Pressure drop in pipes

[lordtemple]

Let's say that we have a 3/4" copper line to supply water. We have a volumetric flow of 20 gpm (0.0445 ft^3/s). With this, we have a velocity of 14.35 ft/s. Now, at the end of the copper line, I want to service two homes, so I split the service. Comming out of the 3/4" copper line, I have two 3/4" copper lines more.

How do I know how much pressure I'm going to loose for every branch that I add? Let say that later on I want to split the service into three or four 3/4" services, comming out of the same 3/4" "main line". I'm trying to apply Bernoulli's equation, but somehow I don't believe my number because, according to my interpretation of my results, the pressure on the branches is higher that the pressure on the "main" line, this is not possible, is it?

 

[sailoday28]

The pressure loss in the original line is dependent upon the flow in that line. Therefore, you must know the flow in the added lines to determine the additional pressure drop.

Are you also assmuming that the pressure source to the original line will remain constant?

 

[lordtemple]

Well... if the main 3/4" line is supplying 20 gpm, if I have 2 lines comming out, I'll have 10 gpm in each line, and if I have 3 lines comming out, I'll have 6.67 gpm in each line, won't I?

Yes, I'm assuming the pressure source will remain constant, thanks for that observation.

I guess something else that can help me is if someone knows if pressure can be calculated knowing only: diameter and flow (ft^3/s). I can calculate velocity, but I can't seem to find a way to calculate pressure on that pipe. Is it even possible?

Thanks

 

[sailoday28]

rdtemple (Mechanical) Dec 28, 2004
Well... if the main 3/4" line is supplying 20 gpm, if I have 2 lines comming out, I'll have 10 gpm in each line, and if I have 3 lines comming out, I'll have 6.67 gpm in each line, won't I?

The flow in your branch lines will depend upon the outlet pressure and pipe resistance of each branch line.

Assuming the 20 gpm in the original line, then the pressure available to each branch remains constant. The length and outlet pressure of each branch will determine branch flow.

 

[TBP]

This sure has the aroma of a homework assignment. Anyway...

Think of piping runs as chains. Each run of straight pipe, each fitting, each valve, etc is a link in the chain. Each will have it's own pressure drop for a given flow. The inlet pressure to one "link" will be the outlet pressure from the "link" ahead of it. There must a pressure drop for there to be flow in a pipe. Pressure drop, flow, and velocity are all related. Change one, and the other two change as well.

Seriously consider picking up a copy of the "Technical Paper No. 410" by Crane. If you're dealing with fluid flow in pipe, this is THE reference.

 

[lordtemple]

jajaja... na... its not a homework problem. I really appreciate your inputs, and I've been told about the Technical Paper a couple of times, so I guess I'll buy it. Now I think of some details that I could've left out, I'm going to do some math and I'll see what I come up with.

Thank you all for your time.