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How soft can soft water be without causing corrosion?
Q. The initial water coming into water plant is 350 ppm hardness. I remember from 'somewhere' that water that is to soft will cause corrosion. My question is what degree of 'softness' (in ppm) should the water not go below to avoid corrosion?
John Owen HarmonCity of Clarksville - Clarksville, Missouri
2005
A. Lets put it in a practical way--Considerably lower than you can afford to go for a municipal water system. 1 PPM will cause no problem.
James Watts- Navarre, Florida
2005
A. Corrosion by water is not purely down to the "softness", but a whole range of other properties, such as pH, temperature, sulphate and chloride content, etc. The hardness can either be permanent or temporary, depending on what causes it. Usually hardness is caused by the presence of calcium carbonate or bicarbonate in the water. These presences will affect the pH of the water and hence the potential corrosiveness of it. Furthermore, the corrosive nature of the water will be related to the material your pipes are made from. In some cases iron pipes are better than copper, in others the reverse is true; when lead pipes have been used, they are more susceptible to corrosion at low pH and soft water, but there are very few lead pipes around nowadays. The best corrosion resistant pipework is plastic pipes. Hope this helps.
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2005
A. Trevor's response is very good, but there is a more quantitative answer. There are actually some quantitative indices to determine the corrosivity or the scaling tendency of water that take into account the pH, hardness, temperature, alkalinity, etc. The most commonly used index is the Langlier Index. This is fairly easy to calculate once you have done it a few times, but a little complex the first time. There is also some software available that will do the calculations if you enter the water analysis.
Your analytical lab can probably provide this information.
consultant - Cleveland Heights, Ohio
2005
Q. Sir
I have two critical cooling tower systems for compressed air. And I'm facing a huge problem of scaling and algae in the heat exchangers. Currently I'm doing chemical treatment, online chemical descaling, yearly heat exchanger cleaning.
Input hardness is around 125 to 150 ppm. Now I'm thinking of putting a softening plant in between.
But I read in many articles, and comments given by supplier also, that Soft water is corrosive in nature. Some articles mention that soft water is not corrosive.
I'm confused,
- Nasik, Maharashtra, India
April 9, 2012
A. The solution to your problems is to keep the hardness and the pH of your cooling water well within your chemical supplier's recommendations. This may require more frequent blowdown of the tower water.
In place of more frequent blowdown, some cooling tower users have used a side-stream softener or soften part or all of their make-up water. However, at the 150 mg/L hardness of your make-up water, this does not appear to be necessary in your case unless you are attempting to to achieve > 4 cycles of concentration.
consultant - Cleveland Heights, Ohio
April 16, 2012
A. Atul,
Get a water treatment company to provide you with a soft water program.
All the makeup water will be softened and chemicals dosed to prevent corrosion. They will also provide a biocide/algaecide.
Regards,
- Tustin, California, USA
May 22, 2012
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