PSI - Issue 80

Sherif Ezzeldin et al. / Procedia Structural Integrity 80 (2026) 195–202 S. Ezzeldin / Structural Integrity Procedia 00 (2023) 000–000

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5.2. Results without Re-torquing

5.2.1. Bolt Loads Figure 4 shows bolt load relaxation at 23, and 60 ◦ C, and the contact stress contour plots post-pressurization and after 1 year of operation. Initial loads vary slightly from 20 kN due to elastic interactions but remain within acceptable limits (Fig. 3). Mean values and load variation are shown. Most relaxation occurs in the first 50 days, consistent with prior studies Jacobsson et al. (2011); Shahin et al. (2021). During heating, thermal expansion and reduced HDPE sti ff ness initially increase bolt loads. However, HDPE stress relaxation soon dominates, causing a sustained decrease until reaching a steady-state plateau (Fig. 4). Following pressurization, the contact area is approximately 45% of the flange surface due to flange rotation. In the subsequent stage, HDPE relaxation occurs. Over the service year, relaxation becomes dominant, accompanied by increased flange rotation and pipe bulging caused by creep under internal pressure. A similar trend is observed at 60 ◦ C, with added e ff ects from thermal expansion.

Fig. 4: Mean bolt load relaxation and contact stress evolution at 23 and 60 ◦ C over one service year.

5.2.2. Contact Stresses To assess the sealing performance of the HDPE flange connection, the average compressive contact stress σ avg c over the contact area is used, defined as:

1 χ 0

1 2 π (1 − χ 0 )

2 π

avg c

σ c ( θ,χ ) d θ d χ

(3)

σ

=

0

where χ 0 is the normalized radial coordinate at which contact stress becomes non-zero. This average stress is compared to the minimum required value of 2 . 85 MPa to ensure leak-tight integrity. Figure 5 shows the evolution of average contact stress σ avg c over logarithmic time under isothermal conditions. Higher temperatures accelerate the decline in σ avg c , particularly over extended periods, indicating reduced sealing performance at elevated ambient temperatures. With an initial bolt load of 20 kN and internal pressure of 0.9 MPa, leak-tightness cannot be maintained indefinitely at any tested temperature. Leakage occurs earlier at higher temperatures compared to 23 ◦ C, underscoring the adverse e ff ects of thermal loading. Re-torqueing is therefore essential to preserve seal integrity over the joint’s service life.

5.3. Results with Re-torquing

5.3.1. Isothermal Conditions Results at constant temperatures of 23 and 60 ◦ C are presented, with re-torqueing applied using the F i values determined in Section 5.1.2. Re-torqueing is carried out when σ avg c ≤ 2 . 85 MPa. At 23 ◦ C (Fig. 6(a)), σ avg c relaxes to near the leakage threshold after 60 days, and a single re-torque maintains tightness for one year. The safety factor