Feedwater Systems, Water Treatment and Blowdown
Feedwater equipment
A deaerator removes oxygen and heats feedwater; feed pumps normally include an automatic standby that starts on low level or pump failure. Feedwater regulators may be single element (level only) or three element (level plus steam flow plus water flow), which responds better to swinging loads.
Chemistry
Scale insulates tubes and causes overheating; oxygen pits metal; high dissolved solids and alkalinity cause priming, foaming and carryover into the steam header. External softening removes hardness; internal treatment adds oxygen scavenger, phosphate or polymer for residual hardness, and amines to protect condensate returns. Monitor pH, conductivity, hardness, iron and silica against the program limits.
Blowdown
Bottom blowdown removes sludge from low points with short, frequent bursts, done at light load with the level at least normal. In a two-valve arrangement the valve nearest the boiler is opened fully first and closed last, with the outer valve used to throttle. Surface or continuous blowdown controls dissolved solids and is adjusted from conductivity readings. Blowdown discharges to a blowdown tank where pressure and temperature are reduced before drain.
Traps and rule of thumb
Wrong answers include long continuous bottom blowdowns, blowing down at low water, and using a globe valve as the boiler-side blowdown valve. Rule of thumb: bottom blowdown is for sludge (short bursts), surface blowdown is for chemistry (throttled continuously).