The train that lost its salt barrier

The incident

A 120 m3/h seawater RO train has run for two years at 45 % recovery on 35,000 mg/L feed. Design salt rejection is 99.5 %, feed pressure 62 bar, pump efficiency 82 %, and an isobaric energy-recovery device is fitted at 80 % efficiency.

The night shift reports permeate TDS of 700 mg/L, up from a commissioning baseline of 175 mg/L. Feed TDS, recovery, permeate flow, feed pressure and differential pressure are all logged at their baseline values.

The online concentrate analyser reads 40,000 mg/L. The laboratory has queried that figure but has not been able to explain it, and the design concentrate value is not in the shift log. The site limit for permeate TDS is 250 mg/L, and the shift supervisor wants a decision before handover.

Objective

Establish which logged value cannot be trusted, reconstruct what the salt balance requires, identify the mechanism, choose the defensible intervention, and state the acceptance value that proves the repair.

Shift log against commissioning

The online concentrate analyser reads 40,000 mg/L. The laboratory has queried it; the design concentrate value is not in the shift log. Site limit for permeate TDS: 250 mg/L.