Pump head pressure question

Swingline77 · Oct 15, 2018

Let's say that we have a return pump with a rated head of one foot and a output diameter of 0.5 inches. According to the laws of whoever (Pascal, maybe?) the pump output can empty upwards into a 0.5" tube or an olympic sized swimming pool; in either case, the vessel will be filled to one foot.

What I wonder about though, is what happens when the diameter of the collection vessel above the pump is decreased beyond that of the diameter of the pump's output pipe? If you have a 1' piece of tubing pointing straight up from the output pipe with a diameter of 0.5", then you have a volume of water of 2.4 inches cubed. This is 39 grams worth of water that is being kept aloft by the impeller. If the diameter of the upright tubing is decreased to 0.25", then 1' of this tubing will only contain 0.6 inches cubed of water, which will have a mass of 10 grams.

So, if the impeller is perfectly capable of keeping 39 grams aloft, shouldn't it be able to have a greater head if the diameter of the output tubing is decreased below the diameter of the pump's output?

Reeferdood · Oct 15, 2018

ca1ore · Oct 16, 2018

Head is the ability to pump against height/gravity; but there are other restrictions like friction. As pipe diameter decreases, friction pressure increases. With the same head, when I swapped to 1 1/2" PVC from 1", I got a net flow increase of 33% (900 to 1,200). Plus, gravity pulls on a broad column equally to a narrow column.

Swingline77 · Oct 16, 2018

Thanks. I heard elsewhere that though doing this will give performance increases, they tend to be modest.

ca1ore · Oct 16, 2018

Think of it this way. Pressure at the bottom of a 6' swimming pool will be the same as at the bottom of a 6' tall piece of 1/2" PVC, even though the water volume is far less. Pump sees the same pressure and will perform the same way absent friction losses.

jda · Oct 16, 2018

I would keep the pipe smaller.

I went to Engineering School, understand math and all of the "laws." However, I put a Mag 12 with a 3/4 pipe up 20 feet and it was able to empty a 55g trash can about twice as fast as the same pump with 1.5" pipe also up 20 feet. I do not know why. I lost a bet on this... I could show why it would not matter on paper since height is the only variable, but the other dude was right and there was something else in play here that I never figured out.

If you go smaller than the output size, you can get a lot more head, but usually less volume.

Swingline77 · Oct 16, 2018

jda said:
I would keep the pipe smaller.

I went to Engineering School, understand math and all of the "laws." However, I put a Mag 12 with a 3/4 pipe up 20 feet and it was able to empty a 55g trash can about twice as fast as the same pump with 1.5" pipe also up 20 feet. I do not know why. I lost a bet on this... I could show why it would not matter on paper since height is the only variable, but the other dude was right and there was something else in play here that I never figured out.

If you go smaller than the output size, you can get a lot more head, but usually less volume.

This is the sort of situation that I was getting at, sort of. I'd guess that "head" is the point at which the pressure generated by the impeller becomes equilibrated with the weight of the water, which would be the mass of the water as acted upon by gravity. This weight would have to be calculated as a force acting over a two dimensional area, such as "pounds per square inch". A 0.5" diameter pipe has an area of 0.2 inches squared. A 0.25" diameter pipe has an area of 0.02 inches squared. I'm guessing this will only be relevant if we're talking about diameters less than those of the output pipe. As mentioned, there are friction losses.

This is all practical, as I have an old pump that could get a job done if I could squeeze a little more height out of it. Otherwise, I'd have to break out the new pump that I was hoping to save.

As for your 55 gallon experiment, here are my layman's thought, for what it's worth. All else being equal, a pump pushing the water in a wide tube would have a lower velocity, but higher volume. Through a small tube, the opposite would be true. This doesn't take into account differing efficiencies at different loads. Maybe the pump was generating more heat with the wider tube. Who knows, it's just a guess.

rockskimmerflow · Oct 17, 2018

jda said:
I would keep the pipe smaller.

I went to Engineering School, understand math and all of the "laws." However, I put a Mag 12 with a 3/4 pipe up 20 feet and it was able to empty a 55g trash can about twice as fast as the same pump with 1.5" pipe also up 20 feet. I do not know why. I lost a bet on this... I could show why it would not matter on paper since height is the only variable, but the other dude was right and there was something else in play here that I never figured out.

If you go smaller than the output size, you can get a lot more head, but usually less volume.

It has to do with centrifugal pump dynamic characteristics based on volute/outlet aperture ratio. The long and short if it is lower power consumption, lower max flow, and higher total head when a given pump is forced to flow through a narrower diameter outlet pipe. Higher power, draw, higher max flow, lower head potential when the same pump is offered a larger diameter pipe. There is a head height in the flow curves where the small pipe takes over in terms of flow.

Kind of like how the laguna 2900 will outdo and SP6 after 11' of head or so due to its 1" outlet vs the SP's 1.25"

P.S. you never replied to my tag regarding this same kind of thing in my thread about the SP4 and 2900. Did you see it? Would still love to know if your 2900 listed 135w tdp or something else. All the new ones I've bought are rated 112w these days and use an SP4 block with an unrestricted volute with an SP6 diameter impeller.

jda · Oct 17, 2018

I missed that. Sorry. What are you looking for? My 2900 is dry now and easy for me to check out and experiment with.

FWIW - on the Mag experiment, both pipe sizes were larger than the outlet of the pump.

rockskimmerflow · Oct 17, 2018

jda said:
I missed that. Sorry. What are you looking for? My 2900 is dry now and easy for me to check out and experiment with.

FWIW - on the Mag experiment, both pipe sizes were larger than the outlet of the pump.

If you can check the quoted wattage total on your pump and the width of the motor block that would be fantastic. They used to rate them at 135w, then 125w, and now they ship then at 112w with an SP4 block essentially. https://www.reef2reef.com/threads/laguna-2900-sp4.448344/

Yeah I get you on the mag experiment. Same principle. You could put that mag on a 2" outlet pipe amd a 1.5" outlet and you'd see a similar, albeit less extreme, discrepancy between the flow curves. It's one of those principles that is more compicated than simple logical math would indicate. I know centrifual pump manufacturers use it to offer 2 models of the same pump all the time. Some of the larger kW+ size iwaki and other industrial pumps will offer a given motor with a different size native flange outlet and different quoted wattage/flow and head specs. Same pump, just different intended applications. A 3/4" vs a 1" outlet pipe run will induce the same effect on a small pump. Basically pushes the flow curve higher up with a lower max potential. This is why @ca1ore saw more flow with an upsized pipe run - his total head was in that lower part of the curve where the larger pipe still retained its flow advantage.

jda · Oct 17, 2018

It says 112w and the top of the pump is 2.75" across the top.

rockskimmerflow · Oct 17, 2018

jda said:
It says 112w and the top of the pump is 2.75" across the top.

Beautiful, thank you - it's an SP4 block after all. Do you know how long you've had it? I only ask cause I've found old product ads data from Laguna and they definitely used to advertise it as a 135W pump as well as a 125W pump for a time. I wonder if they used the SP6 block at that time or if it was just inaccurate and they updated the wattage reading to 112W. I'll have to track down a really old legacy pump to find out for sure.

What is interesting is they currently use the SP4 block, a volute that has no restrictor ring (the SP4 has a molded in plastic ring to restrict the throat of the outlet), but the impeller is the same as the SP6 and fitted to the shorter SP4 rotor. I've made a couple hybrid units now by swapping the rotor assembly into the SP4 block and modifying the SP4 volute with a dremel. With those changes the SP4 becomes a 2900 and basically renders the SP6 irrelevant in anything but about a 4' head or less application. Especially cause the SP6 is huge compared to the SP4. I see why they built the restrictor ring into the SP4 volute tooling. Even with the slightly smaller dia impeller, the SP4 would push enough to kill SP6 sales in almost every case if it didn't have the restrictor. If they put an even wider impeller on an SP6 and gave it a 1" outlet, they'd have a ~150W max pump that could still do 3,200gph or so max and push A LOT of head. That would be a game changer for me. As of now I'm only buying 2900s and SP4's cause they are killer pumps for the size and price.

jda · Oct 17, 2018

I has been a handful of years since I bought it. I have no idea really - I am not the kind of guy who keeps receipts and stuff.

I have a SP4 and it will not pump as much water as this one will.

pfoxgrover · Oct 19, 2018

This is a very interesting discussion. If the 55gallon trash can were repeated with the kind of head preassures expected for an aquarium, wouldn't the results meet expectations? That being that the larger pipe would flow faster. After all that is what we want for out aquariums right? We want high flow, not high max head heights...

jda · Oct 19, 2018

Not if you have a basement sump, or lots of back pressure through a manifold or closed loops.

I was testing for a basement sump application, so this was a normal head pressure for me.

The people who have the typical sump-under-tank with medium flows, then you can use just about any pump and not much of this really matters.

BZOFIQ · Oct 19, 2018

jda said:
I would keep the pipe smaller.

I went to Engineering School, understand math and all of the "laws." However, I put a Mag 12 with a 3/4 pipe up 20 feet and it was able to empty a 55g trash can about twice as fast as the same pump with 1.5" pipe also up 20 feet. I do not know why. I lost a bet on this... I could show why it would not matter on paper since height is the only variable, but the other dude was right and there was something else in play here that I never figured out.

If you go smaller than the output size, you can get a lot more head, but usually less volume.

The Mag 12 has an output of 3/4" in size, not surprised you lost the bet. The water coming out of 3/4" being pushed up the 1.5" pipe starts swirling and that is where friction is introduced. The water starts falling back down and is now "fighting" the water being pushed up by the pump.

You would have won the bet if the pump's output side was at 1.5" in diameter...the 3/4" pipe would have been too restrictive.

pfoxgrover · Oct 19, 2018

I was testing for a basement sump application, so this was a normal head pressure for me.

Oh, That make some sense. I still am convinced that the larger pipes I am installing on my new build are worth the money in the long run, but not easy to quantify.