Before the water is put into the boiler, any impurities are taken out to keep any damage from happening. There are two kinds of water cleaning for this: chemical and mechanical. This article link goes into a lot of information about the deaerator. It’s something you can read to help you understand. This is where 90 to 95% of the water cleaning is done. The chemical process is used to treat the water after the mechanical treatment. Sodium sulphite and hydrazine are used in low and high process doses, respectively. In the next section, we’ll talk about this subject in more depth.

How LP and HP Dosing Work and Why They Matter in Thermal Power Plants

The LP dosing system method is very important in industrial settings, like power plants and factories that use water-steam cycles. Chemicals are added to this system in certain amounts to stop rusting and keep the pH level stable. This whole process is very exact, and each part does a certain job.

LP Dosing Process

This page explains how the LP dosing method works from start to finish. Here are some more details about the main parts and what they do:

LP Dosing Tank:

• Things that are put into the water-steam cycle are kept in this tank.
• A stirrer inside the tank is constantly stirring the chemical solution. This keeps the chemicals well mixed and stops any sedimentation. In this way, the chemical solution stays completely even.

Reciprocating Pump:

• After the tank, a reciprocating pump is put in place to move the chemical from the tank to the output line.
• This pump makes sure that the right amount of chemical is added by putting the solution into the system at a certain pressure and flow rate.

Discharge Line:

• Chemicals are moved from the pump to the boiler feed pump through the outflow line.
• The chemicals can be put directly into the system through this line, which offers a controlled path.

Valve on Discharge Line:

• On the outflow line, a valve is put in place to control the flow of chemicals.
• Its job is to keep the pressure right and make sure the chemicals move right. This valve can change the flow or pressure either by hand or automatically.
• The discharge line is made of full stainless steel (SS) piping.

Boiler Feed Pump:

• The boiler feed pump is a key part that brings chemicals or cleaned water into the boiler.
• This pump keeps the system from rusting and makes sure that the water that is added to the boiler is fully ready for the water-steam cycle.

How does the LP Dosing Process work?

1. Chemical Storage and Mixing: The chemical solution is kept in the LP dose tank, and an agitator keeps mixing it to make sure it stays completely uniform.
2. Chemical Pumping: The agitator mixes the chemical, and then a reciprocating pump takes it from the tank and sends it to the output line. This pump keeps the pressure and flow at the right levels, which is important for adding the right amount of chemical to the system.
3. Flow Regulation: In the release line, a valve is put in place to control the flow of the chemical. This valve changes the chemical’s flow and pressure to make sure the input is even and steady.
4. Injection to Boiler: Through the discharge line, the chemical reaches the boiler feed pump, which then injects the treated water or chemical into the boiler. This step ensures that the boiler and system components remain safe and free from corrosion.

NOTE: For the chemical injection to work, the pressure in the outlet line must be higher than the pressure in the boiler feed pump line. This keeps backflow from happening. The chemical might not be injected properly if the pressure in the discharge line goes below the pressure in the boiler feed pump line. It could leak back into the discharge line or tank.

A higher discharge line pressure keeps the flow going in the right direction, making sure that the chemical is always poured into the boiler without any backflow. This stops backflow, which is important because the chemical could damage the boiler system or throw it out of balance if it runs backwards. Because of this, the pressure in the drainage line is always higher than the pressure in the line that goes into the boiler.

Benefits of the Process:

• Corrosion Prevention: The LP dosing method helps keep the boiler from rusting, which makes boiler parts last longer.
• Efficiency Improvement: By injecting specific chemicals, this process maintains the pH level of the water, which is crucial for system stability.
• Balanced pH and oxygen levels: Chemical dosing controls the pH and oxygen levels of the steam and water, which makes the system last longer.

This process is essential for industrial applications because it keeps the equipment safe and extends its lifespan.

Optimising Boiler Efficiency: The Importance of External and Internal Water Treatment

To fully understand how chemicals are used to treat water, we need to know about each step. If you don’t, I’ll do my best to explain it with as few complex terms as possible. I want to make the idea as easy for you as possible.

First, let’s talk about the fact that there are two types of chemical cleaning for water: external and internal.

External Treatment of Water: This is what is done before water is put to the boiler. Its goal is to get rid of impurities and hardness in the water, such as rocks and gases. This is very important because these impurities can make the boiler work less well by causing rust or scale. For this, filters and other special tools are used.DM (demineralised) water and RO (reverse osmosis) water are examples that can be used in external water treatment:

• RO Water: RO (Reverse Osmosis) water is made by passing water through a special filter that removes most impurities, salts, and minerals. This makes the water purer and safer for use in the boiler, as it reduces the chances of scale buildup.
• DM Water: Water that has been cleaned to get rid of all minerals and salts is called DM water. This water is very clean, and it’s great for boilers because it keeps rust, scale, and other problems from happening inside the boiler.

As part of external treatment to make sure that only clean, pure water goes into the boiler system, both RO and DM water are often used.

Internal Treatment: Once the water gets to the boiler, this cleaning is done inside. Some chemicals are put in the water to keep the boiler safe and stop any damage from happening. These chemicals make sure the water is safe and clean, which keeps the boiler from getting rusty or scaling. All of these treatments make sure that the boiler and the water work properly and without any problems. Here are some simple examples of how the boiler treats water internally:

• Anti-Rust Chemicals: To keep the metal surfaces of the boiler from rusting, certain chemicals are added to the water inside the boiler.
• Scale Preventers: These chemicals help keep hard layers (scale) from forming inside the boiler. Scale can form when minerals stick to the surfaces.
• pH Balancers: Chemicals are added to the boiler water to keep the pH level at the right amount. This keeps the water from becoming too basic or too acidic, which helps keep the boiler safe.
• Oxygen Removers: Some chemicals are used to get rid of any oxygen that is still in the water. This lowers the chance of rusting.

By keeping any problems with the water quality inside the system under control, these treatments help keep the boiler safe, clean, and running well.

Complete Guide to Treating Boiler Water: Key Step-by-Step Procedures

Let’s understand boiler water treatment in a series, covering each aspect step-by-step, such as TDS, oxygen removal, and chemical reactions.

What is TDS (Total Dissolved Solids)?

TDS (Total Dissolved Solids) refers to the total amount of dissolved inorganic salts and some organic matter in water. These salts and impurities dissolve in the water and have a significant impact on boiler systems.

Role and importance of TDS:

1. Impact on Heat Transfer: When the TDS level in boiler water is high, salts and other impurities stick to the sides of the boiler and build up as scale and sludge. This scale stops heat from moving, which wastes energy and makes things less efficient.
2. Corrosion and Scaling Risk: High TDS can change the chemicals in the water, which can cause boiler tubes and walls to rust and scale. These layers can hurt the boiler and make it more expensive to fix and keep up.
3. Blowdown Requirement: A small amount of water and dissolved solids are taken out of the boiler during the blowdown process to reduce TDS levels. Some water and chemicals are lost during this process, but it is necessary for the safety and efficiency of the boiler.

How is TDS Measured?

With a conductivity meter, you can find out how many ions are dissolved in water and measure TDS. The ppm (parts per million) value of the TDS level indicates the precise amount of dissolved solids in the water.

Ideal TDS Levels

• Boiler Feed Water: To keep the boiler system from scaling and rusting, it’s best to keep the TDS levels low, between 50 and 100 ppm.
• Controlled Through Blowdown Process: In the event that TDS levels increase, periodic blowdown is an efficient method for managing it.

TDS shows the dissolved salts and particles in water. These can cause scaling and corrosion in boiler systems. Controlling TDS is important for keeping the boiler working well and lasting a long time.

Deoxygenation means taking oxygen out of water that has dissolved in it. In boiler systems, getting rid of oxygen is important because it can rust metal surfaces, which makes the boiler less efficient and shortens its life.

Chemicals for Deoxygenation:

Sodium Sulphite (NaSO₃) and Hydrazine (NaH₄) are the two main chemicals used to get rid of oxygen in boiler installations. The choice is based on several factors.

1. Sodium Sulphite (Na₂SO₃)

• Use: Mostly used in boilers with low pressure (LP).
• Function: Sodium sulphate, which is not corrosive, is made when dissolved oxygen mixes with sodium sulfite.
• Chemical Reaction:
  Na2​SO3​+O2​→Na2​SO4​

• Quantity Required: Approximately 8 ppm of sodium sulphite is needed to remove 1 ppm of oxygen.

• Why Preferred in LP Systems: It works well at low temperatures and doesn’t cost much, so it’s perfect for low-pressure devices.

2. Hydrazine (N₂H₄)

• Use: Mostly found in boilers that use high pressure (HP).
• Function: When liquid oxygen and hydrazine mix, nitrogen and water are made, and no harm is done.
• Chemical Reaction:
  N2​H4​+O2​→2H2​O+N2

• Quantity Required: Approximately 1 ppm of hydrazine is needed to remove 1 ppm of oxygen.

• Why Preferred in HP Systems: At high temperatures and pressures, hydrazine is more stable and effective. It also makes an oxide layer on boiler tubes that protects them from corrosion. High Temperatures (above 100°C): One place where hydrazine works really well is at high temperatures, like in high-pressure boilers. It breaks down into nitrogen and water, which are both safe for the boiler and won’t damage it. Alkaline Conditions (pH > 7): Hydrazine works well when the environment is alkaline. It helps make a thin, protective oxide layer on the metal sides of the boiler tubes when the pH is above 7. This layer stops corrosion. In these situations, using hydrazine is good for high-pressure systems because it makes the boiler more resistant to rust.

Ideal Hydrazine Dosage

To get the right amount of liquid oxygen, you should use three parts hydrazine to one part air. So, the oxygen is removed effectively without losing hydrazine. Keeping this balance helps lower both costs and the chance of corrosion, which keeps the boiler system safe and helps it work well.

What is hydrazine, and why is it important in boiler systems?

Hydrazine is a clear liquid that melts in water quickly. Its boiling point is 113°C. It is not found in nature; it is made from nitrogen and hydrogen, which is written as N₂H₄. Because hydrazine is a strong oxygen remover, it mixes with oxygen to keep boiler systems from rusting. How it works:

Steps in Deoxygenation:

1. External Treatment: Before sending boiler feed water to the boiler, a deaerator heats the water to remove 90-95% of dissolved oxygen.
2. Chemical Treatment: Any oxygen that is still there is taken away using chemicals after deaeration. Sodium sulphite is used for LP systems, and hydrazine is used for HP systems to ensure effective oxygen removal.

Benefits of Deoxygenation:

• Prevents corrosion: Getting rid of air makes boiler tubes and surfaces less likely to rust.
• Increases Boiler life: Deoxygenation done right makes boilers last longer and work better.
• Saves Energy: Keeping the boiler free of scale and rust makes it work better, which means it uses less fuel and energy.

When and why to use all kinds of chemicals to treat water

1. Trisodium Phosphate (Na₃PO₄), Disodium Phosphate (Na₂HPO₄), and Monosodium Phosphate (NaH₂PO₄)

Even though all three of these chemicals are phosphates, they are not all the same in how well they control pH. They are used to keep scale and rust from forming in boiler water. Let’s look at what they do and how they can be used.

a) Trisodium Phosphate (Na₃PO₄)

• Nature: Strongly alkaline.
• Use: Primarily used in high-pressure (HP) boilers as it effectively maintains pH and creates a highly alkaline environment.
• Function: Helps prevent scale formation and raises the pH of boiler water.
• Reaction with Calcium:
  3Ca2++2PO43−​→Ca3​(PO4​)2​
• How It Works: Trisodium phosphate reacts with calcium ions to form calcium phosphate, an insoluble compound that easily settles as sludge.

b) Disodium Phosphate (Na₂HPO₄)

• Nature: mildly alkaline.
• Use: It is used in boilers with low pressure or anywhere a moderate pH is needed.
• Function: Maintains a mildly alkaline pH and provides a buffering effect.
• How It Works: It balances phosphate and hydrogen ions to regulate pH, though it is slightly less effective at preventing strong scaling.

c) Monosodium Phosphate (NaH₂PO₄)

• Nature: Weakly acidic.
• Use: Used to control pH when the boiler water is overly alkaline.
• Function: Can neutralise the pH slightly and controls excess alkalinity.
• How It Works: Monosodium phosphate provides hydrogen ions, which neutralise excess hydroxide ions (OH⁻), keeping pH in check.

What is Orthophosphate?

Orthophosphate is a word for simple phosphate ions (PO₁³⁻). Scale doesn’t form because it reacts with calcium ions and other hardness ions to make molecules that don’t dissolve and can settle.

• Orthophosphate in Boiler Treatment: Orthophosphate is used in boiler water to capture ions like calcium and magnesium, converting them into insoluble compounds to prevent scale and corrosion.

Sludge Formation and Removal

Sludge is the term for the soft deposits that form in boiler water as impurities or chemicals that can’t be dissolved. This sludge builds up at the bottom of the boiler and is blown out during the blowdown process.

Formation of Sludge

1. Through Chemical Reactions: When trisodium phosphate or orthophosphate reacts with calcium ions in boiler water, insoluble calcium phosphate forms, creating sludge.
2. Reaction with Hardness Salts: Hardness salts like calcium and magnesium react with phosphate to form sludge.

Sludge Removal Process

• Blowdown: Sludge is allowed to settle at the boiler bottom and is then removed through the blowdown process to keep boiler surfaces clean and smooth.
• Frequency of Blowdown: Blowdown is performed at regular intervals to minimise water and chemical loss and to prevent impurities from settling.

In this way, phosphate and sludge control systems manage impurities in the boiler and maintain pH balance.

How Boiler Load Influences pH and Phosphate Levels

In thermal power plants, the load on the boiler changes based on how much energy is needed. The conditions inside the boiler change when the load goes up or down, which affects the pH levels and phosphate balance.

Boiler Load	pH Level	Phosphate Level
Increases	Increases ↑	Decreases ↓
Decreases	Decreases ↓	Increases ↑

When Boiler Load Increases

pH Level Rises: As the load increases, the temperature and pressure in the boiler increase, which can elevate the water’s pH. Phosphate Concentration Decreases: Phosphates are used to stop rusting and keep scale in check. When the load is higher, the boiler water uses more phosphate, which causes the phosphate levels to drop.

When Boiler Load Decreases

pH Level Drops: Temperature and pressure drop when the load goes down, which can cause the pH level in the boiler to drop. Phosphate Concentration Increases: With lower demand for phosphate, the concentration increases as less is consumed.

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