Keys to Successful Feedwater and Deaeration within Industrial Steam-Generating Systems
By Edward Beardwood
Dissolved oxygen (DO) control in boiler feedwater is required to inhibit corrosion of carbon steel and copper alloy materials in industrial steam water cycles. The principles of thermomechanical and chemical deaeration of steam generator feedwaters will be covered in this article. Tray type and spray type deaerators and other degasification equipment will be discussed. The capabilities of available oxygen scavenger/reducing agent products are presented. Methods for general, localized, flow-assisted corrosion control and generating metal surface passivation are discussed. Plant best practices and troubleshooting for operational control to avoid feedwater and overall steam generation system corrosion are also presented.
The feedwater to a steam-generating system is often a combination of condensate steam and pretreated make-up water. Various equipment is used to store both returned condensate and pretreated make-up water for continuous batch treatment, including preheating and degasification. Preheating is required to raise the temperature closer to steam generation saturation to reduce thermal shock without circulation problems and to allow for faster steaming rates to meet demand. Disruption in circulation can lead to the potential for carry-under and/or carry-over.
The finished feedwater blend requires degasification or deaeration of non-condensable gasses. Corrosive gasses such as oxygen, carbon dioxide, hydrogen sulfide, sulfur dioxide, and inert gases such as nitrogen are removed. The corrosive gases will lead to corrosion within the total circuit because of volatility. The affected subsystems include the feedwater transport system, steam generator, steam users and condensate collection and transport system. These volatile non-condensable gases will also affect heat transfer during steam conversion to condensate to produce work. They accumulate in the vapor space next to the heat transfer surfaces and act as insulators. Components that make up the feedwater subsystem include:
- Pretreated make-up storage tank.
- Condensate return storage tank.
- Both low- and high-pressure feedwater heaters, if so equipped.
- Preheater feedwater tank or deaerator, if so equipped.
- Flue gas economizer, if so equipped.
- Connecting piping for the collection, blending, processing, and transporting of feedwater to the steam generator.
Feedwater Systems Corrosion
Common operational conditions that lead to corrosion in feedwater systems are associated with DO pitting when above saturation. General thinning corrosion is caused by low pH associated with non-condensable gases, process in leakage into the condensate returns, and ion-exchange regenerant leakage. Thermo-mechanical deaeration followed by polishing with chemical oxygen scavengers and passivators is practiced to reduce the pitting potential. The use of volatile neutralizing amines and film-forming amines has been employed to decrease the general thinning corrosion and other potential corrosion mechanisms.
Log in or Subscribe to Access the Full Article
To read or download full-length articles you need a subscription to Ultrapure. Please log in or subscribe below.