Water Header Design for Various Users 1

[RJB32482]

Hey,
I am trying to design a pipe size for a header that has many different users on it. The pipe is about 3" Sch 40 SS pipe which is going to need three users with flow requirements of:
-User 1: 20 GPM at 30 eq. feet of pipe
-User 2: 10 GPM at 60 eq. feet of pipe
-User 3: 5 GPM at the end of the main header (3")

I want to size the two branches for the design flow rate. I'm thinking the procedure would be to:
1. Add all the flows to get 35 GPM.
2. Pick Line Sizes for the two branched lines
3. Find the pressure drop for each of the three lines at the design flow rate (for each line).
4. Add all pressure drops up and check to see if pressure at the inlet of the main header is greater than the added pressure drops.


Now one question I have is do I take the pressure drop form P1 to all of the end branches or just from the start of the branch to the end?

Thank you.

 

[Harold53]

RJB32482
May be you can read the Hicks "Standard handbook of engineering calculations". In the Chapter 3 is an easy way to understand how to plot the system-head curve for diverted flows.

 

[RJB32482]

Any way the excerpt or method can be posted here? Can't get to that publication.

 

[katmar]

I assume all your users are discharging to atmospheric pressure.

Pick line sizes for the two branched lines (as per your suggestion).

Start at the far end (the 5 GPM user) and calculate the pressure drop back to where it joins with user 2. Similarly calculate the pressure drop from the user 2 discharge (10 GPM) back to where it joins with user 3. Take the higher of these two numbers as the pressure required at the junction of these two flows. Call this pressure "A".

Now calculate the pressure drop in the header for the combined flows for users 2 & 3 (15 GPM) from the user 2 take off back to the user 1 take off. Add this pressure drop to the pressure "A" and call the result "B". This is the pressure required at the user 1 take off point to satisfy users 2 & 3.

Calculate the pressure drop from the user 1 discharge (20 GPM) back to where it joins with users 2 & 3. Compare the pressure required for user 1 with that required for the combined users 2 & 3 (i.e compare with "B"). Again, take the higher of these two numbers as the pressure required at the junction of all three flows. Call this pressure "C". This is the pressure required at the user 1 take off point to satisfy all 3 users.

Calculate the pressure drop for all three flows (35 GPM)from the user 1 take off back to your supply (pump?) and add this to pressure "C". This tells you the required supply pressure. Compare this with your actual supply pressure and adjust your line sizes and recalculate the whole lot if necessary.

It was probably just your wording that was confusing, but note that you do not add all the pressure drops. At each junction you take the higher value, rather than adding them. The flows are additive, but not the pressures, for diverging flows.

regards
Harvey


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[chicopee]

Harold53, if you dont want to calculate, you can use a hydraulic graph to plot katmar's steps. Graphing is a lot quicker but not as precised.