EngiMetric Arena / Diagnose the Failure
Permeate TDS rose while pressure drop stayed flat. Why?
Competency Fault diagnosis primary Systems and balances supporting Evidence interpretation supporting
What you should take away: Use salt passage, differential pressure and normalized flow together to distinguish membrane integrity loss from fouling or scaling.
The situation
What is happening
A seawater RO train has run for two years. Permeate TDS has risen from 175 mg/L to 700 mg/L at the same feed TDS of 35 000 mg/L, recovery, flow and temperature. Feed pressure and differential pressure are unchanged at 60 bar and 1.5 bar. What is the most plausible diagnosis?
Given
| Quantity | Value | Unit | Note |
|---|---|---|---|
| Feed TDS | 35 000 | mg/L | Unchanged seawater feed. |
| Baseline permeate TDS | 175 | mg/L | About 99.5 % rejection. |
| Current permeate TDS | 700 | mg/L | About 98.0 % rejection. |
| Feed pressure | 60 | bar | Unchanged. |
| Differential pressure | 1.5 | bar | Unchanged across the train. |
| Recovery | 45 | % | Unchanged. |
Assumptions this challenge makes
- Flow, temperature and recovery are normalized to the same baseline conditions.
- The permeate TDS sample is representative and not contaminated by a post-treatment stream.
- The differential pressure is measured across the same membrane array as the baseline.
Your call
Your answer
Hints
- Compare salt passage, not just permeate TDS. Feed TDS is unchanged, so the ratio is meaningful.
- Ask what fouling and scaling usually do to differential pressure and normalized flow.
- A stable hydraulic profile with a sharp rejection loss points away from a deposit and toward a breach.
Your answer Engine result
Where that comes from
Each limit, checked
Why
- Baseline salt passage is 175 / 35 000 = 0.50 %. Current salt passage is 700 / 35 000 = 2.00 %, a fourfold increase.
- Feed pressure, differential pressure and recovery are unchanged, so the hydraulic resistance of the array has not materially changed.
- Fouling and scaling normally raise differential pressure and reduce normalized flow. Their absence makes a deposit a weaker explanation.
- A membrane breach, a failed interconnector or a seal leak can pass feed directly into permeate without changing the overall pressure drop. That matches the data better.
- The diagnosis still needs confirmation: profile the permeate conductivity per vessel, check seals and O-rings, and perform a integrity test before replacing membranes.
What this challenge does not establish
- The data are a single operating point; a full normalized trend is stronger evidence than one sample.
- The analyzer does not locate the failed element; vessel-by-vessel profiling or a integrity test is required.
- Feed TDS is assumed unchanged; a real seawater intake can vary with tide and season.
- The calculation does not distinguish a membrane breach from a seal leak or an interconnector failure.