Valves

Valves [Lesson 2]

Specific Types

1. Gate Valves

Purpose: gate valves are used when a tight shut-off is required. They must not be used for throttling (i.e. must be fully off or fully on) as a restricted flow through a gate valve will erode the seat of the wedge disc.

The wedge-shaped disc is moved up to open the valve by turning the wheel anti-clockwise. To shut off the flow, the wheel is turned fully clockwise until the disc is properly seated and covers the opening.

One common type is the rising stem type. In other designs, the wheel is fixed to the stem and rises with it.

The position of the stem/stem and hand wheel indicates whether the valve is open or shut. If the stem is raised, the valve is open.

2. Ball/Plug Valves
Purpose: Ball and plug valves are used to provide a quick, simple shut-off. They are operated by turning the ball or plug through 90º. Ball and plug valves should not be used for throttling, as a restriction in the flow will lead to erosion of the valve.
Operation: Refer to Figure

The ball or plug has an opening through the centre. When this opening is in line with the inlet and outlet ports, flow will be allowed. When the ball or plug is turned through 90; no flow can take place. Good sealing can be obtained, particularly when special sealing rings made of PTFE/Teflon are used.

Some ball and plug valves are lubricated to provide a seal and prevent wear, and should be regularly lubricated with the proper lubricant.