Phased-Array Ultrasonic Testing for King Bolt and Nut Thread in Pressure Vessels

摘要

The testing for king bolt and nut thread in critical pressure vessels like hydrogenation reactors is very important. If the thread has any cracks, it may lead to sealing surface leakage of pressure vessel, even fire and explosion, which could be disastrous. How to detect thread effectively and find defects timely to eliminate hidden safety trouble? The current standard (DLT694-1999 The Technical Guide of Ultrasonic Inspection for High-temperature Tight Bolts) prescribes that using a single-crystal angle probe or straight probe to perform conventional ultrasonic testing for bolt thread. This method, due to influence of initial pulse, has a big blind zone. Many thread signals are buried in the initial pulse so that parts of thread cannot be detected. These positions are often prone to have defects. There are no related standards about nut thread, hence it is hard to have an effective testing. The testing for king bolt and nut thread in pressure vessels is a technical difficulty that the industry urgently needs a solution. To fix this issue, we have decided to use ultrasonic phased-array technology to handle the existing problem. Ultrasonic phased-array transverse and longitudinal wave 2D imaging technology was applied to test thread defect of king bolts and nuts of different sizes. Finally, it is found that when testing bolts and nuts at the end and cylindrical surface, the blind area of the king bolt thread tested by conventional ultrasonic testing can be avoided, and the bolt cracks at the outboard side can be detected as well. With clear thread imaging, easy identification of cracks, simple and intuitive. The test result is very good in field application. This paper describes using ultrasonic phased-array to test king bolt and nut thread, setting of system parameters, probe selection, artificial defects of test blocks and application examples.