DUPLEX PITTING RATE OF HEAT EXCHANGER TUBES 1

[Cor123]

Dear All
I am keen get your advice or perhaps case studies for potential pitting rate of Duplex X changer tubes with nominal of 1.7 mm. Most of the literature I have seen are based on seawater, which would not fit my conditions
Operating Conditions :
1. Multiphase fluid (mainly oil)with 1.8 mol% CO2
2. Saline water chemistry (85000 ppm chloride and pH 5.5)
3. The well operates cyclically so periods of stagnation.
4. Some evidence of Oxygen (up to 2 ppm).
6.Nominally sand free so erosion not a major issue.
5. Operating temperature of up to 70 degree Celsius.

I know localised prediction rate is fraught with with impossibilities. Nevertheless, I need to a potential rate which can be defensible.
Many thanks

 

[EdStainless]

Which duplex? One of the 25%Cr superduplex grades I hope.
85k Cl, Ph=5.5 and 70C is extremely aggressive.
The Cl level is not too important, but the temp and pH are the real drivers.
Are you sure that the pH is only 5.5 with all of that CO2 available?
A 7%Mo superaustenitic might work here also.

There is no rate, either it is fully resistant and will last 40 years or it will pit and failure will be in something between a few days and 5 years. But if it pits the time does not matter, it will fail and you cannot count on some given time period.

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P.E. Metallurgy, Plymouth Tube

 

[Cor123]

Many thanks Ed
The Duplex is 22 Cr. The HX has been on service for 8 years and not inspected. It recently passed hydrotest (twice the operating Pressure ) hence the need to get some corrosion rate for remaining life calculation.

 

[EdStainless]

There is not such thing as a rate for pitting, if it starts pitting tomorrow it will fail, or it may go another 10 years. It is a probability and you are operating in a region where it will eventually pit.
Of course your hydro didn't tell you if it was pitted or not, just nothing big through wall yet. You should have done Eddy Current testing, or even just used a borescopoe and examined the tube IDs.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

This application requires EC testing to establish baseline data and reinspection to trend pitting damage. You may find you have no issues or the tubes could contain near through wall pits.

 

[Cor123]

Dear All
I totally agree with the facts and that's what I have been saying all along but nevertheless they decided not to inspect despite the RBI interval and scope!
Many thanks.