Guest blog by Dr. Michael Gormley
MSc MPhil PhD CEng MCIBSE MIET FHEA
Dr.Gormley is a specialist in water supply and drainage as well as an electrical building services engineer. Read his full biography here.
The tradition of venting building drainage systems through the roof is as old as modern systems have existed. Most modern appliances and techniques date back to Victorian times in Britain, around the middle of the 19th Century, when there was a flurry of research and development activity – partially fuelled by the industrial revolution, but also driven by the desire for safe sanitary conditions in the rapidly expanding towns and cities at the time. This process continues to this day, however this is now on a global scale, with the recent significant milestone that there are now more urban dwellers than rural dwellers in the whole world.
The Victorian obsession with cleanliness led the development of heavy water usage appliances, with toilets using as much as 40 litres per flush – with appropriately strong names such as the ‘Tornado’, the ‘Dreadnought’, ‘Trident’ and the ‘Deluge’. They really did believe in the old adage that ‘cleanliness was next to godliness’ and cleanliness meant lots of water and open roof vents on the drainage stack.
Some pretty cool facts and a history lesson to boot, but what has this got to do with modern venting techniques? Well, essentially the struggles of the early Victorian plumber and sanitary engineer are the same as their modern day counterparts. Providing safe and efficient sanitation solutions is as challenging now as it has ever been, more so in many cases, as water conservation requirements demand that modern systems do the same job with less water and materials.
While 19th Century researchers made great efforts to understand drainage systems and laid down the fundamental physics of air flow and the importance of friction, there were limitations to what they could do as they only had rudimentary systems with which to work. Some great figures emerged and among those writing on the subject was Osborne Reynolds – famous for his work on fluid mechanics (remember all those equations with Reynolds number from your fluid mechanics classes? That’s him). We truly are standing on the shoulders of giants in this field.
‘Every generation brings advances and ours is no exception – we have a few tricks up our sleeve too.’
Modern mathematical and computer based modelling techniques have allowed us to charge ahead with advances at a rate hitherto unthinkable. This step change has seen a return to look at the fundamental physics of what is going on in there, under the sink, behind the toilet and in your loft. To blow my own trumpet here a bit, and to shout on my own team – central to this new ‘industrial revolution’ of sorts has been the work from Heriot-Watt University, initiated by the late John Swaffield – co-inventor of the P.A.P.A.TM – John’s work on numerical modelling embedded in the computer program AIRNET has allowed any system of any size to be modelled and venting arrangements assessed before the contractor has laid the foundation stone. John’s work is continued by myself, together with colleagues, Lynne Jack, David Campbell and David Kelly.
So, again, where are we going with this?
Well, most of the innovations of the past 20 years have come out of this fundamental research – the P.A.P.A.TM and the DYTEQTA-SYTSTEM to name a just a few – to this list I would like to add ‘Sealed drainage systems’. This innovation was first used in the O2 Dome in London when it was being upgraded to an entertainment arena in the mid 2000s. As a direct result of the developer’s request that the system should not penetrate the iconic tent-like roof structure.
So, with a brief which precluded the use of roof penetrations, in a building with high peak usage capacities the challenge was immense. The sewerage infrastructure wasn’t straight forward either as part of the system was below the river level, so pumping stations were required.
The methodology used to ‘seal’ the above ground drainage system would not be possible without active control of pressure transients. Active control is a method of dealing with pressure transients as close to their source as possible. This is the ideal goal – trying to draw air in to a system through a high friction, small diameter pipe, simply means that the transient will have sucked out the nearest water trap seal long before the air has arrived from the top of the building. In the case of positive transients a similar problem exists in that the protection afforded by parallel vent pipes is negligible and wholly determined by the ratio of cross sectional areas of the stack pipe and the vent pipe. So, traditional venting doesn’t always achieve the goals set out on the design table.
Actively controlling pressure surges is a much more effective way to protect water trap seals.
Air Admittance Valves (AAVs) provide the correct quantity of air to mitigate against negative pressure transients – and this quantity varies automatically with the magnitude of the applied negative pressure transient. In the same vein, positive pressure reduction devices, such as the P.A.P.A.TM (Positive Air Pressure Attenuator) provide an alternative route for positive pressure transients, protecting water trap seals by attenuating the pressure wave in terms of its magnitude and it’s wave speed, slowly releasing the air back into the system in a non-destructive way to ready itself for the next positive pressure event. Taken together, AAVs and the P.A.P.A.TM operate to maintain system pressures at acceptable levels, thereby ‘tuning’ the system to desired pressure levels.
But what if the top of the stack is sealed, will the whole system not pressurise?
The simple answer to this question is no.
The building drainage system is a dynamic multi-phase fluid and solid carrying system. Air currents circulate between stacks, into sewers and around loops. As long as air is provided by AAVs to counteract the negative pressure transients and positive surges can be dealt with by the P.A.P.A.TM then the system cannot pressurise.
The design of the O2 Arena drainage system wouldn’t have been possible without the use of numerical modelling with AIRNET. Simulations of every possible event scenario took several months to complete and produced graphical output running into the thousands. The result was a clean bill of health for the active control solution in the sealed building. By simulating extreme events it was possible to predict how the system would cope, what pressures would be expected and what the limitations were.
So, from Victorian toilets to a state of the art arena in 21st Century London we can see that without innovation we cannot achieve the necessary goals of improved sanitation for all, in any global context. The combination of academic and industrial research; new technologies, techniques and design methodologies, combined with renewed confidence in our own ability to move things forward in this generation, means that we can cope with existing problems, and, more importantly, whatever challenges are thrown at us in the future.
Dr. Michael Gormley
MSc MPhil PhD CEng MCIBSE MIET FHEA
Institute for Building and Urban Design
Senior Lecturer in Architectural Engineering
Room 3.40 William Arrol Building, School of the Built Environment
Heriot-Watt University, Edinburgh, EH14 4AS
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