branching pipe downstream of pump
branching pipe downstream of pump
(OP)
Hi all,
I am trying to size a pump for my system.
The system branches out downstream of the pump discharge (3 branches) and recombines before discharging into a tank that is open to atmosphere.
There is only one pump in my system, this is not a parallel pump system.
How do I determine the system curve to size my pump ?
Thanks





RE: branching pipe downstream of pump
If the three branches vary --- you need to select the one branch that has the higher friction/head loss and use during calculation to determine system head loss.
Is the flow the same in all three branches??
If not then you need to be able to reduce/ throttle flow thru branch which will add loss coefficients to your branches.
RE: branching pipe downstream of pump
Hi Stymidpiper,
All three braches vary and flow varies as well.
This is an existing system but I need to size a new pump for it.
Thanks
RE: branching pipe downstream of pump
The head loss between the junctions is the same as the head loss in each branch.
The total flow rate is the sum of the flow rates in each branch.
Charlie
www.facsco.com
RE: branching pipe downstream of pump
Engineering is the practice of the art of science - Steve
RE: branching pipe downstream of pump
Hi Guys,
What you are saying is also what I read, but I tought it was only applicable to piping with the exact same caracteristics.
So I will need to throttle to get the correct flow into each branch (compressor)
Thank you all for your help
lha:
this is a real life problem, compressor cooling, not homework, I wish it was only homework !
RE: branching pipe downstream of pump
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: branching pipe downstream of pump
There is a typo since parallel resistors add reciprocally so that 1/Req = 1/R1 + 1/R2 + 1/R3 + ...
RE: branching pipe downstream of pump
Hi BigInch and 25362,
Yes this is exactly what I thought about doing last night.
It's just like a parallel electrical circuit, find Req.
If Voltage is Head and Current is Flow, what is Req ?
Thanks
RE: branching pipe downstream of pump
I suppose R=H/Q
where H = head loss and Q = flow
RE: branching pipe downstream of pump
RE: branching pipe downstream of pump
Ohm's law can be compared to a simplified model of laminar flow. Following the Hagen-Poiseuille equation for laminar flow (aka parabolic flow) of Newtonian non-compressible fluids, in pipes of constant cross section (radius =r, length=L), constant viscosity η, the resistance R, in the formula R.Q = ΔP (similar to Ohm's law) would be:
In turbulent flow, ΔP = ƒ(Q), which is not necessarily a linear relationship. For example, when the
Reynolds number for liquids is in the range 30,000-100,000:,
for gases with Reynolds number in the range 100,000 to 500,000:
where ρ is the density. Same formulas can be used for ducts and channels of other cross sections if the diameter D is replaced by the hydraulic diameter Dh.
RE: branching pipe downstream of pump
To be precise, the true water <=> electricity analogy is that head loss corresponds to impedence, since both head loss and impedence are nonlinearly proportional to flow, ie. K * Q^n = Hl, whereas electrical resistance is linear at R * I = E
You can write a system of equations to solve for heads and flows, similar to how Spice solves for electric currents, however, as electrical resistance equations are linearly proportional to current and their simultaneous equations can be solved using linear algebra, but head loss equations are non-linear and the corresponding system of equations must be solved using non-some linear algebraic technique. The non-linear equations are replaced by a linear system, solved and the errors evaluated. Corrections are determined and reapplied to the original linear system and the iteration process continues until (hopefully) it converges to a solution. The Hardy-Cross follows the same principles using a series of step by step iterations, rather than a "simultaneous" solution.
It is possible to program the such a system of equations to approximate a small network (say less than 10 pipes and 2 or 3 loops), even including simplified valves and pumps or compressors, using only Excel.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: branching pipe downstream of pump
RE: branching pipe downstream of pump
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: branching pipe downstream of pump
RE: branching pipe downstream of pump
Not done! The OP is trying to find the system curve. Flowrates depend on the differential head applied... its unknown. He needs to determine the pressure drop through the 3 pipes at any (or all interesting) flowrates first (determine the system curve), therefore he doesn't yet know the required pumping head ...at any flowrate. That's why the equivalent pipe, or the H-C or some other method must be used.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: branching pipe downstream of pump
Rather than solving equations, I tend to plot graphs of the three systems, and add the flow rates together. It's a nice visual way of solving the problem.
RE: branching pipe downstream of pump
This is typical, among others, of cooling water and hot oil systems.