Tee Junction

Definition

A tee has two distinct paths:

• The run — the straight-through path between the two aligned legs.
• The branch — the leg taken off at an angle (commonly 90° for a standard tee, or at a specified branch angle for a wye).

The loss coefficient K for each path is different, and the correct K depends on which leg is designated as the branch. Assigning the wrong leg as the branch applies the wrong K-factors to both the run and the branch paths, changing the predicted pressure drop and — in a connected network — the flow split itself.

The tee loss also differs depending on whether flow is dividing (one inlet, two outlets) or combining (two inlets, one outlet). The same physical fitting can have different loss coefficients in each direction.

Engineering context

In a connected pipe network, the flow split at a tee is not set by the geometry of the fitting. It is determined by the pressure balance across the entire network: flow distributes so that the pressure at the junction is equal for every connected leg. Two outlets that look physically identical will not carry equal flows if their downstream resistances differ.

Four fitting-resistance methods are available in FluidFlow:

• Idelchik — recommended default for most liquid systems. Accommodates unequal pipe sizes and accounts for kinetic energy effects.
• Miller — similar scope to Idelchik; useful for independent cross-validation.
• Crane — simplified industry standard. Assumes equal-sized connections; appropriate for quick estimates or equal-diameter tees.
• SAE — derived from air-flow test data; well-suited to air and gas networks.

For sensitive designs, comparing at least two methods is good practice. For unequal-size tees, use Idelchik or Miller. For gas networks, consider SAE.

The most common cause of the Unable to Prepare Network error in FluidFlow is incorrect branch-pipe assignment or a leg left unconnected. See Tee Junction Flow Distribution: Why Your Flow Split Is Wrong for the setup sequence and the troubleshooting steps.

Related definitions

Branch angle · Cross junction · K-factor (fitting resistance) · Flow regime · Darcy-Weisbach equation

See it in FluidFlow

FluidFlow provides tee, wye, and cross junction elements. The branch leg is designated in the input editor (shown with a red indicator at the node). The solver enforces mass balance at the junction and pressure balance across all legs simultaneously, returning the flow split and per-leg pressure drop. Do not add reducers at the tee node to match pipe sizes — FluidFlow handles diameter transitions internally.