Part 2 of 4: Managing Drum Boiler Blowdown
Challenge: Monitor Blowdown for Maximum Economy
An integral part of boiler operation, blowdown eliminates precipitated sludge and dissolved solids from boiler water before they can reach damaging levels. However, blowdown also results in a loss of heated water and costly treatment chemicals. Economical operation requires careful control of blowdown to maintain safe solids levels while minimizing heat and chemical additive loss.
In continuous blowdown, the rate of solids accumulation in the boiler determines the blowdown rate. As the rate of solids concentration changes, the blowdown rate must also change.
For boilers using intermittent blowdown, solids accumulate to a specific level followed by the removal of a calculated amount of water. Managing blowdown balances sufficient solids removal with avoidance of excessive heat, chemical and water loss. Reducing the need for treated makeup water results in water savings, pretreatment cost savings, along with energy, chemical and wastewater savings. Once reaching the high levels of solids defined by the boiler manufacturer, EPRI and/or ASME, part of the water is discharged (blown down) to waste and replaced with makeup water. Exceeding recommended limitations on solids could seriously impair the integrity of the boiler.
To control blowdown properly, continuous monitoring of control parameters such as silica, sodium, phosphate, pH and conductivity is needed to indicate the effectiveness of the water chemistry program in the boiler. Monitoring of boiler water and automation of the boiler blowdown process through online instrumentation allows plant chemists to see the rate of accumulation of solids and thereby quickly assess the value of any incremental chemistry program changes. At the same time, monitoring minimizes the potential impact of scaling and corrosion to boiler tubes that result in lost efficiency (heat transfer) or potentially localized hot spots.
Continuous on-line monitoring and control of boiler blowdown reduces large swings in boiler chemistries. Reliance on lab testing alone may lead to incorrect or delayed responses to excursions and an inability to track trends such as rates of concentration accurately. As a result, over-corrections or under-corrections of the blowdown rate occur, potentially resulting in excessive solids concentration in the boiler—not enough blowdown—or excessive loss of water/chemicals/heat—too much blowdown. This holds especially true in industrial steam cycles, where percent condensate return rate and quality may fluctuate drastically. Monitoring of a single parameter, such as conductivity, indicates solids accumulation. Monitoring of multiple parameters allows characterization of the solids. The table below outlines key parameters to manage in the blowdown process.