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

Other engineering practices such as selecting less corrosive soils for pipelines are design
considerations for corrosion control.
5.7 WATER SERVICE CONNECTIONS
For standards and guidelines for water service connections, see DSG Chapter 17, Water Service
Connections.
5.8 TRANSMISSION MAIN DESIGN
This section describes transmission main design. Transmission mains are major (16- to 64-inch
diameter) pipelines within the SPU water system. They convey or transport water from a source
to a reservoir and typically do not have any service connections. This section covers the types of
materials used, appurtenances, and restraint systems used in transmission main design. For
detailed information on water storage tanks, standpipes and reservoir design, see DSG section
5.9.
This section of the DSG frequently directs the user to DSG section 5.6, Distribution Main Design.
Many of the elements of transmission main design are identical to those for distribution mains.
5.8.1 Modeling and Main Sizing
See DSG section 5.6.1 for distribution mains. SPU’s contracts with its wholesale customers
specify the minimum hydraulic gradient or head at each wholesale service connection. The
newer wholesale service contracts also specify the maximum flow rate at a given hydraulic
gradient that would be provided for each service connection. Any modification to the
transmission system should consider these hydraulic criteria. While these hydraulic criteria may
be modified if beneficial to the regional system, SPU may make these modifications only once
during any 15 year period provided that 4 years advance written notice is given. At a minimum,
transmission mains must be sized to maintain a pressure of 5 psi or more, unless the mains are
directly adjacent to the storage tanks.
5.8.2 Location
SPU transmission mains are primarily located in the right-of-way (ROW). However, their design
is based on least conflict with other utilities and cost of easements. SPU transmission mains
within Seattle are not moved for other utilities. Outside of Seattle, SPU generally owns the
transmission pipeline ROWs and mains are typically not moved. An exception is some holdings
that belong to WSDOT.
5.8.2.1 Separation from Other Utilities
See DSG section 5.6.2.1.
5-40 SPU Design Standards and Guidelines