Tee Junction Flow Distribution: Why Your Flow Split Is Wrong

Engineering context

Wherever a pipe divides or two streams combine, a tee or junction sets how flow and pressure are shared between the legs. The split is not arbitrary: flow distributes so that mass is conserved at the junction and the pressure is balanced across the connected network.

The loss coefficients for the run-through path and the branch path are different, so designating the wrong leg as the branch changes the predicted pressure drop and, in a connected network, the flow split itself.

A wrong branch assignment does not trigger the Unable to Prepare Network error — that message points to a connectivity or node-initialization fault (broken node snaps, post-undo/redo desync, orphaned connections). A wrong branch assignment instead surfaces as “Tee relationship outside the allowable range or Flow through the Tee is unstable”.

Engineering workflow

1. Place the junction element and connect every leg — all three pipes must be joined with no loose ends.
2. Define which pipe is the branch and which legs form the straight run.
3. Confirm the flow regime: dividing (one inlet, two outlets) or combining (two inlets, one outlet).
4. Choose the fitting-resistance method: Idelchik, Miller, Crane, or SAE (SAE for gas).
5. Assign pipe data to each leg. For unequal pipe sizes, connect the true diameters directly to the junction — do not insert reducers to force a size match; FluidFlow accounts for the size transition internally within the tee calculation.
6. Set the network boundary conditions and the fluid.
7. Solve the steady-state network.
8. Review the split, velocities, and design alerts.

How FluidFlow helps

FluidFlow provides tee, wye, and cross junction elements with Idelchik, Miller, Crane, and SAE fitting-resistance methods. The solver balances mass and pressure to return the flow split and per-leg pressure drop.

See how FluidFlow’s Liquid Flow module handles this.

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