LOGOS Learning · 9
Preferential flow in pump networks: when one branch runs dry
Preferential flow is when the easiest branch takes the flow and a higher branch loses its available pressure: branch B gets water only while H_node > z_top,B. Opening the low branch raises total flow, so H_node = H_pump(Q_total) - hf_suc - hf_main falls.
Open the valve on branch A, which discharges low. Water prefers the easy path, the node energy drops and branch B, up high, runs out of pressure to receive water.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. Partly closing the low branch, the high branch…
2. A design fix for the dry branch is…
3. In a network with branches, water prefers the path…
Why this happens
Arrows compare with valve A closed.
Continuity at the node
All the water that reaches the node leaves through the branches. With B dry, the whole main line goes to A.
Condition for branch B to get water
B only gets water if the node energy exceeds the elevation of the top of its pipe. Below that there is no head to climb up there: flow is zero, even with B’s valve fully open.
Why the node loses head
Opening A raises total flow. The pump moves down its own curve and delivers less head, and the main line loses more. Both pull Hnode down.
How to fix it in design
Throttle A (valve or balancing orifice), enlarge the main line or pick a pump with more head. The LOGOS multi-branch calculator flags the branch as unreachable and can balance the branches.
Teaching pump with an illustrative curve.
Formulas in plain text
- Continuity at the node
Q_total = Q_A + Q_B- Q_total = main-line flow (m3/h) · Q_A = flow in the low branch (m3/h) · Q_B = flow in the high branch (m3/h)
- Condition for the high branch to get water
Q_B > 0 <=> H_node > z_top,B ; H_node = z_top,B + hf,B(Q_B)- H_node = head at the node (m) · z_top,B = elevation of the top of branch B pipe (m) · hf,B = head loss in branch B (m)
- Why the node loses head
H_node = H_pump(Q_total) - hf_suc - hf_main- H_pump = pump head at the total flow (m) · hf_suc = suction loss (m) · hf_main = main-line loss (m)
Frequently asked questions
Why does one branch of my pump network get no flow?
Because the head at the node is lower than the elevation the branch must reach. Another branch, lower or with less resistance, takes most of the flow, the pump drops down its curve and there is no head left to lift water to the high branch.
Is a dry branch a sign that the pump failed?
Not necessarily. The pump can be working exactly on its curve; what is missing is available pressure at that point of the network, caused by the flow split.
How do you fix preferential flow?
Throttle the easy branch with a valve or a balancing orifice, enlarge the main line to cut its losses, or choose a pump with more head. Any of these keeps the node head above the top of the high branch.