A theoretical explanation of the process applied to a real plant with P&Is and useful for a training on the plant. The purpose of this book is to show and explain the process of hypochlorination from sea water, a process used in most of industrial plants (mostly in oil and gas plants). The book has P&Is, drawings and schemes to better understand the presence of the hypochlorination system in a big plant. The theoretical explanation is followed by a practical example of a hypochlorination system, an average hypochlorination system that could easily lead the reader to better understand the features of the system in relation with the features of a real oil plant. This book feature can either help the professional and technichan to design a proper system (being also able to proportion the system to the oil plant production, its needs and following a practical example, generally made up only after years of know-how fails and experience) and a guide for those who have to approach these kind of plants for the first time. A second purpose of the book is to be a "training" book for those who are going to work to this kind of plant ( that's why as example has been choosen an "average" system similar to lots of those in-use in countless oil & gas and energy production plants). This "guide" includes also an innovative "manteinance prevision" system made with the composition of "polarization curves" conceived during the last decade by Engineer Alfredo Mensi after years of experience in the industrial engineering field, in the design (not only in the industrial field, but also in the mechanical, electrical and space engineering) and a huge experience in the pre-commissioning, commissioning and start-up of the systems explained in the book.