After a faulty item of equipment has been repaired it will usually be returned to service (or sent to the spare parts store for future use); the plug is needed to ensure the repaired item is not damaged in transit (and will usually be very similar to the plugs on new items delivered from the factory). For pipeline components, the purpose is to keep dirt out.

'remis en circulation' might also word here - not in the sense the expression is used in respect of motor vehicles, but returning to form part of the circulating equipment stock.