Environmental Engineering Reference
In-Depth Information
the product;
N
c
is the number of load cycles; the nature of crack growth
(stress corrosion cracking or fatigue) is determined from corrosion and other
conditions that are extensively described in scientific literature.
To determine the reliability indices of group
M
of identical (similar)
products (i.e. products that have the same geometrical dimensions, are
made of the same material and by the same technology and operate in the
same conditions) the probability of failure-free operation of the product
P
(
t
)
during operating time
t
is defined for a single product as described above.
The result can be attributed to the entire batch of products, provided that
all the
M
products were subjected to NDT and repaired in accordance with
the NDT results.
More accurate statistical estimates can be obtained on the basis of
statistical estimates of failure-free operation for each of the
M
products:
P
1
(
t
);
P
1
(
t
); ...;
P
M
(
t
)
The statistical estimate of the probability of failure-free operation is
determined by the formula:
1
[
]
++
[5.33]
Pt
( )
=
Pt P
( )
(2)
+
...
P t
( ) .
1
2
M
M
Other options for averaging the results can also be used. For example,
the NDT results can be averaged-out using the:
N
det
(χ) =
N
det.mean
(χ) = [
N
det1
(χ) +
N
det2
(χ) +
N
det3
(χ)+...
+
N
det
M
(χ)] /
M
[5.34]
and then continue with the value
N
det.mean
(χ) as well as for a single product.
Using the determined values of
P
(
t
) for a batch of
M
products other
indicators of reliability, postulated by GOST 27.002-89, can be obtained.
The number of failed products
m
cr
(
t
) according to the criterion of full
or partial failure is service is calculated from the formula
m
cr
(
t
) =
M
[1-
P
(
t
)] =
M
[1-
P
def
(χ ≥ χ
cr
;
t
)]. [5.35]
The number of products
m
def
(
t
), failed in service according to the
criterion of defectiveness, is:
m
def
(
t
) =
M
[1-
P
(
t
)] =
M
[1-
P
def
(χ ≥ [χ]
serv
;
t
)]. [5.36]
The number of vessels or pressure piping showing leaks during operating
time:
m
leak
(
t
) =
M
[1-
P
(
t
)] =
M
[1-
P
def
(
a
≥
s
;
t
)]. [5.37]
The total number of failed product
m
(
t
) =
m
def
(
t
), of which
m
leak
(
t
)
products leaked (if they are vessels or pressure piping) and
m
cr
(
t
) products
fractured.
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