Question 32 1AE01 - First Assistant Engineer

The Correct Answer is A. **Why option A is correct:** Option A states that the pump suction should "be arranged to develop a maximum vacuum of approximately 10" of mercury". This value (or a similarly low value, typically 5 to 10 inches of mercury, or 2 to 5 PSI vacuum) is considered the maximum safe vacuum limit for the inlet of most industrial hydraulic pumps, especially piston and gear pumps. 1. **Preventing Cavitation:** The primary concern in hydraulic pump suction is preventing cavitation. Cavitation occurs when the local pressure drops below the vapor pressure of the fluid, causing vapor bubbles to form and then violently collapse. This collapse erodes pump components (impellers, bearings, gear teeth) and significantly reduces pump efficiency, flow, and lifespan. By limiting the vacuum development to $10^{\text{"}}$ Hg (which corresponds to a relatively high absolute pressure), the system ensures the fluid remains well above its vapor pressure, thereby preventing cavitation. 2. **Maintaining Inlet Pressure (NPSH):** Hydraulic systems require a positive net positive suction head (NPSH) to operate efficiently. A higher vacuum indicates a lower absolute pressure at the inlet. Limiting the vacuum ensures that the fluid can be pushed into the pump by atmospheric pressure (or system pressure) quickly enough to keep the pump fed, maintaining adequate flow. **Why the other options are incorrect:** **B) be arranged to develop the theoretically maximum attainable vacuum:** The theoretically maximum attainable vacuum at sea level is approximately $29.92^{\text{"}}$ Hg (a perfect vacuum). Operating a hydraulic pump near this maximum vacuum would mean the pressure is extremely close to zero absolute pressure (or even below the vapor pressure of the fluid), guaranteeing severe cavitation, rapid pump damage, and failure to maintain adequate flow. **C) be provided with three to five 1/2 inch holes in the vertical, suction line to prevent pump starvation should the strainer become fouled:** Intentionally introducing holes into the suction line is fundamentally incorrect and dangerous for a hydraulic system. 1. **Air Ingestion:** Suction lines operate under vacuum, meaning holes would cause the pump to ingest air (or gas). Air in the hydraulic fluid leads to aeration, which causes sponginess, erratic operation, noise, heat buildup, and often leads to the same issues as cavitation (vaporization/implosion of air bubbles), drastically compromising flow and system performance. 2. **Bypassing Safety:** The strainer/filter is a critical component. If it becomes fouled, the proper procedure is maintenance (cleaning or replacement), not bypassing it with holes, which would allow contaminants to directly enter and damage the pump and sensitive downstream components (valves, actuators). **D) be taken directly off the reservoir bottom without regard to filters or strainers:** Taking suction directly off the bottom of the reservoir is poor design practice because contaminants (dirt, debris, sludge, wear particles) naturally settle there. Furthermore, operating "without regard to filters or strainers" is unacceptable in any modern hydraulic system. Filters and strainers are essential for removing particles that cause wear, ensuring pump longevity, and maintaining adequate fluid quality and system performance.