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

Figure 5-4
Controlled Density Fill
5.6.3.5 Line Pressure
Distribution water mains must be designed to withstand both external loads and test pressure
according to Standard Specification 7-11.3(11)A1.
Most distribution lines serve a portion of the city, and are within a designated pressure zone. In
cases where there is an extreme pressure differential (e.g. downhill pipeline), it may be
advisable to change material thicknesses along the pipeline route and/or install a pressure
reducing valve. Test pressure is measured at the downhill end of the pipe run. Before
considering installation of a pressure reducing valve, the design engineer must coordinate with
USM to ensure the valve will not negatively affect the system.
If a pressure regulating valve (PR) is used, an emergency pressure relief (PRV) must be installed
and set to relieve pressure at 80 psi or maximum zone pressure.
Note: Some current City zones have pressures up to about 120 psi. These zones lie at the
bottom of steep hillside areas. If the site is steep, do not put a PRV where it might cause
unstable soil conditions (i.e. landslides).
Tip:
It is best to install both PRVs and PRs at the lowest point possible. However, sites in
environmentally critical areas (ECAs) may not allow for that.
5.6.3.6 Pipe Supports
A licensed civil or structural engineer must design the pipe supports and review pipe loads and
potential deflections caused by lateral and vertical movement. AWWA M11 (Steel Pipe Design)
and AWWA M41 (Ductile Iron Pipe Design) manuals provide some explanation on how to
5-20 SPU Design Standards and Guidelines