Because they function at very low strain and count on gravity to maneuver fluids (and entrained solids), drain-squander-vent methods use fittings whose interior surfaces are as easy as feasible. The fittings could be "belled" (expanded slightly in diameter) or or else shaped to accommodate the insertion of pipe or tubing http://valves-and-fittings46788.fireblogz.com/17399496/valves-and-fittings-options