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

Chapter 5 Water Infrastructure
5.8.9 Corrosion Control
See DSG section 5.6.8.
5.9 WATER STORAGE TANKS,
STANDPIPES AND RESERVOIR DESIGN
This section describes water storage facility design. Water storage facilities primarily function to
provide adequate flow and pressure for all design conditions where the transmission and
distribution system cannot otherwise maintain the flow or pressure required. Water storage
tanks, stand pipes, and reservoirs are critical infrastructure that directly influences public health
and safety. These SPU facilities must be designed and operated to prevent cross-contamination
of water and degradation of water quality. For more detail on SPU reservoirs, see DSG Chapter
13, Dam Safety.
5.9.1 Planning
USM determines the need for a new or refurbished water storage facility. This planning includes
determining the facility’s general characteristics, size, location, and a timeline for service based
on hydraulic modeling and demand projections. If approved by SPU management, a storage
facility project is incorporated into the Capital Improvements Program (CIP) plan.
5.9.1.1 Service Life
A. Concrete Reservoirs
For new concrete water storage reservoirs, service life must meet the specific project
requirements. Most water utilities use a typical service life of not less than 50 years for
concrete structures. For refurbished existing concrete water storage reservoirs, the
design service life will be established case-by-case based on the specific conditions and
requirements for the reservoir.
B. Steel Storage Tanks
For new steel water storage tanks, most water utilities use a design life of 75 or more
years, assuming that the coatings are well maintained. An economic analysis of coating
and cathodic protection systems should be done to determine the most cost-effective
method for preventing corrosion. For refurbished existing steel water storage tanks,
design service life is established case-by-case based on specific conditions and
requirements for the tank.
5.9.1.2 Hydraulic and Capacity Requirements
Generally, the size of a finished water storage facility must provide sufficient capacity to meet
both domestic demands and any requirements for fire flow.
Specific capacity requirements must meet the applicable elements of the Washington State
Department of Health Water System Design Manual or SPU’s system reliability criteria under
defined emergency scenarios, whichever is less. Storage facilities are expensive to construct,
SPU Design Standards and Guidelines
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