Liquor cooling on the nine Precipitator rows at QAL is achieved by nine Flash Coolers (ISC’s) situated between the fourth and fifth tanks on each row of twelve tanks. Typically, these vessels cool the pregnant liquor by up to 12 degrees. Instability in the operation of these flash vessels was limiting their cooling capability and was also the primary cause of contamination of the cooling water circuit. This paper describes how CFD enabled the cause of the instability to be identified, and a practical solution developed for a fifty year old problem. Implementation of the low cost solution has taken a system that previously experienced over 250 instability events per year to zero since the final vessel implementation in June 2017.