4. Method for determination of the minimum component wall thickness necessary to transfer the actual service loads of the material after long-term service under creep conditions with known creep strength characteristics

The most important element necessary to determine the minimum required component wall thickness goe, which will be able to transfer the required service load (σep, Tep), is to have the characteristics of residual creep strength for the material of test component after service in the form of log σ = f(tre) for the operating temperature Tep. This is the first step in the adopted procedure. Step 2 is to determine the residual creep strength RZe/Tep/tep for the adopted temperature of further service Te and the assumed time of further service te based on log σ = f(tre) at Tep = const. The value of RZe/Tep/tep obtained from the characteristics makes it possible to determine the acceptable stress k for the parameters of further service adopted in this way, which is step 3. To calculate the required minimum wall thickness goe for the adopted parameters of further service, the nominal outside diameter DOut of the component, the operating pressure pep, and the construction weakening coefficient z should be defined, which is step 4. Step 5 is the determination of the required minimum wall thickness goe of the component for the adopted parameters of further service.

And the last, sixth, step is to compare the obtained value of the calculated required minimum wall thickness goe to the measured minimum actual thickness gact min. If for the adopted time of further safe service tep:


4.1 Characteristics of residual creep strength

DOI: http://dx.doi.org/10.5772/intechopen.84986

obtained results are presented in Table 3.

service are provided in Table 4.

Component Designation of

200,000 h service under creep conditions.

Table 3.

63

Figure 13.

properties

Elbow Rze/10,000 105

14MoV6–3 steel after 200,000 h service at 530°C under creep conditions.

Rze/30,000 89 Rze/50,000 82 Rze/100,000 68

Temporary residual creep strength of the material of test elbow and circumferential welded joint made of

Residual creep strength for the adopted levels of temperature Tep = 530, 540°C, and times tep = 10,000, 30,000, 50,000, and 100,000 h of further service of the materials of test elbow made of 14MoV6–3 steel after

4.3 Determination of the acceptable stress

further service Tep = 540°C.

service

The temporary creep strength curve for the material of elbow after 200,000 h service under creep conditions plotted based on results of the tests discussed in Chapter 2 is shown in Figure 13 in the form of lg σ = f(tre) for the temperature of

The Procedure for Determining the Time of Safe Service beyond the Design Service Time Based…

4.2 Determination of residual creep strength for the adopted time of further

Based on the characteristics in point 2, the residual creep strength at 530 and 540°C for 10,000, 30,000, 50,000, and 100,000 h was determined for the materials of test primary steam pipeline components after 200,000 h service. The

The value of acceptable stress k is equal to the value of acceptable lower scatter

k ¼ 0:8RZe<sup>=</sup>Te=te:

The obtained values of acceptable stress k for the adopted levels of temperature Tep = 530, 540°C, and times te = 10,000, 30,000, 50,000, and 100,000 h of further

> Residual creep strength Rze, MPa at the test temperature Te = 540°C

band �20% from the obtained mean value of residual creep strength for the adopted time tep and temperature Tep. It is calculated from the following formula:

If goe ≥ gact min and the component cannot be allowed for further service, a shorter time of service should be adopted and the proposed procedure repeated. It may turn out that with a shorter time of further service tep, for the adopted parameters of further service, the condition of goe < gact min will be met. This will allow the test component to be operated for such a shorter time tep.

The proposed procedure in the form of an algorithm consisting of consecutive steps is shown in Figure 12. Its application is presented on the example of the primary steam pipeline elbow and circumferential welded joint made of 13 HMF steel after 200,000 h service.

#### Figure 12.

Procedure for determination of the minimum wall thickness of a component after long-term service under creep conditions necessary to transfer the required service loads based on the residual life characteristics of the assessed material after service under creep conditions.

The Procedure for Determining the Time of Safe Service beyond the Design Service Time Based… DOI: http://dx.doi.org/10.5772/intechopen.84986
