Environmental Engineering Reference
In-Depth Information
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nas
/
a
aw
/
=
1
.
acn n
asn
/
/
2
a
0
1
+
π
1
/
a
1
The results are presented in Figs. 8.12 and 8.13, where 1 - critical size
cracks the operating mode, 2 - for
p
HT
= 140 kgf/cm
2
, 3 - for
p
HT
= 175 kgf/
cm
2
. Figure 8.12 shows that the size of the family of critical size cracks for
HT mode is smaller than the size of the family of critical size cracks for the
operating mode.
Safety in the test with such pressure is guaranteed because all the critical
size cracks in the HT mode are smaller than the critical size cracks in the
operating mode.
The time of safe operation in the case of successful tests at the selected
pressure will be determined. For this steel and the given operating conditions
the crack grows only under the effect of cyclic loads. Cyclic loading is
produced by loading the pipeline operating pressure and temperature.Crack
growth is defined by
(
)
m
da dN C K
/
=D
/1
−
R
,
1
where
C
1
,
m
are material characteristics and depend on the loading conditions.
In this formula, the coefficients
C
1
and
m
are chosen from Table 1 in Ref.
113, and the stress ratio is zero (
R
= 0), i.e., the cycle is pulsating. The
calculations were performed for various ratios of
a
to
c
. The calculation
results are presented in Figs. 8.14 and 8.15.
The rays diverging from the origin characterise the family of cracks
with the same ratio of
a
and
c
. The first number indicates the ratio of
a
to
c
and and the second number is the number of cycles during which
the critical crack in the HT mode grows to the crack size, critical in the
operating mode.
The calculations show that the minimum number of cycles during which
crack grows to 'dangerous' dimensions in the operating mode is 9. The
duration of safe operation is the time during which the loaded component
enters and leaves the operating mode nine times. For this pipeline two such
loading cycles occur per year, so the time for safe operation is four and a
half years. The following loading with the HT pressure regime is carried
out after 4 years with the pressure being the same as in the initial loading
after the 9
th
cycle of reaching and leaving the working regime andprior to
transfer to the 10
th
working load cycle.
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