2 replies to this topic

[xavio]

Dear members,

I am assigned to design a hydrogenation pilot plant with process conditions as follows: P=70bar, T=230degC, H2 concentration>98%vol.

I have to make decision about material of construction of the equipments. I made reference to API 941 (the Nelson Curve) and found out that 0.5Mo low alloy steel could be used safely. Therefore, I proposed to my superior to use ASTM A-213 Grade T12 for tubing, A-335 Grade P12 for pipe, and A-387 Grade 12 for plate.

However, we wanted to use tubing system for the pilot equipments and I thought low alloy steel tubing might not be easily available, so I proposed the use of austenitic stainless steel (i.e., AISI304L) for the pilot and 0.5Mo steel for future the commercial scale equipments. I argued that using AISI304L would not give too much cost burden, given the small scale of the pilot plant.

There is senior mechanical engineer in my department who did not agree with me. He said 304L stainless steel should not be used for hydrogen service. Stanless steel is even more prone to hydrogen decarburization than carbon steel, he explained.
I presented the API 941 to him and pointed out that there is a note saying: "Austenitic stainless steel is generally not decarburized at any pressure and temperature". So, I could not accept his explanation. Needless to say, he insisted that I should use 0.5Mo in any case.

My questions are:
1. Is austenitic stainless steel safe for my application?
2. Do people choose 0.5Mo over stainless steel only because of cost factor (correct me if I'm wrong: 0.5Mo is cheaper than 304L) and not because of safety consideration?

Thank you.

xavio

Edited by xavio, 25 September 2012 - 12:57 AM.

[Tom the Hydrogen Guy]

Dear Xavio,

I cannot help you much on your selection of piping material. However, I would like to help you with your supply of hydrogen gas for your project.

Let me know if you are interested and with whom I should be speaking to regarding this project.

Good Luck.

[Atttyub194]

Dear all

 

Good day!

1. I would suggest that your choice is apparently right . In my opinion , you can use SS304  safely
2. In the temperature range you indicated, use of low alloy steel is an obvious choice. However selection of material is not always easy. It is seldom only due to economic reason
3. Stainless steels are more prone to SCC, whereas on the other hand SS is preferred as  weldibility of chrome steel is always poor especially after diffusion of hydrogen at welds after few year of service. Theoretically, baking is the solution  but is always cumbersome
4. At temperature range where decarborization can occur you can not even use chrome steel due to temperature limits defined in API
5. With SS 304 De-carborization is a problem at temperature rang of 400 to 700 Deg  C. Please refer ASM handbook and book on Metal dusting by Grabakke
6. Even sensitization will not be an issue in the temperature range you have specified. For details please refer following document   http://www.nickelins...Steel_9004_.pdf
7. Additionally,it would be more wise to go through the document before making the decision  REPORT TO API - THE TECHNICAL BASIS FOR API RP 941
8. I would not recommend to use tubing  with tube connectors for hydrogen service due to the nature of service, I would suggest to using piping with welded joints

Best regards and God Bless You

 

Ahmed Attyub