Water Tube Boilers | Types and Structure

Introduction: Why are Water Tube Boilers the Backbone of Large Industries?

In the oil, gas, refineries, power plants, and advanced process industries, high-pressure, high-temperature steam generation is a fundamental requirement. In these situations, Water Tube Boilers serve as the backbone of steam generation systems.

Unlike Fire Tube Boilers used in small industries, Water Tube Boilers are capable of producing steam at high pressures (up to 300 bar) and huge capacities (up to hundreds of tons per hour). These features have made this type of boiler an undeniable place in thermal power plants, large refineries, petrochemicals, and hydrogen production units.

In this article, we will provide a complete, in-depth, and practical review of Water Tube Boilers — from their working principles to their types, advantages, disadvantages, operational challenges, scale formation, and preventive maintenance solutions using specialized chemicals.

What is a Water Tube Boiler? (Definition and working principle)

In a Water Tube Boiler, water flows through tubes that are exposed to a flame and hot gases. The heat transferred to the tubes turns the water into steam. The steam produced is directed to the upper drum (Steam Drum) and from there it is sent to the consumer systems. (How do water tube boilers work?)

Basic structure of water tube boilers:

• Steam generating tubes: Where water is converted into steam
• Upper drum (Steam Drum): Separates water from steam
• Lower drum (Mud Drum): Accumulates sludge and suspended solids
• Economizer: Preheats the feed water
• Superheater: Increases the temperature of the steam
• Reheater: In power plants, reheats the steam
• Burner: Creates a flame in the furnace

Flow direction in the boiler:

• Water and steam: In the tubes
• Flue gases: In the space between the tubes

Types of Water Tube Boilers

Drum Type Boilers (with Drum)

• Has an upper and lower drum
• Suitable for medium to high pressure
• Common in power plants and refineries

Subcategories:

• Natural Circulation: Water flows naturally (density difference)
• Forced Circulation: With internal pumps (at very high pressures)

Once-Through boilers (without drum)

• Water passes through the pipes once and turns into steam
• No upper drum
• Special for supercritical pressures
• Very high efficiency (up to 45%)

HRSG (Heat Recovery Steam Generator) Boilers

• Burnerless — uses the heat of hot gas turbine gases
• Combined Cycle
• Very common in gas power plants

Advantages of Water Tube Boilers

✅Very high capacity (up to several hundred tons of steam per hour)

✅High working pressure (up to 300 bar and above)

✅High efficiency (up to 90% in modern models)

✅Faster start-up speed than Fire Tube at high capacities

✅Possibility of producing superheated steam

✅Possibility of heat recovery with economizer and superheater

✅Higher safety (the volume of water inside the tubes is less — less risk of explosion)

✅Ability to be used in combined cycles (HRSG)

These advantages have made Water Tube Boilers the main choice in large power plants, global refineries, large petrochemicals and hydrogenation units.

Disadvantages and operational challenges

❌High initial cost

❌Need for specialized manpower

❌Complex and costly maintenance

❌Extreme sensitivity to feedwater quality

❌Scale formation in small pipes with serious consequences

❌Risk of corrosion in high temperature pipes

❌Need for advanced control system

Life of Water Tube Boilers

The life of these boilers is highly dependent on maintenance, water quality, and operating conditions.

Factors that reduce the life of boilers:

• Sediment formation in tubes
• Internal corrosion in the boiler
• Thermal fatigue
• Cracks in drums
• Corrosion due to failure to remove oxygen

The role of feed water in the performance of water tube boilers

The feed water in these boilers must be completely purified. Any impurities can lead to:

• Sediment formation in small tubes (diameter 50-75 mm)
• Reduced heat transfer
• Increased tube wall temperature
• Tube rupture
• Stop production and billions of dollars in losses

Critical parameters of boiler feed water:

Types of deposits in Water Tube boilers

Water Tube Boiler Maintenance and Chemical Cleaning

Feed Water Pretreatment

• Softener + Desiner + Reverse Osmosis (RO) + Deionizer (Mixed Bed)

Periodic Blowdown

• Partial Water Drainage to Control TDS

Chemical Cleaning

• Every 12 to 36 Months Depending on Conditions
• Use Non-Acid Anti-Scaling Solutions

Basic Descaling

• In Cases Where Efficiency Loss is More Than 15%
• With Specialized Anti-Corrosion Solutions

Case Study: Chemical Cleaning in a Gas Refinery

• A High Capacity Boiler Unit
• Efficiency Loss, Increased Fuel Consumption
• Performing Cold Mitreh Solution Washing by a Team of Waterworks Specialists
• Result: Efficiency Returned to 91%, Fuel Consumption Reduced by 19%

Safety Tips for Maintaining Water Tube

• Relieve pressure completely before entering the drum
• Use a respirator and eye protection
• Test safety valves
• Make sure washing solutions are non-corrosive
• After washing, a thorough water rinse and residue test are essential

Why is choosing a washing solution critical?

Using an inappropriate solution can:

• Cause corrosion to the pipes
• Cause cracks in the steel
• Cause secondary deposits

Ideal solution characteristics:

• Non-acid
• Contains anti-corrosion agents
• Ability to dissolve silicates and sulfates
• Completely washable

✅Mitreh products with specialized formulations meet these conditions and have proven their effectiveness in hundreds of large industrial projects.

Summary and operational recommendations

• Water Tube boilers are essential for large industries.
• Preventive maintenance is the main key to increasing life and efficiency.
• Periodic chemical washing with specialized solutions (such as Mitre) can improve efficiency by up to 45% and is very critical.
• It is essential to use a specialized team to carry out the operation.
• Boiler feedwater pretreatment is the first line of defense against scale and is very important.

Credible sources

1. ASME BPVC Section I – Power Boilers

2. NACE SP0106 ​​– Control of Corrosion in Boilers

3. API 560 – Fired Heaters and Steam Generators

4. U.S. DOE – Boiler Efficiency and Maintenance Guidelines

5. ScienceDirect: "Fouling and Cleaning of Water Tube Boilers"

6. Reports and tests of Abrizan Industrial Research Company – Washings performed with Mitreh solution