Hydrazine Dosing Calculation in Boilers: Formula, Examples, and Practical Guide
Introduction
In high-pressure boilers, maintaining proper feed water chemistry is essential for safe operation, efficiency, and long equipment life. One of the critical parameters controlled in boiler feed water treatment is dissolved oxygen (DO) removal.
What is the impact dissolved oxygen in boiler feed water?
Even a small amount of dissolved oxygen can cause severe corrosion in boiler tubes, economizers, feed water heaters, and condensate systems.
Which Chemical is used to remove dissolved oxygen from feed water?
Hydrazine (N₂H₄) has been widely used as an oxygen scavenger in boiler water treatment because of its excellent oxygen removal capability and its ability to form a protective magnetite layer inside boiler systems.
Why it is important to decide the right dosing quantity of Hydrazine?
Proper hydrazine dosing calculation is important because:
- Under-dosing may result in oxygen corrosion.
- Over-dosing increases chemical consumption and can create operational issues.
Correct dosing helps maintain recommended boiler water chemistry limits.
What is Hydrazine and Why is it Used in Boilers?
Hydrazine (N₂H₄) is a strong reducing agent used for removing dissolved oxygen from boiler feed water.
The chemical reaction is:
N2H4 + O2 ——— N2 + 2H2O
One mole of hydrazine reacts with one mole of oxygen.
Molecular weights:
- Hydrazine (N₂H₄) = 32 kg/kmol
- Oxygen (O₂) = 32 kg/kmol
Therefore:
1 kg of hydrazine removes approximately 1 kg of dissolved oxygen.
In practical operation, extra hydrazine is dosed to maintain residual hydrazine concentration in feed water.
Typical residual hydrazine values:
| Boiler Pressure | Recommended Hydrazine Residual |
| Low pressure boiler | 0.02–0.05 ppm |
| Medium pressure boiler | 0.02–0.10 ppm |
| High pressure boiler | 0.05–0.20 ppm |
What are the Factors Required for Hydrazine Dosing Calculation?
To calculate hydrazine requirement, the following input is required:
- Feed water flow rate (m³/hr)
- Dissolved oxygen concentration (ppm)
- Required residual hydrazine (ppm)
- Hydrazine solution concentration (%)
- Density of hydrazine solution
Basic Hydrazine Dosing Formula
Step 1: Calculate Oxygen Load
Oxygen load (kg/hr)={Feed water flow (m³/hr) X Dissolved Oxygen in feed water in mg/L
Because:
1 ppm = 1 mg/litre
and
1 m³ = 1000 litres
Step 2: Calculate Hydrazine Required
1 gram of oxygen requires approximately 1 gram of pure hydrazine.
In practice, operators add 10–20% excess to ensure complete oxygen removal.
Hydrazine quantity required (g/hr) = Oxygen Load (g/hr) X 1.2
For example 100 M3/hr feed water having dissolved oxygen 0.05 ppm will have oxygen load
= 100 X 0.05 =5 g/hr
Any Pure Hydrazine required = 5 X 1.2 = 6 g/hr.
Solved examples:
1-A 100 TPH boiler receives 101 m³/hr of feedwater containing 0.05 mg/L of dissolved oxygen. Calculate the hydrazine requirement per day to remove the dissolved oxygen.
We have, Oxygen load = Feed water flow X Dissolved oxygen in feed water
= 101 M3/hr X 0.05 = 101 X 1000 X 0.05 =5050 mg/hr
Oxygen load = 5.05 g/hr
Oxygen load per day = 5.05 X 24 = 121.2 gram/day
Considering purity of Hydrazine is 100%.
Hydrazine required per day to remove 121.2 gram of Oxygen from feed water will be;
121.2 X 120% = 145.44 gram.
2-A 50 TPH boiler receives 51 m³/hr of feedwater containing 0.01 mg/L of dissolved oxygen. Calculate the hydrazine requirement per day to remove the dissolved oxygen.Assume purity of Hydrazine is 50%
We have, Oxygen load = Feed water flow X Dissolved oxygen in feed water
= 51 M3/hr X 0.01 = 51 X 1000 X 0.01 =510 mg/hr
Oxygen load = 0.5 g/hr
Oxygen load per day = 0.5 X 24 = 12 gram/day
Considering purity of Hydrazine is 50%.
Hydrazine required per day to remove 12 gram of Oxygen from feed water will be;
51 X 120%/50% = 122.4 gram.
Why do in Modern Boilers Hydrazine use is prohibited?
Modern power plants often use alternatives such as carbohydrazide, hydroquinone, or DEHA instead of hydrazine because hydrazine is classified as a hazardous chemical and its use is restricted in many countries.
