energy?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[LittleInch]

Actually continuing ths same analogy, for pump efficiency, think of the first person as you in your physical prime and the seeing person as you now (assuming you're not in t the peak of physical fitness now ), Who would be more knackered after an hour of doing that??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[1gibson]

Step 1: Accept the curve as accurate
Step 2: Plot two points on the curve
Step 3: Analyze the results


There are centrifugal pumps where the highest power requirement is at zero flow; high specific speed (high flow low pressure) axial flow (propeller type) pumps that happen to match your intuition. That doesn't mean your intuition is correct.

You can't skip over the basic principles of a pump, then try to rationalize things with A to B comparison of impeller modifications. Bottom line, look to the curve for answers.

P.S. read Artisi's signature quote.

 

[danp129]

Thanks everyone, I am convinced! Is there a good site for calculating how much feet of head a 90º elbow in 1.25" PVC is equivalent to. At this point I'm going to upgrade my intake from 1.25" to 1.5" because I already cut it, but leave the rest of it alone.

 

[LittleInch]

6.6 feet.

Hope you got it in the end

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[nabilbasha]

I believe there is some confusion on part of danp129 question.
All discussion above is 100% correct for a GIVEN pump in a system.
However, if you are selecting a pump from scratch.
Naturally, 200 GPM @ 60 FEET would have smaller motor hp than one selected for 200 GPM @ 150 FEET as same RPM.
either of these pump once selected will follow the discussion above.

 

[QualityTime]

The pump curve that you show does not show all of the relevant curves that you need to help explain to you the answer you want. You are missing the pump efficiency curve. The further you are away from the best efficiency point, the poorer the pump efficiency. The following equation may better help you in determining the brake horsepower (bhp) of the pump at differnet points along the pump curve.

BHP = (GPM usgpm x TDH ft x sp. gr. of water)/(3960 x pump efficiency)

Basically the equation says the bhp is inversly proportional to the pump efficiency


I have appended a typical pump curve and you can see the "oak tree" efficiency curves. I assume you know how to read the pump curve and bhp curve. You can see that for any given impellor diameter, the further to the right of the curve the more bhp you will need. Plug the numbers from the curve into the equation above and you will get the same answer shown on the pump curve

 

[stanier]

The concept can be simplified by expressing it in terms of mass and energ. A greater flow rate due to the characteristics of the pump results in a greater mass of water being moved. This requires more energy as any increase in mass requires. Expenditure of energy per unit of time requires power. Ergo an increase in flow rate requires more power.

The increase in flow rate comes about becuase the larger pipe provides less resistence. ie a larger plug hole allows the bath to drain more quickly.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[Artisi]

" ie a larger plug hole allows the bath to drain more quickly" provided the connection pipework (drain) is sufficiently sized for the higher flow.

What seems to be missing in all of this discussion (and there has certainly been enough considering it is only a class 101 hydraulic discussion), is the simple overlay of the system curve for the 2 conditions, in this case 1.25" and 1.5" pipe diameters.
For this particular discussion, let us assume there is 50ft of pipework and 10gpm under consideration, the head loss difference will be an unbelievable reduction of approx. 0.5ft by using 1.5" pipe. Also, with a 1hp motor, the likely change of input current will probably be unmeasurable or so insignificant not even worth the effort to measure.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[stanier]

Artisi,

This isnt Hydraulics 101, it barely rates first year high school physics. We are not talking to an engineer here so it needs to be kept basic . No need to introduce pump curves. Simple: If you want to move a larger mass the same distance in the same time the energy/power required is greater. Reducing the friction with a larger pipe means the mass flow will increase for a given pump.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[LittleInch]

Artisis,

What we have all been trying to show danp129 is that for a centrifugal fixed speed pump, the effect of lowering the system resistance is to increase flow. Increased flow at a slightly reduced head = more power. I did a very quick check and reckon for the 50 foot example, even allowiing for some reduction in head (15%), you would get 15 gpm through a 1.5" hose compared to 10gpm though a 1.25" hose. Therefore an increase in motor power absorbed of circa 40%.

In reality the pump might be able to cope for a while but would start to run hot and fail earlier that it would do otherwise.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

I didn't read the 30+ responses above.

Simple answer is, FOR THE SAME FLOWRATE, SUCTION & DISCHARGE PRESSURE, PRODUCT DENSITY, VISCOSITY and PUMP EFFICIENCY , A LARGER PIPE MEANS LESS POWER USED. If you got a different answer one of the "sames" changed too.

Independent events are seldomly independent.

 

[LittleInch]

Big Bro'

You should read them - it's facinating stuff.... As said above, what most of us have been trying to tell the OP is that for a fixed speed centrifugal pump (his piddly little pool pump) is that reducing his system resistance will result in an increase in FLOWRATE. Pump efficiency might suffer as well a bit and discharge presusre also go down a bit, but the net effect is much more flow (my guess is about 50%) hence more power.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

Ya. But over the last 6 or 7 years here I have read and answered all of the "Does increasing pipe size increase power requirements" and "Does VFD save me money", just not this particular one. No time today to do that and I know how it should turn out anyway. I didn't want to miss this one and blot my record.

Independent events are seldomly independent.

 

[Artisi]

LittleInch: think you will find that piddly little pool pumps will operate ok at end of curve as they are designed to be idiot proof, just install and run don't worry about any hydraulic thought.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[electricpete]

At the risk of beating a dead horse, I'll chime on an alternate explanation. Not technically rigorous but maybe intuitively pleasing to op.

Put four cups of water around the perimeter of a bicycle wheel, facing inward so they keep the water held in the cup by centrifugal force as you turn pedal the wheel. The presence of the water doesn't directly translate to any torque requirement while turning the wheel at fixed speed (other than transient force to acceelerate from rest to steady state, and perhaps small air friction... neglected).

Now, put a hole in the bottom front edge of the cup. The water is under pressure due to that centrifugal force. The pressure of the water makes it shoot out in front of the cup as you turn it. This water jet has momentum, so there will be an equal/opposite reaction force (torque, when considering distance) which you will have to match to keep the wheel moving at constant speed. You are applying torque at speed.. expending power to keep the wheel turning while propelling that water in front of it. Open up the hole more and you'll propel more water and have to apply more torque.

Didn't know BigInch had a little brother... cool. Some big shoes to fill.

=====================================
(2B)+(2B)' ?

 

[LittleInch]

electricpete.

I think this horse is now well and truly expired - an "ex horse" to paraphrase the mighty Monty Python -

http://youtu.be/4vuW6tQ0218

You can blame big bro' for getting me involved here.... I told him he was famous and he didn't believe me ;-)

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[1gibson]

And to top things off, we all chose to ignore when he stated that only the intake piping is changing, not the discharge piping. So most of the discussion isn't strictly relevant.