Pressure Standards in Water Distribution Systems: Reflection on Current Practice with Consideration of Some Unresolved Issues

Pressure standards assist in the design of water distribution systems and the assessment of their performance. Although exact thresholds are sometimes rather vague, unusually high and low pressures are widely understood to increase costs and put systems at risk from events like pipe bursts at the high-pressure end to the risk of contaminant intrusion or poor firefighting conditions at the low-pressure end. Interestingly, because the definition of what conditions constitute acceptable pressures differs around the world, a delivery pressure might be considered acceptable in some regions and unacceptable in others. But if a wider range of system conditions is considered, including transient events, an interesting question arises as to what exactly the standards might mean and how violations should be evaluated. Specifically, what kinds of pressure transgressions are most crucial to system performance and economics and what kinds are merely inconvenient? Certainly the issue of evaluating consequences is relevant, but it is also complex because some consequences are not easily attributed to specific system conditions. As this paper considers, the frequency, duration, and intensity of the pressure violation are all relevant, but so is the vulnerability of the system to those violations. Few of these issues have yet received adequate attention, but it is to raising such a discussion that this paper is aimed. There is no doubt that pressure standards can help to assess system performance and to trigger system evolution (i.e.,operational and capital investments). But if the criteria themselves, and the means to evaluate them, are too vague, so will be the corrective outcomes.