Lukoil Neftohim Burgas (LNB), the single refinery currently operating in Bulgaria, constructed and put into operation a new high pressure diesel hydrotreater in 2010. Since its start-up, the unit has operated smoothly by hydrotreating a heavy diesel fraction (200-360 degrees C) with sulfur content of 0.8% and density at 20 C of 0.860 g/cm(3) and producing hydrotreated diesel with sulfur content of 6 ppm and density at 20 degrees C of 0.847 g/cm(3). However, in 2013 a sudden increase of the pressure of the treating gas compressor at the hydrotreater scheme was registered and the unit had to be switched off in emergency. Sediments formed in the tubes of the heat-exchanger, used for heat transfer between the hydrotreater product and hydrogen treating gases, were found. This study presents the results from the determination of the composition and the origin of the deposit, as well as the technological solutions to prevent its formation. According of the data from different analytical techniques applied, the following composition of the deposit was supposed: 46.6% ammonium chloride, 9.6% nitrates, 13.6% organic chlorides and about 30% water, organic impurities (carbonic acids and naphthenic hydrocarbons) and corrosion products. Numerical investigation of the temperature field in the area of the deposits is also presented. (C) 2014 Elsevier B.V. All rights reserved.