A root cause failure analysis was performed on a vaporizer coil removed from a horizontal forced circulation vaporizer. The carbon steel coil was wound in a right-hand helix with a coil centerline diameter of about 2 m. The vaporizer was gas fired and used Dowtherm A as the heat transfer fluid. Design conditions are based on annular fluid flow to cool the coil wall. NDE, metallographic and fractographic examinations were performed. Numerous, circumferentially oriented, OD initiating cracks were found near the crown for two coils near the non-fired end of the vaporizer. The cracking was confined to the inner diameter of the vaporizer coil at positions from 4:00 to 7:00. The cracking was characterized as transgranular and the fracture surface had beach marks. The failure mechanism was thermal fatigue. The heat transfer calculation predicted that dryout of the coil would occur for coils at the non-fired end of the vaporizer during low flow transients. Dryout results in rapid increase in the tube wall temperature. Thermal cycling of the coil is completed by liquid quenching resulting from resumption of normal flow rates and the return to annular flow. The probable root cause of failure was low flow transient operation.