MAINTENANCE & REFURBISHMENT
Never ignore the value of an expansion vessel within a heating system
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When it comes to a commercial heating system, it’s easy to overlook the importance of the expansion vessel – but do so at your peril. Whilst it may be regarded as very basic technology, get your calculations wrong and you are going to be beset with problems sooner rather than later, according to Rob Jacques, Spirotech’s business director UK
ithin an intricate network found in any large commercial system, the expansion vessel is undoubtedly one of the most underestimated
components, yet it plays a pivotal role in maintaining the health, safety and efficiency of the system.
All too often, our technical team find the initial specification for an expansion vessel has been incorrectly calculated, and when coupled with installation mistakes, can result in far reaching consequences. An expansion vessel does what it says on the tin; it is designed to manage the expansion and contraction of water within a sealed system. We all know from our school days that when water is heated, it will expand in a process known as thermal expansion. Therefore, in a closed heating system, the expanding water will have nowhere to escape and will result in a build-up of pressure. Unless there is an outlet for this additional volume of water to be diverted to, the system pressure will exceed safe limits and potentially result in leaks and discharges from valves. Failure to rectify this quickly can lead to even greater and more expensive problems, including ruptured pipes and even boiler damage. To avoid this, it is important to get the specification of the expansion vessel correct from the outset and ensure it is installed correctly.
Selecting the right vessel
An expansion vessel is far from a high-tech piece of kit; it is just a steel container with either a diaphragm or a bag inside made from a type of rubber, most commonly- EPDM (Ethylene Propylene Diene Monomer), Butadiene or butyl. Bags tend to be made from butyl as they don’t need to stretch as much as diaphragms, which are generally made from EPDM. One side of the diaphragm – or within the interior of the bag – is the system water, and on the other (beyond the bag’s exterior) is the air or nitrogen. To determine the vessel size and comply with regulations, the following needs to be established:
• The static height – this is measured from the top of the circuit to the centre line of the vessel
• kW output (heat load of system) – this determines the expansion pipe size
• System volume – this can be calculated or estimated from charts and/or kW output
• Maximum temperature – this is required so that the correct expansion coefficient can be calculated and to assess if intermediate vessels are required
Taking into account the above elements, the calculation ‘Total expansion’ divided by ‘Acceptance’ will identify the next largest available vessel that can be installed. When specifying vessels, there are two British Standards that need to be observed; BS EN 12828 covers guidance for central heating systems in buildings, whilst BS 7074, provides the correct specification for expansion vessels. System designers must always take these two standards into account, including the latest updates, and do the full calculation. When selecting and sizing an expansion vessel, it must be capable of holding the volume of expanded water in the system and cope with the projected thermal expansion. Specify too small a vessel and it won’t have the capacity to absorb the expanded water leading to pressure spikes.
When this occurs, this can result in the safety valve discharging water to reduce the excess pressure, which wastes energy and adds to the running costs.
Location, location, location
Whilst you would expect a heating professional to position the expansion vessel in the right location, we do come across poor choices from time-to-time. An expansion vessel should be installed on the inlet side of the pump. Positioning it wrongly could result in air ingress or water loss.
Regular maintenance
It is crucial that a regular maintenance programme is undertaken with expansion vessels, as failure to do so will ultimately cause
problems. Over time, the rubber diaphragm in the vessel is likely to degrade, air can leak out and this will become an issue. Guidance from BSRIA, the best practice testing consultancy, and CIBSE also recommend that the vessel pre-charge pressure is checked annually for any loss. However, experience has shown that a lower quality fitted vessel requires a more frequent check. A loss of just 15% will start to affect pressure control, and failure to address this could lead to problems just four months after commissioning. The cyclical process of expansion and contraction takes place in the background. Thanks to the expansion vessel, the heating system is not subjected to continuous pressure fluctuations. As the system cools down, the water contracts – resulting in the pressure within the system falling and returning to normal. For the expansion vessel to operate correctly, the vessel pre-charge pressure must be adjusted specifically for the site conditions.
Managing the pressure
When a heating system within a commercial set-up is servicing a larger building, the vessel will be used in conjunction with a ‘make-up’ unit, which combine to form a pressurisation set. The pressure within the system is monitored constantly by the ‘make-up’ unit, which adds refill water to maintain a minimum pressure. It also performs another important function; if the pressure transgresses the design operating range, the end-user will receive an alert. Whilst failure in the ‘make-up’ unit is not ideal, the system can still function. It’s the loss of the vessel that will result in a total loss of control of the system. There are several other key factors to consider when looking at the pressure of the system. The vessel reserve is the minimum amount of water required in the vessel under all conditions. Without water, there is no pressure control. According to British Standard EN12828 the vessel reserve is taken as 0.5% of system volume. This level of water is required to supplement any small losses of water within the system (as well as additional overpressure), but also provides hydraulic stability within the system during operating, particularly with pumps. Lift pressure relates to the safety valve on top of the vessel. A calculation is needed to correctly determine the safety valve lift pressure (PSV). Expansion pipe sizing. This section of the pipe connects the vessel to the system and must be sized to generate minimal frictional loss. If too narrow a pipe is used, it will lead to over- pressure during heating and under-pressure during cooling.
This will lead to unnecessary alarms and unwanted water make-up (introducing fresh oxygen), making it prone to sludging-up and blockage. The pipe size is determined by the rate of volume/temperature change of the liquid – i.e. associated with the kW output of the system. A potential problem occurs when the specifier doesn’t stipulate the size of the pipe on the drawing, as some contractors will look to save money and choose the smallest possible pipe. It’s important to recognise that a relatively small imperfection in the system installation can impact greatly on the system’s overall performance.
Common mistakes
As we have identified, several errors can be made in the specifying and installing of expansion vessels. However, when done correctly, they have an integral role to play in the ongoing performance of heating systems.
Contractors should take the time at each stage to ensure these common mistakes are avoided, in order to reap the rewards in the long-term.
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BUILDING SERVICES & ENVIRONMENTAL ENGINEER FEBRUARY 2025 23
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