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In this paper, stress measurement and analysis were performed on three in-service 1500 m3 spherical petrochemical tanks to test for cracking. Welding residual stresses were measured by X-ray diffraction in both the lower girth weld and the bottom polar plate of the spherical tanks. The results show that welding residual stresses were relatively high in the lower girth weld of the #1 and #2 tanks; the maximum stress was up to 1.06 σs and occurred in the weld zone (heat affected zone or weld seam) on the inner wall. The results reveal that bulk heat treatment was ineffective for the #1 and #2 tanks but was effective for the #3 tank, because its stresses in various zones leading to surface flattening and stresses in both the lower girth weld and the bottom polar plate were less than 249 MPa (0.67 σs) and 208 MPa (0.56 σs), respectively. High welding residual stress (WRS) from poorly applied bulk heat treatment is the principal cause of spherical tank cracking. WRS in the #2 tank decreased noticeably after secondary heat treatment and repair welding were performed. [ABSTRACT FROM AUTHOR]