LOGOS Learning · 21
How to estimate the flow of a new branch on an existing network, step by step
The flow a new branch can draw is the one whose losses use up the head left at the tap: H = P_tap - dz - P_req - h_eq, and v = sqrt(2 * g * H / (f * L / D + sumK)). For a 120 m, 1-1/2" branch with 45 m logged minimum and a 10 % drop margin, H = 19.5 m and the flow converges to 10.5 m3/h.
The network already exists and a new consumer will be tied into it. What is known is the pressure at the tap point, logged over a day. Subtract the elevation, the pressure the consumer requires and the loss across the equipment; what is left becomes branch loss, and the flow is the one that uses up exactly that remainder. A 120 m, 1-1/2" branch will leave an industrial water header to a consumer 4 m higher that needs 15 m of head at its inlet and has a 2 m filter. How much can it draw from the network?
- Step 1 of 10
Read the pressure log
The 24 h log at the tap point gives two numbers. The maximum happens at the network's low-demand valley, the minimum at the peak. The peak is the worst moment for the new branch; the valley is when branch velocity is highest.
Pmáx = 62 mca · Pmín = 45 mca
Formulas in plain text
- Design pressure at the tap
P_design = P_min * (1 - margin)- P_min = minimum logged pressure, at peak demand (m, gauge) · margin = expected drop when the new consumer opens (-)
- Head available for branch losses
H = P_tap - dz - P_req - h_eq- P_tap = gauge pressure at the tap (m) · dz = elevation from tap to consumer (m), positive upward · P_req = pressure required at the consumer inlet (m) · h_eq = loss across the consumer equipment (m)
- Velocity from the branch balance
v = sqrt(2 * g * H / (f * L / D + sumK)), Q = (pi * D^2 / 4) * v- f = Darcy friction factor (-) · L = branch length (m) · D = internal diameter (m) · sumK = sum of fitting K, including the branch tee (-)
- Swamee-Jain friction factor (iterated)
f = 0.25 / (log10(eps / (3.7 * D) + 5.74 / Re^0.9))^2, Re = rho * v * D / mu- eps = absolute roughness (m) · start with f = 0.02 and repeat until f stops changing
Frequently asked questions
Why use a 24-hour pressure log instead of one gauge reading?
Tap pressure changes with everyone else's consumption. The logged minimum is the demand peak, the worst moment for the new branch; the logged maximum is the demand valley, when branch velocity is highest.
What is the drop margin for?
The logged minimum is from before the tie-in. When the new consumer opens, network flow rises and tap pressure falls further; the margin estimates that drop. If the network flow is known, a source curve can replace it.
Is the calculated flow what the consumer will receive?
It is the potential flow with the branch valve fully open. If it exceeds the desired flow, the valve throttles the difference; if it falls short, the branch needs a larger diameter or fewer losses.