EI 11C201
CEMP-E
1 March 1997
now will require the pump discharge valve to be fully closed when the velocity has dropped to
zero. The surge relief valve will be fully open allowing backflow to enter the wet well at a
reduced rate. As before, the relief valve must close slowly to avoid water hammer. Most cases
involving large pump stations with long force mains, which contain several intermediate high
points, will be too complex to solve by hand using conventional methods such as graphical
solutions, arithmetic integration, or water hammer charts. Many computer programs are now
available for water hammer analysis, and are recommended for use in those instances.
7-8. SCREENING AND COMMINUTING DEVICES. Centrifugal pumps are susceptible to
clogging by rags, trash, and other debris normally found in wastewater. To protect pumps from
clogging, equipment will be installed to screen or cut up these materials prior to pumping. Small
pump stations with capacities of less than 13 L/s (200 gpm), including grinder pumps and
pneumatic ejectors, are exempt from this requirement. The types of equipment to be used
include bar racks, screens, and comminutors which are installed in the wet well, or in a separate
influent channel. The design of these facilities is covered in TM 5-814-3/AFM 88-11, Vol. 3. At
most medium to large sized pump stations, the use of mechanically cleaned bar screens or
comminutors will be required. However, at smaller stations in remote areas, manually cleaned
racks may be more feasible. The smallest clear opening between bars is normally 25 millimeter
(1 inch), and spacings of less than 20 millimeters (3/4 inch) will not be permitted. All electrically
operated equipment in wet wells will have explosion proof motors.
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