Flash steam should be separated from condensate while returning the condensate to feedwater tank.
Flash steam is generated when the condensate is moved from a high to a lower pressure. In order to make up for the difference in sensible heat, the excess heat in condensate is used to boil a part of the condensate back to steam. This constitutes flash steam.
If the condensate along with the flash steam that it generates is returned in the same line, i.e. the flash steam is not separated from the condensate, a dual phase fluid flow results in the pipeline. Dual phase flows by their nature increase erosion corrosion in pipelines. This reduces the life of the pipe carrying the condensate. Many a time the damage will be seen with the color of the condensate turning brown as it carries the corroded metal along. It is recommended to separate the flash steam from the condensate to prevent such a dual phase flow in the pipeline to prevent damage to the pipelines. Corroded pipelines are always unsafe and should be avoided.
Additionally, flash steam contains about 20% of the energy content of condensate even though usually 10% by mass. Returning this flash steam to the boiler feed tank or using it locally, reduces the loss by optimizing the utilization.