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Non Return Valves (Check or Reflux Valves)

Non-return Valves (NRVs) protect pumps from reverse flow (and are also used for bypass lines which is discussed later). The NRVs shut soon after flow halts through a pump that has shutdown.

This action frequently occurs without any problems, but in some systems a phenomenon called ‘NRV Slam’ occurs which violently shuts the NRV causing high, localised pressures and damaging pipe movement. This normally occurs when the surge pressures in the system causes flow to rapidly reverse through a still open NRV. This reversing flow forcibly close the NRV causing mechanical forces and also secondary surge pressures. The combination of these can lead to pipe movement, pipe rupture and a much reduced life span for the NRV and adjacent equipment.

Many different types of NRV can be installed, each with different performance characteristics and often with widely different costs. Some of these will prevent NRV slam is some systems and some NRVs alleviate the symptoms. But it is not cost effective to install specialist, costly NRVs designed to prevent the symptoms of NRV slam if the problem will not occur on the system.

The cost difference is enough to justify performing a surge study to accurately determine the performance needed and to determine if NRV slam is even a problem. A project that we recently completed saved almost 20 times the cost of the study in savings on incorrectly specified NRVs.

A surge study will identify any risk of NRV slam and will be able to determine if the risk is frequent or rare.

The speed of closure of an NRV is one of the major factors in preventing NRV slam and our surge studies calculate this closure time so that a more informed and cost effective NRV specification can be agreed with the NRV manufacturers.

Old, legacy NRVs need to be re-studied when the duty flowrate changes or when pumps are replaced. Even small changes to the system can sometimes invalidate the performance of the NRV, so it is always vital to undertake a surge study to assess and confirm the ongoing integrity of the system.