2006 Water Comprehensive Plan - City of Bellevue

SYSTEM SERVICE PRESSURE ANALYSISSystem operating pressures are recorded at each fire hydrant during routine hydrant maintenanceactivities. A review of these hydrant pressures shows that the only hydrants with recordedpressures lower than 30 psi are located along transmission mains where there are no serviceconnections.In order to evaluate the water system’s future ability to provide pressures of at least 30 psi to allcustomer service connections, the system was modeled using build-out demand projectionsduring peak hour demand conditions with all equalizing storage depleted. The results of thisanalysis showed that all areas of Bellevue were provided with at least 30 psi of pressure underthese demand conditions.FIRE FLOW ANALYSISAvailable fire flow is calculated presuming maximum day demand under build-out conditionswith both the storage for fighting the fire and all equalizing storage having been depleted. Asdiscussed previously, no significant changes have been made that would alter the fire flowanalysis results presented in the previous plan update. As a result, fire flow analysis results arenot presented in this plan update. Previous analysis showed that there are single-family areasand nonresidential and multifamily sites in the service area that met fire flow requirements atthe time the water system was constructed but fall short of current fire flow standards. TheUtility and the Fire Marshal have identified and maintain an inventory of these sites. In themajority of these cases, the available fire flow is limited by the presence of small diameter watermain, usually 4-inch, being relied upon to provide fire protection. When adjacent CIP projectsor developer extensions are designed, new pipes are sized with consideration of the need forincreased fire flow to these sites. The Utility does not currently have or recommend a policy toeliminate these "grandfathered" areas. However the Utility does have a policy and CIP programin place to improve fire flows. That program is discussed below.Fire Flow Improvement ProgramIn 1984, Bellevue established a program to replace older, primarily 4-inch, asbestos-cement(AC) water mains which had experienced breaks and/or leaks, shown signs of potential loss ofstrength, or were in areas where scheduled street overlays provided an opportunity forreplacement cost savings.. This program has replaced approximately 20 miles of 4-inch ACpipe over the past 22 years.The primary objectives of this program are two-fold: to proactively address potential pipefailures, and to improve fire flow in areas with undersized water mains. The program currentlyreplaces approximately 2.3 miles of older small diameter (primarily 4" and some 6") pipe each8-4