Boilers, Pressure Vessels, and Heat Exchangers
Know the hardware
Boilers. Firetube boilers (HRT, firebox, scotch-marine) pass hot gases through tubes surrounded by water; watertube boilers pass water through tubes surrounded by hot gas and are used for higher pressure and capacity. Learn the pressure parts - shell, drums, tubes, tubesheets, heads (flanged-and-dished, ellipsoidal, hemispherical), nozzles, manways and handholes, stays and staybolts - and the heat-trapping components such as baffles, superheater, economizer, and air preheater.
Heat exchangers. Shell-and-tube units use segmental baffles to force shell-side fluid across the tubes; baffle spacing and cut affect velocity and pressure drop. Counterflow gives the best temperature driving force; parallel flow gives a lower, more uniform tube-wall temperature. Tube-to-tubesheet joints are rolled (expanded), welded, or seal-welded. Damaged tubes are plugged and counted - plugging reduces surface area and performance.
Pressure vessels. Recognize heads, nozzles, reinforcement pads, and inspection openings, and know that a hydrostatic test is typically run at 1.5 × MAWP (adjusted for temperature per code) with the vessel properly vented and the test water temperature above the brittle-fracture minimum.
Traps. Confusing firetube and watertube construction; confusing a safety valve (opens and recloses automatically) with a relief valve; assuming a safety valve's set pressure equals operating pressure - it is set above it; ignoring that blowdown, water level, and steam separation problems show up as carryover, not just low water.
Worked example / rule of thumb. One boiler horsepower ≈ 34.5 lb of steam per hour at 212°F, roughly 33,475 Btu/hr. So a 500 hp boiler at rated capacity makes about 17,250 lb/hr of steam - useful for checking a specification against a stated steam rate.
Safety note. Never work on a boiler or exchanger until it is isolated, drained, vented, and proven at zero energy; trapped pressure and hot condensate are the classic injuries.