Boiler Water Chemistry — 7 Tests, Treatment & Pressure Proof
Seven titrations decide whether the drum lives or dies — and two pressure proofs decide whether the surveyor signs.
Key Principles at a Glance 5 points
- Seven tests with surveyor-fixed numbers: P-alkalinity 150 ppm, T-alkalinity 300, chloride under 300, phosphate 40–60, pH 10–11, conductivity 3000 low-pressure / 400 high.
- Every titration reads drops × 10 ppm — 100 ml sample, phenolphthalein pink-to-colourless for P, methyl-orange yellow-to-pink for T, chromate-to-brown for chloride.
- Hydrazine eats the last oxygen at 0.1–1 ppm reserve; chloride is removed by blowdown, not chemistry — partial for drift, flash for settled sludge.
- Caustic embrittlement is Na2CO3 hydrolysing to NaOH and then sodium ferrate — P-alkalinity exists to warn of it before steel turns brittle.
- Accumulation test: fires at full, steam locked in, pressure must stop within 10% of working — 7 minutes water-tube, 15 smoke-tube — or the safety valves fail the survey.
1. Seven Tests — Drops × 10 Is the Whole Arithmetic
Boiler water is tested to stop three killers before they start: scale (phosphate guards), corrosion (alkalinity guards) and dissolved oxygen (hydrazine guards) — plus the salt, hardness and solids that feed carryover. Every titration below shares one arithmetic: drops of reagent × 10 = ppm, on a 100 ml cooled sample:
| Test | Procedure in one breath | Surveyor number |
|---|---|---|
| P-alkalinity | 10 drops phenolphthalein → pink in base; titrate H2SO4 till colourless | 150 ppm; warns of NaOH concentration |
| T-alkalinity | Same sample + methyl orange → yellow; titrate till pink | 300 ppm; OH + carbonate + bicarbonate |
| Chloride | Phenolphthalein + H2SO4, chromate drops, titrate silver nitrate to brown | Under 300 ppm — salt load |
| Phosphate | Filtered uncooled sample, molybdate, stannous chloride → blue; comparator | 40–60 ppm; cloudy in 2 min ≈ 70, in 5 min ≈ 20 |
| pH | Dip paper, comparator match | 10–11, alkaline guard against acid gas |
| Hardness | Soap solution stirred in — foam volume reads softness | More foam, less hardness; scale risk if hard |
| Conductivity | Meter, or hardness test as proxy | 3000 LP / 400 HP µS/cm |
2. Treatment — Scavenge Oxygen, Blow Down Salt
Chemistry removes what tests find. Dissolved oxygen pits steel, so hydrazine (or sulphite) is dosed to scavenge it, held at a small reserve so the next ingress is eaten too. Salt cannot be dosed away — chloride leaves only through blowdown:
| Enemy | Weapon | Watch-out |
|---|---|---|
| Oxygen pitting | Hydrazine dosed, 0.1–1 ppm reserve | Zero reserve means the next air ingress pits freely |
| Chloride climb | Partial blowdown to gauge-glass bottom, refill fresh | Complete only for survey/maintenance — it wastes heat |
| Settled sludge | Flash blowdown: ship-side first, then boiler valve cracked repeatedly | Ship-side first proves the line is unpressurised before agitation |
| Oil ingress | Liquid coagulant / sodium aluminate; scum blowdown, ease firing, cut chemicals | Oil arrives with heaters off — foaming, priming and carryover follow |
3. Enemies — Caustic, Acid, Scale, Carryover
Four failure chains the surveyor can ask for by name. Caustic embrittlement is the subtlest: hardness treatment leaves Na2CO3 behind, concentrated carbonate hydrolyses to NaOH, and caustic attacks grain boundaries into sodium ferrate — steel that looks sound and snaps:
Acid + CO2 corrosion
Acid gases dissolve into acidic attack; CO2 rides steam to corrode condensate lines. Alkalinity neutralises both — pH 10–11 is the proof.
Scale + carryover
Hardness bakes into insulating scale; excess phosphate, oil, high level or low pressure foams and primes solids into steam — erosion, water hammer, superheater salt.
4. Level Proof — DP Control, Accumulation Test, Glass Drill
Chemistry means nothing if level lies. Differential-pressure level control reads the drum continuously; the accumulation test proves the safety valves can swallow full firing with nowhere for steam to go; the glass drill proves the gauge itself tells the truth:
Prove the glass. Shut both gauge stop valves, press the spring-loaded test button to bypass shutdown logic, open the blowdown drain — the low-water alarm sounding proves the alarm chain, not just the glass.
Refill and release. Shut the drain, reopen both stops, watch the glass refill and the alarm clear, release the test button — then log time and result.
Trust, then verify. DP control flies the drum day to day; the glass drill and accumulation test are its survey proofs. Level, combustion and draft routines live in Boiler Water, Combustion & Draft.