Page 94 - Handbook of Materials Failure Analysis

88 CHAPTER 4 Creep damage of high alloyed reformer tubes

Table 4.5 Residual Life t res of Reformer Tubes Without
any Safety Factor (Experimental Evaluation)
T (°C) t res (h) t res (Years)
900 88,500 10.1
910 49,950 5.7
920 28,350 3.2
930 16,300 1.9
940 9400 1.1

(Equation 4.2) can be deduced. This rupture time can be considered as the tube resid-
ual life (t res ) in the case of keeping in service.
From Table 4.5, it is evident that the tube residual life is very sensitive to even
small differences of temperature: changes of 10-20 °C inside the tube wall, which are
in the range of measurement errors, may results in significant reductions of creep life.
A theoretical calculation of the residual life can be obtained simply by the dif-
ference between expected rupture time (given by the catalogue minimum curve)
and effective service time. The theoretical residual life is compared with the exper-
imental evaluation of residual life, based on the creep data of deformed samples (type
B1 and B3 samples).
This calculation was performed assuming a pressure of 3.28 MPa (design pres-
sure) and a temperature of 900 °C (operating temperature).
Results, given in Table 4.6, show that the experimental residual life is signifi-
cantly lower than the residual life obtained by theoretical calculation.
This reduction of life is a direct consequence of creep damage accumulation.
Moreover, the experimental residual life drop dramatically when a safety factor
equal to 0.8 is applied on stress, in accordance with the Italian Standards.

Table 4.6 Residual (Life t res ) of Reformer Tubes (Comparison Between
Theoretical and Experimental Evaluation Based on Creep Data of Samples
Taken from Deformed Zones of Tube)
Expected
Service Rupture Residual Residual
b
a
Time (h) Time (h) Life (h) Life (%)
Theoretical calculation 114,800 325,800 211,000 65
(minimum catalogue curve,
safety factor S¼0.8)
Experimental evaluation 114,800 325,800 88,500 27
(no safety factor)
Experimental evaluation 114,800 325,800 17,750 5
(safety factor S¼0.8)
a
From catalogue minimum curve.
b
Fraction of the expected rupture time.