MONITORING THE HYDROTESTING OF A NEW PRESSURE VESSEL BY MEANS OF ACOUSTIC EMISSION N. Parida, B. Ravi Kumar, G. Das, S. G. N. Murthy and D. K. Bhattacharya
National Metallurgical Laboratory, Jamshedpur 831 007, India
Keywords: Acoustic Emission, Pressure Vessel, Prototype Horton Sphere, Hydrotest, Strain ABSTRACT
The application of a multichannel acoustic emission (AE) test system for condition monitoring of a spherical pressure vessel is presented. The spherical pressure vessel on which the AE tests were carried out was a prototype horton sphere of 1 metre diameter with 12 millimetre wall thickness. This was fabricated from a A537 class I steel, a widely used material for fabrication of horton spheres, as per ASME section VII-div. I. Based on this code, the vessel was hydrotested to a pressure of 74.96 kg/cm2, with a periodic load holds of 10 minutes at 50%, 10 min. at 65%, 10 min. at 85% and 30 min. at 100% of 74.96 kg/cm2. The pressure was then decreased to 50% followed by reloading to 98% of the maximum hydrotest pressure with same periodic load holds as the 1st cycle. AE was monitored during both the loading cycle of the hydrotest, by placing eight (8) 150 kHz resonant probe on the sphere surface in two rows (4 probes in each row). The AE signals generated during hydrotest were amplified to a level of 40 dB by preamplifiers and were then fed into the SPARTAN AT-AE system for source location analysis by triangular method. While conducting the hydrotest, strain measurement was also carried out on the outer surface of the sphere at 4 locations near the T- joints using 4 uniaxial strain gages and a strain indicator model P-3500 from M/s. Measurement Group, USA.
The results show that on increasing the pressure the strain increases linearly in all the four cases, however, the maximum strain recorded was less than 0.08% for 1st loading and less than 0.07% for 2nd loading. This indicated that the deformation of the sphere during hydrotesting was well below the elastic limit of the material used for construction of the horton sphere. The amount of AE signals generated during 1st loading cycle was much higher than in the 2nd loading cycle. This was because when a new pressure vessel is hydrotested for the first time, the hydrotest causes relaxation of high residual stresses, induces local yielding in the areas of high secondary stresses and tightens nuts and bolts of the flanges. Moreover, the cluster analysis of the AE source location show that majority of the AE signals have been generated from bottom and top portions of the sphere due to rubbing of the nuts and bolts. These emissions can mask all but the most serious defects. That is why, the reloading cycle was immediately carried following the 1st hydrotest. As the reloading cycle was to 98% of the initial hydrotest load, additional yielding and emissions have been avoided. The amplitude of the AE signals that are generated during the reloading cycle were below the signal amplitudes for crack growth as confirmed by laboratory tests. Moreover, during the load hold at highest pressure of the reloading cycle, no signals were generated. This may be used as an indicator for checking the soundness of the vessels, because any active flaw shall exhibit AE activity during load hold.
From the study the following conclusions may be drawn:
- For AE testing of a new pressure vessel repressurization should immediately start following the hydrotest, and the data from the 2nd loading cycle should be used to evaluate the conditions of the vessels.
- Triangular source location method with cluster analysis can be used effectively for a spherical vessel to find out the location of defects.
- The peak amplitude of AE signals can be used as an acceptance criteria for integrity assessment of containment vessels through AE technique.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 4, pages 2423 - 2426
Publisher: Ashgate Publishing Company