Japanese to English Tech/Engineering Manufacturing Biopharmaceutical production

in context sentence:

配管押し水受入完了後20分以上待ち

Part of manufacturing instructions describing movement of a liquid.

Also, appearing elsewhere in the same manufacturing instructions are:

ラインをエアー押しする
and
ライン押しをする

I realize that エアー押し should probably be the transfer of process material using air (pressurized), but I am not sure about 配管押し, and similarly ライン押し.

Is it transfer due to gravity (pipeline slope)? An answer or other help appreciated.