Civil Engineering Reference
In-Depth Information
would be the pump's maximum capacity, which might not be the same as the plant
capacity.
There may also be recycle flows within the plant that would affect the maximum
flow to a unit. For instance, a backwash decant recycle system may operate on level
control and return to a clarifier periodically at a constant rate. Because this may occur
at a time when maximum flow is being experienced by the clarifier, the recycle flow
would have to be considered in the hydraulic design of the clarifier. Consideration
should be given to such temporary peak conditions when determining the headloss
through a particular unit.
When multiple units operate in parallel and one is out of service for maintenance,
the remaining units must be capable of hydraulically handling the flow without major
disruption to the plant performance. This would apply to all related appurtenances
such as feed pipes, valves, and so on.
Units, such as filters, that normally are taken out of service for backwashing should
be capable of handling the flows with some units out of service for backwashing, as
well as allowing for some units to be out of service for repair or maintenance.
Available Head
Each water plant has unique parameters affecting the available head. These parameters
may influence the design of a process, the plant layout, the type of plant control, and
the method of water transport. The available head will determine the cost of the head-
loss. In mountainous or hilly terrain it may be feasible to design a plant without the
need for pumping the main flow. In such cases the cost of headloss is negligible. In
fact, excessive available head may be used to generate power.
In flat areas or in the treatment of well water, additional head will result in increased
operating costs for pumping. At a power cost of $0.075 / kWh, and a wire-to-water
efficiency of 70 percent, it costs $123 / year to pump 1 mgd (3.78 ML / d) 1 foot (0.31
m). At 8 percent interest over a 20-year design life, the present worth cost is approx-
imately $1,200. Therefore, a 50-mgd (200-ML / d) plant with 3 feet (0.91 m) of wasted
head results in an equivalent additional present-worth cost of approximately $180,000.
When an existing water plant is expanded or upgraded, the available head is known
accurately. Additional new processes in parallel with the existing processes must be
designed to match the available head.
COMPONENT DESIGN CONSIDERATIONS
Hydraulic factors that must be addressed include headloss in pipes, hydraulic elements
within the conveyance system, and special hydraulic conditions associated with certain
treatment processes. The special problems include flow distribution and manifolds,
density currents, and to a lesser degree, wind-inducted impacts. These are discussed
in the following paragraphs.
There are two types of flow encountered in water plant design: pipe flow and open
channel flow. Pipe flow occurs when a conduit flows full, and open channel flow occurs
when the flow has a free surface subject to atmospheric pressure. Figure 26-1 illus-
trates the two types of flow and their components. The hydraulic grade line for pipe
flow is represented by the water level in the piezometer (vertical) tubes shown in Figure
