In general, valves are differentiated according to their design features such as sliders, valves, cocks and flaps. However, in practice, they are often distinguished according to their function, type and function, or combined construction type-function and material. The information provided in this book is intended to be of a general nature as well as that of specific products. Students as well as those fluent with hydrodynamics will find this book useful. SI units are now used extensively world-wide and are used throughout this publication wherever possible. However, where information has been quoted from sources where non-metric units are still in common use, these units may have been retained.