NEMA 250, Type 12 construction and shall have [dual swing-out doors.] [a
single swing out door.] All equipment shall be mounted and wired with
rigid steel conduit and flexible water-tight connectors, all in accordance
with NFPA 70. Main power to the control panel shall be [480] [_____] volts
ac, [3] [_____] phase, 60 Hz, and control power at 120 volts ac, single
phase, 60 Hz. Interlocks shall be provided as required for sludge heater
operation with raw sludge pumps. The following selector switches and
indicating lamps, clearly labeled, shall be provided on the control panel
doors:
- Digester heater switch (manual/automatic/off).
- Fuel selection switch (digester gas/[oil] [natural gas] [LP
gas]/automatic).
- Constant water bath switch (on/off).
- Induced draft fan (automatic/continuous).
- Sludge recirculation pump (continuous/intermittent).
- Low boiler water lamp with alarm horn and silencer.
- Induced draft [or forced draft] fan failure lamp with alarm horn and
silencer.
- Flame failure lamp with alarm horn and silencer.
- Boiler high temperature lamp with alarm horn and silencer.
2.3.5
Digester Temperature Control
Digester temperature control shall be permitted by the manual/automatic off
switch mounted on the control panel. Automatic control of the digester
temperature shall be by means of a thermostat located at the inlet to the
sludge tubes of the heat exchanger. The water circulation pump for sludge
heating and the burner equipment shall be automatically controlled by the
temperature of the sludge passing the thermostat. A repeating cycle time
switch shall be provided for periodic starting of the digester
recirculation pump, with controls arranged such that the recirculation pump
will continue to operate until the digester heating requirements are
satisfied or shall stop after a short cycle if heat is not required by the
digester. Indicating thermometers, with a range from minus 18 to 66
degrees C 0 to 150 degrees F, shall be provided in the sludge inlet and
outlet of the heat exchanger. Each thermometer shall be provided with a
mounting socket that will enable the removal of the thermometers without
draining the sludge tubes.
2.3.6
Sludge Circulating Pump
Pump shall be as specified in Section 11310 PUMPS; SEWAGE AND SLUDGE.
2.3.7
Sludge Piping
Sludge piping shall be arranged to permit heating of the digester contents
by pumping the digester sludge through the heat exchanger and back to the
digester. The piping arrangement shall also include provision for heating
of raw sludge before it enters the digester or for heating a mixture of raw
sludge and recirculating sludge.
SECTION 11380
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