Question

What is the dominant mechanism for boiler tube failure when feedwater contains elevated levels of dissolved oxygen, and how can this be mitigated?

Accepted Answer

The dominant mechanism for boiler tube failure when feedwater contains elevated levels of dissolved oxygen is oxygen corrosion. Oxygen corrosion occurs because dissolved oxygen in the feedwater reacts with the steel of the boiler tubes, forming iron oxide (rust). This corrosion is accelerated at higher temperatures and pressures found within the boiler. The iron oxide layer is not always protective; it can be porous and non-adherent, allowing the corrosion process to continue, gradually thinning the tube walls. Eventually, this thinning leads to tube rupture or leaks, causing boiler downtime and requiring costly repairs. The presence of chloride ions further accelerates oxygen corrosion. Mitigation of oxygen corrosion primarily involves removing dissolved oxygen from the feedwater. This is achieved through mechanical deaeration, where the feedwater is heated and sprayed into a vessel at low pressure, causing the dissolved gases to be released. Chemical oxygen scavengers, such as sodium sulfite or hydrazine, are then added to the feedwater to react with any remaining traces of oxygen. Maintaining proper water chemistry, including pH control, also helps to minimize corrosion rates. Regular monitoring of dissolved oxygen levels in the feedwater is essential to ensure that these mitigation measures are effective. By reducing dissolved oxygen to acceptable levels, the rate of oxygen corrosion can be significantly reduced, extending the lifespan of the boiler tubes.

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What is the dominant mechanism for boiler tube failure when feedwater contains elevated levels of dissolved oxygen, and how can this be mitigated?