Govt. bodies and jurisdictional agencies get involved in FFS and RLA if the welded structure concerned is critical and its failure may cause hazard to life and heath of the people living around. The trend is towards their increased interest in performance-inspection frequency, acceptance standards, repair procedures, and record keeping. In some countries it is manda­tory to establish FFS and RLA after a stipulated service period.

FFS criterion leading to RLA should satisfy the following conditions:

1. Should be sound, practical and based on latest know-how.

2. It should be acceptable to owners and operators both.

3. Acceptable to relevant jurisdictional and certification authorities.

4. Should be based on proven inspection techniques.

5. Should be based on material properties that account for in-service degradation specific to the situation concerned.

6. It should be adaptable to short and long term needs.

12.3.1 Development of Expertise on FFS and RLA

1. Historically industry itself gives top priority to safe operation of process equipment by setting concensus guidelines and implementing various inspection requirements based on existing knowledge and experience available in that period.

2. Over a period of time with increased experience and improved knowledge regarding material behaviour and stress analysis a number of FFS analysis and RLA pro­grammes and guidelines have been developed by individual organisations and by professional and standardisation bodies.

3. While individual programmes and guidelines are being updated periodically, it may take some time before a common set of guidelines based on concensus of all the agencies involved is developed.

12.3.2 Justification for FFS and RLA Studies

Any fabricated metallic component has imperfections/discontinuities as recognised by code of construction which lay down the allowable limits of such imperfections. But there is no con­sensus procedures in industry that categorically spells out the methodology for accurately judging the Fitness-for-purpose for any vessel or piping components with defects beyond the code limits. The important elements of fitness for service approach are as follows.

1. Understanding the origin of the imperfection.

2. Knowing its present status.

3. Knowing the size, orientation, location and other relevant characteristics of the imperfections.

4. Establishing the stress acting at the location of relevance.

5. Characterisation of the material.

FFS and RLA in Presence of Service Induced Defects

Incase the defect is service induced, past records will not provide sufficient justification and safety margins to be employed. For such complex situations a higher level of analysis and data base is needed.

12.3 NATURE OF DAMAGE IN SERVICE

There are various types of damages in service and each type needs to be dealt with separately. In refineries, for example, the following types of deteriorations may be encountered:

• General corrosion

• Pitting attack

• Hydrogen damage (Hydrogen attack—Blistering, sulphide stress—Corrosion crack­ing (SSCC)—Hydrogen induced cracking (HIC) embrittlement.

• Stress corrosion cracking (SCC)

• Metallurgical degradation — Temper imbrittlement

— Secondary precipitation

— Carburisation

— Graphitisation

— Spheroidisation

• Fatigue/corrosion fatigue

• Creep/creep fatigue

• Oxidation

While the nature of the above mentioned damages are different, these can be grouped on the basis of the mechanism by which these affect the health of the equipment. Table 12.2 shows the defect categories and assessment of equipment fitness.