If I understand you correctly, you are asking if it is OK to dump flash steam to an ACC that is at atmospheric pressure. Short answer is NO. The ACC is a heat exchanger. When any heat exchanger is full of air it cannot exchange heat ie. condense steam.
My questions to you are; Why do you want to dump flash steam to the ACC prior to ACC start-up? How long would you want to dump steam to the ACC prior to ACC start-up?
SPX Dry Cooling USA, Inc.