Should you be pre-treating your boiler feedwater by running your makeup water through a dealkalizer? If you are a facilities manager or an engineer with a good amount of boiler system experience, you may know the answer to this question already; otherwise, you may be thinking:
What the heck is a dealkalizer?
Well, before we can easily answer that question for you, we need to first discuss alkalinity. One of the criteria that any successful boiler water treatment program must account for is the alkalinity of the makeup water source. If you are responsible for a steam system in your plant, you should have a basic understanding of what kind of effects alkalinity can have on your system.
First, you should know that alkalinity comes in different forms. Hydroxyl alkalinity (OH-) that comes from soda ash or caustic addition is vital to ensuring the proper chemical mechanics are in place for scale inhibition and corrosion prevention; however, naturally occurring alkalinity in the form of carbonate (CO3-) and bicarbonate ions (HCO3-) found in most water makeup sources can cause the following two issues:
- When alkalinity enters a boiler, it breaks down into carbon dioxide which is released by the steam. Carbon dioxide forms acidic carbonic acid in condensate returns resulting in pH levels usually below 6.0. Condensate treatment, in the form of neutralizing amines or filming amines, must be fed into the steam header to prevent corrosion of the condensate piping. The higher the carbonate alkalinity, the more amine that needs to be fed to counter these issues. For example, a boiler plant that produces 100,000,000lbs of steam per year at 20 cycles of concentration and 20% makeup with a water source alkalinity of 150ppm will require over 1,000 gallons of neutralizing amine per year just to maintain a condensate pH of 8.2.
- Carbonate ions will react with other positively charged ions such as calcium, magnesium, silica, iron and phosphate in the presence of heat and pressure to form various types and degrees of scale. These deposits will precipitate out of solution and attach out to the boiler tubes which will begin to impede heat transfer rates. If left untreated, this can eventually lead to tube rupture from overheating.
For facilities that commonly have low makeup water alkalinity (usually below 50ppm), the above issues can be treated cost effectively by using the proper scale inhibitor and condensate treatment chemistry. However for facilities that have moderate to high levels of alkalinity in their makeup water source (>100ppm), the implementation of a dealkalizer to mechanically remove this carbonate alkalinity may be the most economical and effective means of treating your boiler systems.
A dealkalizer (sometimes spelled “dealkalyzer”) acts and performs very much like a water softener with a few exceptions. While a softener removes calcium and magnesium ions by exchanging them with inert sodium ions when passed over a cationic resin bed, a dealkalizer removes carbonate ions by exchanging them with chloride ions by passing it over an anionic resin bed. Both require solar salt for regeneration of extinguished resin; however, a dealkalizer further requires a supplemental caustic solution to be fed during regeneration to boost effluent pH and enhance the anionic resin efficiency. The vessel, controls and layout are almost identical to that of a softener.
Going back to our steam system example that we listed above, where a boiler plant that produces 100,000,000lbs of steam per year at 20 cycles of concentration and 20% makeup with a water source alkalinity of 150ppm would require over 1,000 gallons of neutralizing amine per year; now with the addition of a dealkalizer removing all but 20ppm of the makeup source alkalinity, we would now see a yearly reduction of amine usage of over 850 gallons per year!
If your facility has high alkalinity in your water source, and you are considering installing a dealkalizer for your steam pre-treatment there are some conditions you need to consider:
- Water entering the dealkalizer must be softened. In order to work properly, a water softener must be in place prior to the dealkalizer.
- The chloride level of your makeup water exiting the dealkalizer will be elevated. You must be aware of and understand any chloride limitations of your feedwater tanks and boilers prior to installing a dealkalizer to ensure this elevation does not damage the metallurgy of your other systems.
- In most cases, the pH of the effluent exiting the dealkalizer will be below 7.0 so you will have to store caustic onsite for regeneration purposes.
As with most mechanical and chemical concerns in your plant, you must weigh the pros and cons of any capital investment such as a dealkalizer. If appropriate to your system, a correctly sized and installed dealkalizer can save you money, time and headaches. A qualified and experienced water treatment company can guide you through this process and help you determine if a dealkalizer is the best option for your site.
Want to take a deeper dive? There is a good article about the different types of dealkalizers on Water Quality Products Magazine’s website.
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