Injection of flash steam and condensate into the feedwater tank should be via a deaerator head. A deaerator ensures mixing of make-up water, condensate and flash steam to deliver uniform temperatures.
Cold water absorbs free oxygen & other gases, which result into corrosion in the boiler & steam plant. This oxygen needs to be removed & can be driven off by heating or by absorbing through chemical process. Oxygen can be removed totally by heating the feed water to 105 deg. C. This heating is achieved by condensate and flash steam recovery.
In feed water tank often hot condensate returned, cold make up water and flash steam are mixed directly. This leads to improper mixing wherein the flash steam does not completely condense by contact with the cold make up water. This improper mixing is indicated by sights of heavy flash steam venting from the feed tank.
A Deaerator head is recommended as good engineering practice for complete mixing of hot condensate, flash steam and cold make up water. Efficient mixing ensures complete heat recovery leading to feedwater at uniform temperatures. Uniform feedwater temperature also avoides thermal shock to the boiler.