Boiler Auxiliaries, Feedwater Systems, and Piping
Domain focus
Questions cover feedwater pumps, draft fans, air preheaters, economizers, deaerators, piping, valves, supports, and water hammer. The exam tests how auxiliary capacity and piping layout affect boiler reliability and safety.
Core rules
- Feedwater must pass through a deaerator or equivalent oxygen removal before the economizer; oxygen corrosion is a leading cause of tube failure.
- Pump curves define head, flow, efficiency, and NPSH required. Available NPSH must exceed required NPSH with margin to avoid cavitation.
- Check valves prevent reverse flow and protect pumps from backspin; isolation valves allow maintenance.
- Piping needs expansion allowance, anchors, guides, and drains; steam lines must be drained and warmed slowly to avoid water hammer.
- Air preheaters and economizers recover heat but can suffer cold-end corrosion if metal temperature falls below acid dew point.
Worked example
A feedwater pump moves 500 gpm against 2,000 ft of head at specific gravity 1.0 and 75% efficiency. Pump power = (gpm × head × SG)/(3,960 × efficiency) = (500 × 2,000 × 1.0)/(3,960 × 0.75) = 336 hp. The reasoning: 3,960 converts gpm-ft to horsepower; dividing by efficiency accounts for hydraulic and mechanical losses. A 350 hp driver is marginal; select a driver with margin and verify NPSH.
Traps
Using total dynamic head as suction head; ignoring NPSH margin; deadheading a centrifugal pump; omitting check valves; and warming steam lines too fast. Water hammer is a piping and procedure failure, not a normal event.