When Is Close Close Enough?
The recent difficulties with the high-tech bridge crossing of the Thames in London, which "swayed" alarmingly on being opened to the public last month, have been the cause of embarrassment to the leading engineers and architects from Ove Arup responsible for its construction. Recent reports on the design process featured the engineers' original design concepts scribbled on a bar napkin-almost a caricature of engineering mythology. However, the ability to draw on the back of an envelope and get somewhere close to the final design is an important but fast disappearing skill amongst engineers. This disappearance is accelerated by two contrasting effects: first, the increased reliance on computational methods and "packages," and a reduction in the mathematics content of engineering courses to make them more attractive in a management-oriented society. There is a need to understand the basic interactions involved and also to recognize when approximations give an acceptable answer. The corollary to the latter point is an understanding of the cost of taking the solution beyond the limits of accurate knowledge of the variables involved. Thus, knowing when to stop is as important as recognizing when an answer is close enough.
These issues were brought to mind recently by two disconnected events. The first was my attendance at a Pressure Surge Conference in the Hague, where I presented a keynote paper on "Unsteady Flows in Building Services Utility Systems," and the second was clearing my office for a paint job to be undertaken while I am away from the University in August.
The Pressure Surge Conference was fascinating for a number of reasons. The most important was that, while I had been heavily involved in the first of the series in 1972, my research from the mid-1970s had taken me into the plumbing area and away from the traditional problems of water hammer, surge protection and hydro-electric plant design. During the late 1960s, I was one of a small group of researchers in the U.K. who seized upon the advantages of computer availability to solve the equations defining water hammer. It is probably difficult for readers of this journal to appreciate, from today's perspective of laptop computing and the Internet, the excitement that could be generated by one computer run a day on a mainframe protected by glass panels and serviced by white-coated high priests who would accept your program cards at set times only.
The use of computational methods to solve water hammer and pressure surge problems was led in the U.S. by Streeter and Wylie at Michigan, and in the U.K. by Fox at the University of Leeds. The first Pressure Surge Conference was held in Canterbury in 1972 and included papers by all the leading exponents of the new art form, along with my paper on "Pressure Surges in Aircraft Fuel Systems." By this date, all the major players had accepted that this was the way forward, and the basis of new industry standard techniques had been laid, drawing upon the method of characteristics, the same methodology we now use to simulate drainage system operation.
Returning to the present and the Hague conference, it was surprising to see how many of the same areas were still being considered almost 30 years later. Admittedly, the computation was faster, and the ability to include greater detail was improved, but I'm not sure that some of the comparisons between theory and practice were any better than we achieved at the first conference. So, perhaps we were close enough some time ago.
Quaint MusingsThe second series of events that triggered these thoughts was clearing old files-always an amusing pastime. "Could that have been what we really believed?"
In looking through the Counseil International du Batiment CIB W62 Water Supply and Drainage for Buildings conference papers from 1975 onwards and more recent ASPE papers, it was apparent that the selection of waste solids for either drain line carry or water closet (w.c.) performance tests suffers from the same circularity as some of the pressure surge discussions.
As far back as 1975, Japanese researchers were advocating PVA sponge waste simulations, while the U.K. Building Research Establishment supported the single 43mm diameter 1.05 specific gravity ball as the best test for w.c. discharge.
By 1982, German researchers had developed a mixable powder, rejoicing in the title "Fakazell," which could be mixed to generate fecal substitutes with variable specific gravity, together with a suitable insertion device to ensure repeatable w.c. flushing assessment.
By the mid 1980s, U.S. codes were involved in both drain line carry and w.c. discharge assessment, closely followed by the German DIN standard and the Australian codes, with their characteristic water-filled sheath materials. The U.S. drain line carry test utilising 50 or 100 1/2-in. diameter balls was an inadvertent throw back to the earliest transport test developed in London for the Metropolitan Water Board in their attempts to lower flush volumes in the 1890s. The discussion progresses with more and more complexities being introduced, from the mixed media U.S. efforts to the simpler Australian/DIN Standard Four solid approach.
In considering "when is close close enough," it is perhaps necessary to ask what is the objective of such testing. I remember talking to a colleague in the mid-1980s who stated that the problem was solved, as an agreed load for a w.c. assessment test had been defined; when asked how this breakthrough had been obtained, he retorted that the load chosen allowed all the w.c.s tested to pass. The objections to the current U.K. w.c. tests being brought in to support the widening of the import base under the 1999 Water Regulations display the same set of concerns. "Tests are okay as long as my product passes."
So, is it necessary to continue the search for a perfect simulation w.c. load, bearing in mind the wide range of parameters whose value we cannot know accurately? Efforts to model the discharge to a w.c. bowl cannot yet be seen as anything but tentative, and any effort to introduce the body forces acting on solids caught up in the flush in three dimensions is clearly beyond the reasonable limit of simulation investment. Therefore, perhaps it might be better to accept that testing should be comparative, allowing manufacturers to cull really poor performing designs before they become available for sale.
Similarly, perhaps code bodies should seek a more simplistic set of criteria, accepting that simulated solids cannot recreate the reality of multi-solid, random order discharge to drains that may well be defective in terms of slope and alignment. It is interesting to note that the only recorded long-term observation of w.c./drain line carry of "real" waste, including fecal material, tissue and sanitary products, due to Boker (fully reported by Swaffield and Galowin in 1992), has never been fully exploited by the community, despite the fact that it clearly identified that the whole range of waste could be simply simulated by commercially available sanitary products of varying cross section and saturated mass. Perhaps no one really believed that Boker could have spent two years in the interfloor voids of a large hospital, monitoring the discharge from both male and female w.c. facilities.
Therefore, it would be useful to pause to review the progress over the past 25 years in the definition of w.c. discharge and drain line carry testing in order to develop more realistic criteria based on comparison between w.c. types and modes of operation with the primary aim of ensuring that the final product is "fit for purpose."
Similarly, the relationship between drain diameter and slope and the ability to transport waste solids with a particular flush volume need to be studied further so that system and appliance designers build a clear understanding of these interrelationships and the consequences of design decisions. Without research and industrial co-operation, none of these understandings will be improved, and this will limit the development of drainage system design.