Expansion vessel – Safety system for thermal power plants

expansion tank expansion vessel

What is an expansion vessel?

» Today where going to talk about expansion tank. An expansion vessel is a component of a safety system that has the role of preventing and stopping the effects of the phenomenon of water expansion, taking over the volume of water generated by temperature variations in the thermal installation.

» In common words, when the water heats up in the thermal installation, it expands and begins to increase its volume. Due to the fact that the thermal installation is a closed circuit, this additional volume of water must be taken over by an equipment and that is the expansion tank.

expansion tank

How does an expansion tank work?

» The principle of operation of an expansion vessel is the following: when the temperature of the heating agent in the heating system increases by 10 degrees Celsius, then its volume increases by 0.3%.

» Due to the fact that the installation is closed, there is an excessive pressure in the system that must be taken over by the expansion vessel. Otherwise, all this pressure would be exerted on the pipes and fittings, which at one point would give way.

» The excess volume and pressure due to the expansion phenomenon is taken over by an air cushion inside the expansion vessel.

» Often this air pillow loses its volume from the factory, leading to pressure increases in the installation. In this case, the volume is re-aired from the vessel and everything returns to normal.

expansion vessel

Types of expansion tank

» According to the specialized books, the expansion vessels can be of 2 types, depending on the centralized heating systems with hot water operating at maximum temperatures of 115ºC, in which they are mounted:

  • Open expansion vessel – used in installations with direct connection to atmospheric air;
  • Closed expansion vessel – used in closed installations equipped with safety valves.

» Because the open expansion vessel is in direct contact with the atmosphere, it allows easy evacuation of the dissolved air in the water, which occurs as a result of the heating and expansion of the thermal agent.

» In this way it blocks the pressure increases from the thermal installation and ensures the minimum pressure necessary to function in parameters of the installation.

» In more academic terms, the open expansion vessel must provide a minimum pressure in the installation, but which will be higher than the saturation pressure of the water vapor at a maximum water temperature in the installation.

» As a rule, an open expansion tank is positioned in the bridge of the house and is covered with a thermal insulation jacket. If this is not possible, you can place it in other rooms, provided the open expansion vessel is located above the level of the heating system.

expansion tank in thermal instalation

» The open expansion vessel must fulfill the following tasks to ensure the safety of the heating system:

  • it must help to remove the air from the installation when filling it;
  • must take over the volume increase in the installation due to the phenomenon of water expansion;
  • it must evacuate the steam from the installation if a power failure occurs and due to the failure of the circulation pumps the boiler starts to “boil”.

» For a proper functioning of the open expansion vessel it is very important to respect certain rules and conditions, such as:

  •     the open expansion vessel must be kept full for proper operation, so it is advisable to check its condition from time to time;
  • the open expansion vessel must be installed at a higher level than the highest consumer;
  • the safety pipes have a slope of at least 3% towards the open expansion vessel;
  • the return safety pipe is connected at least 20mm from the bottom of the vessel;
  • it is necessary to ensure a continuous flow with low water flow in order to provide protection against frost;
  • an overflow pipe is connected to the sewerage system;
  • a signaling pipe is provided that is located at least 200mm from the bottom of the vessel;
  • the open expansion vessel is tested at the pressure of 0.2 bar;
  • it is thermally insulated by a neoprene or mineral wool coat.

» To reduce the rapid corrosion of the vessel, it is recommended to choose an expansion vessel with a height greater than the width.

» In this way we will have a smaller contact surface between water and air, resulting in a smaller dissolution of air in the water.

How to calculate the volume of an open expansion vessel?

»When we talk about the size of an open expansion vessel, the main requirement is that the useful volume of the vessel should be at least equal to the volume of dilated water following its heating from 20ºC to 90ºC.

»The useful volume of the vessel, measured in m³, is equal to 0.05 times the total volume of water in the installation.

Vu = 0.05 x V [cubic meters]

»The height of the expansion vessel is the ratio between the useful volume and the area of the section of the expansion vessel, to which is added a coefficient of 0.25 meters.

H = (0.05 x V) / A + 0.25 [meters]

Safety measures

  • it is recommended to use expansion vessels in hot water heating installations at a maximum use temperature of 95ºC;
  • installation of 2 safety pipes between the boiler and the expansion vessel (return and return);
  • on the route of the safety pipes between the boiler and the vessel there are no closing valves;
  • the safety pipes will be placed in a slope of 3% towards the expansion vessel;
  • the radii of curvature of the safety pipes shall not be less than 3 times the diameter of the pipe;
  • the flow safety pipe shall have a minimum of 1 ″;
  • the diameter of the flow safety pipe will be calculated:

formula calcul conducta de siguranta ducere

  • the diameter of the return safety pipe will be calculated:

formula calcul conducta de siguranta intoarcere

Expansion tank closed

» A closed expansion tank is a cylindrical tank, partially filled with water and air, equipped with an elastic membrane of separation inside, having the role of taking over the expansion of the water following the heating – cooling processes of the thermal agent from the thermal installation.

heated expansion tank

» By the pressure exerted by the air pillow on the water, the closed expansion vessels have the role of maintaining a stable volume of water in the installation.

» The expansion tank for heating systems is designed to compensate for the thermal expansion of the heating agent and to maintain the stable pressure in the closed heating systems.

» There are closed expansion tanks that use instead of air, steam or inert gas (nitrogen). The advantage of using inert gases instead of air is that it reduces the corrosive effect of the installation.

The main component parts

  • steel tank inside which the membrane is located;
  • a rubber membrane having a stepping capacity;
  • flange with lid;
  • threaded fitting for connection.

The main advantages

cele mai bune masini de spalat rufe
  • the thermal agent does not come in contact with the air, reducing the phenomenon of oxidation of the pipes and radiators, which contributes to the increase of their service life;
  • in the case of expansion tanks, the probability of leakage of the thermal agent is zero;
  • the vessels are equipped with membranes that can be replaced;
  • it is manufactured in different shapes, sizes and volumes so that it can serve any heating system;
  • profitability, safety and reliability.

types of expansion vessel

Where is the closed expansion vessel located?

» As a rule, the closed expansion tank is mounted at the bottom of the installation, in the technical room of the boiler, right next to the boiler.

thermal heating boiler

How to calculate the volume of a closed expansion vessel?

expansion vessel


» As we could see, in a heating system, an expansion vessel, closed or open, is a very important element of safety of the installation.

» When purchasing such equipment online or from a store, you must consider the manufacturer and product reviews. Often this plays a very important role in the purchase of quality equipment.

» Even if it costs more, it is worth buying quality expansion vessels, manufactured by well-known manufacturers, in order not to have problems with the equipment functioning in time.

» We hope that through those described above we have managed to make you understand what an expansion vessel is and how it works in a heating system.

» We look forward to your opinions and comments on this subject.


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