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#1
10-15-2010, 05:50 AM
 Unregistered Guest Posts: n/a
Convert feet/second to gallons/minute

I need to convert feet per second to gallons per minute. Doing a flow study in a sewer. I have a meter that shows distance over time (ft/sec) and I need to convert the results to volume over time (gpm). If I know the size of the pipe and depth of water can the converion be made.
#2
10-15-2010, 06:19 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,533 Rep Power: 19
Re: Convert feet/second to gallons/minute

Quote:
 Originally Posted by Unregistered I need to convert feet per second to gallons per minute. Doing a flow study in a sewer. I have a meter that shows distance over time (ft/sec) and I need to convert the results to volume over time (gpm). If I know the size of the pipe and depth of water can the converion be made.
Yes and no. Flow is complicated because the velocity is not uniform across the cross section of the flow. It is zero right at the walls, and maximum in the middle of the flow. The details depend on surface roughness of the pipe as well as its dimensions and on the viscosity and density of the fluid. Results also differ for laminar flow and turbulant flow. However, for many practical problems, velocity may be relatively uniform across most of the cross section, and decrease to zero only in a thin boundary layer at the walls.

If you can find an average velocity for the flow, you can proceed as follows:
1) Multiply the flow in feet per section by the cross-sectional area of the flow (square feet) to get cubic feet per second.
2) Multiply by 60 s/min to get cubic feet per minute.
3) multiply 1728 gallons/231 ft³ to get gallons per minute.
It all depends on finding a measurable velocity somewhere in the stream which reflects the average velocity of the entire stream.

As a minimum, you will need to check velocity variation across the flow channel (and at different flow rates) to see if you can get reasonable accuracy. You may need to consult a flow expert who can model the variations in velocity across the flow at varying conditions. Such a model would still result in an average velocity for which the procedure above would work, but the conversion between measured velocity and average velocity is likely to be condition dependent. Assuming you measure somewhere near the middle of the flow, the peak velocity you measure will ignore the boundary layers and somewhat overstate the flow - actual flow will be less.
#3
10-15-2010, 06:57 AM
 Unregistered Guest Posts: n/a
Re: Convert feet/second to gallons/minute

JohnS thank you for the reply. Based on some assumptions the conversions you suggested should work for this quick study. Thanks again.
#4
08-13-2012, 04:22 PM
 ths2@aol.com Guest Posts: n/a
Re: Convert feet/second to gallons/minute

I have a stream of water 4 ft wide (avg) and 6inches deep (avg) and a float travels 3ft in 8 seconds. how many gallons of water in on min?
#5
08-14-2012, 03:13 AM
 JohnS Moderator Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,533 Rep Power: 19
Re: Convert feet/second to gallons/minute

Quote:
 Originally Posted by ths2@aol.com I have a stream of water 4 ft wide (avg) and 6inches deep (avg) and a float travels 3ft in 8 seconds. how many gallons of water in on min?
6" = 0.5 ft
4 ft x 0.5 ft x3 ft/8 s x 60 s/min = 45 ft³/min
A ft³ = 7,48 gal, so 337 gal/min

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