If I now the pressure and the Pipe size can I calculate the flow rate?

[DrDarrell]

If I now the pressure and the Pipe size can I calculate the flow rate?

I have a piping system with 8 inch Schedule 40 Pipe, and I have a constant pressure of 175 psig can I determine what the flow rate is in gpm? The fluid is fresh water.

Respectfully,
Darrell

 

[BigInch]

Well, you still need a couple of other variables defined to get a technically correct solution, but... we can envelop the extremes.

All these will affect the technically correct answer,
Inlet pressure, 175 psig,
Outlet pressure, ?
Length of the line,
Elevation profile, especially elevation of the inlet and outlet.

With a [red] horizontal [/color] pipe @ 175 psi good targets are theoretically between the following two extremes. The first is keeping the flow fast enough to keep the line clean, the last is to keep the pipe from eroding from particulate matter in the water.

Your 175 psig will allow a maximum rise of up to 400 feet above the point that that pressure was measured, unfortunately without any continuous flow. But you should be able to do most anything between these extremes

1.) 460 gpm with a loss of 1.7 psi)/1000 ft length = total length of about 20 miles.
2.) 1600 gpm with a loss of 17 psi/1000 ft = total length of 2 miles. The 1600 gpm may sound tempting, but you may be letting in other waterhammer gremlins when running at that high a velocity. I suggest a reasonable compromise might be 700 gpm, losing 3.7 psi /1000 ft, for a total distance of up to 7 miles. That would give you a pressure loss of 135 psi, still allowing for up to a 50 foot elevation increase with enough pressure left for a good exit stream.

Decreasing the outlet elevation below the measured pressure point elevation will tend to increase pressure, flow and pressure drop in the line, so do not exceed the pipe pressure rating.



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[vpl]

Yes and no.

If you have a Crane's technical paper 410, there is a table in the back that will give you flow rates for a given pipe size at a standard temperature.

However, if you're wanting to figure it out yourself, you need to know the fluid temperature so that you can find some physical properties. Then you basically do a unit analysis.

I think the following is correct. I'm sure someone will tell me if I'm wrong.

(1) Convert the pounds-force to pounds-mass by dividing by the gravitational constant (32.2). That gives you units of lbm*ft/sec².

(2) Convert your square inches to square feet by multiplying by 144. Your pressure term becomes lbm/(ft*sec²).

(3) Divide by the viscosity (in units of lbm/(ft*sec) for a unit of 1/sec.

(4) Multiply by the density (lbm/ft³). This gives you a unit of lbm/(ft³*sec).

(5) Calculate a unit volume (i.e., your cross-sectional area (making sure it's in square feet!) times 1 ft.) This gives you a value in cubic feet.

(6) Multiply the Step (5) result by the Step (4) result. You now have lbm/sec.

(7) Multiply the Step (6) result by 3600 seconds per hour and you have a flow rate in lbm/hr.

If you want the flow rate in gallons per minute, follow steps (1) through (3) above. Omit step (4). Do step (5). This gives a result in ft³/sec. Then:

(6') Multiply by 7.48 gallons/ft³. Your result is now in gallons per second.

(7') Mulitply by 60 seconds per minute. Your result is in gallons per minute.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

 

[DrDarrell]

Big Inch:

Thank you for your reply.

I'm thinking of more along the lines of a length of 50-200 ft. and an outlet pressure of close to the same as the inlet. The elevation would be nearly horizontal (not more that 10 feet)

Darrell

 

[BigInch]

I kinda' thought so, but wasn't sure. OK.. then stand back, 'cause you'll have pleanty of pressure left over at the outlet at low flow rates, (or even at real high flowrates). Think about a nice valve for the outlet that will give you some good control and make sure the pipe won't move around as it could have around 9000 lbs of force resulting from the water jet, and that's after the initial jump. Open valves slowly.

Since water is a well known fluid and ready-made flow charts exist, I just used this one,

http://www.engineeringtoolbox.com/pressure-loss-steel-pipes-d_307.html

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[MintJulep]

If I now the pressure and the Pipe size can I calculate the flow rate?

No, you cannot.

 

[katmar]

If your outlet pressure is close to the inlet presure then your flowrate is close to zero. More than that you cannot say without a LOT more detail.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[BigInch]

I already said and I think the OP understands,

All these will affect the technically correct answer,
Inlet pressure, 175 psig,
Outlet pressure, ?
Length of the line,
Elevation profile, especially elevation of the inlet and outlet.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com