reverse the action of the vacuum/pressure pump by aspirating air
through a silencer and filter unit and discharging the pressurized
air into the vessel. Part of the air shall be utilized to clean
the filter and part of the air shall pass through the aeration
ring of the vessel to pick up material and convey it under
pressure to the storage silo. The unit shall be furnished
complete with all automatic air control valves to control air flow
to and from the vessel continuously through the two modes of the
operating cycle. Unit operation shall continue automatically
until switched off at the control cabinet. All automatic valves,
interconnecting piping, and the vacuum/pressure vessel shall be
skid-mounted. Vacuum/pressure pump shall be mounted separately.
The control cabinet may be mounted separately or skid-mounted on
the vacuum/pressure vessel skid. Capacity of the unit shall be
approximately [_____] metric tons/hour tons/hour of ash weighing
approximately [_____] kg/cubic meter pcf. Piping sizes for ash
collection system shall be designed to fit the unit supplied. The
vacuum/pressure system shall be used where storage silo is more
than 150 m 500 feet from the boiler plant. A vacuum system should
be used for capacities of less than 45 metric tons/hour 50
tons/hour per system.
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NOTE: Where characteristics of the fly ash require
additional treatment, a water spray shall be
incorporated in the filtering unit. If not
required, the portion included in the brackets shall
be deleted. Air discharged to the atmosphere must
meet the local air pollution standards.
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c.
Pump Unit: Vacuum or vacuum/pressure pump unit shall be sized to
match system design requirements. Pump unit shall be liquid-ring
type having round rotor with curved blades rotating in an
elliptical casing. Water alternately entering and leaving the
chambers within the rotor vanes shall provide the required pumping
action. Water within the casing shall act as an air cleansing
agent and the operation and maintenance of the unit shall not be
affected by dust-laden air. Unit shall be base-mounted with
electric motor drive and all required heat exchangers, separators,
and control valves. The vacuum pump inlet piping shall be
provided with a vacuum filter unit to remove the fly ash obtained
from the economizer ash hopper. The filter unit shall include a
metal housing containing filter bags and an automatic air purge
back-washing system. [A water spray shall be incorporated into
the filtering unit.] The filtering unit shall remove all fly ash
before discharge to the atmosphere.
d.
Control Cabinet: Control cabinet for the complete operation of
the system shall be supplied and shall include all running
indicating lights as required. A push-button switch shall be
conveniently located in the boiler house to start and stop the
system. A vacuum breaker, operating automatically from a timer,
shall be provided in the bottom ash conveyor line to break the
system vacuum.
e.
Controls: Controls for a combination vacuum/pressure system shall
have a selector switch set to automatic position to start the unit
in the vacuum cycle. High-level indicator in vacuum/pressure
SECTION 15559A
Page 92
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