A drainage stack, a roof vent open to atmosphere, a fixture branch with a seal reference and an air-admission path form this lab. Choose the roof-vent opening, the air displacement during a drain pulse and a comparison trap seal head, then watch suction, air admitted and trap margin change during each pulse.
• A real-time 3D view with numbered, clickable parts: drainage stack; roof vent to atmosphere; fixture branch and seal reference; air-admission path; air compliance / pressure station. 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: roof-vent conductance fraction (0–1); air displacement during drain pulse (0.2–2 L/s); comparison trap head (20–80 mm water), plus a show flow/process markers toggle, pause/resume, single-step buttons (0.1 s and 1 s), a playback-speed selector and a restart experiment action. • Live readouts: drain-line suction magnitude; air admitted through vent/leak; active air displacement; trap-head capacity; remaining pressure margin; physical pulse-cycle time. A model response curve is drawn beside the 3D view and updates as you change controls. • A Curves & measurements tab with two live charts, the model equations as written in the simulator and snapshot readouts; an Experiments tab with 2 guided presets (clear vent; obstructed roof opening) plus a model-verification bench, timestamped event log and copyable trial report. • A Learn & assess tab with 3 lessons (follow the system; connect the measurements; interpret the model), a 2-question knowledge check with reset, and a written model-scope statement.
The model treats the drain line as an air compliance. During a pulse, flowing water displaces air at a chosen rate, which pulls pressure negative; vent and leakage conductance admit air and restore pressure. The balance is Cair dP/dt = −Qdisplaced − Gair P with Cair = 0.002 L/Pa and Gair = 0.0002 + 0.008 × opening in L/(s·Pa).
During a constant pulse the suction tends toward −Q/G with time constant C/G, so a clear vent reaches a small steady suction and recovers quickly, while a restricted vent allows much larger suction and slower recovery.
The trap-head capacity for the chosen seal depth is compared with the suction magnitude, and the remaining margin is shown. In the obstructed-roof-opening experiment the suction can exceed a 50 mm seal reference before the pulse ends, which is how venting protects trap seals.
It is an illustrative lumped air-compliance model, not drainage-code sizing or multiphase CFD. A negative margin warns that the model would need seal-breach feedback rather than continuing to assert a safe seal. One animation second represents two physical seconds.
Flowing water displaces or entrains air. Without vent air admission the pressure would swing enough to pull or push water out of trap seals.
Less air can enter, so suction during a drain pulse grows and recovery takes longer. The trap-head margin can go negative.
It follows a first-order air-compliance equation where displaced air lowers the pressure and vent conductance lets it recover, approaching −Q/G during a constant pulse.
No. The compliance and conductance values are teaching parameters, so use the adopted plumbing code for sizing.