Oxygen can cause serious corrosion and deposition problems in boiler feedwater systems. As a result, dissolved oxygen is an important on-line chemistry measurement in all fossil fuel, combined cycle, and other power plants. Plant-specific operating conditions and guidelines dictate where and how frequently to monitor as well as the range, accuracy, and overall performance requirements for oxygen measurement. In each case, a suitable on-line analyzer is essential to verify protection and to reliably indicate when corrective action is required. A basic understanding of oxygen corrosion and dissolved oxygen measurement practices and theory, analyzer selection criteria, installation guidelines, and optimization and verification techniques will ensure reliable and cost-effective operation and response.
INTRODUCTION
The effects of oxygen corrosion and corrosion products in boiler systems represent a great expense to plant management. The destruction of expensive components and the deposition of metal oxide corrosion products, causing inefficiencies, make oxygen a critical parameter to monitor and control. Industry has faced the challenge of minimizing corrosion by reducing dissolved oxygen concentrations to low parts-per-billion levels for decades. Deaerators and oxygen scavenging chemicals continue to perform this function. In some systems, generally higher pressure, with extremely high quality feedwater, control of oxygen within set parameters (“oxygenated treatment”) requires good control of the oxygen residual.
This paper will cover the basic reaction of oxygen corrosion, basic theory of dissolved oxygen analysis, and describe the most common on-line measurement techniques. Guidelines for optimization, some of the factors that affect performance of oxygen measurement and ways to minimize oxygen corrosion are discussed.
Since oxygen sampling is unique in that the air will rapidly contaminate samples, in-line instruments are, by far, most desirable. Spot sampling, using reagent based colorimetric testing or “Chemetrics®” type sampling can be done with proper procedures and care. A detailed discussion of these techniques is beyond the scope of this paper. The reader is referred to the ASTM procedure and manufacturer’s literature for further information.
OXYGEN CORROSION IN BOILER FEEDWATER SYSTEMS
Oxygen is normally measured in the boiler feedwater system after the deaerator, however, oxygen is also measured throughout the system such as in condensate streams and after condensers to check for 02 in- leakage.
Oxygen contamination of boiler feedwater systems can occur due to problems with effective deaeration, ingress from feedwater pumps, vacuum systems, and in-leakage into condensate systems when the condensate is recycled to the feedwater circuit. In addition, oxygen corrosion in boiler feedwater systems frequently occurs during startup and shutdown and while the boiler system is on standby or in storage. This is caused by a large amount of oxygen-laden water contacting the unit in a relatively short period of time. A boiler expected to be out of service for extended periods of time should be prepared for this and closely monitored to guard against corrosion.
The best operating practice for minimizing equipment corrosion requires the use of high-purity water, good chemistry control, and careful monitoring and control throughout the system. System operations vary and usually include periodic startups and shutdowns. During startup and shutdown and at low-load operations, significant ingress of oxygen, metal oxides, and other contaminants can occur. Mechanical, chemical, and operational solutions for this problem include pH adjustment, high le…