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

Chapter 5 Water Infrastructure
5.9.3.2 Sizing Inlet Nozzles for Momentum Mixing
The inlet pipe and nozzle to each reservoir cell should be sized to provide a velocity of the
entering water to enable complete hydraulic mixing throughout the entire cell. Typical time to
mix the cell should be 4 to 6 hours at the designed inlet flow rate for lower-demand periods.
5.9.3.3 Mechanical Mixing Systems
If adequate momentum mixing cannot be achieved using inlet jet velocity (due to flow rates in
relation to reservoir cell size), consider enhancing using a mechanical method. The following are
three mechanical mixing methods:
1. Pumped recirculation. The only form of reservoir mixing SPU currently uses is
pumped recirculation. This system features a pumped recirculation loop with the
suction line from the reservoir and the discharge line entering the reservoir through
a single or multiple ports.
2. Mechanical mixers within the reservoir
3. If higher-pressure water is available, consider a gravity hydraulic mixing system that
pipes the higher pressure water to the tank and mixes through a series of nozzles
attached to a riser pipe in the tank.
A. Recirculation
Recirculation systems should be designed for continuous pumping. A general
estimate for recirculation pump sizing is 1 hp per million gallons of storage
volume.
Provide at least two pumps for full redundancy.
To the extent practicable, select pump sizes and types that are compatible with
recirculation pumps at other reservoirs so that pumps are interchangeable and
can be used as replacements or spares.
The recirculation grid size depends on the size of the storage cell, but should
extend to cover all areas of the cell. Pipe sizes for these grids are typically 4 to 6
inches in diameter.
Orifice sizes and spacing are designed to achieve the velocities necessary for
adequate localized momentum mixing. Typically, the range of velocity needed is
8 to 10 fps at the orifice discharge.
Provide an easily accessible sample collection point on the recirculation piping.
Provide an easily accessible chemical injection point on the recirculation piping.
5.9.4 Water Level and Flow Measurement
Each storage cell must have provisions for online measurement and recording of
water levels between lowest anticipated operating and overflow levels.
Provide a totalizing meter on the outlet side to accurately measure demand from
the reservoir.
SPU Design Standards and Guidelines
5-55