Metal　magnetic　memory　test　(MMMT),　which　is　a　new　subject　in　the　field　of　nondestructive　examination,　can　determine　regions　of　stress　concentration　by　testing　the　distribution　of　the　magnetic　field　of　metal　structures　so　as　to　effectively　diagnose　premature　defects.　MMMT　and　other　test　methods　are　applied　in　the　study　to　put　a　propylene　purifier　of　a　temperature-jump　accident　and　a　leaked　ammonia　vessel　through　safety　evaluation.　Results　are　as　follows:　The　margin　of　safety　declines　after　the　purifier　is　overburnt;　several　stress　concentrations　are　observed　within　the　overburnt　area　and　the　level　of　stress　concentration　rises　after　one-month　operation;　and　overpressure　operation　of　the　purifier　must　be　strictly　avoided　and　carefully　monitored　during　later　operation.　Cracks　are　observed　on　the　ammonia　vessel　after　one　year＇s　service.　Extremely　high　residual　stress　is　the　primary　cause　of　cracks.　After　four　years　in　service,　the　residual　stresses　existing　in　the　area　of　the　base　metal　and　weld　zone　are　still　greater　than　0.5　σ　S,　which　results　in　numerous　cracks　due　to　stress　corrosion.　From　the　MMMT　result　of　the　ammonia　vessel＇s　defects,　it　can　be　seen　that　the　derivative　of　magnetic　density　(dH　p　/dx)　is　an　important　reference　variable.　Within　the　31　leakage　points,　the　one　whose　values　of　dH　p　/dx　are　above　10　occupy　67.7%,　and　the　ones　above　8　share　96.8%.　©　2009　Higher　Education　Press　and　Springer-Verlag　GmbH.