High-Temperature Oil and Gas Flange Assembly: Main Causes of Leakage

Failure of Flange Bolts under High Temperature

The flange bolts are the primary fastening elements in the flange connection system. Under high-temperature conditions, bolt failure not only affects the sealing performance of the flange connection but also significantly impacts the system's economic efficiency and safety. The main failure mechanisms of flange bolts under high temperature include creep, preload relaxation, brittle fracture, and fatigue.

Creep Relaxation Failure of Flange Bolts

Bolts preloaded at room temperature will undergo creep when the temperature and pressure rise to operating conditions and stabilize over time. The amount of creep strain depends on the stress level on the bolts and the operating temperature. Experiments show that with time, due to high-temperature creep, the elastic strain in the bolts gradually decreases, and residual stress diminishes accordingly—this phenomenon is known as stress relaxation in the bolts. When the bolts' clamping force decreases due to stress relaxation, the gasket's compressive force reduces correspondingly. If the compressive force falls below the minimum required sealing force under operating conditions, the flange joint begins to leak, resulting in seal failure.

Influence of Bolt Preload

Improper control of bolt preload can induce additional bending stress. Coupled with fluctuations in temperature and medium load, this can cause premature bolt failure. Insufficient or excessive actual preload on the bolts during high-temperature or pressurized operation can also lead to leakage in the flange connection.

Table 1: Time (t × 10,000 hours) Required for Axial Stress in Bolts to Decrease by Various Amounts Under Initial Preload Reduction

From Table 1, it is evident that the preload at room temperature significantly affects the creep behavior of the bolt material. Maintaining the room-temperature preload under high-temperature service requires re-tightening the bolts at relatively short intervals. For example, with a preload stress of 80 MPa, if a 10% stress drop triggers re-tightening, the interval is approximately 10 months. Under high-temperature operation, preload should not be set too high to avoid the risk of crushing the spiral wound gasket or the need for frequent thermal retightening.

Brittle Fracture and Fatigue Failure of Bolts

Long-term operation of bolts under high temperature can alter the microstructure inside the bolts and rapidly reduce impact toughness. Sudden brittle fracture may occur under abnormal temperature fluctuations or improper startup/shutdown procedures. Without continuous online monitoring or standardized periodic inspection of sealing points, this failure mode can happen unexpectedly, causing severe damage.

Creep and Stress Relaxation of Washer Shim

Improving sealing performance and reducing leakage most effectively depends on enhancing gasket performance. Due to the high compressive forces involved, the washer shim undergoes creep and stress relaxation, which increases in likelihood with rising temperature. During this process, both stress and creep change gradually. In the flange assembly structure consisting of bolts, flange, and gasket, the stress relaxation of the gasket inevitably leads to strain, and creep causes a reduction in bolt preload and sealing capability, ultimately resulting in flange leakage.

Common failure modes of the washer shim include creep relaxation failure, high-temperature rebound loss, and high-temperature strength degradation.

Failure Analysis of Flange Assembly Under Long-Term High Temperature

High-Temperature Creep Relaxation Failure of Flange

Long-term high-temperature service of the flange assembly can cause excessive deformation due to insufficient stiffness. Additionally, differences in the linear expansion coefficients between flange and bolt materials lead to incompatible deformation, causing the flange to undergo slow, permanent, and irreversible plastic deformation, resulting in unrecoverable leakage failure.

Interface Leakage Failure of Flange Under High Temperature

This is usually caused by poor machining accuracy of the flange assembly or improper installation that results in surface scratches. It can also be due to the presence of impurities in the material during startup or frequent temperature fluctuations.

Flange Assembly Insulation

Insulating the flange assembly connection helps prevent heat loss and supports energy saving and carbon reduction efforts. It also protects the flange connection from leakage caused by sudden heavy rain or water spray. However, insulation raises the working temperature of the flange assembly, which over time leads to high-temperature creep and relaxation of bolts, flange, and gasket. The rebound capacity of sealing elements decreases, significantly reducing the gasket's effective sealing stress and ultimately causing flange system leakage failure.

After the flange, gasket, and bolts reach equilibrium under high-temperature operating conditions, creep and relaxation cause the gasket to seal at a lower stress level. In cases of emergency shutdown or sudden cooling from heavy rain spray on the flange joint, if operating pressure remains unchanged, thermal contraction reduces the effective clamping force on the gasket, making leakage highly likely.