Pump output pipe size?

[schatzi]

I'm using a Mag 12 for my return, the output is 3/4" and the overflow returns are 3/4". The pump datasheet says use 1 1/2" pipe, which I'm doing for the pump intake, but doesn't specify if this is suggested for both in & out or just pump in. Should I use 1 1/2" for the output or just stick with 3/4"? Thanks

[billdogg]

your best bet is to use whatever size it is plumbed for - in this case, 3/4"

[Mr.Tan]

my inlet and outlet are both 1/2" on the mag 7, so i use a 3/4" pipe on the inlet and outlet just because ti is what i bought before i read the directions about doubling the size ...if your using a 1 1/2" on the inlet i'd use the same for the outlet.

[schatzi]

Originally Posted by Mr.Tan

my inlet and outlet are both 1/2" on the mag 7, so i use a 3/4" pipe on the inlet and outlet...if your using a 1 1/2" on the inlet i'd use the same for the outlet.

Datasheet said to use 1 1/2 for max flow but it seems odd to size up and then have to size back down. I would think doing that would work in reverse and actually give less flow. But I was looking for what folks are experiencing and using 1 1/2 is not a problem, just wanted to be sure it's worth the effort.

[Brando8719]

I don't think the outlet size will matter. If you get a smaller sized pipe, the water will move at a higher velocity though the pipe. The inlet size is the only size that matters and that is why it is the only one specified.

[schatzi]

Originally Posted by Brando8719

I don't think the outlet size will matter. If you get a smaller sized pipe, the water will move at a higher velocity though the pipe. The inlet size is the only size that matters and that is why it is the only one specified.

I don't think it specified in or out. Maybe I overlooked that though.

[Brando8719]

ah ok. I misunderstood your post. The output is the pump output going back into the tank? That size will not really matter. It will always have the flow of 1200 gph (mag 12 = 1200gph, right?). The smaller the pipe, the faster the water will travel through it. You will just have a more forceful water jet coming out of a smaller pipe.

The problem you might run into though is that your overflow might not supply 1200gph into the sump. You would have to check that out.

[vwwanksta69]

My buddy has the Mag 18 and his pump return is 3/4 to 1 1/4 pvc.

[schatzi]

Yes, from sump back to tank. There are two overflows so I'm sure it will be fine. Sounds like it can work either way, 3/4 or 1 1/2. I planned for 1 1/2 so I'll just do that I think. Thanks for the input

[nmcgrawj]

Originally Posted by Brando8719

ah ok. I misunderstood your post. The output is the pump output going back into the tank? That size will not really matter. It will always have the flow of 1200 gph (mag 12 = 1200gph, right?). The smaller the pipe, the faster the water will travel through it. You will just have a more forceful water jet coming out of a smaller pipe.

The problem you might run into though is that your overflow might not supply 1200gph into the sump. You would have to check that out.

Have you tested this? It doesn't add up to me.

Going up in height puts more pressure back on the pump...back pressure....making it pump less water.

Downgrading pipe size increases back pressure also. This will hurt the pumps performance. According to your theory the pump doesn't care what is resisting the water flow?

Yes, the opening on The pump is 3/4", but a 4ft long 3/4" pipe is a different animal compared to that opening.


They tell you to run a bigger pipe in the manual...and ive seen guys upgrade their pipe size and GAIN flow because there is less resistance.








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[Agu]

Simple plumbing is that your maximum output is from your minimum dimension. If your output is 1/2" and stays 1/2" you get maximum output. If your output is 1/2" and you go to 12" your maximum output is still only 1/2". If your output is 1/2" and you use a valve to reduce that down to 1/4" you get the output of 1/4" plumbing.

That doesn't take into account the effect of elbows and distance that reduce the efficiency of the plumbing. If you have elbows they effectively reduce that 1/2" plumbing because of added friction. Basically adding larger plumbing after the output can reduce friction and improve efficiency, esp if you have long and complicated return lines. Depending on your return plumbing ,enlarging the return lines may have a significant effect on flow, or could just be a waste of money.

[nmcgrawj]

Originally Posted by Agu

Simple plumbing is that your maximum output is from your minimum dimension. If your output is 1/2" and stays 1/2" you get maximum output. If your output is 1/2" and you go to 12" your maximum output is still only 1/2". If your output is 1/2" and you use a valve to reduce that down to 1/4" you get the output of 1/4" plumbing.

That doesn't take into account the effect of elbows and distance that reduce the efficiency of the plumbing. If you have elbows they effectively reduce that 1/2" plumbing because of added friction. Basically adding larger plumbing after the output can reduce friction and improve efficiency, esp if you have long and complicated return lines. Depending on your return plumbing ,enlarging the return lines may have a significant effect on flow, or could just be a waste of money.

Is this theory or tested ?

Its easier to blow through a straw that is 1" long vs. a straw that is 12" long.



Do we a agree that a 4ft section of 3/4" pipe will give more backpressure/friction than a 1.5" pipe of the same length?

[nmcgrawj]

I went with the manufacturers directions for maximum flow. I have a feeling they have done a little testing!







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[ibefishy]

I split the difference and went with 1" pipe with my Mag12.

[Agu]

Originally Posted by nmcgrawj

Is this theory or tested ?

Its easier to blow through a straw that is 1" long vs. a straw that is 12" long.



Do we a agree that a 4ft section of 3/4" pipe will give more backpressure/friction than a 1.5" pipe of the same length?

Originally Posted by Agu
Simple plumbing is that your maximum output is from your minimum dimension. If your output is 1/2" and stays 1/2" you get maximum output. If your output is 1/2" and you go to 12" your maximum output is still only 1/2". If your output is 1/2" and you use a valve to reduce that down to 1/4" you get the output of 1/4" plumbing.

That's why a valve will control flow downstream. As you change the size of the output at the valve you change the flow no matter the dimensions of plumbing downstream.

That doesn't take into account the effect of elbows and distance that reduce the efficiency of the plumbing. If you have elbows they effectively reduce that 1/2" plumbing because of added friction. Basically adding larger plumbing after the output can reduce friction and improve efficiency, esp if you have long and complicated return lines. Depending on your return plumbing ,enlarging the return lines may have a significant effect on flow, or could just be a waste of money.

Agree that resistance affects flow through pipes, that's why head calculators show different flow depending on the run of plumbing.

My point is that greatly enlarging return pipe size will probably not increase flow. You're best off using different dimension in the head loss calculator to determine pipe size.

[nmcgrawj]

Originally Posted by Agu

That's why a valve will control flow downstream. As you change the size of the output at the valve you change the flow no matter the dimensions of plumbing downstream.



Agree that resistance affects flow through pipes, that's why head calculators show different flow depending on the run of plumbing.

My point is that greatly enlarging return pipe size will probably not increase flow. You're best off using different dimension in the head loss calculator to determine pipe size.

"probably not increase flow"....

Have you even run this claim of 3/4" opening = 3/4" pipe in the calculator?


Mag 9
5ft, 1 90*, 1 45*
no valves

Using the calculator....

.75" diameter: 616gph
1": 705
1.25": 746
1.5": 755
2": 762
2.5": 764
3": 765

Maybe 150gph isnt much?


Obviously every set up is different. My point is that a 3/4" opening on the pump is not equal to a 3/4" pipe.

It is more complex than that. The smallest opening will limit the flow, but it won't set the flow. The velocity will increase but more velocity does not equal flow. A valve closing will slow down the flow...adding resistance. A 3/4" pipe with respect to a larger one does the same thing!


Is it a coincidence that mag says to use 1.5"....where the flow pretty much stops increasing at?


If I get time I'll experiment on my mag 18.


I know when I was planning mine a guy said he redid his pipe from 3/4" and increased it....picking up flow. You want to have the least resistance on the pump...as you see u gain flow until a certain point.

Follow what the manufacturer suggests. They don't care what size pipe u buy....



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[Agu]

You're correct, I forgot to put "significantly" at the end of the first sentence. And I've never argued that friction from smaller diameter pipe reduces flow, that's why I recommended using the head loss calculator.

My point is that greatly enlarging return pipe size will probably not increase flow (significantly). You're best off using different dimension in the head loss calculator to determine pipe size.

As your own numbers show going from 1.5 inches to 3 inches increases flow by less than 20 gph.

[nmcgrawj]

Originally Posted by Agu

You're correct, I forgot to put "significantly" at the end of the first sentence. And I've never argued that friction from smaller diameter pipe reduces flow, that's why I recommended using the head loss calculator.



As your own numbers show going from 1.5 inches to 3 inches increases flow by less than 20 gph.

I don't think anyone said to go 3"....

The manufacturer states 1.5". You gain quite a bit up until that point. Then it starts to level off making the extra cost of larger pipe not as beneficial.


.75" vs 1.5" is a completely different comparison than 1.5" to 3". I took 1.5" over .75 as the pipe size being "greatly increased".

The advantages are there. Now if you don't care about losing that flow then that's cool. No harm no foul. Most of us buy pumps for a rated flow we expect, and you've gotta set it up to perform that.





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