Instrument air (IA) system is one of the critical systems in the utilities section of a refinery complex. IA system of this refinery consists of compressor package and dryer package to supply air to refinery header at 0.9Mpa (128 psi) with dew point less than -20degC (-4degF). The 3 stage compressors have intercoolers with extended plate surface fin type tubes. The metallurgy of the tubes is Admiralty brass SB-111 C44300. In this cooler, compressed air is in the shell side and fresh cooling water on the tube side. All 8 intercoolers in the compressor package leaked in a period of 1 month. A failed sample tube was analyzed using Optical Emission Spectroscopy, Stereo Microscope, SEM/EDS and XRD. This paper discusses the findings and the methodology used to identify the reason for failure as Ammonia Stress Corrosion Cracking. Process parameter analysis revealed that the leak from a recycle gas cooler in Hydrocracker unit as the possible source for ammonia in cooling water system. The full-length tube expansion for plate type fins is the source for residual stress. This paper briefs upon the IOW and the monitoring mechanisms for such mixed metallurgy closed loop cooling water system.