I have a client who is having corrosion problems on a stainless steel heat exchanger skid.

At the start the brazed heat exchangers (s/s plates/butterfly valve) were literally being eaten away (secondary side).  I thought this could be a production fault so replaced the unit with another one.  This lasted a mere three months, and even the threaded connections were eaten away.  So I started from scratch, cleaned the system out (just in case) supplied a new unit and replaced all connections.  But now the heat exchanger is fine, but the heat exchanger threaded connections are still being eaten away.  

The system primary side (stream) is 3 Barg LP sat (conditioned) steam @ 275kW and the secondary side (potable tap water) is running 10-65degrees celcius at 1.2 litres/sec.

I thought origonally this could be effect of over chlorination (UK we chlorinate our tap water), but this is the only exchanger on site suffering.

Usually your Heat exchanger should be running in vacuum area due to the fact that the waterside temperature is low. Your steam header is probably working at 3 bar g but after the control valve your pressure should be lower.

For this application you may require additional measures in the condensate discharge or you could possible face condensate retention and this could also  create corrosion.

Which materials are you using on the equipment and piping?
Do you have a pumping system to remove the condensate?
Do you have a check at the discharge?
Do you have a vacuum breaker?
What type of trap do you have?

Check the Langlier Index to find the pH where the water is corrosive.  There are a couple of other indexs which I cannot remember now that also find the corrosive pH.

Are all the exchangers the same SS?  Some SS are more sensitive to chlorine than others.

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