EI 11C201
CEMP-E
1 March 1997
prevent excessive deposition of solids, and (4) provide ventilation of incoming sewer gases. In
smaller stations, bar racks or comminuting devices may be installed within the wet well in order
to reduce costs. Overflows from wet wells are prohibited in all cases.
b. Storage volume. If pumps are of constant or adjustable speed type, the wet well volume
must be large enough to prevent short cycling of pump motors. For pumps driven by variable
speed drives, the storage volume may be small provided pumping rates closely match the
incoming flowrates. The volume required for the wet well will be computed with the following
formula:
V
= tq/4
where:
V
= required volume in liters (gallons) between start and stop elevations for a
single pump, or a single speed step increase for adjustable or variable speed
operation
t
= minimum time in seconds (minutes) of one pumping cycle (time between
successive pump starts), or time required for a speed or capacity change, and
q
= pumping capacity, or increment in capacity where one or more pumps are
operating and an additional pump is started, or where pump speed is
increased, in liters per second (gallons per minute).
Constant or adjustable speed pumps driven by squirrel-cage induction motors will be designed
for minimum cycle times as shown in the following table.
Table 7-1. Minimum Pump Cycle Times
Motor size, kW (bhp)
t, minutes
*
Less than 15 (20)
10 to 15
*
15 to 75 (20 to 100)
15 to 20
*
75 to 185 (100 to 250)
20 to 30
*
Over 185 (250)
as recommended by
*
manufacturer
*
The storage volume calculated for small stations (capacities less than 45 L/s (700 gpm)) which
utilize two identical constant speed pumps, may be reduced one half by providing a control circuit
to automatically alternate the pumps. The storage volume required for variable speed pumps will
be based on providing sufficient time for a change in capacity when a pump is started or
stopped. When a pump is started the motor must be ramped to the desired speed, and the
pumps already in operation must be reduced in speed. The time required for this is usually less
than 1 minute. A considerable amount of storage is normally available in large sewers which
serve stations utilizing variable speed pumps. This volume may be considered in design by
calculating backwater curves for the various operating levels. The maximum retention time in the
wet well will not exceed 30 minutes to prevent septic conditions.
7-6
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