Insulation is widely used in process plants to reduce heat loss of process fluids in piping and pressure vessels. However, insulation is often not installed around Normally No Flow (NNF) line pipe. In a refinery plant, a steam leak incident happened due to a through-wall crack, which was found around the connection between an insulated superheated steam line with insulation and an uninsulated safety valve line. The through-wall crack was identified to be a fatigue crack initiated at the inner surface by fractography. An unsteady Computational Fluid Dynamics (CFD) analysis was performed to identify mechanism of the through-wall crack initiation. Based on the observation of fractography and the CFD analysis, it is inferred that the through-wall crack was induced by a high cycle thermal fatigue phenomenon, so-called thermal striping, due to incomplete mixing of hot and cold fluids. Many thermal striping incidents in nuclear plants and process plants have been reported. In view of the above fact, it is suggested that conventional insulation installation practice for NNF line pipe, in particular superheated steam line, may cause cracks due to thermal striping around the connection between main superheated steam pipe and branch dead-end leg. In this paper, a convenient guideline for insulation installation is proposed for a dead-end leg of superheated steam line to prevent cracks caused by thermal striping. The guideline can be used to judge the necessity of insulation installation, based on degree of superheat of steam.