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appears and not leakage. Obviously, after leakage appearance, the fracture can only be

avoided in case of pressure release or if the leakage is discovered in time and necessary

action, to avoid further growth, were carried out. If anything exists that disrupt leakage or

its detection (like insulation) than safety can not be guaranteed by LBB.

In codes is also mentioned NHLBB (NH = Non Hazardous). In general, an insulated

NHLBB vessel is assumed to leak down to zero pressure without any further cycling,

which assures that the crack cannot subsequently grow to critical size (rupture). Vessels

that can have the pressure cycled or have significant externally applied loads after

developing a leakage are not NHLBB.

Review of the LBB-diagram shows that the application of this concept requires precise

analysis using fracture mechanics methods that include following important aspects:

•

Investigation of conditions that must be fulfilled to apply LBB-concept in actual case.

•

Evaluation of the critical size of the through crack for different crack position, taking in

consideration actual loads and their combinations.

•

Calculation of the leakage intensity starting from the minimal size of the actual crack,

i.e. the control of the crack opening characterizing the leakage rate. The values must be

conservatively evaluated (minimal expected) for leakage conditions to be fulfilled.

•

Cracks discovering through the LBB approach (including proof test)

•

Crack growth analysis.

•

Ascertain the leakage discover in the inspection system.

Discover of cracks and leakage in pipelines is often complicated, as the number of

pipes is high and the elbows additionally loaded by the moments. For the vessels is the

definition of loading simpler and the crack discovery easier. Difficulties, of course,

appear at the connections and supports, especially if the cracks caused by residual stresses

after inspection. However, the LBB-method is of special benefit for the components that

are not accessible for NDI inspection. Although the LBB procedure is now improved,

there are still different problems to be considered in actual application:

Leakage region.

For the evaluation of leakage area local loading and corresponding

conditions must be considered, in the case of elevated temperatures to include also creep

effect. The crack opening under load can be evaluated, e.g. based on EPRI-Handbook

data for plate with mean through crack. For the case of pipe, it can be assumed that the

plate is endless, and this means that the function

is taken for

a/b = 0

(0, )

ch n

δ αε

⎛ ⎞

⎜ ⎟

⎝ ⎠

(9)

for material obeying the law

⎛ ⎞

= ⎜ ⎟

⎝ ⎠

(10)

Crack position.

All positions on component must be considered, but it is not sure

which one is the most important. Situation is complicated, since the position of expected

maximal crack growth in normal operation does not always coincide with the position of

critical crack size for the lowest overloading in operation.

Stress state.

Stresses at the considered positions are based on combination of different

loading that may change in dependence of crack development, especially in creep.