Head Loss through pipe system
Head Loss through pipe system
(OP)
What would be the head loss through this pipe system?
A 3" pipe branches off into 5 - 3/4" pipes all the same length and diameter. 3/4" Pipes discharge to the air.
Just taking into consideration flow through the pipes, is the head loss through the system equal to:
1) (HL in 3" Pipe) + (HL in 1 of the 3/4" Pipes)
or
2) (HL in 3" Pipe) + (HL in all of the 3/4" Pipes)
I'm pretty sure it's 1), but its been a long time....
A 3" pipe branches off into 5 - 3/4" pipes all the same length and diameter. 3/4" Pipes discharge to the air.
Just taking into consideration flow through the pipes, is the head loss through the system equal to:
1) (HL in 3" Pipe) + (HL in 1 of the 3/4" Pipes)
or
2) (HL in 3" Pipe) + (HL in all of the 3/4" Pipes)
I'm pretty sure it's 1), but its been a long time....





RE: Head Loss through pipe system
Mike McCann
MMC Engineering
RE: Head Loss through pipe system
I thought #1 was correct as the 3/4" pipes are in parallel and have the same HL across their length. The flow Q1 is divided into the 5 3/4" Pipes (Q1 = 5*Q2). So HL across the parallel system is equal to 1 of the 3/4" Pipes.
To develop this system curve, which is correct?
1) (HL in 3" Pipe) + (HL in 1 of the 3/4" Pipes)
or
2) (HL in 3" Pipe) + (HL in all of the 3/4" Pipes)
I appreciate your response and apologize if my question still is not clear.
RE: Head Loss through pipe system
but neither is correct
to be accurate, there is additional headloss between each of the tee fittings such that the flow in the 5 3/4 inch pipes are not equal. there will be more flow in the first and less flow in each succeeding branch line.
in addition, I assume you mean friction loss only, not losses due to elevation change?
RE: Head Loss through pipe system
Yes I understand loss at each fitting and elevation head. Just not sure if I was suppose to add up the HL in all the pipes or assume they are in parallel.
RE: Head Loss through pipe system
http://www.mcnallyinstitute.com/11-html/11-12.html
RE: Head Loss through pipe system
Look at it this way. Pushing the liquid through each 3/4" pipe is going to need pressure to keep the water moving through all 5 pipes. To have the pressure you need, you must overcome the resistance (or drag) for each pipe. So you need to have enough head to push the liquid through all 5 pipes.
If you only have enough pressure from the pump to get water through one 3/4" pipe, water would not flow through the other 4 pipes - no pressure.
This is a simplistic illustration. in reality, water would just trickle through each 5 pipes at a very slow velocity because the are all sharing the pressure you provide for that single pipe.
This is an easy problem because all 5 pipes are the same diameter. To prove your solution #1 is wrong, what would you do if all 5 pipes were different diameter?
RE: Head Loss through pipe system
I think Ben Johnsons statement: “…you need enough head to push the liquid through all 5 pipes.” Would be more accurately stated, as “you need enough hydraulic horse power to push the liquid through all 5 pipes,” where hydraulic horse power is the ability for a pump to supply a greater flow rate at a given head.
If increasing pump power can be added to the system with additional branches being added, where the flow rate through each additional branch will remain the same, with the same driving head, then the total head loss through the system will also remain the same even after adding a hundred more branches.
The answer is number 1 without exception for bends, valves and other minor losses (though these things will cause flow in each pipe to not be equal and thus should be considered). It is hydraulic horse power that increase with added branches, not head loss.
RE: Head Loss through pipe system
If you want to guarantee equal flow through all five 3/4" pipes, you need to install some type of pressure device at the end of the 3/4" pipes so that the fluid resistance in the piping is inconsequential.