Based on the past experience on detailed examination of cracks and other damages observed in storage tanks and pressure vessels, the following points in regard to inspection techniques must be considered.

1. Use improved techniques to detect sub-surface flaws, dimensions, locations, depth and number of cracks.

2. Improved technique should be able to :

(а) inspect the entire vessel inside and outside.

(б) inspect it while in operation.

(b) monitor and measure flaw on-line.

(c) have sizing accuracy adequate to identify the margins to critical flow size. Analysis of Available Data on Plant History

1. Analysis of data includes: review of original design, past operating conditions, in­spection and maintenance records. This helps in locating and ranking and analysing the critical areas. For this purpose operators, plant inspection and maintenance staff are interviewed to assess plant and process upsets, fires, modifications, repair. This could affect the residual life assessment.

2. Once the material deterioration mechanism is recognised and state and extent of flaws through appropriate inspection methods have been established, the next step is to establish the critical condition of material degradation beyond which it would be unsafe to operate the structure. Next step is to determine the rate of growth of

flaw/deterioration so that the time period required for reaching the critical limits of

flaw size or material condition could be estimated.

3. A number of approaches to determine the critical sizes of flaws have been developed and are available in ASME Sec. XI approach, BS-PD 6493 approach, and CEGB R-6 Methods. Present metal condition can be established by destructive tests, where fea­sible, or in situ non-destructive metallurgical tests.

4. With these inputs the extent of life spent and the remaining life can be worked out on case to case basis.

12.6 WELD FAILURE

Failure is a term in which a member is subjected to plastic deformation, leading to failure, causing heavy losses to life and property. These losses are of two types: (i) Direct losses, (ii) Indirect losses, as shown in Fig. 12.1 below.