Stagnant water in building water systems can lead to some serious issues. One of the most dangerous is the growth of legionella bacteria that could lead to Legionnaires disease. Legionella can quickly grow and proliferate in water systems where favourable conditions are available.
Legionella can quickly grow in stagnant water ranging between 20°C – 45°C. The chances of legionella growth are even higher in poorly designed and maintained building water systems. One such area where water can stagnate and allow Legionella to multiply even when the system is in use is the dead legs of pipework.
In this blog, we’ll discuss why dead legs are a huge problem for building managers and how to mitigate legionella risk lurking in their water systems.
What are dead legs & why do they form?
A dead leg is a section of piping that is either completely unused or has very little flow running through. The term can also refer to a run of pipework that is only used very occasionally.
Common examples include capped piping, closed cross-connections, low point drains, cooling tower equalization lines, bypass lines, and out of service rooms or equipment. Little used outlets, showers, chillers, heat exchangers, and pumps can also become dead legs depending on how long they are out of service.
Dead leg piping can occur when pipework systems are changed over time, by plumbing faults or changes to the use of the system.
Dead legs may be intentionally installed to facilitate future expansion of a building’s potable water plumbing system to avoid a complete building outage. They may be roughed in to provide options for future construction or anticipated needs.
Why are dead legs a major problem?
While dead legs themselves are not directly harmful, they allow water to stagnate, which can, in turn, support microbial contamination including Legionella.
The stagnant water in dead legs provides ideal conditions for biofilms to form. In these areas of low or no water flow, the residual disinfectant present in the water can be quickly reduced. This allows bacteria to attach to system surfaces to start the biofilm formation process.
A biofilm is formed when layers of microorganisms (typically bacteria) combine and stick to each other. Together they form a tough layer on a surface that encloses colonies of bacteria.
Within the biofilm, bacteria such as legionella need less oxygen and fewer nutrients to replicate. This slime-like layer can also protect micro-organisms from disinfection and biocides by providing a physical barrier from treatments, allowing pathogens like legionella to proliferate.
A build-up of biofilm can also affect heat transfer efficiency and can act as a catalyst for corrosion.
How to manage risks from dead legs?
There are measures for prevention and control of legionella bacteria, dead legs in pipework defy most of these treatment options because of lack of flow. Furthermore, biocides do not come in contact with dead legs.
Although legionella bacteria in dead legs can contaminate the entire water system, the presence of dead legs does not necessarily mean there will be a Legionella problem, nor will removing them necessarily solve one.
Before removing a dead leg, consider the benefits versus the cost. Some dead legs present a greater risk than do others. Some are expensive to correct; others are not.
Lengths of dead leg piping measuring approximately 1.5 times its width should be removed.
The following rules are good practice:
- In equipment rooms and other areas where dead legs are accessible, the cost of removal will typically be minimal, so remove them.
- If a dead leg cannot be removed without tearing out a wall, then leave it in the wall but cut and cap it where it tees into the mains.
- If a dead leg is not accessible, and it cannot be cut at the main, then try other methods of controlling Legionella bacteria before going to the expense of tearing out walls to remove dead legs. The cost of removing dead legs that are behind walls may not be justified without knowing if you have a Legionella problem.
In certain instances, it is not possible to remove dead leg piping. For example, it may not be possible to remove a dead leg if a section of pipework is occasionally used. In this case, you should work with a water hygiene company to ensure appropriate action is taken to keep water systems compliant. In cases such as this, remedial works can be undertaken to reduce the length of dead leg piping and an appropriate flushing regime be set up.
Where new builds or extensions are planned it is important to involve the estates and the operational teams, at the earliest stage of design to ensure the proposed water distribution system and any associated primary heating sources are going to be fit for purpose including the minimisation of the risk of dead legs and blind ends.
Stay Compliant with a Legionella Risk Assessment
If the responsibility to keep your water systems safe lies with you, it is critical to ensure the legionella risk assessment and schematic drawings are up to date and reviewed regularly. Your risk assessment should identify and account for any dead leg piping in your hot or cold water system.
Our team of qualified assessors can identify dead legs in your building water systems and suggest appropriate remedial actions based on the level of risk present. We offer a full suite of services to help your facilities remain compliant.
For more information, get in touch with our team for a free consultation.
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