I received this question today from a friend of mine.
My first suggestion is to see if your customer will allow you to use Ammonia to neutralize Carbonic Acid in the Condensate Lines as is done in Dairies. If so, I would recommend maintaining a pH between 7.0 and 8.0 in the condensate receivers. Ammonia may need to be injected into supplemental feed points in the steam line to achieve this pH level in all or most of the condensate receivers.
Be sure to monitor your condensate for copper and iron and try to move to the pH that minimizes both concentrations in the condensate. You can expect that iron will rise as the pH drops and that copper will rise as the pH rises.
MAKE UP DEALKALIZATION
Another suggestion is to install a dealkalizer after the water softener on the boiler make up line. This will remove bicarbonate ions from the make up water. It is the carbonate and bicarbonate ions in the boiler which generate carbon dioxide gas in the boiler when they are heated. This carbon dioxide escapes into the steam and forms carbonic acid in the condensate. If you can remove all or most of this in the make up you will have little or no carbonic acid generated in the condensate,
The dealkalizer that I am most familiar with has an cationic resin and is used to exchange bicarbonate anions with chloride and hydroxide anions. The ratio is usually 9 parts sodium chloride to 1 part sodium hydroxide as 100% active (dosages are typically 5.4 pounds/ft3 of NaCl to 0.6 pounds/ft3 of NaOH. This is a good starting point. I suggest adjusting the dosage to try to maintain an M alkalinity at 40 ppm coming out of the Dealkalizer, and a situation where P=M=OH Alkalinity. Go higher on caustic if necessary until P, M and OH alkalinity are all the same.
I ran a pharmaceutical plant water treatment program for a company that made an injectable blood product. I was able to keep Condensate pH between 6.5 and 6.8 for years with no Amines and only VERY WELL RUN dealkalization. This results in very good steam purity as well, if that happens to be one of your goals. High steam purity for blood products was very important.
STAINLESS STEEL CONDNESATE SYSTEM
My last suggestion is to replace all of the condensate piping with 316 Stainless Steel. This is very expensive and almost no one does this. But once again, I had customers in the pharmaceutical industry who actually did this and had very good results with their boiler operation.