What Does This Dosing Calculator Do?
Boiler feedwater needs chemicals to stop corrosion and scale. Dissolved oxygen pits feed line and boiler tubes. Hardness forms scale and cuts heat transfer. Low pH causes acid corrosion.
This calculator finds how much chemical to dose. For oxygen scavenger, it uses dissolved oxygen ppm and desired residual. For phosphate and caustic, it uses blowdown rate and desired residual in boiler water. It gives dose in ppm, consumption in kg/h and kg/day as 100% active and as your product solution.
Chemical Dosing Formulas
Sulfite: Dose = DO × 7.88 + Residual (ppm as Na2SO3)Hydrazine: Dose = DO × 1.0 + Residual (ppm as N2H4)Phosphate/Caustic: Cf = Cb × (BD% /100) (ppm in feedwater)Consumption kg/h = Dose ppm × Feedwater kg/h /1,000,000Product L/h ≈ kg/h 100% / (Conc% /100) / density ≈ kg/h / (Conc%/100)
DO is dissolved oxygen ppm in feedwater after deaerator. Residual is extra chemical left in boiler water to handle inleakage. For sulfite, keep 20-40 ppm residual as SO3 in boiler water, 3-5 ppm for high pressure. For hydrazine, 0.1-0.3 ppm residual. For phosphate, 2-6 ppm as PO4 in boiler water. Blowdown carries chemical out, so feed must replace it.
How to Use the Calculator
- Pick chemical. Sulfite for low pressure, hydrazine or carbohydrazide for high pressure, phosphate for scale and pH buffer.
- Enter feedwater flow. Use total feedwater to boiler. If you only know steam flow, add blowdown rate to get feedwater flow.
- Enter DO or residual. For O2 scavenger, enter DO from meter and desired residual. For phosphate, enter blowdown % and desired boiler residual.
- Enter product concentration. If you dose 10% solution, enter 10. If powder, enter 100.
- Click Calculate Dose. You get dose ppm and consumption per hour and per day.
Understanding Results
Dose as 100% Active
This is ppm needed if chemical were pure 100%. Use it to compare products. Multiply by flow to get kg/h.
Consumption as Product Solution
If you dose 10% sulfite solution, you need 10 times more solution than 100% basis. This is what pump actually doses in L/h.
Daily and Monthly Estimate
Daily kg times operating hours per day. Use for purchase planning and storage tank sizing.
Factors That Change Dosing
Chemical dosing is linked to whole boiler system. More feedwater flow needs more chemical, and feedwater flow changes with steam load and blowdown.
Our boiler feedwater calculation system gives you feedwater flow, TDH and power from steam flow and blowdown. Use that flow here to size chemical pump and daily stock. Also check boiler horsepower to steam flow if you only know BHP and need feedwater flow estimate.
- Dissolved oxygen: After good deaerator, DO 0.005-0.02 ppm. After poor deaerator or air inleak, DO 0.05-0.2 ppm and sulfite dose doubles.
- Blowdown: Higher blowdown removes more phosphate and caustic, so you must dose more. If blowdown 5% vs 2%, phosphate dose is 2.5 times higher.
- pH and conductivity: If feedwater pH low, need more caustic or amine. High conductivity means more solids, need more blowdown and more phosphate.
- Pressure: Below 40 bar, sulfite okay. Above 40 bar, sulfite adds solids, use hydrazine or carbohydrazide or DEHA.
Worked Example - Phosphate
Feedwater 20 m³/h, blowdown 3%, desired boiler residual 4 ppm PO4 as TSP, product 10% solution.
| Step | Calc | Result |
|---|---|---|
| Feedwater kg/h | 20×1000 | 20,000 kg/h |
| Dose in feed Cf | 4 ppm × 3% /100 | 0.12 ppm as 100% PO4 |
| Consumption 100% | 0.12×20,000/1e6 | 0.0024 kg/h |
| As 10% product | 0.0024 /0.10 | 0.024 kg/h =0.024 L/h |
| Per day 24h | 0.024×24 | 0.576 kg/day solution |
So very little phosphate needed because blowdown is small. If blowdown rises to 10%, dose becomes 0.4 ppm in feed, 0.008 kg/h 100% basis.
Common Mistakes
- Dosing to steam flow only: Chemical stays in boiler water, not steam. Use feedwater flow, not steam flow for phosphate mass balance.
- Ignoring residual: Dosing only stoichiometric amount leaves no buffer for air inleak. Always add residual.
- Mixing ppm as SO3 vs Na2SO3: 1 ppm SO3 = 1.57 ppm Na2SO3. Our calculator uses ppm as Na2SO3 product. If spec is as SO3, multiply by 1.57.
- Using powder as 100% when it is not: Commercial TSP may have water. Check active content.
Frequently Asked Questions
How much sulfite per ppm oxygen?
7.88 ppm sodium sulfite as 100% per 1 ppm O2. Plus 20-40 ppm residual for low pressure, 3-5 ppm residual as SO3 for high pressure.
What residual for hydrazine?
0.1-0.3 ppm as N2H4 in feedwater after deaerator. Keep slight excess to scavenge any inleak.
How to calculate phosphate dose?
Dose in feedwater ppm = desired residual in boiler water ppm × blowdown % /100. Then kg/h = dose ppm × feed kg/h /1e6.
Can I use sulfite at high pressure?
Below about 40 bar, sulfite is common. Above 40 bar, sulfite decomposes and adds solids. Use hydrazine, carbohydrazide or DEHA for high pressure.
Basis: Stoichiometry Na2SO3 + 0.5 O2 → Na2SO4 (7.88:1), N2H4 + O2 → N2 +2H2O (1:1), phosphate mass balance Feed×Cf = Blowdown×Cb per ASME and Nalco water treatment guides. Residual recommendations per ABMA and boiler maker. Density and conversion uses 1 kg/L for solution, adjust if product density different. For final dosing follow water treatment supplier and boiler maker limits.