When chlorine lines from a new caustic soda plant were added to chlorine lines from an older plant that used the same chlorine compression and liquefaction systems, we encountered abnormal pressure fluctuations on the chlorine side of the new plant. These fluctuations were being transmitted to the chlorine cycle in the older plant, posing a safety hazard in both facilities. After checking the pressure control systems, a design correlation for positive-pressure seal pot was observed and it was found from the inconsistency in the sizes of those equipments in the two plants. The problem was solved by reducing the diameter of the chlorine pipeline in the positive-pressure seal pot of the new plant. After implementing this solution, pressure fluctuations were fully removed, and satisfactory pressure control was attained in both plants. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.