DISTRIBUTION SYSTEM PRESSURE
Maintaining adequate distribution system pressure is critical to ensure both customer service and drinking water quality.
Adequate water pressure enables customers to have sufficient water flow from their household plumbing fixtures and appliances.
In addition, adequate pressure prevents contaminants from entering the distribution system through pipeline leaks and cross connections. In 2004, SPU developed a service level which meets Washington State Department of Health (WDOH) requirements for pressure and provides a method for an economic analysis of supplying higher pressure levels in new and existing areas of the distribution system. This service level establishes a minimum 20 pounds per square inch (psi) service connection pressure standard for the existing distribution system during normal operations and a minimum 30-psi design standard for new distribution system construction, consistent with the Washington Administrative Code (WAC). Current services with less than 20 psi of pressure will be brought up to at least 20 psi through system improvements. Existing services with pressures less than 30 psi will be brought up to a higher pressure when it is economical to do so.
While some level of leakage is unavoidable, it is important to SPU to keep leakage to a minimum because it represents a waste of valuable resources and may result in water damage to property.
WDOH is developing a requirement that would limit distribution system losses from all leaks to 10 percent of the total water delivered to the retail service area. SPU intends to meet the
WDOH requirements with this service level. SPU's water system has had a history of low leakage rates. In 2005, SPU's total non-revenue water was 9.3 million gallons per
day (mgd), or 7 percent of the total 128 mgd produced. Leakage is only one component of non-
revenue water; other components include seepage and evaporation from open reservoirs, water used for flushing and firefighting, as well as meter errors. Current leakage from SPU's
distribution and transmission system is estimated at between 3.3 mgd and 4.8 mgd, or between 5 and 7 percent of the 67 mgd total produced excluding that sold to wholesale customers in 2005.
Approximately 15 percent of the leakage comes from transmission pipelines and water mains, and the remaining 85 percent comes from service connections on SPU's side of the meter.