Parallel Piping Resistence

[eskiba]

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.

 

[JohnGP]

If the parallel runs are the same, then you can size the pump based on the head calculated for one run, believe it or not.

Cheers,
John

 

[rneill]

Not exactly sure of your situation but if you had a total flow of 3 gpm through X number of parallel 3/4" pipes then you would normally assume an equal amount of flow through each line and thus the flow in each line would be X/Y where Y was the number of parallel lines.

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.

 

[eskiba]

consider two situations:

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)

 

[rneill]

In the case where the parallel runs are of unequal length then you will equalize to a situation where the pressure drop across each branch will be equal but the flow will not. In other words, the shorter branches will attract more flow to the point where they end up with the same pressure drop as the longer branches. If the sink is the same then all outlet pressures must be the same and the pump discharge must be the same (since the header is a common point) so the system settles out with different flows in each branch.

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.

 

[eskiba]

I guess the issue stems from when sizing a pump you take the largest pressure drop in the parallel components and that is the head that the pump must over come regardless of what else is in the system.

 

[Latexman]

The pump can be sized based on the head calculated for one run, and for a small inexpensive system like in the OP that may be fine. For a sensitive system or a large expensive one a good engineer would analyze the frictional and momentum effects and design the system so the maldistribution is < 5%. A lot of times this can be done without sophisticated software. Perry's Chemical Engineers' Handbook has a section on fluid distribution.

Good luck,
Latexman

 

[rneill]

eskiba, the problem with just simply taking the largest head and sizing for that is that you may be providing more head than is needed for the other branches in which case you may force more flow down these branches than you intend to the detriment of the branches that you want the flow in. The system has to be properly balanced so that you get the right amount of flow down each line.

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".

 

[MikeHalloran]

Calculate the pressure drop for each run, for an arbitrary flow that's somewhere near realistic.

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

 

[stanier]

Download this AFT tool and play with it.

http://www.pumpsystemsmatter.org/content_detail.aspx?id=110

Then invest in the full blown Fathom software from

http://www.aft.com

 

[MikeHalloran]

If your budget is limited and DOS doesn't scare you, try this:

http://www.atkinsopht.com/eng/strmlins.htm

The software is free; the manual is inexpensive.


( No commercial relationship )



Mike Halloran
Pembroke Pines, FL, USA

 

[stanier]

Use the Hardy Cross method.

 

[BigInch]

Use the EPAnet method. Program's free and so is the manual.

**********************
"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)

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