Engineering design decisions are multifaceted, often based on a balance of probabilities or advantage. Recent regulatory changes in the U.K., reducing water closet flush volume and introducing a wider range of flushing mechanisms from the international marketplace provide an opportunity to identify some of the factors that inform such a decision-making process, namely a consideration of the system implications of any decision, a recognition of the limits imposed on system or appliance operation and the exercise of choice.

The design of building services is all about systems and the way they operate to meet user demand. The operational characteristics of each constituent component must be compatible to ensure user satisfaction and compliance with operational standards, whether safety-based or driven by economic factors. The "systems" approach to design, an overused term, describes the approach required from system designers, component manufacturers and code authorities.

The components within any system must be tested to ensure compliance with pre-set operational standards. However, there are numerous examples where either the component manufacturer or the standards body did not fully understand the effect of the system on the performance of the component.

An example I encountered in the 1970s relates to pressure surge within aircraft fuel systems. Reputable valve manufacturers failed to realize that if a valve closing surge pressure did not exceed a particular psi setting on a given test rig when closure was completed in a pre-set time that this did not guarantee that the surge pressure generated by the same valve closing in the same time in a "real" system would be at that level for the same throughflow. The essential point missed was that valve closure time as an absolute does not determine surge pressure; it is the valve closure time relative to the system pipe period that matters, where pipe period depends upon pipe length and the surge propagation velocity, which depends upon the pipe properties of diameter, wall thickness and elasticity and the fluid properties of density, bulk modulus and free gas content.

Similarly, pumps chosen without reference to the system may be highly unsuitable if their positioning leads to cavitation. A plumbing example would be the difficulty encountered in developing a suitable test criteria for air admittance valves (AAV), where, for reasons identical to the surge example, the presence of the system affects the AAV operation.

Similarly, the solution to induced siphonage in the British Standards Institute code states that if more than five lavatories are connected through a common waste line then the upstream basin must be vented by a 1-inch diameter vent--which is meaningless unless the vent is short, which may not be the case, e.g. in below-ground structures where the vent pipe distance to the surface might be several hundred feet, dictating mechanical ventilation. System performance is dictated by the characteristics of its components as modified by the presence of the system, an engineering version of Heisenberg's Principle.

The laws of physics impose limits on component and system operation. While the best known is the impossibility of velocities greater than that of light--except for "Trekie" readers of this magazine--there are a range of other limitations appropriate to the design of fluid systems, e.g., vapor pressure levels for transported fluids, dissolved gas content and release pressure levels or compressibility effects in high-speed fan designs--a limitation shared in helicopter rotor design, reinforcing the view that all fluid engineering shares a common base from which the separate disciplines have evolved.

Tank-mounted Siphons

A limit of interest in the U.K. in the introduction of new water conservation legislation concerned the design of siphons to initiate water closet flushing. (In the U.K. siphons have traditionally been used to generate flush water flow from the water closet tank to the bowl; in the U.S. this is done with a drop valve and siphonic action is associated with flow in the water closet passages downstream of the trap).

The operation of the tank-mounted siphon involves flush water being lifted over the apex of the siphon to generate siphonic conditions in the downleg, which acts to fully discharge the tank water into the bowl. The siphon is broken by air ingress when the flush volume available is fully depleted. The "limit" is the time taken to generate the siphonic action and the volume of water remaining to be discharged to the bowl as a sufficiently strong flush to be useful once the siphon is established. For a water closet flush to be significant a minimum flush discharge rate of about 20 U.S. gpm is required. Achieving this with a flush volume below 1.6 U.S. gallons is problematic. Although manufacturers confidently speak of super siphon designs, I have yet to be convinced.

This sets a lower limit to water conservation through water closet design; rather a pity as water closet flushing accounts for 35% of domestic and 43% of commercial water usage and is the most hopeful area for a rapid return on design effort. The acceptance, until the 1999 legislation, that only siphon-activated water closet flushing would be allowed in the U.K., maintained a regulatory roadblock to innovative design and the introduction of alternative proven solutions.

Thus, system design requires consideration of the interaction of the system and its constituent components as well as a recognition of the limits set on the performance of both. What remains is choice--choice of component, satisfaction level and indeed evidence.

While emphasizing the importance of the system concept and the physical limits to operation to design decisions, a much more problematic area is the recognition that engineering decisions may be driven by non-engineering considerations, from short term economics to the openly political--surprisingly not always detrimental. An example of the political decision may be seen regularly at Dulles and JFK airports, where Concorde jetliners operates only because the survival of the program in the 1960s was seen by successive British governments as essential to obtaining French approval for, or at least acquiescence in, Britain joining the Common Market.

Numerous negative examples exist, some more mythological than actual. Barriers to trade and protectionism are also examples where the introduction of engineering innovations have been delayed to allow a home industry to survive or adapt. Unfortunately, the mindset that relies upon such regulatory protection is not often sufficiently adaptable and the eventual outcome is often the opposite of that desired.

One or two examples will illustrate the combined importance of a systems approach and a recognition of limits and choice. The water flows within building drainage systems generate air pressure transients, generally suction pressures that put trap seals at risk. The introduction air admittance valves (AAVs) and combined AAV and waterless trap seals, provides the opportunity to revolutionize the design of venting, except that some surcharge conditions may lead to the propagation of positive air pressures. A systems approach indicates that the performance will be improved by the use of air admittance valves, particularly when distributed up the building to provide the shortest relief path. The possibility of positive transient propagation provides a component limit. Choice is represented by the opportunity to either reject AAVs due to their failure to handle positive transients or introduce a secondary vent to handle positive pressures.

A third option would be to initiate alternative solutions based on the AAV principle and an analysis of transient propagation--is it relief airflow or the reflected transient traveling in excess of 1000 fps that matters? The outcome will depend on more than an engineering discussion of the issues.

Dual Flush Standard

The U.K. decision to adopt a 1.6/1.1 gallon dual flush standard from January 2001 will allow non-siphonic flush initiation and demonstrates the importance of the system, limit and choice discussion. A 1.6-gallon flush was chosen due to research indicating that U.K. water use with this conservation legislation would remain at approximately current levels as user aspirations increased, already demonstrated in the Australian introduction of dual flush, alleviating concern on system throughflow. (This does not mean that there will not be isolated systems carrying low flow but it is essential to keep this percentage in perspective). The move to non-siphonic flush initiation was driven by the limit on water conservation inherent in siphon design and choice was illustrated by both the delays in introducing these decisions, when the U.S. had moved in 1992 and the rest of Europe had adopted lower flush volumes in the 1980s and by this legislation in 1999--decisions now supported by surveys confirming that the move to 1.6 gallons in the U.S. was justified.

For the U.K., the reduction in water closet flush volume offers an opportunity for innovative design to ensure that the full benefits of water conservation are achieved. The removal of the limits to development represented by the siphon-operated flush will open the market to improved products from the U.K. and abroad. The decision by the U.K. government to exercise choice in accepting the evidence of water conservation benefits driven by innovative flushing solutions will undoubtedly prove to be correct and the challenge, if accepted, will reinvigorate the U.K. industry as it responds to healthy international competition.