Part 1 of 4: Carryover of Impurities
Protecting the boiler, the most important component of a power plant, presents unique challenges. The quality of the water moving through the boiler is key to both performance and overall boiler life. This series of articles discusses challenges specifically related to water quality in boiler applications:
- Carryover of impurities
- Management of blowdown
- Cooling water in-leakage to condensate
- Water Conditions: Raw Water and Process Water
Challenge: Carryover of Impurities
Impurities in the feedwater system transport to the boiler where they deposit on surfaces. This results in heat transfer loss, reduced efficiency, and an increased risk of boiler tube failure. From the boiler, impurities carry over into steam causing scaling and turbine blade corrosion. Deposits on the turbines reduce efficiency, directly reducing the plant’s power output and resulting revenue.
Potential sources of impurities can include:
- Return condensate
- Makeup water
- Chemical cleaning
- Water treatment chemicals
- Condensate polishers
- Condenser leaks
- Glycol (antifreeze) from plant equipment
- Paints and solvents
- Air in-leakage
- Corrosion products
- Combustion products
Measurements to fight corrosion and minimize carryover include close control of pH and oxygen levels. Silica must be held to exceptionally low levels in high-pressure boilers to prevent carryover into steam and deposition on turbine blades. In addition, ammonia or neutralizing amines may be applied to neutralize CO2, an acidic byproduct that promotes corrosion. Filming amines are often used to form a protective coating on steam system metal surfaces.
Online monitoring of key parameters such as pH, conductivity, Dissolved Oxygen (DO), silica, sodium, and, if applicable, phosphate and hydrazine help maintain optimum boiler chemistry and minimize solids carryover. Refer to the table below for details.
||Maintain pH to reduce corrosion
||8362sc High Purity pH/ORP System with sc200 or sc1000 controller
(Specific and Cation)
|Detect presence of dissolved solids.
||sc200 or sc1000 controllers with GLI D3422A2 high purity conductivity sensors
||Control DO levels to reduce corrosion due to oxidation of metal surfaces.
||K1100 Dissolved Oxygen Sensor
||Confirm target levels; may also be used to detect carryover in the system.
||Series 5000 Phosphate Analyzer (low or high range)
||Identify earliest signs of anion breakthrough in demineralizers and polishers. Monitor in steam to prevent carryover to turbines.
||Series 5000 Silica Analyzer (with sample heater for faster response time)
|DR 5000 or DR 3900, Spectrophotometers with pour through cells for high-throughput testing
||Identify earliest signs of cation breakthrough in demineralizers and polishers. Sodium in steam causes pitting corrosion on turbine blades.
||9240/9245 Sodium Analyzers
||2100Q Portable Turbidimeter
2100AN Laboratory Turbidimeter
1720E Process Turbidimeter.