Oil waste, is one of the most important pollutants in the oil and gas industry. Since the wells' oil has significant saltwater, the effluent amount increases with increasing oil reservoir extraction. Separating the saltwater from the extracted oil before starting the refinery process plays an essential role in reducing the oil costs as well as the useful transfer capacity. This paper presents a new Chapman-Kolmogorov Equation-based (CKEB) method to evaluate a desalination system's availability with three-state equipment and weighted-k-out-of-n configuration. In this system, the equipment is repairable, and each repair facility can repair all equipment types of different sub-systems (pump stations). We consider all failures and repairs to have a constant rate (with Exponential distribution) and use the Chapman-Kolmogorov Equation to drive the system’s availability. Then we validate the presented method using a simulation technique. Finally, the elapsed time of both solving techniques is compared. The results show the superiority of the CKEB technique in terms of computational time. Compared with the simulation technique, the computational time ratio for the CKEB method is between 0.0002% to 0.0058% for the small-size problems, between 0.05% to 0.94% for the medium-size problems, and between 1.31% to 5.39% for the large-size problems.