Water Treatment
Boiler, cooling, potable, FWG chemistry.
Steel and water are not a stable combination. The water on a vessel is a managed chemistry experiment running continuously — lose control of it and the failure mode isn’t a warning light, it’s a corroded tube or a pitted heat exchanger discovered during an expensive drydock.
The four water systems
| System | Critical parameters | Failure mode |
|---|---|---|
| Boiler water | pH, conductivity, hardness, phosphate, chloride | Tube corrosion, scale, foaming |
| Cooling water | pH, conductivity, nitrite, chloride, microbial | General corrosion, biofilm, seawater ingress |
| Potable water | Chlorine, hardness, microbial, salinity | Health hazard, scale, taste |
| FWG output | Salinity, conductivity | Heat exchanger pitting downstream |
Each system has its own spec range, dosing protocol, and consumption profile. The pipeline tracks each independently then integrates findings into the senior review.
Parameter classification — three tiers
For each parameter the analyzer compares the latest reading against the maker/equipment-specific spec:
| Tier | Condition |
|---|---|
| OK | Within spec, trend stable |
| WARNING | Outside spec but within wider safety envelope |
| CRITICAL | Outside safety envelope, or step jump from previous reading |
The wider safety envelope matters: a boiler at slightly elevated chloride is something to watch; a boiler at chloride three times the spec is a corrosion event in progress.
Worked example — MV POSUN
End-of-April water-treatment review:
| System | Parameter | Latest | Spec | Trend | Verdict |
|---|---|---|---|---|---|
| Boiler | pH | 10.8 | 10.5–11.5 | Stable | OK |
| Boiler | Conductivity | 1,650 µS | ≤ 2,500 | Rising | Watch |
| Boiler | Chloride | 42 ppm | ≤ 25 | Step jump from 18 | CRITICAL |
| Boiler | Phosphate | 28 ppm | 20–40 | Stable | OK |
| Cooling | pH | 8.6 | 8.5–9.5 | Falling | WARNING |
| Cooling | Nitrite | 720 ppm | ≥ 500 | Falling | Watch |
| Cooling | Chloride | 45 ppm | ≤ 60 | Stable | OK |
| Cooling | Microbial | 320 cfu/mL | ≤ 1,000 | Rising | Watch |
| FWG | Salinity | 3.2 ppm | ≤ 5 | Rising | Watch |
Chemical inventory:
| Chemical | ROB (kg) | Daily use | Days remaining |
|---|---|---|---|
| Boiler treatment (Drew Liquitreat) | 28 | 1.2 | 23 |
| Cooling inhibitor | 84 | 1.8 | 47 |
| Biocide | 12 | 0.4 | 30 |
| FWG cleaner | 45 | 1.1 | 41 |
Verdict: HIGH.
- Boiler chloride step jump (18 → 42 ppm): feedwater contamination event — most likely a condenser tube leak. Investigation required immediately.
- Cooling pH falling + nitrite falling: dosing rate insufficient or fresh-water leak. Cross-check dosing pump and FW expansion-tank level.
- Boiler treatment ROB 23 days: re-order urgent; next bunker is 14 days away but supplier lead-time is 21 days.
Under the hood
Trend assessment — direction matters as much as magnitude
A three-sample trend per parameter classifies systems as Improving / Stable / Deteriorating:
slope = (x_n − x_(n−2)) / (2 × Δt)For an upper-bounded parameter (chloride, conductivity, microbial count), a positive slope is Deteriorating regardless of whether the current value is in spec — a vessel whose chloride has doubled between samples is heading somewhere bad even if the latest reading is technically OK.
PARAMETER_BOUNDS = { "boiler_chloride": "upper", "boiler_pH": "lower", "boiler_phosphate": "range", "cooling_chloride": "upper", "cooling_pH": "lower", "cooling_nitrite": "lower", "cooling_microbial": "upper", "fwg_salinity": "upper", # …}
def trend_verdict(samples, bound_direction): if len(samples) < 3: return "Insufficient data" slope = (samples[-1] - samples[-3]) / (2 * SAMPLE_INTERVAL_DAYS) if abs(slope) < NOISE_THRESHOLD: return "Stable" if (slope > 0 and bound_direction == "upper") or \ (slope < 0 and bound_direction == "lower"): return "Deteriorating" return "Improving"Chemical inventory — days remaining and re-order tiers
Daily_consumption = (ROB_(n−1) − ROB_n) / Δt
Days_remaining = ROB_n / Daily_consumption| Days remaining | Tier |
|---|---|
| > 60 | OK |
| 30 – 60 | Plan re-order |
| < 30 | Re-order urgent |
| < 7 | Critical — operational risk |
A vessel that runs out of boiler treatment chemical at sea is forced to either secure the boiler or run untreated water — neither is acceptable as a planned outcome.
Equipment-damage risk mapping — specific patterns
Boiler chloride above limit — saltwater contamination of boiler feedwater. Causes pitting corrosion of tubes; in severe cases leads to tube failure and boiler shutdown. Source: condenser leak, distillate carryover, or salt-water bunker fill error.
Cooling chloride above limit — seawater ingress into a closed cooling-water circuit. Cooler shell-side leak is the typical source. Even small ingress drives general corrosion across the entire circuit.
Cooling pH falling — the closed circuit’s buffering capacity is being exhausted. Either dosing is insufficient or a fresh-water leak is admitting make-up water. Drives general corrosion across all wetted metals.
Microbial counts rising — biofilm developing in the cooling circuit. Biofilms shield metal from cathodic protection and corrosion inhibitor — pitting starts under the film. Treatment: biocide dosing plus circuit cleaning.
FWG salinity rising trend — either the FWG demister is failing (allowing salt carryover) or the seawater feed is more saline than the unit can handle. Membrane FWGs are particularly sensitive.
Escalation triggers
| Trigger | Severity |
|---|---|
| Boiler chloride above critical limit | CRITICAL |
| Boiler chloride step jump from previous sample | CRITICAL |
| Cooling chloride above critical limit | CRITICAL |
| FWG salinity rising trend with current value > 7 ppm | HIGH |
| Microbial count above safety limit | HIGH |
| Critical chemical with no re-supply in lead-time window | HIGH |
Data sources and suppliers
| Source | What it provides |
|---|---|
| Vessel-side test forms | Boiler / cooling / potable / FWG chemistry readings filed by Chief Engineer |
| Chemical inventory register (ERP) | Per-chemical ROB, supplier, daily dosing |
| Noon report (water tests) | FWG output salinity, additional spot readings |
| FWG telemetry | Continuous salinity / conductivity logging where instrumented |
Major marine water-treatment suppliers supported: Drew Marine (boiler, cooling, fuel-treatment), Wilhelmsen / Unitor (boiler, cooling, fuel-treatment, cleaning), Vecom (boiler and cooling), Chevron Marine (boiler and cooling).
