Friday, November 23, 2012

Material Selection - Piping Design

Material Selection

Key Considerations

  • Material specification
  • Chemical Composition
  • Mechanical Properties
  • Brittle fracture toughness
  • Carbon equivalent
  • Inspection
  • Repair
  • Welding Procedure
Let’s discuss a couple of these considerations at this time.

Brittle Fracture

Brittle fracture refers to the often catastrophic failure of materials when subjected to stresses at a lower temperature which the materially would normally be able to withstand at higher temperatures.

A “transition temperature” can be defined at the 13.5, 20, 27 J (10, 15, 20 ft-lb) energy level.

Charpy test results for steel plate obtained from failures of Liberty ships revealed that plate failure never occurred at temperatures greater than the 20-J (15 ft-lb) transition temperature.

This transition temperature varies with the material and is not used as a criterion.

Transition Temperatures

The transition temperature establishes the temperature at which a material “goes brittle”. It’s major shortcoming is it’s imprecision and non-repeatability.

Charpy Testing

Impact testing provides a repeatable means to establish the impact toughness capability of a material under temperature. The more common method is the Charpy drop test measurement which determines the energy absorbing capacity of a standard specimen. 

Impact Testing Exemption Temperatures – B31.3

Refer to Figure 323.2.2 in the Code.

This figure provides a correlation between material group, reference thickness and exemption temperature.

Material group is defined in Table A-1. For example, SA 106 B is given a Min Temp rating of “B”. Entering Figure 323.2.2A, this material is impact testing exempt up to a thickness of 0.5” down to a minimum temperature of –20 F. Curve B rises to a minimum temperature of 75 F for a material thickness of 3”.

Minimum Required Charpy V Notch Impact Values (CSA Z 662-1999)

Table 5.1 provides a toughness category matrix. This matrix is somewhat cumbersome to apply as it requires cross referencing to CSA Z 245 and makes use of toughness categories I, II & III. It is not intuitively obvious what these categories represent.

This Table also inherently provides for a risk based approach by bringing in service fluid, test fluid and pipe design operating stress parameters.

Case Study:

Below, the Material Requisition Form has certain boxes marked off to indicate inspection needs. Not all marked boxes are appropriate! Do you know which?

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