Expansion Vessels

Reliance's expansion vessels incorporate an extensive range of sizes and connection formats to suit most uses. The range includes vessels for heating systems and potable water, together with special application vessels for solar systems.

Pressure Vessels

All Reliance pressure vessels are manufactured to comply with UK water regulations and are tested and certified by WRAS.

All Reliance pressure vessels are supplied with a replaceable membrane, which separates the water and air, to prevent contamination of the water system, corrosion of the pressure vessel or pressure loss in the water system.

Typically pressure vessels are used in pumped systems. When the pump starts, the pressure inside the vessel begins to increase. Once the vessel is full and the pump has reached its high pressure setting it will switch off. When water is then drawn off from the system the air pressure surrounding the membrane will force the water out of the pressure vessel and into the system. Once the membrane is fully deflated and the system has reached its minimum pressure, the pump will start on its low pressure setting and will begin the cycle again.

The purpose of this system is to prevent constant cycling of the pump and continuous running when the system is being used.

Pressure Vessels Sizing Guide

The sizing of potable pressure vessels can be calculated using the following formula:

V = K x Amax x ((Pmax+1) x (Pmin+1)) / ((Pmax-Pmin) x (Pprec+1))

K = working efficiency of pump (see table)

Amax = average flow (litre/min)

Pmax = Maximum working pressure of the pump (bar)

Pmin = Minimum working pressure of the pump (bar)

Pprec = Pre-charge pressure of the pressure vessel

Please note – always set the pre-charge air pressure of the vessels 0.2 bar less than the pump pressure.

Expansion Vessels

Reliance manufacture expansion vessels for both potable hot water systems and heating systems.

In the case of expansion vessels for potable hot water they are manufactured to comply with UK water regulations and are tested and certified by WRAS.

All Reliance expansion vessels are supplied with a replaceable membrane, which separates the water and air, to prevent contamination of the water system, corrosion of the expansion vessel or pressure loss in the water system.

The main purpose of an expansion vessel is to compensate for the increase in volume of water due to the varying water temperatures in hot water or heating systems. When water is heated it expands and as water is not compressible this increased volume will create a rise in pressure within the system. As an example, water being heated from 0°C-100°C will increase by approx 4.5%.

The expansion vessel allows for this extra space, as when water temperature increases the membrane inside the vessel inflates to allow the water to fill the vessel. The membrane will continue to expand until the system reaches its maximum temperature. Once this has been reached the membrane will be fully inflated and takes up the capacity of the vessel shell. Gradually the temperature will drop, which will in turn decrease the volume of water. Due to the pressure from the pressurised air surrounding the membrane, water will start to exit the vessel until the membrane is deflated.

Heating Vessel Sizing Guide

The following guide has been prepared as an aid to correctly size a sealed heating system expansion vessel, for further information please contact the Reliance Technical Dept. on +44 (0)1386 712400.

If the system volume is known, expansion vessels can be sized with the formula:

V = eC / (1 – (p1/p2))

V    The total volume or nominal size of the expansion vessel. It is not the acceptance volume. (See product description).

C    The total volume of water in the system (litres).

P1   The fill pressure of the system in Bars absolute (atmospheric or gauge pressure plus one Bar).

P2   The setting of the expansion/pressure relief valve in Bars absolute (atmospheric or gauge pressure plus one Bar).

e    The expansion factor that relates to the maximum system requirements.