Water flow rate through pipes

rohnds

rohnds

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Jun 19, 2004
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My question refers to the above diagram. The water is flowing right to left.

As shown in the diagram, the flow rate via the 2-inch PVC pipe is 300 GPH, which is about 6.1275 ft/s velocity. Water then flows into a 1-inch diameter PVC pipe. My question is what is the new flow rate of water flowing through this pipe?

Can I assume that the water is still has a velocity of 6.1275 ft/s through the 1-inch pipe or has that parameter changed? If the velocity is still 6.1275 ft/s through the 1-inch pipe, then the calculation is simple? But otherwise, I am stumped.

Rohn
 
If the pump can deal with the backpressure, then the flow will still be 300 gph. When the water hits the intersection, it's not like it has anywhere else to go. The velocity will double in the smaller pipe, because you now have the same flow going across half the cross-sectional area.

There will be turbulence at the junction, which will reduce flow, so you might look around for a coupling that is angled rather than straight.
 
If the pump can deal with the backpressure, then the flow will still be 300 gph. When the water hits the intersection, it's not like it has anywhere else to go. The velocity will double in the smaller pipe, because you now have the same flow going across half the cross-sectional area.

There will be turbulence at the junction, which will reduce flow, so you might look around for a coupling that is angled rather than straight.
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cross sectional area of 1"I.D. pipe is .78 & 2" is 3.14 so it is NOT half, it is LESS than half the size. so the flow will be cut more than half along with many other varibles like the pump capabilities as mogurnda stated.
 
If the flow rate is 300 GPH, then it is 300GPH everywhere in the system. You can't get something for nothing. If the flow was 300GPH in the large pipe, where does all the water go if the flow is less than that.

What changes is the velocity of the water in the smaller pipe. It increases.

300GPH/7.5 feetper gallon= 40 cubic feet per hour

1 inch = .0833 feet

A=pi * (.0833/2)^2 =.00545 square feet.

40 cubic feet per hour /.00545 sqare feet = 7340 feet per hour

7340 feet per hour / 3600 seconds per hour =2 feet per second in the small pipe.

using the same calculations, I get that 6 feet per second in the 2 inch pipe is roughtly 3500 GPH, not 300, and at 300 GPH, the flow rate is .5 feet per second.
 
well, joephys when you squeze 300gph from a 2" tube into a 1" tube, it's the pump that dictates how much flow is going through the tube. so is'nt 300 gph through a 1" tube the same as 300gph through a 2" tube? It's just the velocity that changes not how much water moves through the tube.
 
Its going to have more resistance so the flow is going to decrease. The type of pump depends on just how much. No different than adding head pressure to a pump or partialy closing a valve...the flow WILL decrease.

joephys- when you talk of 1" pipe you are talking diameter, when you are working with flow you have to work with the area of the pipe opening & the area of 1" pipe is less than half of 2" pipe. you also have forgotten to figure in resistance. Your figures of velocity is correct if you were working with area & the pump you are working with was capable of pushing that much water through a pipe half its size...which MOST are not.
 
cioutlaw said:
Its going to have more resistance so the flow is going to decrease. The type of pump depends on just how much. No different than adding head pressure to a pump or partialy closing a valve...the flow WILL decrease.

joephys- when you talk of 1" pipe you are talking diameter, when you are working with flow you have to work with the area of the pipe opening & the area of 1" pipe is less than half of 2" pipe. you also have forgotten to figure in resistance. Your figures of if velocity is correct you were working with area & the pump you are working with was capable of pushing that much water through a pipe half its size...which MOST are not.
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I did work with area, where was it that you didn't see that?

A=pi * (.0833/2)^2 =.00545 square feet

thats the area of the pipe opening.

As far as the pump being able to have the same flow rate, yes I agree, that the pump might not be able to push 300 GPH through a 1 inch pipe. How ever, if it is pumping at 300 GPH, the flow rate is the same in both pipes. There is no way for me know know what the pump is capable of doing, so if it is stated that there is 300 GPH going through the 2 inch pipe, then there has to be 300 GPH going through the 1 inch pipe.
 
joephys said:
I did work with area, where was it that you didn't see that?

A=pi * (.0833/2)^2 =.00545 square feet

thats the area of the pipe opening.

As far as the pump being able to have the same flow rate, yes I agree, that the pump might not be able to push 300 GPH through a 1 inch pipe. How ever, if it is pumping at 300 GPH, the flow rate is the same in both pipes. There is no way for me know know what the pump is capable of doing, so if it is stated that there is 300 GPH going through the 2 inch pipe, then there has to be 300 GPH going through the 1 inch pipe.
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sorry..just saw where you converted to feet & missed the sq. feet conversion. you answred the original question correctly. The pumps used in aquarium industry & most pumps in general are going to reduce in flow capabilities when restricted. I guess he didnt give enough information.

Rohn, how do you know what the GPH is going through the 2" pipe? If you are going by what the pump is rated then that is normally free flowing.
 
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