A Remaining Life Assessment of Autofrettaged Tubes from an LDPE Facility
The manufacture of low density polyethylene by radical polymerization regularly subjects components to extreme pressures exceeding 20 ksi and, possibly, to runaway reactions with fluid temperatures exceeding 2000 °F and pressures above 30 ksi. Components are often treated with autofrettage to induce a beneficial residual stress distribution that retards crack growth and increases fatigue life. This paper presents a case-study remaining life assessment of two autofrettaged tubes in accordance with API 579-1/ASME FFS-1. Measurements of the remaining residual stress after 40+ years in service agree with FEA predictions of the initial residual stress, indicating no significant stress relaxation over this time. Nevertheless, the MPC Omega creep model is calibrated to the tube material and used to estimate the potential for stress-relaxation due to creep. The model correctly predicts no stress relaxation for over 40 years of normal operation, but creep damage and stress relaxation are predicted for temperature excursions as low as 900 °F for 10.8 s. ASME FFS-1 procedures for assessing brittle fracture, fatigue, plastic collapse, and creep damage are then adapted for autofrettaged components. It is found that autofrettage increases resistance to brittle fracture and fatigue, does not affect limit load analyses, and alters creep damage distributions.
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