A few years ago, I got a call from an upset Refinery Manager at a large soybean processing plant in Indiana. He wanted to know what was plugging his sodium bisulfite injection quill located in the deaerator.
The customer had been having problems with plugging for quite some time and it was becoming a real aggravation for him and his lead boiler operator. The lead operator went onto say that they never had a plugging problem until they switched over to RO water. This did not make sense to me so I decided to further investigate what was causing the problem.
I first examined the plugged nozzle and sure enough there was a purple colored deposit that was restricting chemical feed in the quill. I was able to scrape enough material for a deposit analysis by a lab in Ohio. When I got the lab result, I was somewhat surprised to find that it contained almost 90% cobalt! We were losing the cobalt sulfate catalyst from the sodium sulfite solution.
The only other chemical (other than some recycled amine from the condensate system) we were adding to the deaerator storage section was caustic soda for pH adjustment. Normally, the pH in the in the bulk boiler feedwater was near 9.0 although there were times when the pH was closer to 10.0. We never expected to have a precipitation problem however it was now obvious that our pH control was not as good as we thought since the caustic was fed continuously and feedwater pH was only checked twice per month during my service calls.
Since the feedwater temperature was usually near 230 degrees F and dissolved oxygen testing was usually around 10 ppb or less, our technical director recommended switching to a non-catalyzed sodium bisulfite solution. This eliminated future plugging problems however it made us more aware of proper deaerator monitoring and the need for more frequent dissolved oxygen testing since the customer had an economizer in the B&W boiler.
We also thought about moving the sodium bisulfite injection point further away from the caustic injection point however since the deaerator was a pressure vessel, a certified welder would have been needed to install a weld-o-let and the customer was not keen on doing this.
It has been several years since I have visited the plant and the service responsibility has been passed onto another service representative. Hopefully, he is aware of the previous history and is continuing to closely monitor the feedwater system.