A sloped drain branch, a drainage stack, vent piping and a fixture trap form this lab. Choose the branch diameter, slope, connected fixture load and whether the vent is connected, then watch how deep the water runs, how fast it moves and how much pressure the trap seal must resist.
• A real-time 3D view with numbered, clickable parts: sloped drain branch; drainage stack and building drain; vent piping and roof termination; fixture and trap seal; building condition readout. Scene tools include home view, focus-selected-part, auto-rotate, expand and show/hide labels, and drag-to-orbit with pinch-to-zoom. • Experiment controls: branch diameter (50–150 mm); branch slope (0.5–4 %); connected fixture load (1–40 DFU); and a vent-connected toggle, plus pause/resume, single-step buttons (0.1 s and 1 s), a playback-speed selector and a restart experiment action. • Live readouts: design discharge; full-pipe gravity capacity; depth of flow; flow velocity; air-pressure swing at the trap; trap-seal pressure margin. • A Curves & measurements tab with two live charts, the model equations and snapshot readouts; an Experiments tab with 3 guided presets (well-designed branch; unvented heavy load; undersized flat branch) plus a model-verification bench, event log and copyable trial report. • A Learn & assess tab with 3 lessons, a 2-question knowledge check and a written model-scope statement.
Drain branches run partly full. The Manning equation, Q = (1/n) A R^(2/3) S^(1/2), links discharge to the wetted area, hydraulic radius and slope. For the chosen load the simulator solves for the depth that carries it, then reports the depth as a percentage of the diameter and the resulting velocity.
Steeper slope and larger diameter both lower the depth for the same load, while a flat or small branch fills up. If the load exceeds what the pipe can carry by gravity, the branch is shown as surcharged.
Draining water drags air with it. With a vent connected, air follows the water and the pressure at the trap stays small. Without it, the pressure swing grows with the load until it exceeds the 50 mm trap-seal head of about 490 Pa and the seal is at risk.
The flow and air-swing relations are simple teaching models, not code sizing tables.
The piping that carries wastewater out of a building by gravity, together with vent piping that admits air so water flows freely and trap seals are protected.
Gravity moves the water, so the branch must fall along its length. Too little slope leaves the flow slow and solids behind.
It lets air replace draining water, which keeps the pressure swing at the trap small compared with the seal head.
No. The relations are teaching parameters, so use the adopted plumbing code and local rules for real design.