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　s　s　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　sigma(s))　and　208　MPa　(0.56　sigma(s)),　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.