Expansion vessels and water tanks. Engineered in Italy, Made in Russia

Expansion vessels

The expansion vessel is a pressurised vessel used when lessening the changes in pressure within the heating system it is used in.

The expansion vessel for heating water systems has two specific functions: from the hydraulic point of view it allows to define a reference pressure for the systems, thermically it welcomes the volume change of the working fluid, usually water thus not compressible, occurring as a result of the thermal expansion of the fluid itself.

There are three kinds of expansion vessels:

  • Open tank: the water is meeting the external environment without shielding. Such kind is historically the first type of solution implemented;such vessels were usually installed on the roof space. Common weaknesses rose from the promiscuous state of water with air: indeed the atmospheric oxygen dissolving in the water can induce corrosion related problems on the pipe work, undermining the soundness of the metal pieces of the system and releasing
    particles in the fluid which could clog the piping. it requires regular draining of the system and air in solution with water increase noisiness and heat loss. Evaporation of the water make it required to cyclically restore the water in the system. Special care must be taken to prevent water losses which could overflow out of the tank.
  • Closed vessel without membrane: closed type of tank in which there is air (or inert gas) and water without any parting mean. It is selfpressurized. Limits of this kind of vessel are similarly linked at the shared environment between air and water: Typical aspect of such tank can be akin the previous kind of vessel. Moreover, the selfpressurized vessel the air or gas share in the tank grows progressively thinner as air goes in solution with water, making it necessary to regularly restore the precharge pressure.
  • Membrane expansion vessel: this device, evolution and improvement of the former, requires an elastic wall shielding water from air made of rubber ( bagshaped or diaphragmshaped). The drawbacks of the other two types are prevented. Moreover, it make it possible to optimize the available volume of use: tuned precharged according to operation allows to minimized the water volume at installation. It is of easier setting up and maintenance compared to the former kinds.


Thermal Expansion coefficient (see table) describes the variation of specific volume of the workign fluid (water usually) with the change of temeperature, as stated:: e = f(T_{massima}-T_{minima})=f(\Delta T) . Change in temperature is with respect to a 10 °C initial temperature.

Therefore, if we consider ten litres of water undergoing a change in temperature of 80 °C corresponding to an expansion coefficient of e = 0.029, (see table), we can compute the amount of operating fluid volume increase as a consequence of the rising of the temperature: the total volume of 10.29 litres is found.

Expansion vessel holds 0.29 litres of additional volume related to the heated fluid.

For many application (such as solar systems) it is employed as thermodynamic fluid a mixture of water and glycol (antifreeze sustance). Such implies a variation of the thermal expansion coefficient with the different percentage of antifreeze substance; With the increase of the percentage of glycol in the mix the same temperature difference is related to a bigger thermal coefficient.

In line vessel

Expansion vessel with membrane for in line application: compact and easy to install.

Colour: ROD (ral 3000)

Precharge: 1.5 bar

Vertical vessel

Vertical expansion vessel with membrane: small footprint and highest reliability allowing the correct operation thanks to the bag shaped membrane.

Colour: ROD (ral 3000)

Precharge: 1.5 bar

Diaphram vessel

Expansion vessel with diaphragm membrane: small footprint with a specially designed diaphragm membrane.

Colour: ROD (ral 3000)

Precharge: 1.5 bar