Maintaining proper boiler water chemistry is essential for the safe and efficient operation of high-pressure (HP) boilers. Among the various chemicals used in boiler water treatment, Trisodium Phosphate (TSP) is one of the most widely used phosphate compounds.
What are the functions of Trisodium Phosphate (TSP) in Boiler water treatment?
- It helps maintain the desiread boiler water pH,
- Prevents scale formation
- Protects boiler tubes from corrosion.
However, overdosing or underdoing TSP can lead to serious operational issues such as caustic gouging, carryover, poor steam purity, and deposition inside boiler tubes. Therefore, boiler operators and water treatment engineers should know how to accurately calculate the required TSP dosage.
What is Trisodium Phosphate (TSP)?
Trisodium Phosphate (Na₃PO₄) is an alkaline chemical added to boiler water to maintain the proper phosphate reserve and alkalinity.
Its primary functions are:
- Maintains boiler water pH between 9.0 and 11.0 (depending on boiler pressure)
- Prevents calcium hardness from forming hard scale
- Converts calcium into soft, non-adherent phosphate sludge
- Reduces corrosion inside boiler tubes
- Helps maintain phosphate residual in boiler water
TSP is generally used in coordinated phosphate treatment (CPT) or congruent phosphate treatment for high-pressure boilers.
Why is TSP Dosing Required?
Even after efficient demineralization (DM) water treatment, traces of hardness may enter the boiler due to:
- DM plant leakage
- Condenser leakage
- Make-up water contamination
- Chemical imbalance
TSP reacts with hardness ions such as calcium and magnesium.
Example reaction:
3Ca²⁺ + 2Na₃PO₄ → Ca₃(PO₄)₂ ↓ + 6Na⁺
The calcium phosphate formed is soft sludge that can be removed through continuous blowdown.
Inputs Required for TSP Dose Calculation
Before calculating the dose, collect the following data:
- Boiler water volume (m³)
- Existing phosphate residual (ppm)
- Desired phosphate residual (ppm)
- Purity of TSP (%)
- Molecular weight of TSP
Generally,
- Molecular weight of Na₃PO₄ = 164
- Commercial purity = 95–98%
Basic TSP Dosage Formula
The required quantity of TSP can be calculated using:
TSP Required (kg)
=Boiler Water Volume (litre) X Increase in PO₄ Required (ppm or mg/litre) X 164/ (Purity% X 1000 X 1000)
Where:
- 164 = Molecular weight of TSP
- Purity (%) of commercial TSP is 95% to 98%
- 1000 X 1000 = Conversion from milli grams to kilograms
Step-by-Step Calculation Procedure
Step 1
Measure the current boiler phosphate residual.
Example:
Current PO₄ = 0 ppm
Desired PO₄ = 10 ppm
Increase required
= 10 – 0
= 10 ppm
Step 2
Determine the boiler water volume.
Suppose
Boiler water volume = 50 m³
Step 3
Calculate TSP quantity.
TSP= 50 X 1000 litre × 10 mg/litre × 164 /(95 X 1000 X 1000)
= 0.863 kg
Approximately
870 grams of TSP
should be dissolved in DM water and dosed gradually into the boiler.
Example:
Given Data
Boiler water volume = 120 m³
Current phosphate = 3 ppm
Desired phosphate = 10 ppm
Purity = 98%
Increase required
= 10 – 3 =7 ppm
= 7 ppm
TSP Required
= 120 × 1000 litre X 7 mg/litre X 164 / (98 X 1000 X 1000)
= 1.4 kg
Answer
Dose approximately 1.5 Kg TSP
Recommended Boiler Water Phosphate Levels
Typical phosphate residual ranges are:
- Low-pressure boilers: 20–40 ppm
- Medium-pressure boilers: 10–25 ppm
- High-pressure boilers: 2–10 ppm (depending on the treatment program and boiler manufacturer recommendations)
What are the Effects of Under-Dosing of Trisodium Phosphate (TSP) ?
Insufficient phosphate can result in:
- Scale formation
- Tube overheating
- Reduced heat transfer
- Higher fuel consumption
- Boiler tube failures
- Increased maintenance costs
What are the Effects of Over-Dosing of Trisodium Phosphate (TSP) ?
Excessive phosphate dosing may cause:
- High boiler water conductivity
- Foaming
- Steam carryover
- Turbine deposits
- Caustic concentration
- Increased blowdown losses
Trisodium Phosphate (TSP) dosing is a critical part of high-pressure boiler water treatment. Its primary purpose is to maintain phosphate reserve, prevent hardness scale, and protect boiler tubes from corrosion.
Accurate dosing depends on the boiler water volume, current phosphate level, desired phosphate residual, and the purity of the TSP used. By applying the correct calculation formula and following good chemical dosing practices, boiler operators can ensure efficient heat transfer, improved steam purity, and longer equipment life.
