By: David Depto
Steam Boiler Water Treatment Basics
From a water treatment standpoint, overall steam boiler operations and maintenance are quite complicated. The intent of this article is to help you understand the basics (free water treatment training), the most important parameters, and different tests to keep your steam boilers working efficiently over colder months.
The most important tests and checks are listed as follows, with a detailed technical description to follow.
Sulfite Residual Test/ Proper DA Operation (taken from boiler bulk water)
Water Softener Hardness Test (taken from output of softener)
Conductivity Control (continuous through surface blowdown)
Daily Bottom Blowdown (done through bottom drains of boiler)
Chemical Scale Corrosion/Scale Inhibitor Test (taken from boiler bulk water)
These procedures and tests take no more than 15 minutes daily once you are proficient and will ensure the efficient use and longevity of your boiler for many years.
Main Steam Boiler Potential Problems:
With such high temperatures and pressures in steam boilers, there are many conditions that can go wrong to inhibit heat transfer across boiler tubes, or eat away at metallurgy inside the boiler. In this section, the most common and severe failures are listed, with some basics as to what causes them.
1) Oxygen Pitting/Corrosion:
Water contains dissolved oxygen. Under the pressures and temperatures in a steam boiler, the remaining dissolved oxygen in the feedwater is very corrosive to metal and will cause pits through the boiler tubes.
Solution: The hotter water is, the less dissolved oxygen it can hold. The use of a properly operating DA tank can eliminate the vast majority of the dissolved oxygen in boiler feedwater. Optimal parameters are 220-225 degrees f at 5-7 psi. If a DA operates at this temperature/pressure, dissolved oxygen is generally reduced to ~ 7ppb in the feed water. A chemical called sulfite is fed into the DA, or feedwater tank under the water level, to scavenge the remaining oxygen. In a well-run system, sulfite residual should test at 20-60ppm to ensure that all of the oxygen has been removed and there is leftover sulfite. If the DA tank or a feedwater tank runs considerably colder, sulfite use becomes extremely high and generally will not remove all oxygen.
2) Boiler Tube Scale:
Most elements are more soluble in solutions when the temperature increases. Think about dissolving sugar in hot tea vs cold tea. However, two elements calcium and magnesium, known as water hardness, are less soluble as temperatures increase. Therefore, they are likely to plate out as scale on the hottest internal boiler surfaces, usually the tubes. Once scale forms, the heat transfer of these surfaces is drastically reduced and requires much more boiler fuel to achieve the same result.
Solution: A properly working water softener will reduce the hardness in boiler feedwater to less than 1ppm. In almost all systems with softener upsets, scale is forming. It will get worse with more hardness and time. The real solution here is to test your softener daily (1-minute test) and ensure there is proper maintenance of the unit over time. With even a small amount of scale, the fuel use increases significantly. You can see in the figure to the right that even .15” of scale can increase fuel use by 25%!
3) Wasting Boiler Heat/Water/Chemical:
You spend a lot of money to properly prepare your boiler water to heat your buildings. The input heat, water, and chemical treatment are valuable. In fact, it is estimated that 60% of all utility costs in your building are tied to heating and cooling systems.
Solution: It is very important to blowdown your boiler efficiently. We use conductivity as a measure of the dissolved minerals in the boiler water. If we run this too high, we risk issues such as scale, corrosion, and foaming. However, if we blowdown too much, we are wasting the heat, water, and chemical that is in the water. Proper conductivity control is essential to saving money and efficient operations. Whether you have automated blowdown or it's manual, the trick is to stay within the proper range set by your water treatment professional.
Conductivity control is achieved most efficiently from surface blowdown, as that is where the highest concentration of dissolved minerals lie. However, we must do daily bottom blowdown as well, to purge the system of solids that accumulate in boilers. Both are needed.
4) General Scale/Corrosion:
With high temperatures, pressures, and a lot of dissolved minerals, general scale and corrosion can happen inside your steam boiler. To mitigate problems and efficiently maintain your system, we feed a chemical into the water that helps to minimize these conditions.
Solution: The best way to know if there is the proper amount of chemical in your boiler, is to do a daily test in the boiler bulk water. Too much chemical is wasteful and an extra expense, while too little won’t protect the metals as well as it could.