Denver Water's All-pipes (136,377) Model (APM) For Comprehensive Fire Flow Modeling
Denver Water (DW) serves over 1.1 million customers and supplies about 500 million gallons per day on a maximum day basis. The DW system includes three treatment plants, 17 major pump stations and over 160 pressure zones with over 400 pressure reducing valves. DW's primary responsibility is to supply high quality water to its customers at the lowest cost possible. This article presents the interim results of DW all-pipes distribution model fire flow analysis.

In 1997 DW began a process of creating steady-state and extended period simulation (EPS) transmission models for a comprehensive Capital Improvement Program (CIP). This CIP planning was completed in late 2000. The transmission model was expanded in a second phase by creating a steady state all-pipes model in 2002. This more detailed model allows hydraulic analysis of smaller distribution mains. The current effort will transform this steady-state model into an all-pipes EPS model. This phase will verify the model with two system-wide tracer tests at maximum day and fall average day conditions. The all-pipes EPS model will allow accurate determination of detention times within the distribution system as well as determining efficient water quality sampling locations to meet regulatory requirements. The EPS model will also be used to refine operational energy requirements.

As a result, these three models will allow DW to efficiently perform the following:
  • Continual monitoring of DW's 10 year Capital Improvement Program with the transmission models.
  • Review fire flow, planned developments, pressure zone studies and track the replacement/rehabilitation program with the steady state all-pipes model.
  • Comply with continually changing regulatory requirements as well as improving operational efficiencies with the EPS all-pipes water quality model.

APM Fire Flow Summary

Now that the third phase of the EPS APM is being completed, DW began implementing fire flow analyses using the interim steady state APM. As a coincidence, one of the most severe droughts of the century hit Colorado, with that along went our field testing for fire flow availability. Fortunately, DW steady state APM verification was completed and ready to be tested under fire.

DW began the process of assigning fire flow demands to each of our 25,205 fire hydrants. This was a simple process since DW had all the hydrant nodes flagged and simply created a query to assign the demands. DW also needed to know the available fire flow at a 20 psi residual pressure to assist in determining rehabilitation and replacements of our distribution system. All that was left to do was to push a button and wait. In a demanding test of its exceptional power and stability, the all-pipe (136,377 pipes) H2ONET model was run continuously more than 50,000 times and consistently produced exceptional results in a timely manner throughout the process, completing each run in less than four seconds.

DW than began verification of the flows with our over 900 field results and determined that the flows and pressures were too high. This made DW realize that the minor losses at the fire hydrant weren't accurately modeled and could be improved on. DW then added an equivalent minor loss coefficient representing a 4 ½ -inch hydrant nozzle to our fire hydrant branch lines and re-ran the analysis. Again the analysis was successfully run with comparatively better results.

Conclusion

With an accurately verified and validate hydraulic model, results of a comprehensive fire flow analysis will now allow DW to report "calculated" available fire flows at very specific locations and minimize the public relations nightmare of water waste during a drought year. In the past DW would report fire flow availabilities using field tests conducted nearby adding uncertainties to the users needs. This also adds to DW understanding of system weaknesses that aren't readily apparent.

This example also underscores the practical approach of developing an APM for EPS analysis. Not only will the EPS model be beneficial for future water quality analysis so will the interim steady state model that was used to develop the EPS model.







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