Water treatment: Getting it right in the boilerhouse

One of the most important principles of good water treatment is to provide the boiler with the best quality feedwater that we economically deliver.

07/11/2013


Courtesy: Spirax SarcoWater softener: Why use it?

One of the most important principles of good water treatment is to provide the boiler with the best quality feedwater that we economically deliver. Feedwater is made up from a mixture of site supply (make–up) water and pure condensate. The level of impurities in the make-up water is the key deciding factor in minimizing deposit build-ups in the boiler. The most common impurities are calcium and magnesium which when present are collectively known as “hard water.” Left to accumulate to uncontrolled levels in a boiler, these “hardness” contaminates will form thermally insulating boiler scale deposits at boiler hotspots such as boiler tubes. This results in rapidly increased fuel usage and boiler operating costs as the boiler struggles to burn more fuel under reduced efficiency to meet steam demand. The resulting increase in fuel costs often exceed the cost of maintaining a chemical treatment program by a factor of ten or more over a 12 month period.

Water softeners remove hardness from the feedwater to produce “soft water” and thereby minimize the risk of scale in boilers.”

Selecting a softener?

The most commonly used softener is the zeolite type that consists of a softener exchange vessel, control valves and piping, and a system for bringing, or generating, the resin.

The softener tank is usually a vertical pressure vessel with an inlet distribution system, free-board space, a regenerant distribution system, ion exchange resin, and a resin-retaining underdrain collection system. Selecting the correct softener for your boiler feedwater involves considering the following:

  • Sizing to handle peak water flow demand
  • Testing make-up water to ensure adequate softening capacity
  • Adequate regeneration options to reflect plant operating loads

While easy to maintain, and replenish with salt, softeners require regular monitoring to ensure peak performance and maximum resin life. In areas where make-up water is hard, temporary softener failure can lead to rapid scale formation on boiler tubes, so proper softener selection and monitoring is essential.

pH control of boiler water

Mild steel is an “amphoteric” material meaning it will be reactive with both acid and highly alkaline water. To minimize corrosion, boiler pH is controlled in a range between pH 9.0 - pH 12.5 depending on the make-up water characteristic, boiler pressure and metallurgy. A localized area of either low or high pH in a boiler is the second most common cause of boiler corrosion behind oxygen corrosion.

At boiler temperatures and ideal pH conditions, mild steel passivation results in the formation of magnetite, a black iron oxide film that acts as a barrier between boiler water and steel. The corrosion reaction stops after a uniform magnetite layer is formed.

Rapid general corrosion can happen if this protective film is disrupted, so water chemistry must be carefully controlled to maintain the film. An acidic condition can destroy the magnetite film, while a highly alkaline condition can cause caustic gouging. A “pH excursion” will often show itself as black boiler water in the sight glasses as the magnetite layer is lifted off the tubes.

Control of pH is important for the following reasons:

  1. Corrosion rates of metals used in boiler systems are sensitive to variations in pH
  2. Low pH or insufficient alkalinity can result in corrosive acidic attack
  3. High pH or excess alkalinity can result in caustic gouging/cracking and foaming with resultant carryover
  4. Good pH control is essential to form the protective magnetite film

Content provided by Spirax Sarco, originally published in Steam News Magazine.



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