TM 5-814-2/AFM 88-11, Vol. 2
available for water hammer analysis,
and
are
The types of valve operators most often utilized are
recommended for use in those instances.
hydraulic, electric and pneumatic. Valves and operators
specified for use will be fully adjustable for closure times
ranging from t to 4t minimum. In some large pumping
devices.
5-6.
Screening
and
comminuting
stations, the use of automatically controlled valves
Centrifugal pumps are susceptible to clogging by rags,
alone will not be sufficient. Extremely long force mains
trash, and other debris normally found in wastewater.
(over 1 mile) may require very long valve closing times,
To protect pumps from clogging, equipment will be
and thus result in excessive backflow to the wet well and
installed to screen or cut up these materials prior to
reverse rotation of the pump and motor. To solve these
pumping. Small pump stations with capacities of less
problems, a pump bypass with surge relief valve will
than 200 gpm, including grinder pumps and pneumatic
generally be required. Valves used for surge relief will
ejectors, are exempt from this requirement. The types
be automatically controlled cone or butterfly valves,
of equipment to be used include bar racks, screens, and
similar to the pump discharge valves. Normal operation
comminutors which are installed in the wet well, or in a
upon pump shutdown now will require the pump
separate influent channel. The design of these facilities
discharge valve to be fully closed when the velocity has
is covered in TM 5-814-3/AFM 88-11, Vol. 3. At most
dropped to zero. The surge relief valve will be fully
medium to large sized pump stations, the use of
open allowing backflow to enter the wet well at a
mechanically cleaned bar screens or comminutors will
reduced rate. As before, the relief valve must close
be required. However, at smaller stations in remote
slowly to avoid water hammer. Most cases involving
areas, manually cleaned racks may be more feasible.
large pump stations with long force mains, which contain
The smallest clear opening between bars is normally 1
several intermediate high points, will be too complex to
inch, and spacings of less than 3/4 inch will not be
solve by hand using conventional methods such as
permitted. All electrically operated equipment in wet
graphical solutions, arithmetic integration, or water
wells will have explosion proof motors.
hammer charts. Many computer programs are now
5-9
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