FFS Forum: Unraveling Type A, B, and C Components

What Are Type A, B, & C Components?

Component “Types” provide a simple way of categorizing the simplicity (or complexity) of the loads and stresses in a component. They are used in some API 579 assessments to identify components that are eligible for Level 1 or 2 assessments.

Definition

API 579, Fitness-for-Service (API 579) identifies four component types:

• Type A
• Type B, Class 1 (or, B-1)
• Type B, Class 2 (B-2), and
• Type C.

These four component types are defined as follows:

• Type A: has a design equation that directly relates pressure to wall thickness (i.e., t = f(P)).
• Supplemental loads can also be present, but the design equation has to include the supplemental loads, and
• The wall thickness has to be controlled by the pressure.
• Type B-1: is the same as Type A, except that pressure + supplemental loads govern the wall thickness.
• Type B-2: does not have a design equation that directly relates loads to wall thickness, t ≠ f(P), but has a Code design procedure.
• Type C: does not have a design equation that relates load to stress or a Code design procedure.

In all cases there is some mention that, for a storage tank, fill height would substitute for pressure.

Supplemental Loads

The presence or magnitude of supplemental loads is important in distinguishing between Type A and Type B-1 components. In Type A, supplemental loads are negligible; in Type B-1, they may be controlling. Thus, it is worth a moment to clarify what is meant by “supplemental loads.”

Supplemental loading is, essentially, something other than internal pressure. Supplemental loading can induce bending or axial load in the component. It can also induce hoop or torsional stress, but those are probably less common. The resulting stresses act in addition to the longitudinal and circumferential membrane stresses caused by internal pressure. Stresses, or material thickness requirements, for supplemental loading are not addressed by the standard pressure-stress formulas such as Barlow’s formula or the formulas in UG-27 of Section VIII, Division 1 of the ASME Boiler and Pressure Vessel Code (Div 1). Some additional thickness is required to accommodate the supplemental load.

Paragraph 2C.2.7.2 of API 579 (2016) provides a good list of many sources of supplemental loads. I’ve included this list in Table 1 for reference.