In the U.K., and most developed countries, the spectre of environmental degradation looms large, spawning a whole new vocabulary. The most popular concept at present is "sustainability," leading to the question, "What is the sustainability content of this new course, research project or governmental policy?"

In many cases, sustainability was an established criteria that went unrecognised. For example, how many remember the UN Decade of Water? It was the 1980s, and it was meant to provide drinking water supplies to populations within developing countries. Clearly this project, which would now be part of a sustainable agenda, was not wholly successful.

In contrast, there have been successful water conservation efforts within our industry over the past 20 years, particularly in the setting of w.c. flush volumes, that contribute directly to a sustainable environment, though they were never labelled as such. Similarly, water reuse and collection systems predate their labelling as grey water reclamation or rainwater harvesting. Thus, in adopting new terminology, we must not assume that we are the first to identify the problem we seek to describe anew.

Similarly, within the educational process, the term "transferable skills" has become a crucial criteria for course approval. For those unaware of the relationship between the universities and the professional institutions in the U.K., a word of explanation is probably necessary. The professional engineering institutions, including the Institutions of Civil and Mechanical Engineering, form an Engineering Council, whose approval, or accreditation, is necessary for university courses whose graduates wish to proceed through postgraduate professional training to the attainment of Chartered Engineer status.

These institutions have a long pedigree, their historic importance being indicated by their prestigious buildings some 200 yards from both Parliament and Downing Street--translated to a Washington, DC, location, it would be somewhere along Pennsylvania Avenue between Congress and the White House, but I'm not sure the analogy holds. While transferable skills content is now sought, it may be argued that it has been there for a very long time, represented by the training necessary to facilitate clear report writing, communication and presentation skills, and an awareness of the application of modern technology.

Researching Pressure Surge

A suitable example in our own discipline concerns our attempts to deal with the many manifestations of pressure surge or water hammer within building utility services--efforts that are based on the multinational research that led to our current knowledge.

The terms pressure surge or water hammer are used to describe flow conditions that change with time in any fluid carrying system. In reality, these "unsteady conditions" are the norm in most applications.

The basic mathematical description of unsteady flow was developed by French mathematicians in the 19th century. The first recorded systematic experimental investigation of water hammer was undertaken by Joukowsky at the Imperial Water Works inMoscow in 1900. Joukowsky went on to do fundamental work in aerodynamics, perhaps reinforcing the points made concerning transferable skills.

Joukowsky identified the fundamental relationship linking pressure surge to fluid density, the wave speed or acoustic velocity in the fluid, as modified by the elasticity of the pipe walls and the instantaneous velocity change that triggers the phenomena. The Joukowsky equation indicates that pressure surge is the product of fluid density, acoustic velocity and instantaneous flow velocity change. (Care must be used, as most induced velocity changes are not instantaneous).

Joukowsky's work was translated in the U.S. in the early part of the 20th century and was independently replicated by Allievi in Italy. By the mid-1930s, Schnyder and Bergeron, German and French respectively, had developed graphical analysis techniques that allowed a wide range of surge phenomena to be predicted. Initial areas of interest were understandably in water supply, the scene of Joukowsky's work, and hydro-electric power plant design. Load rejection generates large transients that have to be controlled via surge shafts.

Joukowsky and Bergeron's work found application in the U.S. during the 1930s expansion of hydro-electric power. Pump failure, possibly due to power outage, generates large negative pressure surges that may reduce the line to air release or vapour pressure levels with consequent implosion failures of the conduit. Air chambers and inwards relief air valves have long been the solution of choice in such cases.

Calculating Surge

The speed of computing, even in the 1960s, allowed the original equations describing unsteady flow to be revisited without the restrictions inherent in the graphical methods. Friction was incorporated, and the range of systems analysed expanded to include such diverse applications as surge in engine exhaust systems and aircraft fuel systems, as well as the traditional areas of water supply, hydro-electric plants and long distance pipelines. New areas included nuclear plant flows and sub-sea oil pipelines.

The mathematical description of these applications proved that "size doesn't matter," as pressure surge in all cases was described by the fundamental equations developed in the 19th century. The introduction of the method of characteristics in the 1960s, predominantly by Streeter at Michigan and Fox in the U.K., led to an industry standard for surge simulation and prediction.

The range of pressure surge applications extends well within the remit of this journal. A quick calculation shows that an initial flow of water at 1 m/s at a density of 1,000 kg/cubic m and with a wave speed reduced by the elasticity of the pipe wall to 1,000 m/s yields an instantaneous pressure rise of 100 m head. However, Joukowsky's initial expression for pressure change did not limit the fluid to a liquid state. A similar reduction of 1 m/s instantaneously in an entrained airflow in a building drainage vent system, where the air density is 1.3 kg/cubic m, and the acoustic velocity, unaffected by the effectively rigid pipe wall, is 320 m/s, yields a pressure rise of around 40 mm water gauge. Both calculations have application within building water supply, drainage and vent systems.

Air Admittance Valves: The Solution?

Air pressure transients are central to the next improvement in drainage vent system design, which addressed the way to deal with positive pressure excursions as a result of drain surcharge. It is clear that Air Admittance Valves, available and accepted in Europe, solve many of the negative surge problems and allow reductions in vent sizing via parallel path air flow, thereby reducing trap seal loss. However, the AAV cannot deal with positive surge pressures and may even exacerbate the situation by trapping pressure within the network.

Perhaps the solution lies within the body of research that over the last century involved representatives of all the major developed countries in the application of a truly transferable skill to solve real engineering problems.

And Finally...