The importance of the water trap seal
Being the only barrier between the drainage system and the living and/or working space, it is critical that the water trap seal is maintained at all times. This protects the occupants of the building from the potentially harmful gases and pathogens which within the drainage system. All buildings, regardless of whether they are low rise or high rise, need correct drainage ventilation to allow the essential water trap seals to be maintained.
In high rise and complex drainage systems there is additional stress placed on the water trap seals due to the increased loads into the system, and the distances of the relief of the pressure regime is greater in time and distance in taller buildings.
Conventional thinking in drainage venting is to deal with the negative pressure. This originates from the studies carried out by Hunter in 1920 (Hunter’s Curve), which only looked at steady state systems. The established thinking is that water trap seals are depleted due to siphonic action. The most common causes are self siphonage and induced siphonage.
A negative pressure transient occurs when there is a discharge of fixtures to which the water trap seal is connected. This can have the effect of reducing the trap seal (or pulling the trap). This occurs as the momentum acquired by the waste passes through the fixture and down the trap seal. This momentum is transferred directly into the trap seal and trap seal loss occurs – commonly known as “self siphonage”.
Other fixtures discharging in the building can also affect the trap seal. This occurs when there is a pressure fluctuation caused by a discharge of another fixture in the system other than the fixture to which the trap is connected. This is commonly known as “induced siphonage”, which is very common in multi-storey buildings.
Maintaining the water trap seal
Plumbing Regulations call for the water trap seal on sanitary fixtures to be maintained at all times. This is accomplished by the introduction of atmospheric air into the plumbing drainage system.
Venting with pipes has limitations, as open vents require penetrations through the roof to allow the atmospheric air to balance the pressure transients within the sanitary drainage system, as well as the requirement for the pipe to carry this air. Research is proving that the current practice of using a 50% smaller vent pipe network is unsafe as this can lead to water trap seal depletion. The only correct and safe way to install a passive pipe vent system is where all vent pipes must be larger than the waste carrying pipes.
Alternatively, Air Admittance Valves (AAVs) can be installed at the “point of need” near the water trap seals that require protection. This has the benefit of eliminating the vent pipe network, the space required, the roof penetrations, and greatly simplifies the design of a large plumbing drainage system.
Whilst AAVs are very suitable to vent the negative pressures that are generated in the system (particularly with regard to branch venting), special consideration must also be given to positive pressure which occurs typically in high rise buildings. Traditional standards call for relief vents and open stack vents to provide positive pressure relief. “Active ventilation” is an alternative solution which uses a combination of AAVs and the P.A.P.A. (Positive Air Pressure Attenuator).
The active drainage ventilation principle works by local intervention to remove or attenuate an incoming transient that, if left, would lead to trap seal depletion. This is achieved by placing AAVs and the P.A.P.A onto the standard pipes of the system to limit the surge pressures adjacent to the traps by reducing the rate of local changes in flow conditions. The placement of the AAVs and the P.A.P.A must be positioned between the source of the transient and the base of the stack; offsets are typical points to generate positive transients. The pipe calculations for loadings and gradients do not change when installing active drainage ventilation, so BS EN12056 can still be used for the sizing of the system.
Active drainage ventilation reduces the effects of the negative and positive transients, balancing the pressure within the drainage system – if the system pressure exceeds +/- 40mm WG (400Pa) the water trap seals can be lost by induced or self siphonage:
- Negative pressure (transients) are dealt with through the introduction of local airflow using AAVs on the branches. Air is allowed in through the AAV as required, which then seals tight to prevent sewer gases from leaking out into the habitable space.
- Positive pressure (transients) are absorbed by the P.A.P.A., slowing them down from the speed of sound (320m/s) to a harmless 12m/s, which is then released back into the system to naturally dissipate.
Active venting versus traditional “passive” venting
Active venting should be considered as functionally “superior” over traditional “passive” venting:
- The removal of long, and possibly, convoluted vent connections to atmosphere reduces the time taken before local relief can be applied – allowing the pressures to be balanced quicker with active venting.
- Local suppression prevents transient propagation throughout the network prior to relief – removing the risk of the siphonage of multiple traps.
With traditional “passive” venting, reliance on roof penetrating open terminations allows transients to travel the whole system prior to any remedial action. The fundamental issue being that a transient should be dealt with between its source and the first appliance trap seal in order to prevent trap seal depletion.
Scientifically and empirically proven
Extensive research by leading industry academics has repeatedly proven that active drainage ventilation (utilising the combination of AAVs and the P.A.P.A.) protects the water trap seals of high rise buildings without any of the limitations of the “passive” vent pipe solution.
Anyone wishing to see first-hand both systems operating is invited to attend a “Seeing is Believing” test tower demonstration. Located at Studor’s Northampton facility, the test rig replicates the drainage system of a 40 storey building and incorporates valves to switch between an active vented system and a traditional passive vented system. More information and to register: www.studor.net/test-tower
Written by Steve White, Studor Technical Director