Allowable creep stress for ASTM material. 1

[hjkoors]

We got a process pipeline with a design temperature of 510 °C (950 °F), for the pipeline material A335 Gr.P11 is chosen.

We can find the allowable stress in tension in ASME B31.3 Appendix A, which is the code that we are using in this case.

Here in Denmark we are used to European code, and if we compare A335 Gr.P11 with a European materiel it would be similar to 13CrMo4-5.

Here starts my misery.

The guys from operation asks me. If this pipeline was made according to European code we had to use allowable 200.000 h creep stresses for 13CrMo4-5 at 510 °C (950 °F)? Meaning that the pipeline got a 200.000 h lifetime.

What is the lifetime for our A335 Gr.P11 pipeline with a design temperature of 510 °C (950 °F) according to the ASME code?

Is the lifetime according to ASME code infinite, as it would be using non creep stresses?


Thank you in advance
Hans Koors

 

[metengr]

No, it is not creep life time based for ASME B&PV Code. The ASME method for determination of allowable stress for time dependent service does not look at creep rupture life directly. The allowable stress used by ASME looks at creep deformation over a service life of 100,000 hours or rupture based on the requirements below from ASME Section II, Part D, Appendix 1

(b) At temperatures in the range where creep and
stress rupture strength govern the selection of stresses,
the maximum allowable stress value for all materials
is established by the Committee not to exceed the
lowest of the following:
(1) 100% of the average stress to produce a creep
rate of 0.01%/1000 hr;
(2) 100Favg% of the average stress to cause rupture
at the end of 100,000 hr;
(3) 80% of the minimum stress to cause rupture
at the end of 100,000 hr.
Stress values

For example, for design purposes, a creep deformation rate not to exceed may be used to determine allowable stress if this is the most conservative stress value using the above requirements. This means that after 100,000 hours at x temperature, there will be y% deformation, which is still considered acceptable for service but is used for design purposes. This value does not indicate failure or rupture. Keep in mind that the allowable stress for expected creep deformation has a design margin based on review of creep rupture and deformation data when material is approved for use by ASME.

So, this material may actually have a creep rupture life of several orders of magnitude beyond 100,000 operating hours.
Review carefully, Section II, Part D, Appendix 1 for complete information.

 

[hjkoors]

Thank you very much Metengr

From a maintenance point of view, would you incorporate the pipeline in the existing maintenance plan with a certain expected lifetime?

Thank you in advance
hjkoors

 

[metengr]

hjkoors;
No. What you really need and what ASME has done is allow fitness for service or condition assessment monitoring techniques to be used for creep service. What I mean by this is you can periodically perform metallurgical evaluations of the pipe base material by monitoring creep deformation over time (% swell), along with hardness testing and surface replication to evaluate the microstructure. The % creep swell is an easy and cheap method to evaluate creep rate, where once the pipe reaches 2% swell, this could indicate creep damage is starting to accumulate at an accelerated pace and supplemental testing is required.