Prior to it entering the boiler, the plant manager has the opportunity to control both the boiler feed water temperature as well as its quality. This involves introducing treatment and heating that will provide optimal conditions for a long boiler life.
An “Improperly Heated Feedwater” condition occurs when the feedwater coming to the boiler is not adequately heated. Improperly heating feed water results in lower boiler efficiency, incomplete mechanical deaeration, and can cause cracking and catastrophic failure to the feedwater lines, boiler shell, and boiler tubes through thermal shocking.
Low temperatures can affect the boiler feed water quality as well, as it will result in higher levels of dissolved gases, oxygen and carbon dioxide, that will corrode the internal metal surfaces the water comes in contact with.
Thermal shock also occurs any time there is a sudden change in temperature of a metal or uneven temperatures throughout the boiler. Metal expands and contracts in relation to temperature and temperature fluctuations may result in cracking at local sites of temperature swings, boiler tube leaking, cracking of the tube sheets, or catastrophic boiler failure.
Pre-heating the feed water to achieve an ideal boiler feed water temperature will allow the boiler to run most efficiently as the heat input required to raise the temperature of the water up is reduced. The difference between 140°F and 180°F in boiler feed water temperature equates to about a 3-4% difference in boiler efficiency. Feed water preheating typically occurs in either the feedwater tank or deaerator using supplementary steam.
Pre-heating the feedwater is the most basic method of mechanical deaeration or the process of removing dissolved oxygen from water. According to the National Board, 70°F water at 0 psig contains approximately 8.6 ppm, 150°F water at 0 psig contains approximately 4.3 ppm, and 212°F water at 0 psig contains approximately 0.0 ppm.
To avoid the “Improperly Heated Feed Water” condition described above, the boiler feedwater should be heated to at least 180 °F when using a feedwater tank and 227 °F when using a deaerator. This will minimize the chemical treatment required, provide adequate mechanical deaeration, and greatly reduce the thermal shock experienced on the internal components of the boiler.
Although temperature plays a vital role in boiler feedwater quality, it’s important to remember to maintain your boiler feedwater’s chemistry at acceptable levels. Untreated water is full of minerals, gases and particulates that can cause critical damage to the health of both firetube and watertube boilers. Unless your boiler receives water of proper quality, the boiler’s life will be needlessly shortened.
The removal or otherwise ‘treatment’ of each of these is critical to efficient boiler operation for different reasons. Each water supply source requires an independent analysis. Depending upon this analysis, various pretreatment methods may be employed to prepare makeup water for your boiler feedwater system.
Maintaining appropriate feedwater temperature and boiler feed water quality will help to ensure the boiler remains safe, operational, and failure-free for many years of service.