Piping system is used to move fluid from one place to another desired. Piping system has the potential to occur in a critical pipeline. Critical pipeline is a path that is suspected of experiencing excessive stress and is likely to experience a leak. Caused include large diameter pipes, high pressure and high temperatures. Line 170-ACF0001-A2A3R-18 "-ST RFCC at PT Pertamina (Persero) Refinery Unit IV Cilacap has a diameter of 18" pipe, design temperature of 250 ° C and pressure of 3.5 kgf / cm². This line is a critical pipe, so a stress analysis of the pipe is needed to ensure the pipeline is safe. Pipe stress analysis using Caesar RI 2013 software. The data entered in the form of static load include thermal load, weight and internal pressure as well as dynamic loads including eartquake loads. The data is obtained from isometric drawing, and pipe specifications. The pipe stress analysis is continued based on ASME B31.1 code standards and flange leak checks based on the ASME B16.5 standard. The result of analysis on line 170-ACF0001-A2A3R-18 "-ST RFCC at Cilacap PT Pertamina (Persero) Refinery Unit IV Cilacap produced a stress ratio of 207,74% in nodals 1128 and 160,93% in nodal 1060 which exceeds the permissible levels. The maximum deflection of the DX is 104,1670 mm and DZ is -907968 mm. A flange leak occurred with a ratio of 155,28% (L3), 156,91% (L24) and 157,98% (L25). So it is necessary to modify the replacement elbow with pipe band 5D on nodal 1128, design changes and addition of components to nodal 1060. Replacement flange with rating 300#. After modifying the L24 stress ratio to 87,05% and L26 to 96,21% and the flange did not leak with L3 ratio to 44,48%, L24 to 45,31% and L25 to 45,62% so the path can be said to be safe.