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

5.6.3.1 Minimum Pipe Size
The standard water distribution main size is:
8-inch-diameter pipe for residential areas
12-inch diameter pipe for industrial and commercial areas
Dead-end distribution mains typically include fire hydrants and must be at least 8-inches in
diameter. Typically, feeder mains are 16- to 30-inch diameter pipes. Some SPU feeder mains are
more than 30 inches in diameter. Pipes more than 30 inches in diameter must not have service
connections.
5.6.3.2 Material Types
All new or replaced water pipe in the City of Seattle must meet the standard material types
shown in Table 5-4.
Table 5-4
Standard Materials for SPU Distribution and Feeder Mains
Structure
Pipe
Bends and Fittings
Joints
Casing
Material
2-inch diameter pipe must be Type K copper (when allowed by SPU).
4-inch diameter and larger pipe must be ductile iron pipe, class 52 or thicker, cement
lined-double thickness.
Feeder mains larger than 20-inch diameter must be ductile iron or steel.
Typically, bends and fittings must be the same material as the pipeline.
Fittings for 2-inch copper soft coil must be brass, either flared or compression.
Joints for ductile iron water mains must be restrained joint, slip joint, or mechanical
joint.
Joints on steel casing pipe must be welded and conform to AWS D1.1 Structural
Welding Code, Section 3, Workmanship.
Whether installed above- or below-grade, casing pipe must be smooth steel, with the
diameter and wall thickness specified in the drawings.
All joints must be welded by qualified operators. Steel casing pipe is discussed in Std
Spec 9-30.2.
Casing seals and spacers must be per Std Spec 9-30(15).
If specified in the contract, the space between the carrier pipe and casing pipe must be
filled with sand, grout, or some other material.
Non-standard mains less than 8 inches in diameter and approved by SPU, must be ductile iron,
except for 2-inch pipe, which must be Type K copper.
5.6.3.3 Pipe Cover
Depths of cover for water mains are shown on Standard Plan No. 030. The depths vary
depending on size of pipe. Mains larger than 12 inches in diameter typically use butterfly valves.
Butterfly valves require less cover due to their shape and allow large mains to be buried at
shallower depths. Generally, SPU attempts to bury the pipes as shallow as feasible for ease of
installation and maintenance, but no less than 35 inches deep except in special cases as directed
by SPU. The depth to the pipe invert should be kept to less than 6 feet to reduce the need and
cost for excavation and shoring.
5-18 SPU Design Standards and Guidelines