Vibration isolation test method of large non-constrained metal bellows expansion joints

In the vibration isolation testing of large non-constrained metal bellows expansion joints (hereinafter referred to as expansion joints), due to their easy instability under filling pressure, test devices that balance the huge blind force generated by the internal pressure cannot be completely fixed at both ends of the expansion joints, making testing difficult and leading to the lack of a standard test method. For this reason, a device is designed to test the vibration isolation effect of expansion joints.

Double-metal expansion joints are supported against each other with both ends fixed by the pedestal support, achieving axial stabilization. Two different limiting methods（i.e., bearing cable and airbag) are designed to prevent radial instability. The axial and radial excitation of the expansion joints under pressure is carried out using a shaker, the acceleration response in the frequency range of 10 Hz to 315 Hz under the two limiting methods is collected, and the difference in vibration level between the input end and output end is calculated.

The total vibration level drops measured by the bearing cable and airbag devices respectively decrease with the increase in the internal pressure of the expansion joints, and the total vibration level drop of the airbag device under internal pressure of 0P, 0.8P and P increases by 8.59 dB, 5.70 dB and 4.80 dB respectively in the axial excitation direction of the bearing cable device. In the radial excitation direction, it increases by 4.32 dB, 3.57 dB and 4.95 dB respectively.

The designed test device can measure the vibration isolation performance of metal bellows expansion joints. As the airbag limiting method has less influence on the vibration level drop than the bearing cable limiting method, the device for testing the vibration level drop of expansion joints should adopt airbag radial limiting to obtain better measurement results.