2013 Water System Plan, Volume II - Seattle City Clerk's Office - City ...

The major supply transmission pipelines from the Cedar and Tolt sources deliver water to
wholesale customer master meters and intertie locations, such as the City of Tukwila.
Transmission pipelines deliver water to various tanks, standpipes, and reservoirs throughout the
system and on occasion directly to the distribution system. Twelve reservoirs and 14 tanks
comprise the in-town storage facilities. Water from the in-town facilities is distributed to
customers via the distribution system.
5.2.3.2 Distribution System
The SPU water distribution system contains more than 1,690 miles of water mains. These mains
vary in diameter from 4 inches to greater than 30 inches. Most SPU water mains are unlined or
mortar-lined cast iron, ductile iron, or steel pipe.
Seattle’s water distribution system also includes 15 pump stations and more than 180,000 water
service lines and meters serving residential and non-residential properties. Generally, both
transmission and distribution mains passing under railroads or similar facilities are encased.
Most pipelines do not have corrosion protection. See Chapter 6, Cathodic Protection.
See DSG Chapters 11 and 17, respectively, for standards for pump stations and water service
connections.
For more information on the history and condition of the water distribution system, see the
2007 Water System Plan.
5.2.3.3 Valves
SPU owns about 21,500 valves of various types that support the SPU Water System. SPU installs
most valves where needed for ease of operation and system redundancy. District valves are
installed on the distribution system to separate pressure zones. Pressure valves regulate flow
between pressure zones. Both transmission and water mains have blow-off valves at low points
to drain pipelines and air valves at the high points. Air valves either release trapped air from the
pipelines when under pressure or allow air into the pipelines while being drained to prevent a
vacuum in the pipeline. Some blow-off valves are installed at dead ends for water quality
flushing. Inside major facilities, SPU uses blow-off valves for dewatering and flushing operations.
Other valves function as bypass, altitude control, or pump control.
5.2.3.4 Infrastructure Elements
The following are key components in SPU water system infrastructure:
Infrastructure
Water Main
transmission main
feeder main
distribution main
Description
Large diameter (generally >3 ft) pipeline that transfers water from source to
feeder mains or storage tanks. There are no service connections on
transmission lines, except for purveyors.
Smaller diameter pipeline (generally