What is an Expansion Vessel?

In this short article we look at expansion vessels and their uses. An expansion vessel is a small tank used to protect closed water heating systems and domestic hot water systems from excessive pressure caused by water hammer and thermal expansion of the water inside the sealed system.

An expansion vessel is usually split into two compartments divided by a diaphragm. One side is connected to the heating systems pipework and is filled with water; the other side contains a gas such as nitrogen under pressure. As the water pressure in the heating system increases the diaphragm will push outwards, compressing the gas and therefore absorbing the excess pressure. The diaphragm arrangement helps to prevent the undesired transfer of air, and helps maintain low levels of oxygen within the pipes, reducing corrosion in the system reducing maintenance requirements and increasing the service life. Using nitrogen rather than compressed air in the expansion vessel prevents the permeation of the air through the membrane which can reduce the pre charge pressure. The oxygen in the air also oxidises the membrane in the vessel, causing under-inflation and deterioration of the rubber. Nitrogen by comparison maintains proper inflation pressure for longer and will not corrode the inside of the vessel – therefore prolonging the life expectancy of the component.

At Mibec we can supply a full range of expansion vessels for heating systems, or for potable water applications. Mibec vessels can range in size from 2 litre for use in instantaneous water heater applications, all the way up to 2000 litres or even more for use on larger biomass or commercial applications. We supply expansion vessels from leading brands including Reflex, Zilmet and Varem, all from stock here in the UK. All vessels have an outer epoxy coating for protection and are approved to latest European standards such as PED 97/23/EC Directive and BS EN 13831:2007 ‘Closed expansion vessels with built in diaphragm for installation in water’.

We offer a full free of charge specification service covering the whole of the UK, designed to support architects, specifiers or contractors. We can give you knowledgeable, yet impartial advice on selecting the right expansion vessel to meet your projects needs. Please feel free to email or call our support department on 0330 053 880 (local call rate) where one of our trained advisers will be more than happy to help you.