What Is an Expansion Tank?

An expansion tank is equipment that, in closed-loop mechanical piping systems, compensates for the volume increase caused by the temperature change of water.

In heating systems, water expands as it heats up, and this increases system pressure.

If there is no expansion tank, the following problems can occur:

• system pressure rises excessively
• the safety valve constantly discharges water
• the pump and boiler may be damaged
• faults occur in the piping system

Therefore, using an expansion tank is mandatory in modern mechanical piping systems.

What Is a Steel Expansion Tank?

A steel expansion tank is the classic type of expansion tank in which the system water and the air inside the tank are in direct contact.

In these tanks, generally:

• water is at the bottom of the tank
• air is at the top

As system pressure rises, water enters the tank and the air is compressed, balancing the pressure.

Advantages of the Steel Expansion Tank

Steel expansion tanks offer several advantages.

Durable Structure

They are long-lasting due to their thick steel body.

Large Capacities

Large-volume tanks can be used in industrial systems.

Simple Design

The operating principle is quite simple and maintenance costs are low.

Disadvantages of the Steel Expansion Tank

These tanks also have some disadvantages.

Air Loss

Over time, the air inside the tank can mix with the water and system pressure can drop.

Constant Need for Air Recharging

These tanks need to be recharged with air at regular intervals.

Corrosion Risk

Since water and air are in contact inside the tank, corrosion can occur.

What Is a Membrane Expansion Tank?

The membrane expansion tank is the most commonly used tank type in modern piping systems.

In these tanks, water and air are separated from each other by a flexible membrane.

Thanks to the membrane:

• water does not come into contact with air
• system pressure is more stable.

Advantages of the Membrane Expansion Tank

Membrane tanks offer many advantages.

Stable Pressure

Thanks to the membrane, system pressure is more stable.

Less Maintenance

Air loss is much lower.

Compact Design

Provides high performance in a small volume.

Disadvantages of the Membrane Expansion Tank

In some cases, there can also be disadvantages.

Membrane Rupture

Over time, the membrane can become damaged.

Capacity Limitation

In very large industrial systems, a steel tank may be preferred instead of a membrane tank.

Differences Between Steel and Membrane Expansion Tanks

Which Tank Is Used in Which Systems?

Central Heating Systems

Generally, the membrane expansion tank is preferred.

Booster Pump Systems

Membrane tanks are widely used in booster pump systems.

For example:

• Grundfos booster pump systems
• Wilo booster pump systems
• Etna booster pump systems

work together with a membrane tank.

Industrial Systems

Steel expansion tanks can be used in large-volume boiler systems.

How Is an Expansion Tank Calculated?

The following factors are considered when calculating expansion tank volume:

• system water volume
• temperature difference
• system pressure

The general calculation is done as follows:

The expansion tank volume is generally taken as 8 – 12% of the system water volume.

Expansion Tank Brands

There are many quality brands in the mechanical piping sector.

The most preferred brands are:

• Reflex expansion tanks
• Zilmet expansion tanks
• Varem expansion tanks
• Cimm expansion tanks
• Aquasystem expansion tanks

Choosing the right brand is important for system safety.

Expansion Tank Prices

Expansion tank prices depend on the following factors:

• tank volume
• membrane quality
• brand
• working pressure

Tanks used in residential systems are more affordable.

Conclusion

Steel expansion tanks and membrane expansion tanks are two different tank types with different operating principles.

Generally:

• membrane tanks are preferred in small and medium-sized systems
• steel tanks are preferred in large industrial systems

Choosing the right tank is of great importance for system safety and efficiency.