API Standard 650 material groups

[rww88]

I thought I would try my luck here in the Metal and Metallurgy engineering forum on this one. It has been five days without a single response since I posted this same question in the Storage Tank engineering forum where I surmised it to be more appropriate: What is the underlying factor or factors that differentiate the API Standard 650 material groups? This question was raised in my mind when I observed that a flush-type clean out fitting's maximum opening dimensions are dependent on material group number. I cannot find a simple definition of material group other than the API Standard 650 table that merely lists the various acceptable material specifications by group number.

 

[mk3223]

Please clarify what's the issue of the "material groups" from?

Is it about the material groups regarding the minimum design metal temperature without impact testing per Fig. 4.1a & b?
Similar to the ASME Codes of the pressure vessel and piping, the materials are grouped by the thickness vs. minimum design metal temperature based on the material testing and experience.
For the tank material selection, if the design temperature is below the specific Group curve, one can either perform an impact testing as required, or select another material with the lower design metal temperature for the specific thickness.

 

[rww88]

Thank you for your response.

To clarify, from the Standard regarding flush-type cleanout fittings:
"When the shell material is Group I, II, III, or IIIA, the width or height of the clear opening shall not exceed 1200 mm (48 in); when the shell material is Group IV, IVA, V, or VI, the height shall not exceed 900 mm (36 in.)." Because Standard 650 apparently does not offer an official definition for material group, I was having difficulty relating the dependency of a material group with tank opening dimensions.

Based on your response is it safe to conclude that the reasoning behind the specified dimensional limitations is based solely on design metal temperature and material thickness? Moreover, is it correct to make the following statement about material groups:

Allowed API 650 storage tank shell materials are divided into groups based on their minimum permissible design metal temperatures before impact testing for toughness and ductility is required. Tensile, yield, and allowable design stresses are not a consideration in material groupings.

 

[rww88]

Finally the answer: The API Standard 650 material groups are differentiated by the average values of longitudinal and transverse notch toughness impact values.