Chilled Water System Corrossion Protection

[batman77]

We have a closed loop chilled water system. This system was filled with demineralised water. The main components in the system are:

Heat Exchanger Tubes made of copper.
Circulating pumps made of cast iron.
All pipework made of stainless steel 304.
Evaporator at chiller made of copper tubes, cast iron end plates and the body I believe being steel.

The water fed to fill the system is of ph 6.5 and conductivity of around 120 microSiemens.

The system has been in operation for about 4 years and no dosing of any chemical has been carried out. The results of a sample taken this last year gave a conductivity of around 200mS and a pH of 9.5.

My questions are the following:

1) Any indication what should be (or need to be) the range of pH for such a system? I could not find information of the water quality requirements for the chiller.

2) Any indication, or explanation, what could be the the reason for this change in pH and what has happened?

We replaced around 25% of the total water volume with fresh demineralised water but the results remained the same.

 

[SMF1964]

If you have a conductivity of 200 milliSiemens and a pH of 9.5, then you do not have demineralized water in your system, even though you started with demin water initially.

I suspect that with the mixture of alloys in your system (copper, stainless, cast iron, steel) that you have a galvanic corrosion process occurring in your system which is why the conductivity has jumped 1000-fold over the past 4 years. If you are going to maintain a demineralized water requirement in the system, you're going to need water treatment equipment like an RO system or demineralizer (cation/anion/mixed bed systems) in the system. I would recommend contacting an industrial water treatment company for suggestions on additives in case there are olternatives to trying to maintain demin conditions. Ten years ago, I would've said to contact someone like Ecolochem or Nalco, but I'm not sure who is bought up by whom now.

 

[batman77]

I'm very sorry but I did a typo error. The value is 200microSiemens which means that the conductivity practically remained unchanged after 4 years. It's the pH which went up. sorry again.