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Sensitivity of leakage detection.
Different conditions affect leakage detection. Leakage
effect may be reduced if the fluid evaporates or enters to the chemical reaction.
Material data.
Frequently the material data are insufficiently known, especially for the
welded joints where the safety evaluation by crack opening becomes more difficult.
Anyhow, LBB method is successfully applied in general, because it offers important
technical advantages and economy.
Stored energy in a vessel under pressure is calculated based on formula
1
1
1
sp
u
u
p
P V E
p
κ
κ
κ
−
⎡
⎤
⎛ ⎞
⋅ ⎢
⎥
=
− ⎜ ⎟
⎢
⎥
− ⎝ ⎠
⎢
⎥
⎣
⎦
(11)
where
p
u
and
p
sp
are internal pressure and pressure acting outside vessel, respectively,
V
is
vessel volume and
κ
gas constant (ratio of specific heats
p
v
c
c
=
, for air 1.4). The
containment is a pressure vessel if the calculated energy is 20 kJ or more,.
5. CONTROL AND VALIDATION OF CALCULATION RESULTS
Understanding of the basic relations of the critical structures integrity was the stimulus
in the development of engineering analysis methods starting with Galileo’s investigation
of beam bending resistance in 17
th
century. One of the most important goals in modern
metallic structures design is the optimisation of useful weight and the application of high
strength materials in order to achieve high economic efficiency. Besides this, certain
structures, as for example space vehicles, can be only realised if, based on available
driving power, are capable to carry, not only dead weight but also the weight of necessary
fuel and useful load, and this means, they should be light structures. It is possible only
applying new developed analysis methods, in which fracture mechanics has the decisive
role in assessing reliability and residual life of structures (power plants, transport systems,
including pipelines, process equipment and mechanical plants of different kind, during
their life subjected to complex loading and conditions). Common problem of such
structures is large financial load to community of their retirement. Having in mind
shortage in capital, and the growing risk caused by the aging of this infrastructure, ability
to fast and efficient assess structural integrity and the priority of repair and retirement
with the minimum costs at acceptable risk is of great importance for national economy.
As by all calculation tied to the structure integrity, also the fracture mechanics data
must produce some incertainty. For that it is recommended to apply the data that, with
possible deviations, in all cases produce conservative results. Still it is necessary to apply
some safety margin calculated as the ratio of maximal load (force) that structure can carry
without failure and the real applied maximal loading of the structure. The safety factor in
fracture mechanics can be assessed using FAD, as ratio between lines OB/OA
(Fig. 9).
This leads to conservative results, because it is usual to select minimal values of
material characteristics. According to the MIL-standard, for example, two options are
allowed, “A” or “B”. “A” value corresponds to the material characteristic that must be
fulfilled in 99 % cases with the confidence level of 95 %, and are applied if the fracture of
considered element leads to the overall system failure. If the fracture of the element leads
to the load redistribution to other joined elements it is possible to apply “B” value, with
the probability of 90 % (with same confidence). The selection is based on the experiments