Because of the diversity of the types of systems, fluids, and environments in which valves must operate, a wide variety of valves have been designed to meet specific needs. Some valves are capable of throttling flow, other valves can only stop flow, others work well in corrosive systems, and others handle high pressure fluids. Each valve type has certain inherent advantages and disadvantages. Understanding these differences and how they affect the valve’s application or operation is necessary for the successful operation of a facility. Although all valves have the same basic components and function to control flow, the method of controlling the flow can vary dramatically. In general, there are three methods of controlling flow through a valve. 1. Move a disc or plug into or against an orifice (globe valve or needle type valve). 2. Slide a flat, cylindrical, or spherical surface across an orifice (gate valve or plug valve). 3. Rotate a disc or ellipse about a shaft extending across the diameter of an orifice (butterfly valve or ball valve).
