Back to Basics: An Overview of Plumbing Engineering
This perspective leads to caution in addressing the title “Back to Basics.” In the U.K., and probably other western democracies, this phrase represents the graveyard of many political aspirations. The first step in this overview therefore is to define “basics” and probably more importantly to determine what is not included under this heading.
The fundamental requirement of a plumbing system, including both water supply and drainage, is that it does no harm. The public health imperative is supreme. This is often encapsulated within codes through an apparent obsession with such issues as back-siphonage. Drainage systems must carry away waste without posing a hazard to the user or providing any long term hazard within the areas through which the drainage system passes. Habitable space must be protected from the incursion of foul odor through the provision of venting systems. These requirements are basic and form, in the U.K. at least, the backbone of the Victorian drainage and vent systems installed from the 1880s and which became the precursor of similar systems within the developing Western world at that time.
In the U.K. in the mid 19th century Chadwick and other social reformers introduced many of the urban water supply and drainage system techniques now taken for granted––effectively the Industrial Revolution spawned both the problem and the socially aware groups that would provide its solution. To these requirements must now also be added the need to conserve water, not only as an economic and political imperative, but also to meet the demands being placed on water supply systems by increases in population, particularly in the cities of the developing world, and the rising expectations of those fortunate to live in developed cities.
What’s Not IncludedSo much for the definition of “basics.” The consideration of what should not be included within this title is more controversial. “Back to basics” does not mean a return to some golden age when rule of thumb was supreme, when venting was so excessive that traps never oscillated and flush volumes so great that imperfections in system design and installation went unnoticed, and each community applied its own codes and standards developed from limited experimentation and observation, unsubstantiated by any degree of engineering analysis or rigor. In the U.S. the design of water supply and drainage systems may be traced back to the fundamental work of Hunter.
However, Hunter recognized in his definitive 1940 paper that the solutions he proposed were limited by the analysis techniques available to him. Referring to the design of building drainage systems he observed that “...the conventional pipe formulae apply to the irregular and intermittent flows that occur in plumbing systems only during that time (usually very short) and that section of pipe in which the variable factors involved (velocity or volume rate of flow or hydraulic gradient and hydraulic radius) are constant.”
Hunter recognized that the basic physics underlying water supply and drainage, and, in particular, drainage, as this arm of the subject offers particular challenges to the analysis due to the free surface nature of the flow, which may also be multi-phase due to the transport of solid material and the possibility of entrained air, is identical in Seattle and Sydney, Hoboken and Helsinki or even Albuquerque and Auchtermuchty. Yet each of the states or nations represented in this list has its own code or standard. The European Community has struggled for nearly 20 years to generate a common plumbing code––unsuccessfully.
Thus, the predominant issue for plumbing engineering at the end of the 20th century has to do with education––the need to stress that physics defines operation and mathematical simulations can function without rule of thumb overrides.
Water ConservationWater conservation offers an example for the interaction between practitioners and those involved in developing both products and system design simulation processes. A review of the water usage within developed countries indicates surprising similarities in the percentage use of domestic water for a whole range of common requirements––approximately 30 to 40% of the drinking-quality water is used to flush toilets. Similar figures are found in toilet and urinal usage in commercial buildings. Careful monitoring of usage has provided this data, and to introduce effective water conservation measures it is imperative that water closet flush volumes decrease.
This has been a continuing thrust for the whole of this century, a century which opened with a disagreement between the London Metropolitan Water Board and the ceramic industry over the 10.5 gallons proposed for water closet flushing and closed with arguments in similar arenas as to the acceptability of 1.6 gallons for flushing devices. Similarly, the century opened in the U.K. with the Institute of Health in London proposing a drain line carry test using half-inch diameter balls and closed with an extremely similar test probably facing demise within the U.S. water closet.
Reducing water closet flush volume is imperative. Good design can deliver. The introduction of dual flush (i.e., a lower flush volume for urine removal, particularly significant in commercial buildings with a high female population), offers further opportunities for conservation. Dual flush was first introduced to the U.K. in the 1980s, where it was unsuccessful due to a lack of clarity in operation. It will be reintroduced in the 1999 Water Regulations, encouraged to a large extent by the successful use of 1.6 and 0.8 gallon dual flush in Australia. The introduction of non-siphonic flushing devices within the U.K. Water Regulations from January 1, 2001, will allow a simpler and unambiguous two-button mechanism which will ensure that the system in understood by all users.
Drainage SystemsHowever, there is a need to recognize that drainage networks are a system. Alteration to one element in isolation may lead to possible problems. There is a need to recognize that reducing flush volume should be accompanied by possible reductions in drainage diameters, particularly for isolated water closets or increases in slope. Similarly, the decay of the flush wave needs to be recognized and modeled. Hunter recognized the importance of wave attenuation but was unable to model it due to the lack of computing power in the 1940s. The modeling method to ensure that these considerations are fully investigated at both the code and design stage exists, developed initially through initiatives at National Bureau of Standards, now NIST, and propagated in the U.S. through ASPE conferences over the past 10 years.
Similarly, water closet design can, and will, benefit from the introduction of modern technology. The application of computational fluid dynamics to the flow regime within water closets has already been demonstrated by such industrial organizations as Toto in Japan. While there is a need to approach with caution the boundary conditions which must determine the accuracy of any such CFD model, the introduction of such models is a major step forward in the development of water closets.
Mathematical simulations can inform the designer when the item is acceptable. Mathematical models can provide the targets to which design should aspire or confirm the appropriateness of a given set of performance code criteria. This approach would bring the plumbing engineering industry in its broader sense in line with those other industries which depend on a fundamental understanding of fluid mechanics.
Thus, this overview of plumbing engineering stresses the importance of water conservation and highlights its growing importance in the coming century. There will be a need for the developed world to reduce its usage of water while at the same time being able to provide low water use solutions to those countries still developing and whose cities are severely taxed by the overuse of water for purposes that could be achieved at lower cost.
In defining the basics of drainage design, the prevention of odor ingress was highlighted. The Victorian concept that smell equalled disease led to extremely complicated venting systems, known in the U.K. as two-pipe networks where each individual appliance was separately vented to a vent stack and black and grey water were separately taken away from the building through two vertical wet stacks. The development in the 1930s of the one pipe system in the U.S. and, in particular, the introduction of the single stack system in the U.K. from the 1950s on, led to considerable savings in terms of the cost of plumbing installations. In the U.K. in the 1950s such reductions were important in the post-war housing rebuilding process. However venting systems are still over-provided.
Modern technology allows the analysis of vent system operation and allows the identification of means by which pressure excursions may be limited. The introduction of pressure relief valves or air admittance valves and the opportunity for distributed venting up the whole height of a multi-story building offers tremendous advantages and savings for vented system design. In the U.K. the development of waterless trap seals that also act as air admittance valves provide exciting possibilities for system designers in the future. In this context “back to basics” does not mean adhering to complicated vent systems when modern technology could offer simpler and more elegant design opportunities.
In the area of rainwater drainage, which varies considerably due to geographic location, similar advantages may now obtained through the introduction of systems relying on a fundamental understanding of system flow. Siphonic rainwater systems applied to large buildings, such as airports and retail outlets, offer tremendous cost advantages, at the expense of ensuring that the mechanisms necessary are fully understood.
Siphonic rainwater systems require that the rainwater pipes run full, creating sub-atmospheric pressures to increase the drainage rate from the roof during design rain storms. System design is related to the design storm and these must be matched. Over-provision in the drainage system will lead to under usage as the design storm may not occur for several hundred years. Under-design of the network may lead to excessive suction pressures as the design storm may occur and be exceeded on many occasions.
The siphonic nature of the system means that pipework is depressurized and there is a possibility of system collapse under particular flow conditions––flow conditions that can be properly analyzed and predicted so that the dangers can be minimized. These innovations emphasize the need for plumbing engineers to fully understand the fluid mechanics of their subject area and confirm that designers and code specifiers understand that rule of thumb safety factors may, in drainage systems, lead to failure, rather than safety.
Water ReuseWater conservation may be viewed as a source of new supply. One of the most hopeful areas for development is in the reuse of wastewater or the collection, or harvesting, of rainwater. In the U.K. this is an area being given considerable attention. However, there are legislative areas to be addressed concerning the quality of the collected water. While rainwater harvesting may be an attractive solution in some areas, as demonstrated at the AWWA Conserv99 conference, water quality must be suitable and there must be no possibility of cross contamination between the contributing water sources, e.g., water closet flushing where rainwater and reused bath or sink water could be collected together in one holding tank also provided with a mains top up supply. There may have to be treatment if water resides within the system longer than the minimum period of time. Developments in on-line control, sensors and tariff structures will allow the benefits of water reuse to be realized.
Recurring IssuesA number of recurring issues are identified by this overview. First, the need to fully understand the fluid mechanics of plumbing system operation will require a shift in the way in which the industry responds to research. However, the research community must understand that its products must be made accessible and understandable to the industry its serves. Therefore, dialogue is necessary between the plumbing engineering industry and the research community that serves it.
Second, it is necessary to address the issue of code specification. All of the developed countries take codes forward in much the same way, by setting up committees which discuss and agree on code specifications. These committees are often dominated by interested parties and the input of fundamental engineering, or a full understanding of the fluid mechanics of the process, may be lost through a perceived need to protect industry from encroachment.
There is a need to reevaluate the manner in which codes are developed and it may be that code proposals should be subjected to theoretical analysis based on system simulations before acceptance to highlight any unnecessary code restrictions. Traditionally, international codes are extremely difficult to generate. However, industry, particularly in the product area, is increasingly becoming international. The purchase of national ceramic manufacturers by overseas bidders has led to a reduction in the overall number of organizations now producing ceramic sanitaryware in the developed world. While individual names may remain, companies have merged and are becoming large multi-national organizations. Codes should recognize this and should be written in the next century to allow freedom of trade.
No overview of plumbing engineering can avoid the problems of operative training. In the U.K. the traditional training routes for plumbers have been severely reduced. Similarly regulation and enforcement have been weakened in the 1980s and 1990s by privatization. Performance codes alone are insufficient if there is no degree of regulation and enforcement to ensure that installations are fit for purpose and meet the basic requirement that plumbing systems shall do no harm. It is likely that both of these issues will be addressed and that there will be a degree of reintroduction in these areas.
Advances in sensor technology applied to water use monitoring will allow water providers considerable freedom to devise tariff structures to control water usage. Complex tariff structures will allow off peak water use and the introduction of a cost regime which will not penalize those less able to pay. These are issues which the industry will have to address, although technically not engineering issues in the sense of the title of this overview.
In conclusion, this overview has tried to identify the range of problems facing the plumbing engineering industry at the end of this century. The advances since 1900 have been considerable, not only in the obvious areas of materials but in the fundamental understanding of the fluid mechanisms of water supply and drainage. The integration of this understanding with the design process has changed the design and installation of building water supply and drainage and vent systems.
There has been a continuous process of water conservation, particularly by reduction of water closet flush volumes, culminating in Scandinavian dual flush water closets that utilize 1.0 and 0.5 gallons as optional flush volumes.
Similarly, there has been a continuous simplification of vent systems, a process advanced in the last 10 years by the introduction of air admittance valves and the possibility of distributed venting. The reduction in complexity of vent systems within buildings is desirable and will replicate to some extent the reduction in material costs seen in the U.K. during the 1950s when the single stack system was first developed.
Perhaps the greatest advance has been in the availability of computer power. Hunter, whose definitive 1930s work laid the basis for drainage system design, recognized that his solutions were limited by his ability to analyze numerically the unsteady flow conditions present in building drainage networks. The explosion of computer power over the last 20 years has made this a reality so that calculations, whose mathematical basis have been known for some 200 years, are at last within the range of drainage designers through system simulations mounted on laptop computers. This offers the greatest possible potential for future development.
No review of plumbing engineering can avoid the changes that have come about through the political pressures placed on the industry. Privatization, deregulation and the reduction in the traditional training for plumbers leaves the industry with considerable problems at the end of the century. In order that the basic principles of drainage design, that the system shall do not harm, can be maintained there may well have to be a reevaluation of these issues.
It is likely that an overview of plumbing engineering in the future will see the emphasis on water conservation in this last decade as being the precursor for major changes in the industry. If this is linked to improved education across all of the constituent parties then the plumbing engineering industry has a successful future to look forward to.