Question 67

The Correct Answer is D **Explanation for Option D (Correct Answer):** P-traps (or S-traps, though P-traps are generally preferred and more effective in modern plumbing) are U-shaped or S-shaped sections of piping installed directly beneath a drain fixture like a sink in a ship's head (bathroom). The primary function of this configuration is to retain a small amount of water within the bend after the fixture is used. This retained water forms a crucial barrier, known as a water seal, which prevents foul-smelling and potentially dangerous **sewer gases** (which originate from the sewage holding tank or discharge system) from rising back up the drainpipe and entering the habitable compartment. **Explanation for Other Options (Incorrect):** * **A) to reduce water velocity and minimize erosion:** While the trap might slightly impede flow, its design is not primarily aimed at velocity reduction or preventing erosion. Erosion is a concern for long, straight pipes under high-velocity flow, not short trap sections. * **B) to create a strong siphon effect:** A siphon is created when the outlet of a tube is lower than the input, causing continuous flow once initiated. Traps are designed to *prevent* a full siphon effect (which would empty the water seal) and are usually vented (in non-marine applications) to maintain the seal. Their purpose is retention, not continuous flow generation. * **C) to provide a cushion of water to reduce the effects of water hammer:** Water hammer (a shockwave caused by sudden valve closure) is typically mitigated by air chambers or dedicated arrestors near supply lines, not by the small amount of standing water in a drain trap. Drain traps handle waste outflow, not pressurized water inflow.