good reference to size the pipe diameter against flow coming from a pump

[AskTooMuch]

Say I have a pump with 45,000 gpm capacity and I want to size the stormwater steel pipe diameter. What's a good reference for this? I'm familiar with manning's open channel but in this case there is a pump involve so there maybe pressure involved.

This is just high level check. I'm not going to do the detailed design of this.

 

[Bill R]

Pressure will be involved, so the Hazen-Williams equation is a common one to use in this case. As there are a lot of unknowns such as downstream pressure, length, etc, just checking the pressure loss on a certain size of pipe at your rated gpm is a good start. You can quickly do some checks using the calculator linked below. Just throwing in 140 for the coefficient, 45000 gpm, and 100 foot pipe length, we can see the pressure loss per 100' of pipe. Here are the results for some pipe sizes:

12" diameter: 113 (psi/100 ft pipe) head loss
18" diameter: 15.7 (psi/100 ft pipe) head loss
24" diameter: 3.86 (psi/100 ft pipe) head loss
30" diameter: 1.3 (psi/100 ft pipe) head loss
36" diameter: 0.54 (psi/100 ft pipe) head loss
42" diameter: 0.25 (psi/100 ft pipe) head loss

https://apps.engineeringtoolbox.com/head-loss-water-pipe-a_15.html

As you can see, the pressure loss is huge below 24" diameter, but gets a lot better in the 30-42" range. Once you have the rest of the givens in the design and have your acceptable design pressure loss through the pipe run, you'll choose the pipe size to accommodate.

 

[cvg]

it is likely that with a stormwater pump discharge line, the flow will not maintain full pipe flow to the end unless it is a short pipe. You may need to have a larger pipe flowing partially full. try and maintain a reasonable velocity (say below 10 feet per second) to minimize head loss and to maintain sediment movement through the system. this will also reduce scour at the discharge point

 

[AskTooMuch]

Thanks for the help.
The pipe length is 1500', assuming it's flat and I need 45000gpm discharge, what is the formula/reference to calculate the pipe diameter including head loss with maximum velocity of 10 ft/sec?
The calculator only calculate head loss.

 

[cvg]

suggest you start with tailwater conditions and work backwards. for a storm drain this large and long, a hydraulic model is typically recommended. a concept model could be done very quickly and shouldn't be a large effort

https://www.engineersedge.com/fluid...ulation/partiallyfullpipeflow_calculation.htm

 

[AskTooMuch]

Using Q=V/A since I have Q from the pump of 45000gpm or 120cfs if I use 3'dia of pipe that would result in V=17ft/s.
Since 10ft/s is the maximim ideal, I need to increase the pipe diameter?

 

[AskTooMuch]

From quick google search, I can have a faster velocity as long as there is a proper riprap if it's goes into a pond?

 

[cvg]

17 feet/sec is far to high for riprap and more likely you would need an impact basin

https://usbr.gov/tsc/techreferences/hydraulics_lab/pubs/PAP/PAP-0269.pdf

 

[AskTooMuch]

Cvg, even if I have pumps total capacity of 45000gpm, the pipe will not have Q that much right cuz of losses? I don't how much the total losses, it will be calculated later by others in detailed design.
But if Q at exit is much less than 45000gpm then V is less as well.

 

[FacEngrPE]

Q in will always equal Q out, unless your pipe leaks, which is not good.
Losses in your pipe are head or pressure losses.
Depending on the size of your pond, 10 ft/ second might be too high at the discharge point.