Impact of interruption of antiscalant dosing or cleaning balls circulation during MSF plant operation

MSF desalination plants occasionally experience interruption of antiscalant dosing or circulation of cleaning balls. The interruption may be either due to sudden system failure or the system is to be isolated for maintenance works. In such situations, specific operational procedures are normally adopted to counter the consequences of mal-operation of antiscalant dosing or cleaning balls systems. The various corrective measures that are normally adopted by SWCC MSF plants to mitigate the consequences of malfunctions of the antiscalant dose or the on-line ball cleaning systems are reviewed. The aim of this study is to establish operating conditions and consequences when antiscalant dosing or ball circulation is interrupted. The established operating conditions are intended to be used as a guide to recommend appropriate lines of actions to be followed by SWCC MSF plants to counter the impact of interruption of antiscalant dosing or cleaning balls circulation. Two series of evaluation tests were carried out. In the first series, extensive pilot plant tests were conducted to obtain preliminary assessment on the impact of sudden or gradual interruption of antiscalant dosing and the consequences when operating without cleaning balls circulation. In the second series, an evaluation test was conducted in one of the MSF distillers of Al-Khobar Phase-II plant to explore the consequences when antiscalant dosing was suddenly interrupted and stopped completely. The pilot plant experimental results revealed that when antiscalant dosing was suddenly interrupted or stopped, the plant could safely operate without scale formation for a maximum period of 24 h. When the MSF pilot plant was operated at TBT of 90°C and brine recycle concentration ratio of 1.4 without circulation of cleaning balls, the experimental results revealed that the effectiveness of a polycarboxylate based antiscalant in inhibiting scale formation was quite different compared to that of a phosphonate based antiscalant. Ball cleaning operation is highly influencing and effective when phosphonate based antiscalant is used. It has also been found that the threshold inhibition of the phosphonate based antiscalant with no ball cleaning at a dose rate of 1.0 ppm was more effective than that when using a relatively high dose rate of 3.0 ppm. Over-dosing of phosphonate based antiscalant with no ball cleaning was responsible for formation of excessive sludge. Based on the preliminary tests which were carried out in the MSF pilot plant, it is recommended that plants which are using phosphonate based antiscalant have to review the corrective actions which they normally follow in case of interruption of ball circulation. Al-Khobar Phase-II plant evaluation test revealed that it is quite safe to operate the MSF distiller without antiscalant dosing for a maximum period of 3 h.