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Multiple lines feeds

Multiple lines feeds

Multiple lines feeds

(OP)
It's been awhile since I've done these calculations, please help.

I have a 2" line feeding 3 other lines (I'll call them A, B, and C).

Line A is a 1" line that has 6 branches coming off of it. Each branch must have an output of 2.5 GPM, 55 PSI.

Line B is a 1" line that has 32 branches coming off of it. Each branch must have an output of 0.8 GPM, 55 PSI.

Line C is a 1" line that has 6 branches coming off of it. Each branch must have an output of 0.8 GPM, 55 PSI.

I need to figure out what size pump, pressure, flow rate I will need to operate. Also, a plate and frame heat exchanger size. I'm guessing I have to work backwards on this.

RE: Multiple lines feeds

If you know the pump's suction and discharge head, you can work in the downstream direction.  

If you know the outlet pressure needed at the end of the branch with the most head loss, you work upstream from that point.



http://virtualpipeline.spaces.msn.com

RE: Multiple lines feeds

(OP)
I'm stuck. I'm not sure if I'm doing it the right way.

For example:
Line A: Each branch coming off of it has to have 55 psi and a flow of 2.5 gpm. So I'm thinking that I need a total pressure, at the beginning of Line A, of 55 psi*6=330 psi. The same for the flow... 2.5 gpm*6=15 gpm.

Line B: 55 psi * 32 = 1760 psi (@ beginning of Line B), 0.8 gpm*32=25.6 gpm.

Line C: 55 psi*6=330 psi, 0.8gpm*6=4.8 gpm.

Line 2: Since lines A, B and C are connected to Line 2 (the main line), Line 2 must provide enough pressure (330+1760+330=2420 psi) and flow (15+25.6+4.8=45.4gpm) so each branch can have the desired output, am I right?

Or am I doing this entirely wrong?

RE: Multiple lines feeds

Yes, you're doing this almost entirely "wrong".

Try plotting the Hydraulic Grade Line (HGL) from the discharge points back through the system to the pump or source of flow.  Use the Bernoulli Equation and account for friction losses in the lines using whatever information you have. Those sources may be friction loss tables, the Darcy Weisbach Equation, Manning's Eq., or even the Hazen williams Eq. (if the fluid is water) .

You can also model this in the free program EPANet, or any pipe network tool you may have or can get.

If greater precision is required you can even model it physically at "bench scale" or actual scale.

good luck

RE: Multiple lines feeds

Assuming some pressure drops (dP) between branch outlets (+)
and  55 psig at any outlet. (You must have AT LEAST 55 psig at all outlets, so put 55 psig at the branch with the greatest pressure drop between the inlet and that branch, and you should have something that looks like this.  If you know the pressure at the last branch, >=55 psig start there and add back to the inlet (->).  That will give you the inlet pressure to Line A.

Line A

 Segment  Segment  Segment  Segment Segment Segment
   A1       A2        A3       A4      A5      A6
->=====+=========+========+=======+=======+=======+
Q   15     12.5       10      7.5      5      2.5
                                        outlet 55 psi
dP  20psi   18 psi    15 psi   9       6       3

P 126   106       88      73       64      58       55     
  psig  psig      psig    psig     psig    psig    psig

Line A inlet pressure required is 126 psig.
Do the same for each Line B and C.
Take the highest Pressure at the inlet of any Line A, B, C

Now do the same for the line with A, B and C coming off of it, to get the pressure at the pump.  

http://virtualpipeline.spaces.msn.com

RE: Multiple lines feeds

(OP)
Here is what I've done...hopefully it's a lil more correct. Please let me know. I wasn't so sure about the velocity, so I did it at 2.5 gpm (individual branch) and 15 gpm (all branches=2.5*6=15). The total pressure at the head of this line is 55.84 psi (when you have a drop of 0.04 psi between branches) or roughly 76psi (when Pdrop=1 psi), right?

RE: Multiple lines feeds

Doesn't sound like it.  Please post a diagram of your pipe configuration, include flows, lengths and diameters.  Maybe I still don't understand what your system looks like.

When you go down line A for example, passing each 2.5 gpm outlet, the flow in each segment of line A between outlets drops by 2.5 gpm, right?

Line A

segment1   segment2   segment3  segment4  segment5       
===15===|===12.5===|===10.0===|===7.5====|===5===|===2.5==|
             2.5                   2.5                     2.5              2.5               2.5               2.5
             out                   out                out              out               out               out

If each segment of line A is the same length and diameter, you must have greater head loss in segment 1 than 2,
a greater head loss in segment 2 than 3, 3 than 4 etc.
'cause there's more flow in segment 1 than there is in segment 2, right?

http://virtualpipeline.spaces.msn.com

RE: Multiple lines feeds

(OP)
here is a quick sketch

RE: Multiple lines feeds

Thought so.

Take the end of each branch as a control point where you must have a minimum of 55 psig discharge.

Find the flow losses for each segment, adding the flows in each segment as you go upstream past each outlet cluster.

The flow losses get higher in each 15 ft segment... the flows get bigger.

Sum all the flow losses and add to 55. That's the minumum feed pressure for each branch.

Do for all branches.

The highest feed pressure is what you need to feed any intersection of branches.

Keep going upstream, adding flows and calculating pressure losses.

Take the highest of the minimun feed pressures at any intersection of 2 or more branches.

When you get to the end, you will finally have your feed pressure for the system.

Like this....

http://virtualpipeline.spaces.msn.com

RE: Multiple lines feeds

Once you have that, hold it and go in the downstream direction.

Subtract each flow loss from your system inlet pressure.

At each branch, go down the branch subtracting each flow presure loss.

Outlets will have pressures greater than 55, but that's OK because 55 is the minimum pressure you need.

When you get to the ends of all branches, one will have the 55 psig outlet pressure value.  That's the "system control point".

http://virtualpipeline.spaces.msn.com

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