A Bridge Too Far?
One of the recurrent themes of these columns over the past couple of years has been the need to encourage the use of modern simulation and analysis tools to aid in the modernization of building drainage system design. It is essential that new solutions be found for some of the oldest problems faced in system design; however, it is also essential that these new solutions be accepted in practice. Perhaps the greatest challenge comes in the introduction of new appliances that imply changes in long accepted design approaches. While our industry rightly considers safety its prime objective, it is important that such concerns are not used to block the discussion, investigation and subsequent possible acceptance of new approaches.
VentingProbably one of the areas where there is the largest divergence between U.K. and U.S., and possibly European, design and installation practice lies in the area of building drainage network venting. Current U.S. and European practice still requires far greater venting provision than that acceptable in the U.K. In the early 1960s, research work at the U.K. Building Research Establishment, the U.K. equivalent of the Center for Building Technology that was part of NBS, now NIST, in Washington, introduced the concept of single stack drainage for multi-story buildings. This dispensed with direct appliance venting, relying on limiting horizontal branch slope, length and diameter, as well as the appropriate sizing of the vertical stack, to ensure trap seal retention. A "modified one-pipe system" evolved, with intermediate cross connections between the wet stack and a parallel vent stack. Real savings in installation cost and materials resulted, with no degradation in safety or system capability, representing a difference of approach to European and U.S. practice.
AAVs: Positives and NegativesThe advent of the air admittance valve in the1980s was welcomed in the U.K. as it removed the necessity to penetrate the roof structure to provide an open termination to atmosphere. Adherents of the air admittance valve claimed that it solved all the venting problems commonly associated with trap seal loss and the ingress of foul air into habitable space. While accepting the value of air admittance valves as a proven means of offsetting trap seal loss due to negative or suction pressures propagating within the drainage system, these were inflated claims. The provision of an air admittance valve provides protection against trap seal loss due to negative transients--it does nothing to prevent trap seal loss due to the arrival of a positive transient. Design guides require an open termination to atmosphere on the main stack in order to protect against the effects of positive pressure transient propagation. Ideally, an air admittance valve can provide local suction pressure protection to groups of appliances connected to the stack.
Positive pressure transients are generated when stack surcharge cuts off the entrained airflow path. Once propagated, the positive transient will travel throughout the network, being both transmitted and reflected at any branch or branch-to-stack junction. Arrival at a trap seal will initiate displacement of the trap water up into the appliance and possible passage of foul air. Similarly, arrival at an air admittance valve results in AAV closure and a positive reflection of the transient, as the closed valve appears as a dead end. Arrival at last at an open termination generates a negative reflection or relief pressure wave that propagates back through the network. However the positive pressure transient has already done damage, as its passage affected all the system traps to the open termination--which would normally be remote from the most common surcharge site at the base of the vertical stack or at any offsets in the stack.
Positive pressure transients obey all the laws of wave propagation. They travel at the acoustic speed in air--around 320 m/s or 1050 ft/s--and their magnitude following an instantaneous stoppage of airflow travelling at 1 m/s would be in excess of 40 mm water gauge. Thus, a surcharge that destroys an entrained airflow in excess of 8 liter/s in a 100 mm diameter stack will put at risk trap seal retention.
New Solutions for Pressure SurgeWhile lecturing on water hammer and pressure transient courses in the wider area of engineering applications of pressure surge, I used to "wind up" my students by suggesting that the best way to take out a pressure surge was to drill a hole in the pipe. While not a practical solution, this is a route to a solution of the positive pressure transient discussed here and a potential new solution to one of drainage design's oldest problems. If that hole is not to atmosphere but to a collapsed bag within a containment vessel, then the transient is intercepted and trap seals lying between the surcharge site and the open termination of the network will be protected.
Ongoing research at Heriot-Watt has led to the development and patenting of such a device. A 4-liter collapsed bag provides the necessary protection, inflating to absorb the pressure transient. Severe transients would be handled by multiple attenuators distributed up the stack. That is the easy part. Whether the introduction of such appliances will be acceptable lies in the future. Acceptance can only follow site trials and lengthy discussions with code bodies and will require an understanding that new solutions are necessary if old problems are not to remain a permanent part of the drainage system design landscape. It remains to be seen whether such an understanding is a "bridge too far."