Does increasing pipe size increase power requirements?
Does increasing pipe size increase power requirements?
(OP)
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-t5...
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!
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-t5...
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!





RE: Does increasing pipe size increase power requirements?
I can understand your question, but first you need to understand some key points. Many people believe that centrifugal fixed speed pumps like your simple pool pump provide a fixed flow regardless of the pressure or head that exists. Pumps are actually chosen in conjunction with a system curve which is the head required at different flow rates and is a curve in the opposite form to the pump curve, i.e. starts low and gradually increase on a squared basis a friction is proportional to the square of the velocity/floodgate for a fixed diameter system. Where the two curves intersect is your flowrate.
When you change the system by increasing the diameter of the piping you change that system curve by reducing the required head for the same flow. So the sturm curve is a shallower curve which then intersects the pump curve further to the right. Although the head is reduced the flow increases more and power is proportional to head and flowratr. What your pump curve doesn't show is that the redirect of the pump changes as flow changes and pumps, at least bigger industrial pumps, are normally chosen to have their required duty at the highest efficiency, called the BEP, the best efficiency point. As you change the flow through the pump this efficiency can reduce therefore power goes up due to increased flow and decreased efficiency.
It can be a little difficult to get your head around, but a fixed speed electric mirror will take whatever power it needs to maintain that fixed speed.
So what are your options? On an industrial pump you could use a VFD, variable frequency drive, which reduces the speed of the pump and hence bring your flow back to the same flowrate for your lower friction system. Or you could reduce the diameter of the impellor which results in a similar curve, but has lower head for the same flow and hence the point where the pump curve intersects your system curve moves to the left and hence flow and power reduce. Or just buy a smaller pump.
So in summary, matching the right pump to the right system is not guesswork but can be calculated to get the flow you want for the least overall cost. If you then change one part of the system, then it has impacts on the design.
Hope this helps and sorry i can't upload a curve vs system as I'm on a tablet, but if you search pump and system curves either here our on google you will probably find what I'm going on about.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
Why you want to increase pipe size - just because you can or do you have a valid reason?
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.)
RE: Does increasing pipe size increase power requirements?
Let an electrical guy like me try. I had to learn this stuff for modeling building energy consumption, but I don't have the type of understanding that the folks above do. An ME gave me this as an explanation:
Add system resistance with a smaller pipe and flow goes down = less power needed. Decrease system resistance and flow goes up = more power needed. Why? How? Why should I believe that?
A swimming pool pump will have isolation valves on it, you can go prove it to yourself with a clamp-on ammeter, or even better a wattmeter with clamp-on current transformers. With your meter in place and the pump running, close an isolation valve partway but don't shut it. Watch the current or power decrease. You added system resistance by restricting flow with the valve. It's intuitive that the flow decreased. You know that from your garden hose and spigot valve. Closing the valve partially is equivalent to using smaller pipe. They both increase head on the pump and reduce flow.
Or you can bypass the sand filter and disconnect the discharge from the pump so that it blows water like crazy. I bet the circuit breaker trips in a short time. Moving more water = doing more work.
Here's a not-totally-technically correct, but less complex explanation of what happens on the pump curve. Check out the curve in the link, which is typical. No matter where your operating point is, when you increase flow (heading right on the chart), your system curve is and new operating point move in the direction of crossing the horsepower lines in the more-power direction, even though your head decreases.
http://kb.eng-software.com/images/WhitePapers/Pump...
Best to you,
Goober Dave
Haven't see the forum policies? Do so now: Forum Policies
RE: Does increasing pipe size increase power requirements?
I know it's not exact and doesn't include the efficiency of the pump issue and the fact the head falls off a bit as flow increases, but I think it sometimes helps get over the principle??
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
Because the power curve is continually rising, anytime you move the vertical line to the right (by reducing the pipe restriction, which lowers head at the pump outlet, and increases the flow) your power will increase. Not all centrifugal pump curves are like that, but most of them are, especially small ones.
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
My theory (true or not, I am sure this is somebody else's theory too!):
If water is moving through the pump at low rate, the inner impeller (which requires less torque from the motor) is actually contributing to the movement of water. At high flow rates, the inner impeller does not move fast enough to help push the water and could even hinder the movement of water at this rate. Only the outer parts of the impeller are actually moving fast enough to keep up with a high flow rate and this outer part requires more torque to turn.
Is my theory correct? If so that makes it so much easier for me to understand!
RE: Does increasing pipe size increase power requirements?
Read the posts carefully.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
The major concept you are missing is "water horsepower." Google for a definition, it should make things click in your head.
RE: Does increasing pipe size increase power requirements?
Power = flow x head / hydraulic efficiency.
so if flow is 1 and head is 2 and efficiency is 50%, then power = 4
increase pipe size so that flow is 1.5 (increase) and head is 1.5 (reduction) and efficiency is 0.5, then power = 4.5
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.)
RE: Does increasing pipe size increase power requirements?
When getting wrapped up in system curves, friction losses, curve intersections and the like, trying to understand centrifugals; it helps me to remember that the pump doesn't know anything that happens beyond its flanges. If you can just think about the data at the flanges and that all the crap going on downstream only has its effect on the pump at the discharge flange, it may be easier to "get it".
RE: Does increasing pipe size increase power requirements?
If you increase the flow more than you lower the pressure, then HP increases.
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
In one sense you are correct, less resistance will mean less power, but only if the flow rate doesn't change. If you could turn your pump down, e.g. plug it into 110 v instead of 220, you might pump the same amount of water for less energy. However with a fixed speed centrifugal pump less resistance just means the pump can pump more water, hence more power.
If you had a fixed speed piston pump then this would pump the same amount of water for less energy. But you don't. If you can't grasp that a centrifugal pump works differently to a piston type pump then I don't think anyone here can help you much more than we already have.
I really hope this makes sense.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
Sorry to be pedantic, but motor does not turn faster. It's a fixed speed pump. I understand it's not easy to get it, but that's the way it is.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
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?
RE: Does increasing pipe size increase power requirements?
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
RE: Does increasing pipe size increase power requirements?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
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.
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
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
RE: Does increasing pipe size increase power requirements?
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.
RE: Does increasing pipe size increase power requirements?
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
RE: Does increasing pipe size increase power requirements?
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/
RE: Does increasing pipe size increase power requirements?
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.)
RE: Does increasing pipe size increase power requirements?
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/
RE: Does increasing pipe size increase power requirements?
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
RE: Does increasing pipe size increase power requirements?
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.
RE: Does increasing pipe size increase power requirements?
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
RE: Does increasing pipe size increase power requirements?
Independent events are seldomly independent.
RE: Does increasing pipe size increase power requirements?
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.)
RE: Does increasing pipe size increase power requirements?
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)' ?
RE: Does increasing pipe size increase power requirements?
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
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
Concerning the subject of power vs flow rate, and since it is generally accepted that axial-flow pumps are members of the centrifugal (rotodynamic) pump family, have a look at thread407-27055: shape of bhp vs flow curve for radial, axial, mixed flow pumps.
RE: Does increasing pipe size increase power requirements?
'nuff said - we all need to do some work now......
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
Independent events are seldomly independent.
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
Here there are at least 3 variables. P = Q * H / Efficiency.
"Learning about axial pumps using less power at higher flow rates". That's like saying a car will use less power at higher speed, while forgetting that it is only possible when going down a steep hill. News. A parachutist doesn't require any power at any speed during the fall back to Earth.
EFFICIENCY is another story, since we know efficiency affects power directly, whether either Q or H varies, or both. If you want to talk about power consumption in terms of one variable while allowing two to vary, talk efficiency. A change in efficiency will have the same effect on power no matter what you do with both Q and H.
Independent events are seldomly independent.
RE: Does increasing pipe size increase power requirements?
Axials pumps prove this is not an acceptable answer.
Had I seen a pump curve for an axial before posting I would likely have understood that it was just a difference in how the different types work and never asked anyone. For those of you that have heard my original question before and find it exhausting trying to explain, you might start off by showing the curves of both axial and radial pumps and explaining why those have very different characteristics. Explaining why max head requires the most power for axial but the lowest power for radial would be the best place to start after showing the pump curves. It would likely not need any further discussion. I'm not talking about explaining it by getting numbers from the pump curves, what I mean is explaining why the two different pump designs cause the pump curve to be remarkably different concerning bhp in relation to head & flow. Pointing to efficiency is not helpful unless you are going to explain why one design is more efficient at max head (or 1gpm) vs the other. I find it much easier to understand an axial requiring full power at max head than a radial requiring very little at max head. I'm not saying this to be a smart ass to anyone, I say it to save you time in your next attempt to answer this question in the future.
I do thank everyone for their time, I have learned a few things.
RE: Does increasing pipe size increase power requirements?
Whether running on a test bench or not, you cannot get any other answer other than what you get from,
Power = Flow x Head / Eff
With Head equal to a very small value, or zero, of course power is nill.
Let's, for talking purposes, assume you have constant efficiency over BEP flow to Runnout Flow (max flow, where head also equals zero, right end of the curve).
When you couple a pump to a real system, off the test bench, the resistance to flow (the head) increases exponentially with flow. It does not drop. Now with Head increasing exponentially, not dropping, on account of the pipe and valves and equipment attached and the pump operating at the intersection of pump and pipe system curve, with increasing flow, you get a very big number required for H. Now with increasing flows, Q, Q x H = very very large. Divided by E, even greater. Both centrifugal radial and centrifugal axial machines work in that manner. There is no theoretical difference at that level, although there may be other reasons, maximum flow capacities, maximum working pressure ranges, cost, internal configuration of bearings, pressure balances, good wide efficiency ranges, etc. that might influence your choice of one over the other of the two types, BUT whatever, more favorable power consumption, ie. lower power at higher flowrates, does not exist in any situation away from the test bench.
Independent events are seldomly independent.
RE: Does increasing pipe size increase power requirements?
Axial pumps would seem to have a very large turbulence effect at no / low flow cuasing the vanes to thrash around in very disturbed water as it flows back past the blades which doesn't happen in centrifugals which are much more static in terms of back flow through the pump.
http://www.webbpump.com/technical-support/centrifu...
Danp129, I hope you enjoy your pool and don't spend too long working out how its pump works whilst relaxing in it.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
I did find that site illuminating, I found it after 1gibson mentioned axial pumps. It was a good idea to post it because it does help and made even more sense to see it again after BigInch made me memorize P = (Q * H) / E.
I think a better question to get the answer I've been looking for is... Why are radial pumps so good at giving head while axial pumps suck at giving head?
RE: Does increasing pipe size increase power requirements?
Increase diameter, increase velocity at the vane tip, that increases the pressure it can generate.
Increase hydraulic passage area (the "height" between the impeller shrouds at the outlet), increase the amount of fluid that it can move, increase flow.
The power curve will start uphill, flatten out, then start going downhill as you go from low specific speed (pancake) to high specific speed (propeller.)
Check this out:
http://www.mcnallyinstitute.com/07-html/7-03.html
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
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.)
RE: Does increasing pipe size increase power requirements?
Ted
RE: Does increasing pipe size increase power requirements?
Knowing the formula for calculating power WHILE looking at the axial and radial pump curves together helped explain the power issue, which helped me form a better question. And 1gibson's most recent post answered my last question as to why axial and radial pumps have such different characteristics.
So thanks everyone for being patient with me, I do understand everything I ever wanted to know about centrifugal pumps!
RE: Does increasing pipe size increase power requirements?
Independent events are seldomly independent.
RE: Does increasing pipe size increase power requirements?
I played and coached baseball for years. The most valuable baseball lesson I was ever given was in Pee-Wees: keep your eye on the ball. Nothing could be more central to the game, yet 99% of coaching is spent on secondary/tertiary minutia.
My favorite simple truth in the pump world: A centifugal will ALWAYS operate at the intersection of the performance curve and the system curve. ALWAYS. EVERY TIME.
Doesn't matter, some will still ignore, try to go around, or argue the point to the contrary.
RE: Does increasing pipe size increase power requirements?
Independent events are seldomly independent.
RE: Does increasing pipe size increase power requirements?
Favorite Dylan line: The only thing I know for sure is that nothing will ever turn out the way you had it planned.
RE: Does increasing pipe size increase power requirements?
I've just spent a day lecturing students telling just this exact same thing and I actually don't think they believed me....
Do you think we can get this one to 100 replies?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Does increasing pipe size increase power requirements?
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.)
RE: Does increasing pipe size increase power requirements?
RE: Does increasing pipe size increase power requirements?
Independent events are seldomly independent.
RE: Does increasing pipe size increase power requirements?
Looking at Baldor motor data as a reference and to satisfy my own curiosity, a 1hp 1ph motor at 50% load has a current draw of 5.3 amps, 75% is 5.8amp and 100% is 6.5 amp.
So, back to the question, a change of pipe diameter will / should / could / maybe result in an increase of power input however, in the case of the OP, how much extra current are we talking about - maybe 0.2 or 0.3 amps, so was it worth raising the issue other than from an academic aspect, it also and makes you wonder who "they" are, -- "They backed up that claim .."
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.)