Hydrocarbon Processing - August 2021 - 66

Maintenance and Reliability
ment of the value of 1 to X for Tcreep < T < Tdes is for increased
safety, since creep is occurring in this range, but the lifetime is
affected by a factor of less than 1. This can also be used to count
the number of hours in creep regime, and it is compliant to the
normal control routines made by Italian creep control bodies.
The value of the X parameter for T > Tdes is equal to the ratio
of the design time and the remaining life at the temperature
(T). In the example, if the remaining lifetime is 10,000 hr and
the design is 100,000 hr, then the X parameter is 10. This means
that 1 hr at T is equivalent to 10 hr at Tdes. An example of how
the program works is shown in FIG. 2.
1,000
950
900
850
800
750
700
650
600
550
500
450
400
350
300
250
200
150
100
50
Tcreep
Creep
10
8
9
7
6
5
4
3
2
1
Hr
Tdes
T
X
FIG. 2. An example of how the creep calculation program works.
FABRIZIO D'ANTONIO is the Asset Integrity Manager at Raffineria di Milazzo.
He has been responsible for several units at the facility during his time there.
He earned an MS degree in engineering and has also been a contract professor
at the Università di Messina for a course on crude oil refining.
By integrating the X parameter, you get the total
equivalent creep time spent. In this example, overtemperature
lasting 2 d, with a peak temperature of nearly 20°C above
the design limit, reduces the life of equipment by a factor of 2.5.
The 48 hr of overtemperature have reduced the equipment lifetime
by 48 × 2.5 = 120 hr.
Due to this exponential behavior, it is easily seen that, even
with relatively low overtemperatures, the lifetime reduction is
important. This provides extra value to the importance of properly
monitoring metal temperatures.
Takeaway. The Raffineria di Milazzo has developed this application
to achieve the following advantages:
* Precisely controlling total creep hours
* Providing the ability to estimate the impact of
temperature excursions.
Another advantage is that this method is easily applicable to
all equipment that undergoes creep. This can greatly help to
prepare effective NDT control plans, focusing on the sections
that show real problems. In addition, cost reductions are possible
for sections in which temperatures are far below the design
limits.
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66 AUGUST 2021 | HydrocarbonProcessing.com
Temperature, °C
1
7
13
19
25
31
37
43
49
55
61
67
73
79
85
91
127
133
139
145
151
97
103
109
115
121
157
163
169
175
181
187
193
199
X parameter
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