I am trying to size a gravity fed pipe that takes water from a small creek and transmits it down 7000 linear feet of pipe, at a fairly consistent slope of 4%, resulting in 300 feet of total head. I am deciding whether to run 8" pipe, 6" pipe or a combination of both. I've used Manning's formula for full pipe flow at that slope, and Hazen/Williams for head loss due to friction at that length of pipe, but niether calculation takes into effect the total drop (head) of 300' and it's effect on flow rates and pressure??? What am I missing here? Thank you.

You can use pressure drop calculator.

Here are results that I have for your inputs:

1. volume flow rate (q):

q : 365.5706 m3/h

2. weight flow rate (w):

w : 365570.6 kg/h

3. pipe length (L):

L : 7000 ft

4. pipe diameter (D):

D : 8 in

5. pipe roughness (kr):

kr : 0.1 mm

6. density (ρ):

ρ : 1000 kg/m3

7. kinematic viscosity (ν):

ν : 1.006 mm2/s

8. dynamic viscosity (μ):

μ : 0.0010060001 Pas

9. K factor - minor losses coefficient (K ):

K : 0.0

10. velocity (V):

V : 3.1313229 m/s

11. cross section area (A):

A : 32429.277 mm2

12. friction coefficient (f):

f : 0.01742538

13. Reynolds number (Re):

Re : 632489.8

14. pressure on the pipe start (p1):

p1 : 333.886963 ftWS

15. pressure on the pipe end (p2):

p2 : 101325.0 Pa

16. pressure drop (p1-p2):

p1-p2 : 300 ftWS

----------------------

Pipe pressure drop calculator

1. volume flow rate (q):

q : 171.63454 m3/h

2. weight flow rate (w):

w : 171634.53 kg/h

3. pipe length (L):

L : 7000 ft

4. pipe diameter (D):

D : 6 in

5. pipe roughness (kr):

kr : 0.1 mm

6. density (ρ):

ρ : 1000 kg/m3

7. kinematic viscosity (ν):

ν : 1.006 mm2/s

8. dynamic viscosity (μ):

μ : 0.0010060001 Pas

9. K factor - minor losses coefficient (K ):

K : 0.0

10. velocity (V):

V : 2.6136415 m/s

11. cross section area (A):

A : 18241.469 mm2

12. friction coefficient (f):

f : 0.018759523

13. Reynolds number (Re):

Re : 395943.3

14. pressure on the pipe start (p1):

p1 : 333.886963 ftWS

15. pressure on the pipe end (p2):

p2 : 101325.0 Pa

16. pressure drop (p1-p2):

p1-p2 : 300 ftWS

--------------

I have assumed that p2 is atmospheric pressure and I used p1 as 300 ftWS gauge pressure to simulate geodetic height difference.

Hope it can give you some help.

Here are results that I have for your inputs:

1. volume flow rate (q):

q : 365.5706 m3/h

2. weight flow rate (w):

w : 365570.6 kg/h

3. pipe length (L):

L : 7000 ft

4. pipe diameter (D):

D : 8 in

5. pipe roughness (kr):

kr : 0.1 mm

6. density (ρ):

ρ : 1000 kg/m3

7. kinematic viscosity (ν):

ν : 1.006 mm2/s

8. dynamic viscosity (μ):

μ : 0.0010060001 Pas

9. K factor - minor losses coefficient (K ):

K : 0.0

10. velocity (V):

V : 3.1313229 m/s

11. cross section area (A):

A : 32429.277 mm2

12. friction coefficient (f):

f : 0.01742538

13. Reynolds number (Re):

Re : 632489.8

14. pressure on the pipe start (p1):

p1 : 333.886963 ftWS

15. pressure on the pipe end (p2):

p2 : 101325.0 Pa

16. pressure drop (p1-p2):

p1-p2 : 300 ftWS

----------------------

Pipe pressure drop calculator

1. volume flow rate (q):

q : 171.63454 m3/h

2. weight flow rate (w):

w : 171634.53 kg/h

3. pipe length (L):

L : 7000 ft

4. pipe diameter (D):

D : 6 in

5. pipe roughness (kr):

kr : 0.1 mm

6. density (ρ):

ρ : 1000 kg/m3

7. kinematic viscosity (ν):

ν : 1.006 mm2/s

8. dynamic viscosity (μ):

μ : 0.0010060001 Pas

9. K factor - minor losses coefficient (K ):

K : 0.0

10. velocity (V):

V : 2.6136415 m/s

11. cross section area (A):

A : 18241.469 mm2

12. friction coefficient (f):

f : 0.018759523

13. Reynolds number (Re):

Re : 395943.3

14. pressure on the pipe start (p1):

p1 : 333.886963 ftWS

15. pressure on the pipe end (p2):

p2 : 101325.0 Pa

16. pressure drop (p1-p2):

p1-p2 : 300 ftWS

--------------

I have assumed that p2 is atmospheric pressure and I used p1 as 300 ftWS gauge pressure to simulate geodetic height difference.

Hope it can give you some help.

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So both the 8" and 6" are going to generate the same pressure at the end of the pipe? 14 psi? Using other pressue drop calculators, and head loss calculators, as well as pvc flow charts, I was coming up with 60 psi at discharge end with 8" pipe. To get this 60 psi number a specific flow had to known, and given to calculate the friction loss? To get this "flow" number, Manning's equation is used which only uses slope of pipe, not total head??? Still lost on this one. Thanks

You will have different flow rate for fixed pressure difference. As I understand you have flow between two water levels. And water is under atmospheric pressure. Pressure difference in the calculation is just representing difference in height - 300'. Based on that pressure (head) difference flow is established. So if you have bigger pipe, flow will be higher in order to "use" available 300' of head compared with smaller pipe that will have lower flow for the same head available.

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Ok, I've been studying this calculator and have found that it also, just like the rest of the hydraulic equations I have found is only using the slope of the pipe to calculate flow. Unfortunately I am not educated in hydraulics but have been trying to educate myself over the past week. I know that I cannot get the required amount of water for my irrigation system with only 10' of total head and a short amount of pipe. I need to go further up the creek to gain more head to get the pressure needed for the system. I can go a maximum of 7000' feet up the creek, resulting in 300' of total head. In using the pressure drop calculator, I will get the same flow 3500' up the creek resulting in 150' of head because the slope is still the same; 4%. And the same result only 100' up the creek with 4' of head, because once again a 4% slope. ??????????

As pressure drops linearly with pipe length, with constant pipe slope, head gained due to height difference is compensated with pressure drop due to friction keeping same flow rate for given pipe diameter.

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boonesfab wrote:I am trying to size a gravity fed pipe that takes water from a small creek and transmits it down 7000 linear feet of pipe, at a fairly consistent slope of 4%, resulting in 300 feet of total head. I am deciding whether to run 8" pipe, 6" pipe or a combination of both. I've used Manning's formula for full pipe flow at that slope, and Hazen/Williams for head loss due to friction at that length of pipe, but niether calculation takes into effect the total drop (head) of 300' and it's effect on flow rates and pressure??? What am I missing here? Thank you.

Mannings formula is helpful if you don't have a full pipe or you have two-phase flow. That doesn't sound like your case. I'm assuming you'll have a flooded pipe.

To calculate pressure drop you will need to assume a flow rate. Pressure drop is directly proportional to the square of the velocity of the fluid in the pipe. Pressure in the line is not taken into account in a pressure drop calculation. It's all based on friction and velocity. Pressure drop is the result of forcing water (or other fluid) through the pipe. It's really immaterial what the line pressure is for a non-compressible fluid (water). If you look at Hazen-Williams you'll see that fluid pressure isn't even taken into account.

You may want to assume a usage rate, say 500 gpm, and calculate line losses. Using the on-line calculator, I get a pressure drop of 58.3 psig (134.4 ft WC) in 6 inch pipe with velocity of 5.7 ft/s . Using 8 inch pipe the line loss is 13.6 psig (31.3 ft WC) with a velocity of 3.2 ft/s. These values were based on a density of 1000kg/m3 (water density at 68 deg. F) and 7,000 ft. of pipe with default roughness values and kinematic viscosity of 1.006 mm2/s.

As mentioned earlier, pressure drop is a function of the approx. square of the velocity. If you double the velocity, you will quadruple the pressure drop (see Darcy-Weisbach equation).

You may want to consider the flow in the stream to determine how much water you can get from it. Size the line based on water usage (flow requirements), the pressure drop you will be able to tolerate, and the cost of the pipe.

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