Application notes: The Sievers InnovOx TOC analyzer: boiler water monitoring

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Courtesy of GE Analytical Instruments

Background

A boiler system is a semi-closed loop system in which water is heated and converted into steam to power a generator. The steam is then allowed to form a condensate and the process is repeated. Boiler water chemistry directly impacts boiler efficiency and fuel use. Improper water treatment allows scale formation and corrosion. Boiler scale and deposits form when impurities precipitate on the hot boiler tubes. Scale insulates boiler tubes and requires excess fuel use to generate the same amount of steam. For example, a moderately scaled 250 HP boiler consumes thousands of dollars more fuel annually than the same “clean” boiler. Corrosion reduces equipment life expectancy and requires capital expenses for repair.

Corrosion in boiler systems can quickly result in tube failure and plant shutdown. A properly-operating deaerator and an appropriate chemical program will greatly extend boiler life by addressing corrosion problems. The solution to boiler corrosion is both mechanical and chemical along with proper monitoring. One common monitoring technique is measurement and control of incoming water hardness and iron. Maintaining optimum water chemistry minimizes the potential for deposits and scaling. If you are not sure how your boiler chemistry is performing, then better monitoring and control should be implemented.

A boiler system consists of several key components susceptible to corrosion. If corrosion occurs in any of those components, it leads to boiler inefficiency. One leading measurement to indicate corrosion is the presence of organics. By monitoring total organic carbon (TOC) in the boiler water, the system integrity can be monitored and corrected before any detrimental consequences occur.

Most plants set maximum limits for TOC in boiler feedwater with the level depending on the pressure of the boiler. Typically, the lower the pressure, the lower the purity requirements. The majority of organic compounds found naturally in water can be removed by ion exchange or physical processes like ultrafiltration. Some organic compounds, however, require additional steps for their removal or destruction.

One of the most common causes of boiler corrosion is carbon dioxide (CO2). Carbon dioxide can enter a condensate system as a dissolved gas or it can be chemically combined in the bicarbonate or carbonate alkalinity of the feed water. Generally, dissolved carbon dioxide is removed in the deaerating heater. The following reactions show the breakdown of naturally occurring bicarbonate and carbonate alkalinity to carbon dioxide.

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