Question 19 1AE01 - First Assistant Engineer

The Correct Answer is C **Why Option C is Correct:** A "waterlogged" hydro-pneumatic header tank is one that has lost its necessary volume of compressed air (the air cushion) and is nearly completely filled with water. The purpose of the air cushion is to act as a spring, storing hydraulic energy and dampening pressure fluctuations, thereby providing a significant buffer between the pump's cut-in (start) and cut-out (stop) pressures. When the tank is waterlogged, the volume of air is minimal. Water is nearly incompressible. As the pump runs, the system pressure rises almost instantly because there is no air cushion to compress. Once the pressure reaches the cut-out point, the pump shuts off. However, even a small demand (e.g., opening a faucet) causes the pressure to drop almost instantly because there is no stored energy (no air cushion) to absorb the small loss of water volume. The pressure rapidly falls to the cut-in point, and the pump immediately restarts. This results in **very short and frequent running and idle periods** for the potable water pump, often referred to as "short cycling." **Why Other Options Are Incorrect:** * **A) Failure of the potable water pump to stop with zero demand on the system:** The pump stopping mechanism (the pressure switch) is designed to cut power when the upper pressure limit is reached. A waterlogged tank accelerates the achievement of the cut-out pressure; it does not prevent the switch from activating. This failure mode is typically associated with a faulty pressure switch or a stuck check valve/relief valve, not a waterlogged tank itself. * **B) Failure of the potable water pump to start with demand on the system:** A waterlogged tank rapidly drops pressure when demand occurs, which is exactly what triggers the pressure switch to start the pump (cut-in). Failure to start is usually caused by electrical issues, a completely broken pump, or a faulty pressure switch failing to make contact, not tank waterlogging. * **D) Extremely long running and idle periods for the potable water pump:** Long running periods typically occur when there is a major leak in the system, forcing the pump to try and maintain pressure against continuous flow, or if the tank size is correctly matched and the demand is low and sustained. A waterlogged tank minimizes the stored volume, leading to the opposite effect: short, rapid cycles, not long cycles.