The only impact that hardness has is in relation to scaling tendency.
It has nothing to do with corrosion of SS, unless you allow scale to form and then crevice corrosion under the scale does become an issue or if the pH is low(from acid that you add to suppress scale).
Typically 316L is used in water up to about 200ppm Cl. If the system is very clean and free from biofilm and scale you can push that higher, perhaps to 500ppm. but the higher you go in Cl, lower in pH, and hotter the greater the risk for corrosion.
You need to look up scaling index, there are a few different versions of this. Various saturation indexes are called Langelier, Ryznar, and Stiff/Davies .
You basically use the water chemistry, all of the various ions, and the max service temp to calculate a tendency to scale. Some of the scale formers are more soluble in hotter water, and some are less.
Some of machine manufacturer recommend Total hardness below 100 mg/l and some between 70-600 mg/l like Evaporative Condenser and others 200 mg/l. I totally confused. please make it more clear for me. Many Thanks
Total hardness is only one of four or five critical factors, if people are telling you that you can control just on hardness then they either think that you are too stupid to do it correctly or they are too stupid to know the difference.
Use Google and read, I prefer Langelier.
One thing to watch is that some people like to use these indexes to estimate corrosivity (mostly to steel). What you want is a balance. Too far to one side and scale forms, then water is not very corrosive to plain steel, but you will get under scale attack of stainless. Too far to the other side and you will never get scale, but any plain steel in the system will be corroded.
Most systems use some water treatment. In a closed system (not open cooling tower) you can use very expensive polymer additives because they will not be lost. In open systems it is more common to use sulfuric acid to lower the pH and prevent scale formation. In open systems you also need to be very careful about biological control, so biocides are a critical part of the treatment system.
One of the most usable books is published by Nalco, the water treatment company.
If you contact a Nalco rep they may be able to provide your with one for free.
There are a number of other good references on this subject, but try starting with Nalco.
