FFS Analysis By 8 Step /Step 4- Assessment Techniques and Acceptance Criteria

FFS Levels

Three Levels of assessment are provided in each damage that cover assessment procedures FFS. 

1. Level 1 Assessment

The assessment procedures included in this level are intended to provide conservative screening criteria that can be utilized with a minimum amount of inspection or component information. This assessment is used for Type A Components which the pressure and thickness are linear proportional and pressure design thickness are govern, such as cylindrical shell under internal pressure. 

a.Type A Components:

component that has a design equation that specifically relates pressure (or liquid fill height for tanks) and supplemental loads, as applicable, to a required wall thickness, and the supplemental loads in combination with pressure do not govern the required wall thickness, i.e. the required thickness is based on pressure only. Examples of Type A components are shown below:

Pressure vessel cylindrical and conical shell sections with dimensions that satisfy the criteria in Following Figures:

?

Spherical pressure vessels and storage spheres.

?Spherical, elliptical and torispherical formed heads.

?Straight sections of piping systems and elbows or pipe bends that do not have structural attachments that satisfy the temperature criteria in preceding figure .

?Cylindrical atmospheric storage tank shell courses.

b-Type B Components:

There are two classes of Type B Components.

1)Type B, Class 1 components have the same geometry and loading conditions as described in a) above but are not classified as Type A components because supplemental loads in combination with pressure may govern the required wall thickness. Examples of such Type B, Class 1 components are shown below.

?Pressure vessel cylindrical and conical shell sections that are not classified as Type A components in accordance with the criteria in a) above.

?Piping systems that are not classified as Type A components in accordance with the criteria in a) above.2)Type B, Class 2 components do not have a design equation that specifically relates pressure (or liquid fill height for tanks) and/or other loads, as applicable, to a required wall thickness. These components have a code design procedure to determine an acceptable configuration. Type B, Class 2 components typically exist at a major structural discontinuity and involve the satisfaction of a local reinforcement requirement (e.g. nozzle reinforcement area), or necessitate the computation of a stress level based upon a given load condition, geometry, and thickness configuration (e.g. flange design). These rules typically result in one component with a thickness that is dependent upon that of another component. Design rules of this type have thickness interdependency, and the definition of a minimum thickness for a component is ambiguous. Examples of Type B, Class 2 components are shown below.

?Pressure vessel nozzles, tank nozzles and piping branch connections.?The reinforcement zone of conical transitions

.?Flanges.

?Cylinder to flat head junctions.

?Integral tubesheet connections.

c)Type C Components – A component that does not have a design equation which specifically relates pressure (or liquid fill height for tanks) and/or other loads, as applicable, to a required wall thickness. In addition, these components do not have a code design procedure to determine local stresses. Examples of Type C components are shown below.

?Pressure vessel head to shell junctions.?Stiffening rings attached to a shell

?Skirt and lug-type supports on pressure vessels.?Tank shell bottom course to tank bottom junction.

2. Level 2 Assessment

The assessment procedures included in this level are intended to provide a more detailed evaluation that produces results that are more precise than those from a Level 1 assessment. In a Level 2 Assessment, inspection information similar to that required for a Level 1 assessment is needed; however, more detailed calculations are used in the evaluation. The level 2 assessment is used when on of the following occurs:

·      The acceptance criteria of the level 1 is rejected

·      For component Type 1B or 2B which the pressure thickness are not linear function and the component has a pressure temperature rating such as flanges. The component type 1B calculation are provided at the construction codes and type 2B are not. 

The following picture is an example of pitting evaluation data gathering by Pitt Couple method base on ASME FFS-1 chapter 6.

3.Level 3 Assessment

In a Level 3 Assessment the most detailed inspection and component information is typically required, and the recommended analysis is based on numerical techniques such as the finite element method or experimental techniques when appropriate. A Level 3 assessment is primarily intended for use by engineering specialists experienced and knowledgeable in performing assessments FFS. 

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