Parallel Piping Resistence
Parallel Piping Resistence
(OP)
Hopefully you guys can help me here. When calculating the head to size a pump how to you combine the pressure drop through a parallel bank. Say you have 3gpm through 6ft of 3/4in pipe. I know you cannot just add them up but I have been told that you can just size the pump based on one of the runs. I refuse to believe this is correct.





RE: Parallel Piping Resistence
Cheers,
John
RE: Parallel Piping Resistence
You would calculate the pressure drop through one of the lines with flow X/Y and this would be the pressure drop across each of them (since they are in parallel). The pump would need to be capable of the total discharge of 3 gpm but with only the output head calculated for the single line (since they are in parallel).
The difference is that if you had only one line (instead of X) then the calculated pressure drop would be much higher since it would see all the flow instead of X/Y flow and thus the pump would need much higher head. As you add parallel branches, the head requirement drops since the pressure drop across the system drops.
RE: Parallel Piping Resistence
1.) All // runs have the same flow and known pressure drop. The combined pressure drop (for sizing purposes) is the pressure drop across one collector.
2.) The // runs are of unequal length and therefore have uneven pressure drop. To size the pump, the max head overcome is the pressure drop of the longest run (longest resistance)
RE: Parallel Piping Resistence
However, in the real world, you might have different discharge conditions as well as different lengths so the calculation of how much flow would go to each branch as well as what the pressure drop would be might become complex necessitating the use of some network analysis software.
RE: Parallel Piping Resistence
RE: Parallel Piping Resistence
Good luck,
Latexman
RE: Parallel Piping Resistence
If the system is not inherently balanced then you would provide valving to allow manipulation of the flow in a branch line.
Anyway, it is a bit difficult to figure out what's important in your system as we don't really have any information on where the flow is going and how important it is to have a specific amount of flow. For example, is this a heating or cooling system in which each branch is providing heating/cooling flow to a different "user" and then returning back to the source in which case there will definitely be different pressure drops and flow requirements in the different branches and it would be important to make sure that each branch got the flow it required. In a system like this, I would design for the largest pressure drop and then install a valve on every branch so that I could "pinch" back as necessary to balance the system. It would also be common to install automatic control valves that regulated to achieve the specified degree of heating or cooling by monitoring of the temperature in the "users".
RE: Parallel Piping Resistence
Then compute a Cv for each run:
Cv = gpm/sqrt(psid)
Then add the Cvs to get a composite Cv, which you can plot in Excel as your system curve. Superimpose your pump curve on the same plot, and you've got your OP.
It gets more complicated if there are elevation differences among the distal ends, but you get the idea.
Mike Halloran
Pembroke Pines, FL, USA
RE: Parallel Piping Resistence
http://ww
Then invest in the full blown Fathom software from www.aft.com
RE: Parallel Piping Resistence
http://www.atkinsopht.com/eng/strmlins.htm
The software is free; the manual is inexpensive.
( No commercial relationship )
Mike Halloran
Pembroke Pines, FL, USA
RE: Parallel Piping Resistence
RE: Parallel Piping Resistence
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/