The closed expansion vessel can be mounted at any point in the installation; However, if there is no problem, it is advisable to place it in the suction of the pump, since this will be the area with the lowest pressure in the entire system and the connection point of the glass is constituted as a zero pressure point or reference pressure of the circuit.[6]​.

The red line in the attached figure shows the pressure values ​​throughout a heating installation. The line begins at the connection point of the expansion vessel, which corresponds to the reference pressure, and ends at the same point, since it is a closed circuit. In the pump, the pressure goes from a value lower than the reference[7]​ at the suction until it reaches the highest point of the circuit at the pump outlet. Subsequently, the pressure is slowly reduced due to pressure losses in the pipe until it reaches the emitters where it suffers a sudden drop due mainly to the restrictions in the inlet and outlet valves, since the radiator itself hardly has any pressure loss. After the emitters, the pressure continues to decrease through the pipe, until it suffers another vertical drop due to the loss of boiler pressure and ends up again at the reference.

Any installation that must work in a hermetic circuit with respect to the atmosphere must be equipped with a safety valve for protection against overpressure and a pressure gauge to control the pressure at which the system is located.

Filling the installation must be done from the bottom of it, very slowly and with all the purges open, which will close as the water reaches them.

Prior to the definitive filling of the installation, successive fillings and emptyings must be carried out until all residues, such as filings or welding remains, that may have remained inside the circuit as a result of the installation work, are eliminated. Next, the pipe tests required by the RITE will be carried out.[8]​.

Once all this has been done in accordance, you can proceed to warm up the entire circuit until the maximum expected working temperature is reached and maintain it for 7 or 8 hours. It must be verified that the pressure acquired by the installation when the temperature has stabilized does not exceed the maximum pressure considered.[9]​

During heating, repeated air purges must be carried out, since the air dissolved in the water is free when it is heated.

Finally, let the installation cool until it reaches a temperature of approximately 40 °C. The pressure must drop to 0.1 or 0.2 kp/cm² above the pressure equivalent to the static head. If the final pressure is lower than indicated, the installation must be refilled. It is not advisable to overfill,[10]​ since the evacuation of excess water can cause limescale deposits on the seat of the safety valve in the long term.

The method of calculating the expansion vessel is established in the UNE 100155 standard.

For a closed expansion vessel, with fluid in direct (without diaphragm) or indirect (with diaphragm) contact with a pressurized gas, the total volume of the vessel will be calculated using the following equation:.

The expansion coefficient is always positive and less than unity; it obviously represents the relationship between the useful volume of the expansion vessel, which must be equal to the volume of expanded fluid, and the total volume of fluid contained in the installation:

The expansion coefficient of water between the temperature of 4 °C, which corresponds to the minimum specific volume, and the maximum operating temperature of the system can be expressed theoretically by the following relationship (valid up to 210 °C):

where the denominator temperature function can be expressed by the fourth order polynomial:.

with a maximum percentage error of less than 1%.

The pressure coefficient for calculating the total volume of closed vessels without transfer of fluid to the outside of the system is found based on the equation of state for perfect gases, considering that the volume variation takes place at constant temperature (Boyle and Mariotte law). This coefficient, positive and greater than unity, represents the relationship between the total volume and the useful volume of the expansion vessel:

in the case of vessels with diaphragm:.

The minimum operating pressure will be chosen so that, at any point in the circuit and with any operating regime of the circulation pump, the existing pressure is higher than the atmospheric pressure or the saturation voltage of the water vapor at the maximum operating temperature, whichever is higher.

In particular, the minimum pressure in the vessel must be such that cavitation phenomena in the pump suction are avoided; To do this, it must be verified that the NPSH available at the location of the pump is higher than the NPSH required by the pump manufacturer. In any case, a safety margin must be taken that is greater the higher the operating temperature, with a minimum of 0.2 bar for systems at temperatures below 90 °C and 0.5 bar for systems at higher temperatures.

The maximum operating pressure will be slightly lower than the setting pressure of the safety valve, which, in turn, will be lower than the lowest of the maximum working pressures of the equipment and appliances that are part of the circuit.

Naturally, the minimum and maximum pressures, established as indicated above, must be corrected according to the geometric height of the expansion vessel location.

In summary, the calculation of a closed VE will be done by following the following steps:

Contenido

Los sistemas de expansión se clasifican en:.

No mass transfer to the outside of the circuit

The open expansion vessel, as stated above, is not recommended and its use was prohibited in the last edition of the RITE in 2007. In reality, it does not specifically prohibit such a thing, but rather it literally says: *Closed circuits of water or aqueous solutions will be equipped with a closed type expansion device. It is in the Comments to the RITE where its interpretation is clarified and reaffirmed.[11]​.

It is the system described in this article as most commonly used in heating and cooling systems.

With mass transfer to the outside of the circuit

They are used in high-power systems with a large volume of water. There are two types:

They are compressor-controlled systems&action=edit&redlink=1 "Compressor (machine) (not yet written)"). The tanks have a large capacity and can reach up to 20 m³. The increase in water volume due to the increase in temperature is collected inside the membrane, while the air contained between the membrane and the tank is evacuated to the outside through a purge valve. Due to this reduction of the air cushion, the tank can absorb an expansion volume practically equal to its total volume, maintaining the pressure of the regulated installation within a very narrow range. When the drop in water temperature causes a decrease in its volume and a drop in pressure, the compressor automatically supplies the necessary air to the tank so that the membrane, when inflated, returns the water to the installation and continues to maintain a constant pressure.

In installations of 30 MW and up to 250 MW, the mass transferred is water, whose volume is controlled by an automatic pumping system. With these systems, the fluid is transferred from the circuit to an external storage tank or vice versa, depending on whether it is the expansion or contraction phase, maintaining the pressure of the circuit practically constant, well understood within the limits imposed by the proportional band of the control devices. It is highly recommended to use an inert gas (nitrogen) to fill the mattress above the free surface of the water.[12]​ In this case, there may be no separation membrane between the water and the pressurization gas.

The expansion vessel, as can be deduced from everything already said, is one of the key elements in the operation of a heating circuit. In fact, many of the most common breakdowns in these systems come from the lack or malfunction of this element.

It is very common, for example, in heating installations, for the fill indicator needle to drop and the installation has to be refilled with new water every day. When this happens, the most common thing is to blame the problem on a leak. Well, in almost all cases, this is due to a lack of pressure in the expansion vessel.

If, when cold, the gas pressure in the vessel is low, the water pressure in the installation is already deforming the membrane and therefore reducing the gas volume. As the temperature of the installation increases, the pressure will increase more than expected and the set pressure of the safety valve will be reached during operation, which will spit out a quantity of water until the pressure is restored to the maximum allowed. This hot water is what will be missing the next morning with the cold installation. If it is not replaced, the air that replaces it is distributed throughout the circuit, causing circulation defects and unsuspected effects on the terminal elements, which sometimes causes hasty and certainly not necessary decisions.

Systematic daily refilling is not a good solution either, due to the deposition of new water and because the risk of corrosion is considerably increased.

It is advisable to check the condition of the expansion vessel from time to time. To do this, press the core and check if air comes out. If what comes out is water, it means that the membrane is pitted and the water from the installation is passed into the air chamber. In this case you have to change the membrane, if it is replaceable, and if not the glass.

If air comes out and we notice a drop in the water level, the boiler is emptied until there is no water in the glass and the air pressure is measured with a manometer. If this is low, air must be injected until the pressure corresponding to the geometric height of the installation is reached.

The closed expansion vessel can be mounted at any point in the installation; However, if there is no problem, it is advisable to place it in the suction of the pump, since this will be the area with the lowest pressure in the entire system and the connection point of the glass is constituted as a zero pressure point or reference pressure of the circuit.[6]​.

The red line in the attached figure shows the pressure values ​​throughout a heating installation. The line begins at the connection point of the expansion vessel, which corresponds to the reference pressure, and ends at the same point, since it is a closed circuit. In the pump, the pressure goes from a value lower than the reference[7]​ at the suction until it reaches the highest point of the circuit at the pump outlet. Subsequently, the pressure is slowly reduced due to pressure losses in the pipe until it reaches the emitters where it suffers a sudden drop due mainly to the restrictions in the inlet and outlet valves, since the radiator itself hardly has any pressure loss. After the emitters, the pressure continues to decrease through the pipe, until it suffers another vertical drop due to the loss of boiler pressure and ends up again at the reference.

Any installation that must work in a hermetic circuit with respect to the atmosphere must be equipped with a safety valve for protection against overpressure and a pressure gauge to control the pressure at which the system is located.

Filling the installation must be done from the bottom of it, very slowly and with all the purges open, which will close as the water reaches them.

Prior to the definitive filling of the installation, successive fillings and emptyings must be carried out until all residues, such as filings or welding remains, that may have remained inside the circuit as a result of the installation work, are eliminated. Next, the pipe tests required by the RITE will be carried out.[8]​.

Once all this has been done in accordance, you can proceed to warm up the entire circuit until the maximum expected working temperature is reached and maintain it for 7 or 8 hours. It must be verified that the pressure acquired by the installation when the temperature has stabilized does not exceed the maximum pressure considered.[9]​

During heating, repeated air purges must be carried out, since the air dissolved in the water is free when it is heated.

Finally, let the installation cool until it reaches a temperature of approximately 40 °C. The pressure must drop to 0.1 or 0.2 kp/cm² above the pressure equivalent to the static head. If the final pressure is lower than indicated, the installation must be refilled. It is not advisable to overfill,[10]​ since the evacuation of excess water can cause limescale deposits on the seat of the safety valve in the long term.

The method of calculating the expansion vessel is established in the UNE 100155 standard.

For a closed expansion vessel, with fluid in direct (without diaphragm) or indirect (with diaphragm) contact with a pressurized gas, the total volume of the vessel will be calculated using the following equation:.

The expansion coefficient is always positive and less than unity; it obviously represents the relationship between the useful volume of the expansion vessel, which must be equal to the volume of expanded fluid, and the total volume of fluid contained in the installation:

The expansion coefficient of water between the temperature of 4 °C, which corresponds to the minimum specific volume, and the maximum operating temperature of the system can be expressed theoretically by the following relationship (valid up to 210 °C):

where the denominator temperature function can be expressed by the fourth order polynomial:.

with a maximum percentage error of less than 1%.

The pressure coefficient for calculating the total volume of closed vessels without transfer of fluid to the outside of the system is found based on the equation of state for perfect gases, considering that the volume variation takes place at constant temperature (Boyle and Mariotte law). This coefficient, positive and greater than unity, represents the relationship between the total volume and the useful volume of the expansion vessel:

in the case of vessels with diaphragm:.

The minimum operating pressure will be chosen so that, at any point in the circuit and with any operating regime of the circulation pump, the existing pressure is higher than the atmospheric pressure or the saturation voltage of the water vapor at the maximum operating temperature, whichever is higher.

In particular, the minimum pressure in the vessel must be such that cavitation phenomena in the pump suction are avoided; To do this, it must be verified that the NPSH available at the location of the pump is higher than the NPSH required by the pump manufacturer. In any case, a safety margin must be taken that is greater the higher the operating temperature, with a minimum of 0.2 bar for systems at temperatures below 90 °C and 0.5 bar for systems at higher temperatures.

The maximum operating pressure will be slightly lower than the setting pressure of the safety valve, which, in turn, will be lower than the lowest of the maximum working pressures of the equipment and appliances that are part of the circuit.

Naturally, the minimum and maximum pressures, established as indicated above, must be corrected according to the geometric height of the expansion vessel location.

In summary, the calculation of a closed VE will be done by following the following steps:

Contenido

Los sistemas de expansión se clasifican en:.

No mass transfer to the outside of the circuit

The open expansion vessel, as stated above, is not recommended and its use was prohibited in the last edition of the RITE in 2007. In reality, it does not specifically prohibit such a thing, but rather it literally says: *Closed circuits of water or aqueous solutions will be equipped with a closed type expansion device. It is in the Comments to the RITE where its interpretation is clarified and reaffirmed.[11]​.

It is the system described in this article as most commonly used in heating and cooling systems.

With mass transfer to the outside of the circuit

They are used in high-power systems with a large volume of water. There are two types:

They are compressor-controlled systems&action=edit&redlink=1 "Compressor (machine) (not yet written)"). The tanks have a large capacity and can reach up to 20 m³. The increase in water volume due to the increase in temperature is collected inside the membrane, while the air contained between the membrane and the tank is evacuated to the outside through a purge valve. Due to this reduction of the air cushion, the tank can absorb an expansion volume practically equal to its total volume, maintaining the pressure of the regulated installation within a very narrow range. When the drop in water temperature causes a decrease in its volume and a drop in pressure, the compressor automatically supplies the necessary air to the tank so that the membrane, when inflated, returns the water to the installation and continues to maintain a constant pressure.

In installations of 30 MW and up to 250 MW, the mass transferred is water, whose volume is controlled by an automatic pumping system. With these systems, the fluid is transferred from the circuit to an external storage tank or vice versa, depending on whether it is the expansion or contraction phase, maintaining the pressure of the circuit practically constant, well understood within the limits imposed by the proportional band of the control devices. It is highly recommended to use an inert gas (nitrogen) to fill the mattress above the free surface of the water.[12]​ In this case, there may be no separation membrane between the water and the pressurization gas.

The expansion vessel, as can be deduced from everything already said, is one of the key elements in the operation of a heating circuit. In fact, many of the most common breakdowns in these systems come from the lack or malfunction of this element.

It is very common, for example, in heating installations, for the fill indicator needle to drop and the installation has to be refilled with new water every day. When this happens, the most common thing is to blame the problem on a leak. Well, in almost all cases, this is due to a lack of pressure in the expansion vessel.

If, when cold, the gas pressure in the vessel is low, the water pressure in the installation is already deforming the membrane and therefore reducing the gas volume. As the temperature of the installation increases, the pressure will increase more than expected and the set pressure of the safety valve will be reached during operation, which will spit out a quantity of water until the pressure is restored to the maximum allowed. This hot water is what will be missing the next morning with the cold installation. If it is not replaced, the air that replaces it is distributed throughout the circuit, causing circulation defects and unsuspected effects on the terminal elements, which sometimes causes hasty and certainly not necessary decisions.

Systematic daily refilling is not a good solution either, due to the deposition of new water and because the risk of corrosion is considerably increased.

It is advisable to check the condition of the expansion vessel from time to time. To do this, press the core and check if air comes out. If what comes out is water, it means that the membrane is pitted and the water from the installation is passed into the air chamber. In this case you have to change the membrane, if it is replaceable, and if not the glass.

If air comes out and we notice a drop in the water level, the boiler is emptied until there is no water in the glass and the air pressure is measured with a manometer. If this is low, air must be injected until the pressure corresponding to the geometric height of the installation is reached.