Many times steam is thought of as a very pure vapor containing little, if any, contaminants. Unfortunately, this usually is not the case. Frequently, boiler water dissolved solids and even suspended solids are carried over in the steam. These contaminants can cause post-boiler problems by leaving deposits or causing corrosion in critical areas of the steam system. Examples would be turbines, superheaters, valves, steam headers, and process equipment. If the impurity level is significant, the deposits that are formed can result in loss of efficiencies or severe equipment damage. Poor quality steam can also affect process and product quality.
The general term used to describe this entrainment of impurities in stream is “carryover”. There are various causes of carryover but they can be classified into two major categories, chemical or mechanical. These two causes of carryover can be controlled through proper monitoring and operation of boiler water chemistry and steam generating equipment.
Modern high efficiency steam generators are built with highly efficient steam separation equipment and usually deliver steam purity of exacting standards. The equipment is necessary to prevent damage to modern high speed turbines and other ancillary equipment. However, even with this highly efficient separation equipment, impurities cannot be completely eliminated. Even the best designs of boilers and separation equipment still deliver about 20 to 50 ppb (parts per billion) of impurities under normal operating conditions. However, as discussed earlier, mechanical and chemical problems may result in much higher levels of impurities