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

Chapter 5 Water Infrastructure
levels may need to be considered when designing air gap height. Air gaps are usually found at
the end of the discharge line on pipeline and tank blow-offs.
5.6.4.4 Blow-offs
Blow-offs are outlets located at low points in a pipeline profile. They consist of a tangential
outlet off the bottom of the pipeline with an isolation valve and riser located in an access vault
opening to grade. Blow-off valves are used when pipelines need to be drained or flushed for
repairs. Drain rates are limited by the receiving utility or water body. A tap to add sodium
thiosulfate or ascorbic acid for dechlorinating water can be provided at the blow-off, but is not
common.
All blow-off outlets must have an air gap between the pipeline discharge point and the ground
(or surface water elevation of the receiving body) equal in length to a minimum of two discharge
pipe diameters. See example plans in the Virtual Vault 776-227 and 776-203. Both show
examples with sacrificial throttling valves. Example plan 776-203 also shows a pipeline-topipeline
pumping connection.
For detail on design of small blow-off valves, see Standard Specification 7-10.2(11) and Standard
Details 340a and 340b.
Fire hydrants can also be used as blow-offs on dead-end mains.
5.6.4.5 Flow Meters
Flow meter selection is covered in more detail in DSG Chapter 10, Instrumentation & Control.
When selecting a flow meter, make sure to follow the upstream and downstream clearances
requested by the manufacturer. In general, there should be smooth straight pipe (no
appurtenances, bends, fittings, etc) for the equivalent length of 10 pipe diameters upstream of
the meter and the equivalent length of 5 pipe diameters downstream.
5.6.4.6 Fire Hydrants
Fire hydrant installations must be in accordance with the Standard Specification 7-14 and
Standard Plans 310a, 310b, 311a, 311b, 312, 313, and 314.
Hydrants must be located in areas that are accessible and approved by the Seattle Fire
Department or other fire departments with jurisdiction.
5.6.5 Rehabilitation of Existing Mains
SPU rehabilitates existing water distribution main either through slip-lining the pipe or re-lining
the large diameter pipe. Additionally, cathodic protection can be used to prevent further
corrosion of the buried exterior. For detailed information on cathodic protection, see DSG
Chapter 6.
5.6.5.1 Slip-lining
In slip-lining, a pipe of smaller diameter is installed within the original larger pipe. Usually sliplining
is performed on distribution or feeder main pipes 30-inches or larger in diameter. The
existing pipe becomes the casing for the new pipe. It is often a cost-effective no-dig alternative
to traditional open-cut installation of pipe. Typically, slip-lining is used as a structural repair.
However, it cannot be used if there is substantial damage (crushed or misaligned joints) to the
SPU Design Standards and Guidelines
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