A boiler is a heat exchanger that doesn't get to be wrong. Once a tube has scaled, the metal under the deposit runs 30–60°C hotter than design, creep takes over, and you're shutting down for a tube failure within months. Once a tube has pitted, you're already past the point where chemistry can save it.
Most Pakistani industrial boilers — IPP HRSGs, refinery package boilers, fertiliser CO-boilers, textile-mill steam plants — are running on chemistry that was specified at commissioning, never reviewed, and is no longer correct for current makeup quality or operating pressure. Here is the framework.
Boiler treatment by pressure class
| Class | Operating pressure | Treatment regime | Critical chemistry |
|---|---|---|---|
| LP package | < 20 bar | Phosphate, polymer, oxygen scavenger, amine | Hardness leakage tolerance highest; coordinated phosphate-pH workable |
| MP industrial | 20–60 bar | Coordinated phosphate-pH (CPT), polymer, scavenger, condensate amine | Demin or two-stage RO + softener makeup; phosphate hideout becomes a constraint |
| HP utility / HRSG | 60–125 bar | All-volatile treatment (AVT) or oxygenated treatment (OT) | Demin makeup only; ppb-level conductivity, cation conductivity < 0.2 µS/cm; no solid chemistry permitted in feed |
| Supercritical | > 220 bar | Oxygenated treatment (OT) with ammonia | Ultra-pure feed; controlled ppb O₂ addition; no contaminant tolerance whatsoever |
Match the chemistry to the pressure class, not to the salesperson's stock list.
The four chemistry roles you must cover
1. Oxygen scavenging
Dissolved oxygen pits boilers and economisers. A deaerator should pull DO to < 7 ppb at full load; chemical scavenger handles the rest. For LP/MP boilers, hydrazine (N₂H₄), DEHA, carbohydrazide, or erythorbate work — dosed to maintain 5–20 ppb residual at the economiser inlet. For HP boilers under AVT, residual is held at single-digit ppb. For OT systems, oxygen is deliberately added (50–250 ppb) and scavenger removed.
2. Internal scale and deposit control
For LP/MP boilers with softened or RO makeup, coordinated phosphate-pH treatment is the standard. Tri-sodium phosphate is dosed to maintain 3–10 ppm phosphate residual and a pH between the coordinated and congruent control lines (typically pH 9.0–10.5 at 20°C). A polymer dispersant (polyacrylate or polymaleic) is co-dosed at 3–10 ppm to keep iron and any hardness leakage in suspension.
For HP utility boilers, AVT runs phosphate-free; pH and conductivity are controlled by volatile chemistry (ammonia + amine) only. Solid chemistry is forbidden because it deposits in low-flow zones and causes phosphate hideout.
3. Condensate corrosion control
Steam condensate is corrosive — carbonic acid forms from CO₂ released when bicarbonate makeup is heated, and pH drops below 5 in copper-iron return lines. Neutralising amines (morpholine, cyclohexylamine, MEA, AMP) are dosed to the steam header at 1–5 ppm to maintain condensate pH 8.5–9.2. Filming amines (octadecylamine-based) are an alternative or supplement for systems with long return lines.
4. Polymer dispersant
Polymer dispersants keep iron oxide, residual hardness, and silica in suspension instead of letting them settle on hot surfaces. Polyacrylate-based dispersants (low MW, 2000–5000 Da) cover hardness and silica work; polymaleic anhydride covers iron-oxide dispersion. Most commercial boiler-treatment products are blends of phosphate + polymer + scavenger pre-formulated for a given pressure class.
The makeup quality problem in Pakistan
Most LP and MP industrial boilers in Pakistan are still running on softened makeup. That worked when boilers ran at 10–15 bar, makeup hardness was <1 ppm as CaCO₃, and silica was a non-issue. It does not work when:
- Operating pressure has crept up to 25 bar after a turbine replacement
- Softener regeneration is irregular and slip events deliver 3–8 ppm hardness episodically
- Feedwater silica from tubewell makeup runs 15–30 mg/L and concentrates 30–50× in the boiler
The fix is upstream — add a polishing RO stage or a mixed-bed demineraliser — not downstream by chasing scale with more chemistry. We have seen multiple plants increase phosphate dose to 30+ ppm trying to outrun a hardness-leakage problem, then experience phosphate hideout under deposits, alkalinity excursions, and caustic-induced stress corrosion cracking.
Control limits — a starting framework for industrial LP/MP boilers
- Feedwater: hardness < 0.1 ppm CaCO₃, DO < 7 ppb, iron < 20 ppb, silica < 0.3 ppm
- Boiler water: conductivity < 3500 µS/cm (LP) / < 1500 µS/cm (MP), phosphate 3–10 ppm, OH alkalinity per coordinated diagram, silica per (steam-pressure × distribution-coefficient) calculation
- Steam: silica < 20 ppb (for downstream superheaters or turbines), sodium < 5 ppb
- Condensate: pH 8.5–9.2, iron < 20 ppb, conductivity < 5 µS/cm
What we actually do on a Pakistani boiler audit
- Pull 30 days of plant log: makeup hardness, DO, conductivity, phosphate, alkalinity, pH, blowdown rate.
- Pull boiler manufacturer's chemistry envelope and current insurance / inspection requirements.
- Compare actual chemistry vs envelope. Almost always there is one parameter trending toward limit, one off-limit, and one not being measured at all.
- Re-specify chemistry — usually a change in phosphate-to-polymer ratio, a scavenger residual target, and a blowdown control philosophy.
- Set up a weekly chemistry review with the operator until the new programme is steady.
For a boiler-chemistry audit or a re-specified dosing programme tied to your operating pressure and makeup chemistry, contact us. See also our feedwater audit service and power-sector applications.