Apr 20, 2006

Optimization of the Pumping/Storage/Piping Relationship in Large Water Systems for Effective Master Planning

Zone 2W is the largest area considered in this study, and comprises most of the lands in the central/west part of Ottawa. Two large reservoirs, 8.8 million gal and 4.9 million gal in volume, are located in this pressure zone, primarily to provide balancing storage.

Zones 3W and Barrhaven are remote from the central urban area, and are each fed through single large diameter (48 in.) pipes and booster pump stations from Zone 2W. These single feed pipes each extend over 2.8 miles across the National Capital Commission’s Greenbelt lands.

It was evident that a significant amount of infrastructure would be required to service the new accelerated growth, particularly in the two outlying pressure zones. Aside from the need for additional major system feedermains, additional pumping and storage facilities will likely be required in the next 20 years. This assignment required several investigations and various analyses to determine the optimum relationship between pumping, storage, and piping to meet the increasing future needs. In optimizing the infrastructure, several key factors needed to be considered with respect to the new infrastructure, including capital and operating costs, impacts on water quality, water supply reliability, as well as supplying peak summer demands and fire flows.

Infrastructure requirements

The general approach to developing the infrastructure requirements involved a multi-step process. These can be summarized as follows:

1. Conduct a Value Engineering Session to establish general system design issues, targets for reliability, level of service needs, customer expectations, and operational opportunities and constraints.

2. Develop future water demand projections including fire flows, basic winter day, maximum day and peak hour demands, as well as diurnal water demand characteristics for each pressure zone.

3. Confirm existing individual pump capacities; total, firm, and diesel pump station capacities; storage operating levels; major pipe capacities; minimum and maximum hydraulic grade lines in each pressure zone; pipe travel times; and storage detention times.

4. Determine minimal additional firm and diesel pumping needs with varying amounts of storage to meet fire demands by assuming (for the city of Ottawa) that 60% of storage is available for fire supply and other emergency purposes.

5. Develop an empirical formula for “Storage/Pumping and Storage/Piping Relationship” curves based on historical diurnal water demand curves for each pressure zone. These graphs indicate, by year, the storage needed at different pumping rates to meet the balancing requirements. The curves also include a baseline for existing storage and for various incremental volumes in the future.

6. The Storage/Pumping curves are then used to develop “Pumping and Piping Requirement Curves” for each zone. These graphs provide curves of pumping needs for different storage volumes through time (with Year on the x-axis and Pump Rate or Pipe Flow on the y-axis). These curves reveal the impact of different balancing storage volumes on the required pumping or piping rates and allow for a direct evaluation, at each future year or future demand level, of the pumping/storage relationship.

7. Develop a “Prioritization of Reliability Needs Table” for the entire system. These tables combine the expected probability of various system failures, the impacts of the resulting outages on supply in terms of magnitude and duration of interruption, and the number of persons/businesses affected. This ultimately results in a “Shortage Potential” value for each failure scenario.

The Shortage Potential value provides a comparative estimate of the overall impact of a failure on the customers, and thus allows for easier targeting of the most critical areas of the system in terms of reliability.

Critical items

The most critical items required to implement the above process involve the development of the “Pumping and Piping Requirement Curves” and the “Prioritization of Reliability Needs Table”. These curves simplify the overall master planning process and lead quickly to the optimal solution, particularly in water systems with multiple pressure zones with related infrastructure. These integrate the normal supply requirements with the system reliability needs.

For example, in the Ottawa system it was found that although pumping is generally much less costly than storage, small storage volumes can be extremely effective in lowering peak pumping rates and associated piping needs. As well, a small storage facility provides some relief for certain system failures, and thus considerably improves system reliability. The results also indicate that it takes a significant amount of storage to have a meaningful impact on piping needs or to provide a secondary source of water for long duration system failures. Storage also allows for some major infrastructure elements to be deferred, and these must be considered in the overall cost analysis (i.e. present value capital and operating costs should be considered).

The process discussed above can easily be used to develop and optimize the piping/pumping/storage/pumping relationships for simple or complex water supply systems.

About the author

John D. Krug, P.Eng. is a Principal with Stantec Consulting Ltd. He can be reached at 613/724-4395 or by e-mail at [email protected]

Ziad Ghadban, P.Eng. is a Senior Program Manager for the City of Ottawa. He can be reached at 613/580-2424 or by e-mail at [email protected].

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