Expansion vessel

A trapped volume of pressurised air or gas inside a steel vessel to allow for the expansion that accompanies temperature increases.

Any fluid filled sealed circuit system, which subjects the fluid contain within it to temperature change, needs to have the capacity for dealing with expansion as the temperature increases. The expansion vessel in a solar hot water system has to be able to operate at higher temperatures than those a standard heating vessel are subjected to.

An expansion vessel consists of a trapped volume of pressurised air/gas inside a steel vessel which contains a rubber bag or diaphragm. This rubber bag, referred to as a diaphragm, contains fluid which is inside the sealed circuit system. The vessel is connected by pipes to the sealed system so that when the fluid in the sealed system expands the diaphragm within it stretches and compresses the gas within the vessel. The pressure increase in the gas is disproportional to the pressure increase that would have occurred in the liquid if the provision for expansion were not there. The liquid pressure is maintained, or the change of pressure at least limited, by this expansion vessel as the volume of fluid in the system fluctuates due to changes in temperature.

Unvented cylinders also require provision for expansion as they have non-return, pressure regulation and pressure relief valves. They are not open to the atmosphere and unless a tap is open/on, the water contained in the cylinder and pipe work effectively behaves as a sealed system. Any volume which is not allowed to expand when it goes through temperature change will result in a potentially damaging or dangerous pressure change. If the diaphragm in the vessel has breached then liquid will fill the gas side and result in the expansion provision being eliminated.

As water in a sealed system expands it is pushed into the expansion vessel, compressing the air inside the diaphragm. As the air is compressed its pressure – and also that of the water in the liquid system contents – increases. The larger the expansion vessel, the smaller the increase in pressure. The appropriate size should be fitted, and the correct pre-charge pressure needs to be set. The pre-charge in the expansion vessel is crucial to it operating correctly and an understanding of ‘head’ as well as ‘applied’ pressure in needed to determine what the correct pre-charge should be. The size of an expansion vessel is relative to the volume in the system, working pressure, and temperature change the system is subjected to during its normal duty.