It’s quite common in boilers with base-exchange water softeners, the effects are worsened by having hard water and/or high make-up rates.
If you have a base-exchange softener the scale forming calcium is taken out but the carbonate remains, this doesn’t harm the boiler but instead breaks down to form CO2. This CO2 leaves the boiler with the steam, and then dissolves in the condensate to form carbonic acid which erodes the condensate pipework forming a metal salt (Iron carbonate, copper carbonate, etc.). The carbonate returns to the boiler via the feed system where it once again breaks down to form CO2 and the cycle begins again.
Copper and zinc are a particular problem as the metal ion left over from the dissociation plates-out on the steel boiler surfaces; as the steel and the copper are dissimilar, a galvanic cell is formed and, as steel acts as the anode, the steel is eroded rapidly local to the copper deposit.
There are a number of possible solutions depending on how deep your pockets are:
- Remove the copper pipework (remember – it is corroding right now so you will have to do this at some point anyway) and;
- Replace with steel (this won’t stop the corrosion of the condensate lines but it will stop the galvanic corrosion in the boiler) or;
- Replace with stainless steel (this will stop both the damage to the boiler AND the condensate lines).
Soluble copper usually results from carbon dioxide, oxygen or ammonia in the condensate system.
In order to deal with these problems, the common solution is to eliminate or neutralise the sources of these gases.
With all steam systems, it is recommended that you carry out a boiler water treatment risk assessment in accordance with BG04, considering the suitability of introducing chemical product(s), in order to boost pH and reduce free CO2 (and therefore reduce the damage to the condensate line). This boiler water treatment risk assessment should consider the destination of the steam and what it might contact.
It should also consider the suitability of the pre-treatment plant currently in operation.
From the conclusions of the boiler water treatment risk assessment, carried out in accordance with BG04, you can then consider your pre-treatment options.
Usually, there are at least two options;
Install a reverse osmosis unit after your existing base-exchange softener, which will not only remove carbonates but almost everything else too, producing ultra-pure water.
Alternatively, install a dealkalisation plant, which will remove the carbonates only. Both systems will have the potential added benefit of removing make-up water dissolved solids, thereby consequentially reducing blowdown and energy costs.
You will need to talk to a water treatment company, proficient in boiler water treatment, about these options and they should be able to then put an economic case together for you, to put to management.
It is very important when dealing with copper pitting in boilers that the whole steam system is sampled and considered, since there are other routes to copper corrosion, which are very specialist and as such are not considered here.
We would suggest contacting a specialist in steam boiler water chemistry, for further advice with respect to this.
The boiler maker however, will no doubt refer to BG04 Boiler water treatment guidance for shell boilers, coil boilers, steam generators and hot water boilers or BS 2486 Recommendations for treatment of water for steam boilers and water heaters in their technical documentation. Both will recommend keeping the alkalinity below 25mg/kg in the feed water to keep the amount of CO2 released in the boiler to a minimum.
