Manual valves may be grouped according to the way the closure member moves onto the seat. Four groups ofvalves are thereby distinguishable.

1. Closing-down valves. A stopper-like closure member is moved to and from the seat in the direction of theseat axis.

2.Slide valves.A gate-ike closure member is moved across the flow passage.

3. Rotary valves. A plug or disc-like closure member is rotated within the fow passage, around an axis normalto the flow stream.

4.Flex-body valves. The closure member flexes the valve body.

Each valve group represents a number of distinct types of valves that use the same method of flow regulation, butdiffer in the shape of the closure member. For example, plug valves and butterly valves are both rotary valves,but of a diferent type. in addition, each valve is made in numerous variations to satisfy particular service needsFigure 3-1 illustrates the principal methods of flow regulation and names the types of valves, which belong to aparticular valve group.