MONITORING DISSOLVED OXYGEN IN HIGH-PRESSURE BOILER SYSTEMS
Many power plants use fossil fuels or nuclear power as sources of heat to boil purifi ed water for the production of steam. The pressurised steam drives turbines to produce electricity. The quality of boiler feedwater must be carefully controlled throughout operations to optimise system performance. Proper water quality maintenance maximises operating effi ciency and the lifespan of boiler equipment.
Maintaining Water Quality in High- Pressure Boilers
Dissolved oxygen, or DO, is one of the most important water quality parameters to control in a boiler system. It is the primary corrosive agent of steam-generating systems. Even low concentrations of DO can be highly destructive, causing localised corrosion and pitting of metal system components. Pitting is a concentration of corrosion in a small area of the total metal surface, effectively drilling a hole in the metal. Over time, DO can cause an oxygen tubercle, or scab, to form over the point of original attack. Once the scab forms, the corrosion will continue, even if the system is then properly maintained. The high temperatures and pressures in steam-generating systems accelerate the rate of corrosion. Improper water quality management results in not only ineffi ciency, but also costly repairs and downtime. Operators of high-pressure boilers try to eliminate DO entirely from feedwater.
Mechanical deaeration is commonly the fi rst and most economical technique employed by plant operators to remove DO. Deaeration equipment heats the feedwater and vents the released gases, including oxygen. Properly maintained deaerators can typically reduce DO levels to as low as 10 ppb (parts per billion).
Plant operators supplement mechanical deaeration with chemical treatment, using oxygen scavengers such as hydrazine, DEHA and carbohydrazide to consume remaining DO. Theoretically, maintaining a high residual of an oxygen scavenger would consume all dissolved oxygen. In reality, competing chemical reactions between oxygen and boiler surfaces are more likely to occur, resulting in signifi cant corrosion within the system.
Consequently, maintaining high scavenger levels may not provide adequate protection. Routine monitoring of DO levels is crucial to confi rm suffi cient removal of DO within the system.
Common Methods for Monitoring DO
Various kinds of dissolved oxygen monitoring equipment are commercially available. Probes and analysers with galvanic, polarographic, and optical sensors are often mounted permanently inline. Portable colorimetric test kits that are sensitive enough to measure low levels of dissolved oxygen are also routinely used in boiler applications. Plant engineers consider equipment performance, reliability, measurement frequency, and maintenance costs to determine the most suitable methods for monitoring DO in their systems.
Online analysers offer continuous DO measurement, but require routine calibration of the sensors. Calibration is typically based on measurements of water-saturated air, which can be accomplished by placing the probe in the air space above the water level in a closed container of water. Calibration frequency depends on vendor guidance, the type of sensor, and the water quality conditions to which the probe is exposed. With the use of either a sensor or an analyser, it is recommended that plant operators periodically compare online data to results obtained with an alternate test method, ensuring that equivalent results are obtained. This helps to identify calibration drift, sensor corrosion, or other factors that could compromise the validity of online results.
CHEMetrics is the only manufacturer of a portable test kit for detecting trace levels of dissolved oxygen in boiler applications. CHEMetrics®
Test Kits feature the convenience of “snap and read”
self-fi lling ampoules, offering plant operators a rapid, reliable, maintenance-free means of determining ppb levels of DO within one minute. Over the course of nearly 50 years, CHEMetrics has earned a reputation for providing quality DO test kits to this marketplace.
Test kits provide all the components necessary for analysis and do not require calibration by the operator. CHEMetrics ampoules
IET MARCH / APRIL 2022
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