12.1 INTRODUCTION

1. All welded structures are expected to have an estimated service life. The actual service life may be more or less than the estimated period.

2. To ensure safe service and avoid unexpected failure, it is customary to inspect the welded components/structures at regular intervals.

3. Welded structures suffer from defects/discontinuities leading to failure.

4. The defect which most commonly leads to failure is some or the other form of crack, which when attains a critical length runs at unbelievably high speed leading to catastrophy.

5. Once a crack has been detected, it is imperative to repair it.

6. If repair is not possible steps are taken to assess the residual life of the component/ structure so that steps are taken to replace it quickly before its life expires.

7. If unexpected failure occurs, causes are investigated, so that steps are taken to eliminate such causes from future structures.

There are two aspects of the problem for structures in-service with cracks having initiated in them viz.

1. Residual life Assessment

2. Failure analysis.

12.2 RESIDUAL LIFE ASSESSMENT OF WELDED STRUCTURES

Chemical process plants and power plants are constructed in accordance with some construc­tion codes and tested according to the relevant inspection codes.

Construction and inspection codes for major components of chemical and power plants are given in the following table (Table 12.1).

These codes do not talk about guidelines to assess the fitness of the equipment or deter­mining its remaining useful life. They provide only the design rules and method of con­struction and inspection.

It has been found that a large proportion of process equipments have failed in service due to manufacturing defects or severe working environment.

12.2.1 Fitness for Service (FFS)

It is the ability of a structure to serve satisfactorily under a given set of process conditions for a reasonable period economically. This means the determination of accepable critical sizes of cracks (or other defects) or extent of material deterioration beyond which equipment cannot be adjudged as suitable for continued service.

Residual Life Assessment (RLA)

It is the time period during which the equipment shall retain the fitness-for-service characteristics. Fitness-for-service thus becomes very important for residual life assessment.

Extensive and expensive inspection programs are undertaken, in addition to routine inspections, to monitor the extent of in-service deteriorations. These inspections are more rigorous than routine ones and are needed with a view to find out whether a particular material condition was service induced or existed since the structure was built.

Deterioration of the material properties which is important for assessing the safety and reliability, must be assessed before an effective analysis for FFS or RLA is considered.