A Guide to Expansion Vessels

We talk every day about expansion vessels in one form or another, this could be giving part codes to customers to sizing vessels for installers – but what is the purpose of an expansion vessel and what are the differences between all the different types, let’s take a look.

What is an Expansion Vessel?

An expansion vessel is a pressurised vessel that incorporates a bladder or diaphragm in its design and is installed in unvented heating, cooling, potable water and solar heating systems. These are also referred to by some manufacturers as Expansion Tanks or Pressure Vessels.
It consists of a fabricated pressure vessel, water system connection, air valve and a membrane (diaphragm) or bladder. The membrane or bladder separates the pre-charged air or nitrogen from the system water.
Membranes are usually retained centrally in the vessel by crimping the two halves together. Bladders are usually retained by the water system connection.

 

What are they for?

It is fairly simple and self-explanatory really, they are there to take expansion within a system to prevent over pressurising thus causing damage, and this could be internal damage to components or pipes blowing off.

 

What do they do?

The expansion vessel accommodates any increases in water volume which occurs during heating or cooling, keeping the pressure of the system stable by absorbing the extra volume. This helps maintain a constant pressure which can aid a reduction in energy consumption.

 

Why is this important?

Within a pressurised heating or cooling system, it is important to maintain the pressure within the required limits, therefore reducing the risk of excess pressure or to maintain a minimum pressure to reduce the risk of vacuum or cavitation in the system (prevalent in superheated water and solar heating systems). It is also important to prevent negative pressure at high points to reduce the potential for air pockets or to compensate for variations in volumes due to temperature fluctuations.

 

How does it work?

A pressurised membrane (also called a bladder or diaphragm) allows the ingress or egress of water from the system to maintain a constant pressure.

As water cannot be compressed in a closed water circuit, any increase in temperature needs to be accommodated and this is done by the expansion vessel. When cold the pre-charged pressure allows the bladder to collapse and it remains in this state until the pump is switched on which then causes the bladder to inflate. As the water temperature increases, there is an increase in volume and pressure. The increased volume of water enters the bladder which lowers the pressure.

As the temperature reduces, the pre-charged pressure forces the water from the bladder and back into the main water circuit.

Expansion vessels are also available which specifically combat water hammer. In these types of Expansion Vessels, the liquid is contained within the rubber bladder while the air (nitrogen) is trapped in the space between the outside of the bladder and the wall of the vessel.
The liquid is therefore not in contact with the steel walls of the vessel and since the water is contained in a minimum permeability butyl rubber bladder, there is no air dissolution.

Once installed, commissioned and set at the adequate pre-charge pressure, the bladder will help dampen any water hammer pressure waves which may result resulting from quick opening or shutting of valves.

 

Where should the expansion vessel be installed?

Expansion Vessels are typically used in unvented central heating systems in domestic and commercial projects. The expansion vessel is normally installed on the cold return pipework to the boiler, as this is typically less than 71°C. It is important to ensure that there is no obstruction between the expansion vessel and the system. The maximum allowable temperature of the diaphragm should be checked for suitability.

Depending on the design selected, expansion vessels can be suitable for horizontal or vertical installation. Expansion vessels installed in the vertical orientation should be located so that the length of the connecting pipework is kept to a minimum. Ideally, these should be installed downwards to avoid any potential sediment build up.

In solar applications, it is recommended that the expansion vessel is installed in the return pipeline from the solar collector. If the return flow temperature is more than 70ºC then it is recommended that an intermediate vessel is installed before the expansion vessel.

Expansion vessels can be installed on pressure booster systems in order to reduce pump starts at small draw-offs and help reduce pump wear and extend the pump life. In this case the expansion vessel is installed on the discharge side of the booster system. Maintenance of the vessel can also be carried out without shutting off the water supply. They can also be installed on the suction side, if the water pressure from the mains is too low which helps reduce the risk of cavitation in the pump or the creation of vacuum on start-up.

 

How should I size or select the expansion vessel?

There are expansion vessels available for various hot water systems. These need to be sized correctly for the capacity required. Different sources of heat can be applied to unvented hot water systems and it is important to control the temperature within normal limits.
Installations where the heat source and input temperatures could be uncontrolled such as in solid fuel or solar thermal systems, require specific expansion vessels designed for these applications. Larger capacity expansion tanks are available where irrigation pumps, centrifugal pumps or booster sets are installed. To correctly size an expansion vessel, you will need to know the highest water temperature (flow temperature), the static height, the safety valve setting and either the system volume or the boiler or chiller power rating.

Altecnic provide expansion vessel sizing calculators for potable and heating systems FOC. Request yours now: https://www.altecnic.co.uk/vessel-calculator/