corrosion mechanism 1

[TechnicalServices]

There is a condensate collection tank. Due to some reason it was overflpowing now and than.In a life span of about 2 years the tank wall thichness has been reduce substancially. I hope this is due to outside corrion but wanted to know the mechanism.
The tank was insulated.

 

[TD2K]

You really need to inspect the tank to identify whether you have internal or external corrosion.

External corrosion could occur if the insulation was wetted during the overflows. The wet insulation and likely high temperature in this service up against the steel (I'm assuming) shell could result in fairly agressive corrosion. Underinsulation corrision has been a signficant problem in some plants and service.

 

[MJCronin]

tech sevices,

I am assuming you are talking about a condensate collection system as would be used in a District Heating system for a college or a hospital....

I believe TD2K may be wrong in guessing abou external corrosion...... This type of tank almost always fails via interal corrosion.....

Condensate collection tanks are the "classic" location for concentrated corrosion of a carbon steel system. As the condensate returns from the collection piping,it is typically about 160-200F and has absorbed oxygen producing "carbolic acid" (as I recall). This means we have a thin-walled carbon steel tank contacting a hot, oxygenated, weak acid. Most typically, the bottom nozzle corrodes away. Needless to say, the relacement market for these tanks is a good one.....

If you check with some of the vendors of this type of tank, you will find out that they offer premium interior coatings, heavy duty wall constuction and even an "all stainless" version of this tank.

If you are replacing a tank of this type, you may also want to inspect the impellers of the pump drawing on this tank. They also become corroded away if they are of cast iron construction and have been in service any length of time.

Some websites to consider are:

- Burnham Boiler ( they make a Deaerator/collection tank combination)

- Cleaver Brooks ( deaerators)

- Armstrong International

Please give us more details!!???...... How old is the tank ?!! are there pump problems Etc.... etc



MJC

 

[TD2K]

And, what is your condensate treatment program. For example, do you add neutralizing amines?

Carbonic (carbolic acid is a phenol based material) acid is produced by the water picking up CO2, not O2 (minor point, not to be confused with criticizing MJCronin's good points). The water will also pick up O2 mark you and that is a good ingredient to have, if you WANT corrosion.

Do you routinely check the pH of the returning condensate? If it is much below 7.0, you will have significant corrosion occuring of the steel piping from carbonic acid.

 

[TBP]

Actually, most of the CO2 is the result of alkalinity in the boiler make up water. It breaks down into CO2 under boiler conditions, and gets carried out with the steam. It dissolves in the condensate, and results in carbonic acid of about the same strength as soda pop. This problem can be dealt with by the use of amines, and/or installing dealkalizers to treat the make up water. Corrosion from this source shows up as an overall wasting of the metal. Condensate lines typically "groove-out". The top half is full wall, while the bottom half is so thin you can poke your finger though it.

Condensate can indeed pick up oxygen from the air, but usually most of the oxygen corrosion (localized pitting) is the result of undeaerated water being added to condensate tanks. Some plants do this to help eliminate peak loads on the deaerator, and it will do that. However, unless you've got a lined or SS tank & piping, the corrosion is just spectaclar. I've seen mild steel tanks in applications like this fail in 18 months due to the dissolved oxygen in water added for make up or cooling purposes.

I've also seen leaks or overflows that have been allowed to go on for extended periods corrode tanks from the outside in.