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!

 

[25362]

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.

 

[LittleInch]

I re-read the OP and he doesn't say that, only once we all finished telling him it wouldn't work he was only going to change the inlet piping "because I already cut it, but leave the rest of it alone.", so clearly had an intent to change the entire system.

'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

 

[BigInch]

Increasing suction pipe diameter will normally tend to reduce suction pressure and increase discharge pressure. That will tend to increase flowrate, or if discharge pressure, or flowrate is limited, will reduce power consumption.

Independent events are seldomly independent.

 

[danp129]

Learning about axial pumps using less power at higher flow rates did the most to help me understand that it is all just a difference in how radial pumps work. Thanks guys!

 

[BigInch]

Generic power consumption characteristics of any pump cannot be discussed in terms of flowrate only, which is why you only think you understand just 1/3 of the problem, but even have that backwards. Power consumption varies DIRECTLY with Flow, not inversely, BUT STILL ONLY WHEN head and Efficiency are held constant. Power consumption will never be lower at higher flow when head and eff are held constant. Power consumption varies directly with HEAD, not inversely, BUT STILL ONLY WHEN flow and eff are held constant. If you let two variables vary, you must stop talking about things in terms of ONE VARIABLE!

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.

 

[danp129]

"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.

I was quickly trying to say that comparing the pump curves of axial and radial pumps is what best helped me understand that it's just a difference in pump characteristics and tell people thanks. Yes, I forgot to mention increased flow due to reduction in head. In general I would say it is characteristic of an axial pump to use less power as flow increases due to head reduction. I would also say in general, it is characteristic of radials to use more as the flow increases due to head reduction.

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.

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.

 

[BigInch]

You have to look at who's answering the question sometimes. A lot, but not all, "pump guys" answer that question solely from the perspective of the pump running on a test bench, not hooked up to any pipe, or any piece of equipment. When a pump is running on a test bench, the head, flow and power all follow the precise curves drawn up by the pump manufacturer and these are with NO consideration for what the attached pipe, valves and other equipment might be doing. Working only with the pump curves on a test bench, at runout (maximum) flow, by definition, head = 0, and 1 Billion x 0 = 0.

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.

 

[LittleInch]

At the risk of elongating this post even further, I have learnt more about axial pumps than I did before as I haven't really come accross them before, but clealry work best on high flow low head applications where you don't go and close the discharge valve.... As they are essentially ducted prpellors, they work in a completly different way to centrifugal / radial pumps. I found the site below which has these curves and other info which I think danp129 may find illuminating - I wish I'd found it earlier in this post. I don't think all of it is completly correct, such as the throttling curve which IMO should be a vertical line down from the pump curve to meet the system curve, but I suppose it depends on where you place the throttling valve.

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/centrifugal-pump-basics/pump-and-characteristic-curves.php

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

 

[danp129]

You have to look at who's answering the question sometimes. A lot, but not all, "pump guys" answer that question solely from the perspective of the pump running on a test bench, not hooked up to any pipe, or any piece of equipment.

I agree, this is more of a physics teacher question the get the answer I'm looking for in the way I was originally asking.

I found the site below which has these curves and other info which I think danp129 may find illuminating - I wish I'd found it earlier in this post.

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?

 

[1gibson]

There are obviously more details to hydraulic design (number of vanes, vane overlap, inlet/outlet angles) but generally speaking, wide and flat (pancake) is high pressure, narrow and tall (propeller) is high flow.

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

 

[DubMac]

HP = ( GPM * TDH ) / 3960* EFF

 

[DubMac]

I just wanted to make the 50th post and the HP equation is the most meaningful thing I could think of left to say on the subject.

 

[Artisi]

DubMac, but do you think it will it ever sink-in after being sighted so many times by some many of the posters.

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.)

 

[hydtools]

Axial pumps do not have the outward throw to generate pressure like radial pumps have. Axial pumps are used to move lots of flow against very little head.

Ted

 

[danp129]

DubMac, but do you think it will it ever sink-in after being sighted so many times by some many of the posters.

Actually it did sink in after being drilled into me multiple times, but only while having it in the back of my mind while looking at axial and radial pump curves at the same time did it explain (or more likely "click") why axial pump curves tended to show less power required at greater flow.

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!

 

[BigInch]

It won't stay that way for long. Enjoy it while you can.

Independent events are seldomly independent.

 

[DubMac]

Artisi, I do find it peculiar that most of us humans keep trying to find "ultimate truth" in the most obscure places and have a hard time accepting that it is usually held in very simple concepts right in front of our faces.

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.

 

[BigInch]

One of the few times that "Always" can be said with a high degree of certainty, unless the pump is on the test bench. Opps.

Independent events are seldomly independent.

 

[DubMac]

Yes of course BigInch. I am speaking about the theoretical world where numbers always add up and work. The real world and test benches do everything they can to disobey.

Favorite Dylan line: The only thing I know for sure is that nothing will ever turn out the way you had it planned.

 

[LittleInch]

Dubmac - unless it trips before it can get there.... But mostly that is true.

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