Effects of Constant and Alternating Temperatures on Sealing Performance of Threaded Pipe Joints over a Broad Temperature Range

Due to improper assembly and varying work conditions, gas leakage frequently occurs in threaded pipe joints. This work investigates the degradation of sealing performance in threaded pipe joints in rail transit vehicles for a broad temperature range arising from ambient environmental variations. By combining experimental investigations with numerical simulations, the effects of constant temperatures on torque decay and leakage rates across five nominal sizes (DN8, DN10, DN15, DN20, DN25) were systematically examined, as well as the influence of alternating temperatures (−30°C to 70°C) on DN25 joints. Mechanical tests were conducted to characterize the silicone rubber gaskets, followed by long-term torque decay monitoring and sealing performance tests. The results indicate that alternating thermal cycles induce significantly more severe torque decay and sealing degradation than constant temperatures, and the degradation becomes more severe as the frequency of temperature fluctuation increases. Under the specified tightening torque, the DN8, DN10, and DN25 joints maintained leakage rates within the sealing requirements after 2 months at room temperature, whereas DN15 and DN20 exceeded the safety threshold at a medium pressure of 0.7 MPa. Both low (−30°C) and high (70°C) constant temperatures substantially reduced the service life of the joints. These findings underscore the critical importance of accounting for thermal cycling in joint design and operation and provide practical recommendations for enhancing sealing reliability in pneumatic systems.