For fluid piping systems, valves are the controlling elements. valves are used to isolate equipment and piping systems, regulate flow, prevent back flow, and regulate and relieve pressure. The most suitable valve must be carefully selected for the piping systems. The minimum design or selection parameters for the valve most suitable for an application are the following: size, material of construction,pressure and temperature ratings and end connections. In addition, if the valve is to be used for control purposes, additional parameters must be defined. Theses parameters include: method of operation, maximum flow capacity requirement, pressure drop during normal flowing conditions, pressure drop at shutoff and maximum and minimum inlet pressure at the valve. Theses parameters are met by selecting body styles, material of construction, seats, packing, end connections, operations and supports.