Relief Valves

Valves [Lesson 6]

5. Relief Valves
Purpose: Relief valves are used to protect systems from over-pressure or to control processes by allowing flow to commence when a certain pressure has been reached.
Operation: Refer to Figure

A spring holds the valve disc in place against the seat. The valve, therefore, will not open until the force exerted on the valve disc by the fluid pressure exceeds the force exerted by the spring. When this occurs, flow can take place through the outlet port until the fluid pressure is reduced to below the valve operatingpressure. The spring force will then release the valve.

Relief valves operate automatically and are usually pre-set to a specified relief setting by the manufacturers or adjusted when in use, if required. Re-calibration is then required

Butterfly Valves

Valves [Lesson 4]


4. Butterfly Valves

Purpose: Butterfly valves are used for controlling flow and can act as a shutuoff ~va1ve, if the sealing arrangement is designed accordinglyThe disc or wafer rotates about a vertical axis and can be turned through 90º.

The disc seals against the opening to cut off flow and can be positioned at a point between fully closed and fully open as required.

Some butterfly valves have a direct acting lever, others are operated through gearboxes when finer control is required. In most cases, a clockwise movement will close the valve

Globe Valves

Valves [Lesson 3]

2. Globe Valves
Purpose Globe valves are used to control flow as they can operate quite safely at part openings. A good shutoff can also be achieved if required
Operation: Refer to Figure

When the hand wheel is turned clockwise, the disc is against a seat, stopping the flow. Turning the hand wheel anti-clockwise lifts the disc from its seat and allows flow to continue.

The high pressure is usually on the bottom of the plug, so that the stem, seal, etc. are not under continuous pressure.

Applications are widespread, including domestic water taps