"Purge" is bleeding, i.e. removal of air from a circuit intended to contain a liquid only.

"Chasse" is flushing, supposedly emptying, draining. Obviously draining a circuit will do the very opposite of removing air! So I can only assume that they mean not draining but filling at high flowrate. For whatever reason this is prohibited. Maybe because, if using a coolant, it results a large quantity of spillage (large quantities of liquid being expelled from the system), with financial and environmental consequences.

Maybe the bleeding operation is to be done slowly and carefully, progressively closing drains and vents as the liquid first starts seeping from them, then moving to the next highest point in the system and repeating the procedure.

Unless they mean that fresh liquid is not to be used to drive old liquid out (possible risk of contamination, with some of the old liquid remaining), but rather that the old liquid must be drained off, (the pipes cleaned), and new liquid fed into the system.

Not at all clear to me, in the detail, but "bleeding by means of flushing" is probably just as precise as the French!

CHASSE : VIDANGE A GRAND DEBIT d'un réservoir ou d'une canalisation.
http://www.basse-normandie.sante.gouv.fr/drass/environnement...

FLUSHING
Flushing is the process of running the water until all the water standing in the system has been replaced with fresh water. It usually takes a few minutes and uses one to two gallons.
http://www.purdue.edu/dp/envirosoft/leadenv/src/glossary.htm

FLUSHING – The procedure of CLEANING THE INSIDE OF A WATER MAIN by moving LARGE AMOUNTS OF WATER through a hydrant or fire pump. ...
www.hosemonster.com/glossary.html

FLUSHING - Legionella Glossary



Flushing - The process of draining and cleaning a cooling system, including all associated pipework.

FLUSHING - The process of DRAINING AND CLEANING A COOLING SYSTEM, including all associated pipework
http://www.legionellacontrol.com/flushing-glossary.htm