A.when the boiler has been secured long enough for most solids to settle
B.when the boiler has been cooled to ambient temperature
C.only after raising the water level to within 0~5 inch of the high water cutout only
D.after bypassing the low pressure

A.completely drain the boiler in an emergency
B.prevent sludge from forming in the drum
C.remove floating impurities from the boiler water surface
D.remove settled solids from the water drum