Demin Water on carbon steel 1

[vmonceller]

I need advise how to deal with demin water (chemical additive??) so that it will not be too harse on carbon steel piping, without using stainless steel as an option?

 

[cub3bead]

Degas it. If you remove all of the dissolved oxygen and carbon dioxide you can use carbon steel successfully. However, you have to keep the O2 and CO2 from being re-absorbed from the atmosphere.

 

[vmonceller]

Thank you for your input!

Demin water is used as make-up for the cooling tower water. What is the process of removing O2 & CO2 from the water and preventing them from being absorbed again by the recirculating water? Thanks again!

 

[cub3bead]

Why are you using demineralized water for cooling tower makeup? That is a very expensive way to operate a cooling tower.

You will not be able to prevent the demin water from reabsorbing O2 and CO2 from the air in a cooling tower, sorry.

CO2 removal is the easy part. CO2 is converted into HCO3 and removed by the anion resin inyour demin system. O2 can be removed by vacuum degassing, chemical scavenging using hydrazine or a hydrazine substitute ( I assume that you would not want to add additional TDS, so you couldn't use sulfite), or by using a gas transfer membrane swept with N2.

 

[countryham]

The earlier responses to your message were unquestionably "on Target"... I never heard of anything like that! and I've been to a whole bunch of county fairs......There is one thing for certain that has been well researched back to the pre 1940,s............The combination of carbon dioxide and oxygen are sygergesically more agressive when combined in solution than when each are added separately under identical conditions.... I can refer you to the research work, if you wish..............During recent years there has been a new approach to corrosion control......Now there seems to be a general agreement that 50-200ppb of dissolved oxygen is acceptable in the absence of carbon dioxide if the oxidized barrier is not disturbed.....I know of one instance where my client felt that he had to use hexametaphosphate in 17 deep well supply lines because "He had always done it that way other places"....The water was saturated with bicarbonates that released carbon dioxide, especially when the lines were exposed to summer heat.I performed on site corrosometer tests to prove to him that the phosphate made the water MORE corrosive upon addition because it (I assumed) had a detergent effect in removing the protective oxide barrier where there was insufficient calcium to form a calcium phosphate barrier.(This approach was patented by Calgon fifty years ago)........CONCLUSION.......HE STILL USED THE PHOSPHATE.---CONFIRMATION OF MYTHS--- FROM-- "IMPROVING CHEMICAL ENGINEERING PRACTICES",by(Kletz,T.A.) 1.They are not completely untrue; there is usually a measure of truth in them,but they are not completely or literally true either.1.They were often more true in the past than they are now.3.They are deeply ingrained.When our reasons for believing in them are shown to be invalid,we look for other reasons,or continue to act as if the myths were still true.Once in the mind,they are there to stay...Countryham

 

[waterexpert]

vmonceller,

you can assess the aggresivity of demin. and other waters by means of the Langelier Index. This is a technique for comparing the pH at which Calcium Carbonate begins to precipitate (pHs) with the actual pH. If pHs < pH, water is in a scaling condition, and has a low corrosion capacity. If pHs > pH, water has an aggressive nature. The condition of the water may be determined thus:

pH + TF + CF + AF - 12.1 < 0 implies aggressive water

pH + TF + CF + AF - 12.1 > 0 implies scaling water

where : pH = Measured pH Value

TF = Temperature Factor

CF = Calcium Hardness Factor

AF = Alkalinity Factor

A Table of values of the various factors to assist with this can be seen at point 9 on the following page:

http://www.wastewatertreatmentplantdesign.co.uk/IIR.htm

Seán Moran

 

[cub3bead]

waterexpert,

Something is wrong with the link you provided, would you post a coorected link?

cub3bead

 

[waterexpert]

Sorry, it's .html not .htm, thus:

http://www.wastewatertreatmentplantdesign.co.uk/IIR.html

Seán

 

[waterexpert]

It's probably worth mentioning that the Langelier Index, and the other similar indices, of which there are a few, (Singley,Ryznar,Larson,Riddick,Driving force,momentary excess,aggressive) are:

1. Empirically determined
2. Qualititative, rather than quantitative, representing the probability of the water being corrosive.

Nevertheless, correction of Langelier index is quite widely use in the UK water industry for correcting problems with aggressive waters.

Seán

http://www.seanmoran.co.uk

 

[cub3bead]

Waterexpert,

The problem with using the simplified calculation method you show on your webpage is that there are cases which do not apply.

The computation method for the Langelier Saturation Index requires six bits of information:
1. Total Alkalinity
2. Calcium Hardness
3. Ionic strength or TDS
4. pH
5. Temperature
6. pH at saturation equilibrium

The first five items can easily be measured but the saturation pH must be calculated and that involves solution of a quadratic equation. In the case of water with low calcium hardness and low alkalinity where it cannot become saturated with respect to calcium carbonate at any pH; the calculated hydrogen ion concentration at saturation equilibrium becomes imaginary. Demineralized water falls into this category of water so the use of LSI to characterize it scaling or corrosive tendencies is meaningless.

Another area where the simplified equations fail is at or above the bicarbonate-carbonate equivalence point (pH ? 10.3 – 10.5) because the negative root of the quadratic equation applies and the definition of LSI must be reversed.

Your method works in most cases involving natural waters, however, you have to be very careful in using it with waters that have been chemically transformed.

cub3bead

 

[waterexpert]

cub3bead,

the use of indices it is certainly controversial in some quarters, but I still think the simple technique given is useful for all waters. I think it would be a minority opinion that it is totally meaningless. I think it's actually better to use the simple tabular technique for this purpose than the algebra, as the result is quantitatively meaningless. It is however qualitatively useful. DI and RO waters can be remineralised in accordance with the predictions to give reduced corrosivity.

A detailed discussion of the various indices by people far more knowledgeable than me can be seen at:

http://www.corrosion-doctors.org/NaturalWaters/corrosivity.htm

Hope this helps

Seán

http://www.expertise-limited.co.uk

 

[GarySCWSVI]

Count me in on that "minority" of believing indices of any type like LSI has any value whatsoever in determining if a water is scale forming or secondarily corrosive. Scale forming indications are best determine by just plain looking at the water analysis.

In addition, water that contains very little or none of the necessary ions required for the LSI type indices, the results are meaningless.

Gary Schreiber, CWS VI
The Purolite Co.

 

[cub3bead]

waterexpert,

What we have to remember about forums such as this is that there maybe readers lurking without the training or experience to make informed decisions about the applicability of our discussions.

When I do remineralization calculations I pick a desired hardness and alkalinity level and then calculate the LSI to check whether the blend is corrosive or scaling. I do not start with a desired LSI and use it to calculate the Ca hardness and alkalinity required for a stable water.

I am a believer in going back to the basics and understanding the chemistry involved.

cub3bead