Myth on increasing pipe diameter?

[jda]

Not at all. For low/no head, bigger pipe is better, but so is more pump. For beating gravity, then weight matters and you size down pipe, or increase pump. 2 different solutions and probably a variation of the two for each level in between.

Use 3/4" schedule 40, if you want, instead of garden hose... it will still flow better at max head than 2" schedule 40.

 

[Aqualund]

Not at all. For low/no head, bigger pipe is better, but so is more pump. For beating gravity, then weight matters and you size down pipe, or increase pump. 2 different solutions and probably a variation of the two for each level in between.

Use 3/4" schedule 40, if you want, instead of garden hose... it will still flow better at max head than 2" schedule 40.

You are wrong.

When the inside diameter is made larger, the flow area increases and the velocity of the liquid at a given flow rate is reduced. When the velocity is reduced there is lower head loss due to friction in the pipe. On the other hand, if the inside diameter of the pipe is reduced, the flow area decreases, the velocity of the liquid increases and the head loss due to friction increases.

 

[BeanAnimal]

Not at all. For beating gravity, then weight matters and you size down pipe, or increase pump.

No... That is simply not correct. You are very confused with regard to the difference between weight and pressure. The context here is fluids, not solid mass.

You are glossing over the interactions between friction, fluid flow, turbulence, velocity, Reynolds number in various parts of the cross section, vertical head, orifice interactions at reductions, etc.

Nonetheless, as stated over and over, the weight of the water in the pipe is not one of the factors involved.

 

[Aqualund]

I just saw erotic and orifice and my brain shut down.

 

[atreis]

I used to think as many people here seem to. I was trying to pump water from the basement to my tank on the first floor for top-off. I didn't need a lot of flow for this, so figured if I was close to max head, that was actually okay. I pulled 70 feet of 3/4" diameter reinforced tubing (non-kinking sort) through the wall, over the ceiling (most of the length of the run was horizontal), to the tank and turned on the pump. Nothing came out. I could clearly see water making it a little past the point where the hose started heading upwards through the floor. So, figuring I could get just a little more (hopefully enough) by going with a smaller diameter and lowering the weight of the water in the pipe, I pulled all that out and replaced it with silicone airline tubing. The water went almost, but not quite, as high up and stopped.

My wife is a Chemical Engineer... Unfortunately, I didn't consult her until AFTER I had gone through all this effort. She knew it wouldn't work, and she knew the water wouldn't go as high with the smaller diameter tubing.

Just because something seems like it ought to be right intuitively doesn't mean it will work that way in the real world.

BTW - she also mentioned that changing the pipe diameter abruptly (as opposed to a smooth gradual transition) can cause a pressure drop and decrease the head by introducing turbulence, whether you're going from smaller to larger or vice versa. (Again, nothing to do with the weight of the water in the pipe.)

As for my top-off: since neither worked, I left the airline in and replaced the pump with a really high head model (and fabricated a part to take the water from the outlet to the airline tubing - no parts available for that off the shelf).

 

[BeanAnimal]

I just saw erotic and orifice and my brain shut down.

So did mine... Clearly I have no business using an iPhone to post...

 

[SGT_York]

But you cannot sit there and use entirely different materials and pumps in your quest to show a .5" diameter pipe can push more flow than a 1" diameter pipe on different pumps, pumps that arent designed for it, different piping, different elevations etc, because then you're changing too many variables...and of course the math won't fit around the one singular subject of the discussion.

You missed my experiement

Still no one has been able to explain why when you take a Mag 7 it will pump more at 15 feet with 3/4" pipe than with 1.5" pipe. Same pump same PVC style. Pump impeller is the variable I think is being left out of the equations to show if increased diameter will increase flow at all height levels. Nothing was changed just used a 3/4" to 1" thread to slip then 1" to 1.5" slip to 1.5" pipe. on the 3/4" just used a threaded to slip coupling.

 

[BeanAnimal]

I am not sure what variables cam into play in your experiment, but there is certainly a variable not being considered.

A centrifugal pump is purely a pressure governed machine. The difference between intake and discharge pressure equates to the amount of work done, that difference is the governed by the total head at each port.

Given any of the accepted (proven) models (Fanning, Hazen-Williams, etc) for pipe flow, we can come fairly close to a friction head loss for a given piping system.

In your case, using Hazen-Williams and a Mag drive near shutoff (lets say 10' at 60 GPH = 1GPM) You would have .3 feet of head loss per hundred feet or .03 feet of head loss

The same vertical head with 1.5" pipe yields a number too many significant digits from zero to count as anything but 0.

In other words, for that small of a pump at a flow that low, both pipes are roughly the same within significant digits and any number of other variables can easily and greatly affect the total head. They may be turbulence at the transition, a few inches here or there of pipe difference or the oddity of the pump curve vs power factor at or near the shutoff head.

As you move down the pump curve (lets say the same 2 pipe systems and vertical heads but on a MAG 15) the increased velocity and the effects on pressure and friction (Fanning, Reynolds, etc) all become rather significant between the piping systems and the effects of the larger diameter pipe becomes very measureable.

You can't cheat the physics, but if you play on the margin of error or significance, things are not always what they appear to be.

 

[ATLL765]

So I think we can safely say it's totally not a myth, LOL.

 

[Lavoisier]

So I think we can safely say it's totally not a myth, LOL.

Exactly so! I missed a detail at the beginning: turbulence may make it unprofitable over a 2 or 3 foot length to go from 1.5" to 2" but once the turbulence is resolved a 2" pipe provides more flow.

 

[SGT_York]

Certainly not a myth for normal tanks with sump under the display. I'm not convinced that there isn't a tradeoff if your sump is located significantly below (basement) I'll run my Panworld through some tests once the weather heats up and see how it performs in comparison with my MAG 7.

If both pipe dimensions have the same max head then that would be significant noted the larger diameter will take longer time to reach max head.

If one pump does and the other doesn't I'll have some photos to support my hypothesis. Should have taken some the last test years ago.

Anyone else want to give it a try, or think of a logic flaw?

Need to use the same water storage container for pump input. (tricky external vs internal.

 

[N1ceDrgn]

Wow! and Thank you, is all that comes to mind for this thread. I've learned similar concepts through engineering classes and conversations with other people in the hobby. It's nice to come across a conversation like this.

 

[Aqualund]

As I stated earlier, I was suffering from nearly a 50% loss in GPH using a 1.5" diameter pipe trying to pump 12,000gph to nearly 20ft ...when I switched to 3" pipe, I went up to 11,700 gph...so it was a dramatic increase and improvement.

like beananimal said, the significant figures are so small in your experiment that it doesn't make enough of a difference to consider the argument, and it make it more difficult to identify the factors which should also be included into the equation in order to arrive at the result mathematically that your getting empirically.

 

[jda]

I will take a video this time when the weather warms up (might be Spring at this rate)... I can rig up a pair of 75G tanks both with Mag 12s and 15 feet of vertical pipe - one with 3/4 and one with 1.5 - and side discharge. You can easily watch the Mag 12 with the 3/4 inch pipe drain it's 75G tank about three times as fast as the one with the 1.5" pipe at this height.

Then, we can switch pumps and do it again to rule out that one pump was not better than the other.

I also did it at 5 feet and the 1.5 and 3/4 were about the same at this height.

I talked to my friend again and he reiterated that simple bernoulli and H-W is not enough when overcoming gravity with pumps. Volume (weight) will factor into velocity and some other force that I don't understand which will make for more waste in between the impeller blades themselves as well as the impeller blades and the housing - it is harder to move water exposing the inefficiency in the pump. Only with infinite power and a no loss design (impossible, he said) does none of this matter... as well as other things that I do not understand. This is why even 100' across water towers pump water up and over the top with 3-6ish inch pipes instead of hooking the same pumps, that can support at least 100' of head up to the bottom of the tank.

 

[Aqualund]

For this experiment you should be comparing .75" to 1.25" pipe to see the difference before diminishing returns from cavitation reduce the gain in head from a larger diameter.

The funny part is that I went through this same journey you're on right now a few months ago, and I'm a physics major, that's why it was so embarassing. And even after I showed you the thread where this all happened, and even with Beananimal telling you, you still don't believe it. But that's the good thing about science, there's no belief, just many ways to prove yourself wrong. I do it several times a day in many ways.

 

[jda]

I did this already. I have the gear... but I do need to get a 75G tank back off of loan.

If the volume/weight does not matter, then who cares about the 1.25 to 1.5 difference... or even 3"?

 

[tektite]

That is damn near genius. I have been doing custom installs for 15 years and have never heard of this. I just use spa flex instead, which still works well. I have heated pvc and bent it, but never used the sand idea.

I do this quite frequently, the sand keeps the pipe round when bending it, works great. No need to glue the caps on the end though, just pushing them on so they keep the sand in or even tape works too, just if there will be a fitting near where you'll be bending the pipe its good to have it on first as heating the pipe can make it come out of a perfectly round shape.

 

[Aqualund]

I did this already. I have the gear... but I do need to get a 75G tank back off of loan.

If the volume/weight does not matter, then who cares about the 1.25 to 1.5 difference... or even 3"?

Do you even read what I write?

Look, just reserve your opinions until you do it with 1.25" pipe, not 1.5" and then we can talk...because apparently it doesnt matter what people tell you...you gotta see it for yourself...so please go see, otherwise were just wasting time here.

 

[jda]

So now cavitation matters? Bigger is not better? My whole point all along is that, in the real world, bigger pipe is not always better when you are pumping at high head... but I was wrong on page 3. Now, I am wrong by trying to use bigger pipe since smaller pipe will perform better? I have always been taking about real world performance, not numbers on a web site.

I don't think that my mind will change. I will still think that bigger pipe is not always better when pumping up high distances - I have seen it. I am sure that the drop off in pipe size is different for each pump, but if there is a drop off of any kind, then it is a myth that increasing pipe size is always better. If you are saying that for my particular pump (the mag 12) 1.25 will perform better than 1.5, then we are in agreement and might just be wasting time,mixing terms or arguing over semantics.

 

[Aqualund]

1.25 will perform better than 1.5, then we are in agreement and might just be wasting time,mixing terms or arguing over semantics.

in this instance, like I have always stated, and others have stated, 1.25 is bigger than the outlet of the mag 12 which is .75. Therefore, 1.25 (bigger) is better than .75, and will always be better.


It has always been clearly laid out that there is always an accounting for when bigger is better, and how much bigger is better...with the general knowledge that after a certain point, larger diameter was a hindrance...but not because of the weight of the water over the aperture.

These have always been clear distinctions made in every example posed to you.

You come here looking for answers and decry the ones given to you based on your own tests which negate every fact necessary to allow for the result you want.