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

Chapter 5 Water Infrastructure
Valve
Combo Air Release/
Air Vacuum
Use in SPU Water System
Combination air release/air vacuum valves are air valves that have both large orifices and small
orifices and provide the combined functions of both types of valves. These valves must comply
with the requirements of ANSI / AWWA C512. They are sized per AWWA M51 based on 2-
psi pressure differential for filling and draining. Air release valve orifice sizes are chosen based
on water main operating pressure and air release rate, which must be selected to avoid abrupt
pressure surges.
Air vacuum and air release valves must be equipped with isolation valves and be installed in
vaults. Discharge from all air valves must be plumbed to the surface. Washington State DOH
regulations for cross connections do not allow discharge into the vault.
On large pipelines, open air stand pipes are sometimes used in lieu of a combination valve.
B. Bypass Requirements
Bypass assemblies are used on larger valves to ease valve opening. When a line has
been closed, and one side of the pipeline drained, pressure on the valve becomes
unbalanced making it very difficult to open the valve. A bypass line allows water to
backfill the unpressurized side of the valve, which equalizes pressure on the valve
allowing it to open more easily. Bypass assemblies also allow the operator to fill an
empty pipe more precisely. Bypass assemblies must be installed on all valves 16-inches
and larger in diameter. A typical bypass assembly is included in the standard line valve
vault detail. Typically, SPU will use a hydrant or an adjacent active line with a PRV and
temporary piping to do this. See Figure 5-5 Typical Layout of a 16-inch or Larger Line
Valve.
C. Valve Turns
The number of turns to close a valve is very important. Rapidly closing a valve can create
a surge pressure wave in the pipeline and damage the line and appurtenances. See
Standard Specification 9-30.3(4).
D. Valve Spacing and Location
Line valves are to be located at grid junctions in the distribution system, such that each
of the converging main segments can be independently isolated. Additional
intermediate valves may be required between grid connections, such that any single
shutdown segment will be no more than one block (approximately 300 to 500 feet) in
length within commercial/multi-family residence zones.
On primary distribution mains and feeder mains larger than 12 inches in diameter,
valves are to be located where these mains intersect with other mains larger than 12
inches, such that each of the converging large diameter main segments can be
independently isolated. Additional intermediate valves may be required between large
diameter pipe junctions, such that any shutdown segment of a main larger than 12
inches in diameter will be no more than 1,300 feet in length. The actual number and
positioning of intermediate line valves on a main larger than 12 inches in diameter is to
be guided by an analysis of secondary outage impacts associated with the isolation of
individual segments of mains larger than 12 inches in diameter. As a general guideline,
the total outage area resulting from the isolation of any one main segment including all
of its subordinate dead-end systems (if any), should include no more than 130 accounts
or 20 fire hydrants or 200 gpm of average daily consumption. Placement of line valves
SPU Design Standards and Guidelines
5-33