Traps, Interceptors and Special Wastes
The trap seal
The trap holds a water seal - commonly 2 to 4 inches - that blocks sewer gas. Everything in this domain protects that seal or protects the sewer from what the fixture discharges.
What loses a seal
- Siphonage (inadequate venting or an oversized trap arm for the drain)
- Back pressure (a blocked stack or surging flow)
- Evaporation (unused floor drains - use a trap primer)
- Capillary action (a rag or string draping over the weir)
- Diffusion (long-idle traps)
Prohibited arrangements
S-traps, drum traps, bell traps, crown vents (a vent taken within roughly two pipe diameters of the trap weir), double trapping of one fixture, and two fixtures sharing one trap except where the code expressly allows a continuous waste.
Interceptors
Grease interceptors for food service, oil/sand interceptors for garages, lint traps for laundries, and neutralizing or dilution basins for corrosive wastes. Sizing is based on flow rate and retention time; the interceptors must be accessible for cleaning, and their vents must be independent of the sanitary system in many designs.
Worked example
A 2-inch trap arm is limited (commonly to about 8 ft in the model code tables; 1½ in ≈ 6 ft, 1¼ in ≈ 5 ft). If your run measures 10 ft with two 90° fittings, relocate the vent rather than assume "2 inch pipe is big enough."
Traps the exam sets
Confusing trap-arm length with developed length; installing a grease trap without a flow-control fitting; and forgetting that special wastes may not discharge to the sanitary system at all.