Boiler Water Problem

Water hardness would tend to result in scale formation on the water side of the tubes. This doesn't seem to be a problem with your softened water. Instead, your problem sounds more like dissolved oxygen in the water, which can cause pitting-type corrosion. There is a chemical treatment for this, but I forget the name. Also you could use a deaerator in the feed water system.

It is not what you can do now, it is "how long has it been working without corrosion protection"

The problems you have may have started a long time ago. They may be general and you have the first two failures. Others will follow.

Suggestions: Find out condition of tubes. You may be able to remove a failed one and see condition. Special ultrasonic thickness detector may be easier if you have that service available.

Using boiler chemicals can help but where corrosion already started it is likely to proceed up to failure. Or you may chemical/acid clean the boiler--with some risk of more failures.

If failures are costly to your production get good advice from some service people who may quote for the job. You pay nothing and learn a lot until you decide.

Majid...

Do you have a deaerator in your system ?

Do you periodically test for dissolved oxygen in your system ?

You mention that you have had your feed water tested but have you tested the boiler water for dissolved solids, salinity, etc?

Other things that can cause problems with boiler water are:

Scale forming salts

Sludge

Metals and metal oxides

Acid water

Dissolved gases (particularly oxygen)

Excessive alkalinity

Oils

All of these things can have a detrimental effect on your boiler tubing.

Why didn't you take a section of one of the tubes that you had to blind and check how bad the problem of scaling was?

Do you ever clean the the boiler intenally and externally?

Majid

Your feed water needs better analysis. Total hardness as CaCO3 is important but so are many of the other parameters like total alkalinity, pH, conductivity, Total Dissolved Solids, and silica. Check with your boiler manufacturer for blow down rates and TDS tolerances. Just based on the hardness of 850 mg/L, I would guess the conductivity of this water at close to 1900 μS/cm (assuming hardness is the majority of cations present) and possibly much higher. The feed water has excess salts and may make your blow down times very frequent. Is there any way you can improve the feed water? Ion exchange or reverse osmosis may be used to remove much of the TDS present. Salty water (or high TDS water) can prove to be both scale forming and corrosive at the same time. Remember, a boiler converts water to steam and leaves behind the TDS. If you are starting with a high TDS I am guessing you are already starting with limited uses. Your report of Halite (NaCl) suggests that there may be an already high level of Na ion present in the raw water before softening. It really sounds like you need a better quality feed water and that is why I suggest treatment such as deionization or reverse osmosis. Be sure to discuss the problems with your manufacturer. They should know what are the operation limits.