Environmental Engineering Reference
In-Depth Information
critical crack size is calculated for this higher pressure. And so on, until
all the critical crack sizes for the pressure in the HT become smaller than
all of the critical crack sizes for the operating mode. This pressure is then
chosen as the HT pressure.
The time of the next HT test is then defined for the HT pressure
determined by this process by the time of the growth of critical cracks for
the pressure in the HT mode to the critical crack size for the pressure in
the operating mode of operation.
In addition to the stresses caused by pressure, pipes and pressure vessels
are also loaded by general or local bending stresses whose magnitude
in operation and in HT differs. The magnitude of bending stresses is
determined by weight and thermal loads, as well as by the possible time-
dependent thermal effects in operation.
A pipeline of a power plant is shown in Fig. 8.11 will be taken as an
example. The inner diameter of the pipe is 500 mm (DN500), wall thickness
s
= 32 mm. The pipeline has straight sections and bends, is produced
from 0Cr18Ni10Ti austenitic steel, operating temperature = 300°C, HT
temperature =20°C, working pressure = 125 kg/cm
2
. The stress during
operation from internal pressure in the axial direction is σ
p
= 451 kgf/cm
2
,
in the tangential direction σ = 902 kg/cm
2
, while in the axial direction there
are general bending stresses of σ
b
= 60 kg/cm
2
. The yield strength of the
material at room temperature σ
y
= 220 kgf/cm
2
, at the service temperature
σ
p
= 190 kg/cm
2
.
To determine the HT pressure
p
= 140 kgf/cm
2
is selected, and the stress
in the axial direction σ
p
= 506 kgf/cm
2
, i in t h e t a in g e in t i a l d i r e c t i o in σ =
1012 kg/cm
2
, and bending stresses are equal to 5 kgf/cm
2
.
It should be noted that the material is in the viscous state in both the
test regime and the operating mode.
We define the critical crack sizes for the operating mode and for the
HT mode:
- for transverse cracks (the safety margin is equal to 1; if there is no
safety margin, the size is critical, and if there is a margin, the cracks do
not cause failure):
{
}
(
)
[ ]
[ ]
T
s =
2 /
R
π
2sin
nas n
g−
/
sin(
ϕ
) ;
B
F
a
ϕ
1
1
(
)
[ ] [ ]
g =
1/ 2
nasn
π−
/
ϕ − πs
/
R
T
;
a
ϕ
mF
1
1
- for axial cracks (safety margin 1):
(
)
T
s+ s= −
where σ
B
in the general bending stress,
R
T
is the half sum of the yield limits
and ultimate strengths of the material, φ is the length cracks in radians,
S
is
the pipe wall thickness, σ
m
is membrane stress,
n
a
,
n
φ
are the safety margins
for the crack size (in this case
n
a
=
n
φ
= 1 );
0.67
R aw
1
/
,
m
B
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