There is a lot of misinformation and incorrect guidance being offered within the industry regarding the maintenance of steam boilers in non-scaled and non-corrosive conditions.
Due to the addition of make-up/feed-water that is not deaerated (pre-treated mechanically to remove dissolved oxygen), boilers require a chemical additive to tie up or neutralize free oxygen molecules. If these molecules are left untreated, oxygen-pitting-type oxidation will manifest itself as craters on internal metal surfaces.
Included in most boiler water treatment products or programs is an additive called sulfite (SO3) which, when kept at a modest residual of 30–60 parts per million (ppm) in the boiler water, will convert the free oxygen into sulfate (SO4) which is a soluble salt that can be removed through boiler blowdown. So long as there is a residual of sulfite present in the boiler water, there cannot be a simultaneous presence of free oxygen. This control limit offers reasonable assurance for continuous protection should there be a rapid infusion of free oxygen.
Typically built in with treatments is either a polymer or a phosphate constituent. In the presence of proper alkalinity levels, it will either prevent scale crystallization or precipitate out the calcium and magnesium hardness into a non-adhering sludge. This sludge can be removed from the boiler by routine blowdown.
Comprehensive treatment products or programs also include a volatile amine return line treatment, an ingredient which exits the boiler with the steam (unlike the other treatment constituents). This neutralizes the acidity often found in the after-boiler section, which includes the steam lines and condensate return line plumbing.
Boiler treatments are best added continuously to the feed-water line near the entry point into the boiler to ensure a consistent level throughout the operating day. In smaller applications, it can be inserted directly into the feed-water condensate return tank. Slug feeding will cause proper residuals to spike either too high or too low and is not an ideally safe or effective method.
Underlying proper treatment procedures and residual controls are the precise blowdown methodologies.
Vaporization of boiler water into steam causes the minerals, salts, and chemical treatments that are dissolved in the boiler to remain behind and concentrate, while make-up water additions are continuously made. Imagine adding sugar to a teapot as it is boiling. At some point, the water in the vessel will saturate, and the sugar will precipitate out to create a deposit on the bottom of the pot at the hottest location. Similarly, the boiler water contaminants (measured as total dissolved solids) will begin to drop out of the solution, no matter how well the boiler is being treated. If the concentrated solution is not diluted through intentional discharge (blowdown) and replaced with lesser solid-laden water (make-up/feed-water), these solids will form scale on the hottest internal surfaces where maximum heat transfer occurs.
If there is too much blowdown, one will waste valuable BTU-rich hot water and treatment, which makes it more costly to treat the boiler. Too little blowdown will allow for the possible formation of difficult-to-remove insulating scale buildups that can occur. There is also the potential for boiler water carryover (surging) wherein solid-laden steam is sent through the lines to contaminate traps or live-steam process applications.
Below, we address below several questions that often come up in discussion with boiler owner/operators when discussing treatment options or justification:
Q: My boiler hasn’t been treated in many years. My mechanic says that scale is likely holding it together and that applying a chemical treatment program will remove this scale and cause the boiler to leak. Is this true?
A: No, definitely not — scale does not hold a boiler together! The more scale film you have, the more heating fuel will be consumed to produce the same amount of steam in the boiler operation.
Q: I have a water softener treating my boiler feed-water, so do I really need a treatment additive?
A: Yes, absolutely! Water softeners are an excellent method of removing water hardness before it enters the boiler; however, most softeners will allow some leakage of hardness. A treatment additive will prevent this from leading to scale formation. Additionally, water softeners do not remove oxygen from the feed-water. Oxygen pitting and general corrosion of metal are accelerated in the presence of softer water.
Q: My mechanic says I only need to blowdown once or twice a week, and only when the boiler is at low pressure. Is this true?
A: No, blowdown is ideally performed once or twice each day, the frequency and duration will vary depending upon water quality and steam-load. The constant buildup and concentration of solids and formation of sludge require at least some discharge of the solid-laden boiler water each day. There are many large steam generating plants where the boilers run continuously 24/7 at high pressure and they are being blown-down through metered valves to keep the dissolved solids in check. Therefore, don’t believe that a boiler can’t be blown-down at high pressure. There is nothing wrong with blowing down the boiler first thing in the morning before it reaches full pressure to aid in the removal of sludge that has settled to the bottom before it is dispersed throughout the boiler due to the turbulation of high-pressure operation.
Don’t slug feed a daily dosage of chemical and then blow down massively afterward. This wastes treatment chemical and will cause a low reading later in the operating day.
There is a lot more we can add to the discussion of proper boiler water treatment programming which can be addressed directly with our field service engineers at any time.
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