Water is the basis of daily life and industrial development for all communities. Water desalination plants emerged as one of the most important alternatives to overcome the shortage of water resources especially in desertified countries like Libya. Boilers are the main part in thermal desalination plants which depends on steam as the working fluid to heat and evaporate the seawater. with a capacity of 80 tons/hr of superheated steam at 220℃ and 15bar. Due to operating conditions, there was a decrease in steam boilers productivity and low efficiency. Heavy black smoke in chimneys was also detected. This work investigates the performance of the above mentioned boilers to determine the actual causes of these negative results by comparing the design values with different operating readings data. The results show a decrease in the efficiency of the boilers for low loading rates, which is affected by the air/fuel ratio. The deviation of this ratio from the design values leads to low efficiency and the emergence of heavy smoke in the chimneys, which causes deposits on the boiler pipes reducing the effectiveness of heat exchanger and therefore the thermal efficiency. The study also shows that the long operation suspension of the boilers in the first years of its life, due to the lack of discharge network and electricity cutoffs resulted in bad effect on the pipes conditions. The last led to the pipes deterioration resulting in water leaks and thus low boilers evaporation rates.