Calculation of Corrosion Allowance for Carbon Steel Material 1

[Venkatachalam Ponnusamy]

We are working on one of the project in India for Gas Sweetening by Amine (Removal of CO2 & H2S) & Gas Dehydration Unit, we have to Design the equipments and pipings for 6 Years of Design Life and accordingly the materials needs to be selected.
In the inlet Gas we have 11.61% of CO2 & 100 PPM of H2S is present & 85.01% of Methane. Gas Flow rate is 2.5 LSCMD
Considering such high concentration of CO2 in natural gas i have to calculate corrosion allowance for Carbon Steel MOC for the design life of 6 Years.
I have also having a option that i can go for SS or DSS, but i wanted to go with Carbon steel with calculated corrosion allowance for being economical & competitive.
Anyone Can you help me how to calculate the corrosion allowance and suggestions for the above.
Regards
Venkat

 

[LittleInch]

1) What is LSCMD? Lakhs? Much better to use numbers most other people recognise. Lakhs and Crore are not used much outside India.

So this would be 250,000 SCMD??

2) You need to use a materials engineer for this. Wet CO2 at that percent (you also need to specify pressure and temperature), will eat Carbon Steel. Anything more than 6-7mm CA is unfeasible.

With H2S as well the material requirements can be quite stringent.

This is a forum for engineers to ask questions, not for free engineering.

There is also a corrosion engineering forum.

Have you done ANY searching on this topic? CO2 corrosion is a well understood mechanism so it should take you long to find the basics.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[0707]

Do a PWHT to your CS pipes and consider a corrosion allowance of 6mm.

luis

 

[SJones]

If the dehydration unit is upstream of the sweetening plant, what is the problem?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

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

 

[Venkatachalam Ponnusamy]

To : LittleInch

Thanks for your response & suggesitions.

Yes it is 2,50,000 SCMD of Gas.

BASED ON DEWAARD-MILLIAMS equation, i have calculated the corrosion rate for 6 Years of equipment design life as our client want's the equipment design life should be minimum 6 years since the project is going to be on Hiring Basis, the calculated corrosion rate is coming around 10.47 mm, but i understand that maximum 6 mm Corrosion allowance is advisable for Carbon Steel?.

As u suggested we will approach a Material Engineer for the results we wanted.. Once again really appreciate your inputs.

Regards,
Venkat

 

[Venkatachalam Ponnusamy]

To luis:

Appreciate your suggestion, does PWHT alone will suffice to satisfy the design life of 6 Years ? since heavy concentration of CO2 present in the Gas (11.61%) so material study needs to be done to select CA & MOC.

Any how Amine is lethal hence PWHT is mandatory.

Regards
Venkat

 

[Venkatachalam Ponnusamy]

To: SJones,

Appreciate your response.

if so even in GDU Unit the CO2 & H2S presence is at high concentration, so the material selection & corrosion allowance is highly depends on presence of CO2 & H2S.

In our case before entering the gas in to Dehydration unit all acid gases are removed (CO2 & H2S)by amine based sweetening unit, so we can go for Normal Carbon Steel for most of GDU equipment's with 6 mm of CA to meet the design life of 6 Years. If GDU comes upstream of Sweetening unit then we have to go for Stainless Steel Materials for most of the GDU equipment's which will increase the CAPAX we feel.

We wanted to be economical & as well as we have to meet the client specifications for being competitive.

We can appreciate if you could help on selection of material & corrosion management.

Regards
Venkat

 

[LittleInch]

VP,

At 11.6% assuming water saturated gas going into the amine unit, 10.5mm (~2mm/yr) sounds quite low.

The >6mm bit I will leave for others like Steve or your materials engineer to elaborate, but my understanding is that beyond that (6mm) the corrosion pit (because CO2 corrosion isn't usually a uniform thickness) starts to accelerate corrosion and any Corrosion Inhibitor is less effective.

Also these calculations, IMHO, are more guides than precise numbers and the actual corrosion pitting could very easily be deeper in isolated locations than your calculation.

10.5mm over 6 years tells me you would probably be lucky to survive more than 3 without some sort of high corrosion in one location where some other effect such as erosion or swirl or something else is happening.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.