Expansion vessels
Introduction
The purpose of the expansion vessel is to absorb volume variations of a fluid contained in a closed circuit by varying its temperature, maintaining the pressure between pre-established limits and preventing, at the same time, losses and replacements of the fluid mass.[1].
For a long time, a tank open to the atmosphere through a tube or overflow and located in the highest part of the circuit was used for this purpose (the name "glass" comes from this type of tank, which may seem absurd in closed types). Although the tank was sized to contain the increase in volume of the fluid, on many occasions, part of it was expelled outside, which implied a lack of cold filling with the corresponding entry of air into the circuit and the operating problems that this entails, in addition to the imminent danger of oxidation.
Advantages of the closed glass
The introduction of the closed expansion vessel has been a great advance due to its obvious advantages over the open vessel:
The use of an open expansion vessel is therefore inadvisable under any circumstances,[2] even in installations where there is a possibility that the expected average temperature will be exceeded, as is the case of boilers that use solid fuels. In this case, excess heat accumulation or dissipation systems must be provided and if, despite this, the pressure reaches the safety value, the safety valve or valves will open (preferably two in series. See Comments on RITE[3]).
Working principle
The closed expansion vessel for installations with a hermetic circuit consists of a sheet steel container with two compartments, separated by an elastic membrane; One of them contains air or nitrogen and acts as an elastic element that absorbs the volume variations experienced by the water that fills the circuit, the other compartment is in contact with the installation and therefore full of water. The cup is normally made of steel according to DIN 17100 and welded in an inert atmosphere. The membrane is generally made of neoprene, highly elastic, resistant to heat and low permeability to the dinitrogen molecule. The nitrogen chamber is separated from the outside by a small non-return valve (tire core), which allows part of the gas to be evacuated or recharged in case of lack of pressure.