What is the function of bleed valve?

I'm sure there are tons of applications. Here is one definition:

"Bleeder Valve - A small valve on a pipeline, pump, or tank from which samples are drawn or to vent air or oil; sample valve."

Bleed - To drain off liquid or gas, generally slowly, through a valve called a bleeder. To BLEED DOWN, or BLEED OFF, means to slowly release the pressure of a well or of pressurized equipment.

http://www.lloydminsterheavyoil.com/glossary.htm

T "bleed" valve is a valve installed for the intensional purpose of releasing vapor or liquid from a trapped space or container.

If you work on a car's brakes, you need to "bleed" the air out of the brake fluid system so the brakes work properly.

If you work in a process plant and you shut-down a part of a system for maintenance, you need to "bleed" down the pressure and maybe a hot fluid so the work can be done.

"Bleed" valves are installed for operation, maintenance and for safety.

I hope this helps.

Thank you!!!