Flange Isolation Kit 1

[79ron79]

Gents,

I would like to know when a flange isolation kit is required when installing a SS valve into a CS line. The system is Hot Process Water built to B31.3. The valve is 18" butterfly.

Regards
79ron79

 

[Duwe6]

They are not needed.

 

[yyulius77]

Duwe6,

Can you please explain why not? People here seem to believe when a CS flange meets SS, we need an insulation kit there due to dissimilar metals, hence galvanic corrosion.

Cheers,
YY

 

[Duwe6]

Galvanic corrosion between s/s valves and c/s pipe seems to only be a theoretical problem. In the 'real world', that combination works well. Don't know why. But it works.

 

[MJCronin]

I agree with Duwe6..... An isolation kit is not necessary because of the dissimilar metals

But, the reason for this is contained in the position of the metals in the galvanic series.

Carbon steel/stainless steel are considered to be "not that far apart" galvanically.

Whereas copper/zinc are considered "significantly far apart"......and you can make a battery.

http://www.corrosionsource.com/FreeContent/1/Galvanic+Series

MJCronin
Sr. Process Engineer
Venture Engineering & Construction

http://www.VentureEngr.com

 

[LittleInch]

Also there's something about the relative mass and surface area. I think the Stl Stl is the cathode and the carbon the anode, but because there is so much more C STL the galvanic effect is minimal.

However if you put a small piece of C Stl connected to a wholly Stl Stl system then it could suffer significant galvanic corrosion.

I might have got the anode and cathode but the wrong way round, the theory is still valid.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

The anodic index illustrates the electro-chemistry of LittleInch's reasoning.

Ag = -0.15V
SS = -0.50V plates all the above
CS = -0.85V plates all the above
Al = -0.95V plates all the above
Galvanized Zn = -1.2V plates all the above
Zn = -1.25V plates all the above reasonably well
Mg = -1.75V plates all the above very well

Difference between CS and SS is 0.15 or greater, so they really should not be placed together, however if they are, electron flow will be from most negative to less negative, the CS will plate the SS. If you put the two together, put a small bit of SS in contact with a large surface area of CS.

Lesson learned. DO NOT put SS "silverware" in the dishwasher with the real silverware. Both will not come out looking very nice.

I hate Windowz 8!!!!

 

[SJones]

There are a number of other factors involved beyond the simple differences in electrode potentials as evidenced by the large number of erudite publications on the subject. Other factors that will come into play will be: geometric factors, electrolyte factors (conductivity, ions), flow rate, surface condition of the two metals, amongst others. The key question to ask will be: is there an expectation of significant metal loss in the carbon steel system itself, disregarding any stainless steel coupling, i.e. has more than a token corrosion allowance been applied. If there is corrosion of the carbon steel, there will be some enhancement from coupling the stainless steel, but will it be enough to warrant isolation and the ensuing inspection and maintenance issues? The answer may be derived by experience of the system and inspection records, or by risk assessment - how critical is it that the water stays on the inside and that carbon steel replacement activities are avoided.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[BigInch]

As always, but which out of all of those is the most important?

I hate Windowz 8!!!!

 

[SJones]

Depends what's in the 'process water.'

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[BigInch]

Hot it is at that.

I hate Windowz 8!!!!

 

[ripaldee]

*Not recommended when the service temperature is above the limitation of the non-metallic gasket application.
*Not recommended in flammable services.