Materials of Construction for compressor casing

[jiang1976]

guys, I need your suggestion on the materials selection for compressor caseing.

At the begining of the design, the minimum operating temperature is -2C and MDMT is -25C. The vendor proposed SA516-55 for the compressor casing.


Now, the minimum operating temperature of the compressor is -28C and MDMT is -45C. The vendor did not recommend SA516-55 for low temp and changed the materials to SA352 Gr.LCB.

To me, it looks like Impact tested SA516-55 can be used to -45C, which is also specified in API617.

My questions are:
(1) Can I use SA516-55. If not recommended, why?
(2) What is the differece between these two materials, when used at lower temperature.

Thanks.

 

[gr2vessels]

jiang,
There must be some other condition you haven't mentioned, perhaps some corrosive gases or hydrogen, etc..The impact test temperature for grade 55, thickness less than 2" is listed in SA 20, table A1.15 as -50 deg C. However, from the mention of -45 deg C, I presume the casing plate thickness is over 2" (is that correct?). For that thickness, the carbon content could exceed a limit of say 0.20 and therefore the hardness limit in weldment and HAZ, some 22 HRC or 248 HV, might be exceeded (perhaps he doesn't even have a valid PQR for that hardness and that grade of carbon steel plate). However, the SA 352 LCB seems to me an even lesser quality material than the grade 55 ( is impact tested at only -46 deg C and the carbon content of 0.30 is even higher that grade 55!!?)
Reply to Q1 - The grade 55 can be used for operating temperatures down to -50 deg C, let alone MDMT -45 deg C.
Reply to Q2 - The grade 55 seems to be more suitable than the grade LCB for low temperature application, particularly if hardness limits are also imposed.
cheers,
gr2vessels

 

[jiang1976]

gr2vessels,

Thanks for your reply.

This compressor also has H2S and CO2, but no free water.

The thickness of SA516-55 plate is about 3". The client's specifications request 200HB Maximum hardness for carbon steel. After PWHT, the hardness of weld, HAZ and base metal should be less than 200HB. Right?

 

[gr2vessels]

Hi jiang,
Yes, it is correct, to the extent of 200HB seems a bit unreasonably tight requirement. This is not required in any wet H2S environment (refer to NACE MR 0175). From this point of view, I hardly believe that the fabricator will achieve the required hardness using the standard SA 352 and I strongly suggest to stick to the SA 516 grade 55, max C=0.22% and CEmax.=0.45%. Perhaps the plate should be quenched and tempered and the welding procedure qualified on the actual material (gr.55). If there is any chance of negotiating with the Client, to raise the hardness limit to 248HB, it will help the fabrication and it will avoid lots of headaches when negotiating non-conformance notices of hardness.
cheers,
gr2vessels

 

[SJones]

Ever thought that the switch to a casting was because the supplier would not then have to perform any welding? You might also take into account that HIC requirements could rear their ugly head with a plate material too. Jiang should enquire about A352 LCC rather than LCB - the carbon is lower and the required toughness higher. Nevertheless, both are listed in Annex 1E of API 617.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[jiang1976]

gr2vessels, Steve Jones

Thanks a lot.