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4 Essential tips to optimise the correct use of Non-Return Valves in drainage design

October 19, 2023

Optimising the correct use of non-return valves in drainage design:  4 Essential Tips

Non-return valves play a key role in drainage design and the flood protection of buildings.

Whilst they have been used for many years, the growing demand for urban development, the impact of climate change and the strain on sewer infrastructures, have made the correct use of non-return valves more important than ever.

Our engineers at Civilistix have shared 4 essential tips to help ensure non-return valves perform as intended and effectively mitigate the risk of flooding from sewers:

  1. Location: safeguarding against sewer and rainwater intrusion.

Non-return valves are typically installed to prevent sewer backup during flash floods. However, it is often overlooked that once the valve is shut to protect the system from sewer water, it hinders the outflow of rainwater from roofs and external areas which can then build up in the system.  Incorrect valve placement can result in water accumulation within low points of the local drainage network, generally in undesirable locations such as lightwells, internal gullies or even toilets or kitchens. When choosing the location, it is critical to understand where the water will go once the valve is closed. This is particularly significant in older combined drainage systems where rainwater downpipes share drains with the foul drainage. To address this issue, consider installing an external gully lower than the finished floor level. This offers a safe route for excess water to exit, as stated in Part H of the Building Regulations.

  1. Choose the right non-return valve for the level of protection needed.

There is a wide range of valves available, varying in price and design. Each product has different applications and the choice of product should align with the level of protection needed. The following need to be considered when deciding on which non-return valve to use – the level of risk, the installation requirements and how the valve will perform once installed. For example,  flap push fit valves are less expensive and offers flexibility as they can be installed in the chamber inlets. They only need a small amount of flow to open. However, as they are closed until water drains down, there is a risk of blockage under heavy foul flows and they may not be suitable for high-pressure conditions. On the other hand, a cast iron floating ball system remains open by default and has a more robust closure device. This reduces the risk of a blockage. They are required to be installed within an accessible manhole but might face durability issues.

There are many NRV options available, including products with a membrane that can be installed in the outlet pipe thereby protecting the entire system with one valve, or built-in valves in small chambers that can raise the pipe against the cover when water flows from the sewer. Whichever valve you use, ensure it is compliant with the BS EN 13564 standards and it provides the level of protection needed for your system.

  1. Don’t delay valve decisions at early design stages.

In construction, it is common to provide notes in drawings indicating the requirement for a non-return valve, leaving the selection and specification for the contractor to choose. This may lead to coordination problems and project delays, as the works required for the valve may include the construction of a new chamber near foundations or in areas with limited accessibility. To speed up the construction, the contractor may choose an inadequate valve type and location thereby turning the protection measure into a liability. To streamline the process, specify the valve type and location during the early design stages. 

  1. Special consideration for Basements and lower ground refurbishments (Low points).

It is usually an advantage when the basement can be connected to the sewer without a pump and can run under gravity. However, as mentioned earlier, once the valve is shut and the system is full of water, appliances at lower levels may lack the necessary pressure to drain against the accumulated water. Where there is a higher risk of flooding from sewers, the installation of a pump might be imposed in the planning consent conditions. If not, consider non-return valves with small built-in pumping systems and emergency closure mechanisms that can be operated remotely.

By following these four tips you can optimise the effectiveness of non-return valves in your drainage design, enhancing flood protection and safeguarding your building against sewer-related flooding events.

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