Recently we received the following question from a prospective customer.
We are trying to decide whether to purchase stainless steels or lined carbon steel vessels for our industrial water softeners. We currently have old stainless steel units we inherited from a sister plant. These tanks are corroded. What is Res-Kem's position on buying stainless steel versus lined carbon steel tanks for our water softeners?
Res-Kem is of the opinion that a stainless steel vessel is "usually" the more cost effective alternative when compared to a lined carbon steel vessel for water treatment applications. This assumes that the end user has the financial capacity to handle the higher upfront cost and that the vessel internals are properly designed. We added the word "usually" as the possibility of stainless steel chloride corrosion can be problematic in high temperature applications or those with improperly designed vessel internals.
Used with properly functioning internals, stainless steel vessels will last for a very long time on a simple water softening application. A carbon steel vessel on the other hand is very much dependent on the condition of the vessel lining. Eventually every lining will develop a pin hole failure leading to corrosion, a subsequent leak, and full vessel failure. Obviously this is not a concern with unlined vessels constructed of stainless steel.
In the case of the corrosion of the existing stainless steel vessels at this customer, there are two explanations for the corrosion they are experiencing. For starters they are not diluting the regenerant brine. This only increases the chloride levels during the regeneration cycle. It's worth mentioning that ordinarily any water softener is only subjected to high chloride levels during the 30 minutes or so as brine is introduced to the vessel. At this customer, not only is the brine saturated, but there is little assurance that the design of the internals gets all the brine out during the following rinse steps. Should this be the case, the bottom dome of the vessel would have a stagnant brine solution remaining in it well after the regeneration cycle is finished. This would particularly enhance the corrosion process especially if the water contains an appreciable amount of dissolved oxygen. One can only assume this condition applies at this customer and things are exacerbated with the higher concentration of brine being used.
It's worth pointing out that General Water, the service DI arm of Res-Kem has a rental fleet of well over 200 skid mounted DI exchange vessels. All are 36" diameter and all are constructed of stainless steel. The decision to purchase stainless vessels over lined carbon steel was not done because we like to spend money. It was done as it's a good investment over the long haul.