RRB ALP Plumber Trade — Solved Practice Question Bank

Q: Which pipe material is suited for hot water lines — standard PVC or CPVC? Sol: CPVC (Chlorinated PVC) — it is specifically rated for hot water service, unlike standard PVC which is not designed for sustained hot water exposure. Q: Why must drainage pipes maintain a specific gravity slope, unlike pressurized water supply pipes? Sol: Drainage relies on gravity alone to move waste — an inadequate slope causes waste to move too slowly (risk of blockage) or too fast (risk of solids separating from liquid and settling); supply pipes are pressurized and don't depend on slope for flow. Q: What is the purpose of a backflow prevention device in a water supply system? Sol: To prevent contaminated water from flowing backward into the clean potable water supply, protecting public health.

Q: What is the function of a P-trap beneath a sink? Sol: It retains a small amount of water forming a seal that blocks sewer gases from entering the building through the drain opening. Q: Why should a gate valve not be used for throttling/partial flow control? Sol: Partial opening of a gate valve causes high-velocity flow across a small gap, eroding/damaging the gate and seat over time — gate valves are designed for fully open or fully closed use only; globe valves are the correct choice for throttling. Q: Why must a centrifugal pump be primed before starting? Sol: It cannot draw water on its own without liquid already present in the casing/suction pipe — priming ensures liquid is present to generate the centrifugal force needed to create flow and suction.