Purge
Purge turns up in 4 fields: tribology, adhesives, welding, boilers. Here is what it means to the people in each one.
Purge in tribology
To pump fresh grease into a bearing until the old grease is pushed out of the relief port or seal.
To flush a bearing housing with new grease, driving out old, contaminated or incompatible grease until clean product appears at the relief or drain. Purging is the standard way to change grease types or recover from water ingress. It is done with the relief plug removed and ideally with the machine running, or the pressure will blow the seals, and motors are left running with the drain open afterward to shed the excess.
"Pull the drain plug and purge it till clean blue comes out, then let it run open for half an hour."
Purge in adhesives
To run adhesive through a mixer or dispense valve and discard it, clearing old, unmixed or off-ratio material.
Operators purge after fitting a new static mixer, after breaks that approach pot life, after drum changes and before a ratio check. Hot melt and PUR systems are purged to clear charred or cured material. The purged adhesive goes into a waste cup, and many lines specify a purge weight or bead length before the first production part.
"Purge two bead lengths into the cup before you hit the first part, that new mixer still has unmixed resin in it."
Purge in welding
Inert gas flowed inside a pipe or behind a joint to keep the back of the weld from oxidizing.
Displacement of air from the root side of a weld with argon, or sometimes nitrogen for suitable alloys, so the backside of stainless, nickel alloy and titanium welds stays clean and corrosion resistant. Dams or water-soluble paper block the pipe, and oxygen is brought down to a low level, often below 0.1 percent, checked with an oxygen meter before welding. Without it, the root oxidizes into sugar.
"Purge meter reads 40 ppm, start the root, and keep the flow low so you don't blow it concave."
Purge in boilers
Blowing air through the furnace and flue before ignition or after shutdown to clear out unburned fuel.
Prepurge runs the fan with dampers open long enough to replace the furnace and flue gas volume several times, so any leaked fuel is diluted and removed before a spark is struck. Commercial burners commonly purge for 30 seconds to a few minutes at high fire air flow, proven by an air switch and damper position switch. Postpurge clears the furnace after the flame goes out. In hydronics, purge also means pushing air out of piping with water.
"Don't jumper the high fire switch to skip the purge, that's the one minute that keeps the boiler in one piece."