A duplex double-acting reciprocating feed water pump has how many steam pistons and what is their relationship to the water piston?

• A. One Piston
• B. Two Water Pistons Equal in Size to Steam Pistons
• C. Two Steam Pistons Larger in Area than the Water Piston
• D. Three Pistons

Answer: (C)

In a duplex double-acting reciprocating feed water pump, two steam pistons drive a single water piston. To push water into a boiler that is under pressure, the steam side must generate enough force. Having the two steam pistons larger in cross-sectional area than the water piston ensures that, for the same steam pressure, the hydraulic force delivered to the water side is sufficient to overcome boiler pressure and maintain a steady flow. The larger steam-area pistons also help provide smoother, more continuous pumping by alternating strokes, while keeping the water piston compact for efficient displacement. So the arrangement of two steam pistons larger in area than the water piston best fits how this pump is designed to operate.