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Electrode Boiler
High voltage feedback circuit
This circuit shows whether the high voltage circuit breaker is open or closed by lighting either the
"HIGH VOLTAGE ON" or "OFF" light on the boiler control panel. It also performs an ancillary control
logic function (see control wiring schematic).
Conductivity controller and control circuit
During normal boiler operation the conductivity controller will continuously sample the conductivity of
the boiler water. The conductivity is sensed by the conductivity cell as the sample water passes from
the sample cooler, and reads out on the conductivity transmitter in micromho/cm. Conductivity high
and low are set to the desired levels. If a high or low limit is reached, the conductivity control circuit
will activate the "HIGH CONDUCTIVITY" or "LOW CONDUCTIVITY" alarm in the PLC. If the high
conductivity limit is reached, the circuit will also open the surface blowdown line and bleed off some
water so that the water in the system can be diluted with makeup water to bring conductivity down.
In a low conductivity condition, the conductivity control circuit will signal the addition of the
appropriate amount of chemicals to the water in order to bring the conductivity up.
Load and pressure control circuits
These circuits handle the main load and pressure control functions, checks, and limits in the boiler.
The primary logic circuits are incorporated here. Current feedback is monitored in these circuits.
A current transformer in the high voltage power circuit measures the current being drawn and feeds
that information back to the load control as an indication of boiler power drawn. If the system demand
for steam is higher than output, an instruction is sent to the control circuit to lower the regulating shield
for the output needed. If a lower output is required, the load control raises the regulating shield for the
lower demand.
The load control also allows the circulating pump and load control system to be tested with the high
voltage power off. On the boiler control panel, the "Test" function allows the load control to drive the
regulating shield to its minimum position and permits the circulating pump to be activated. The
regulating shield can then be manually moved to the maximum load position. If the pump and load
control system are functioning properly, the test is complete and mode may be returned to 'Auto-Run'.
Standby control circuit
This circuit will place the boiler in Standby mode. The regulating shield will drive to minimum load
position and the circulating pump will stop. The "STANDBY" banner on the boiler touch screen will
come on, as will the standby heater (if present). The regulating shield will stay in the minimum load
position as long as the boiler is in standby. This circuit uses the control pressure transmitter and is set
at the desired standby pressure.
Circulating pump control circuit
If the boiler is ready for operation and placed into 'Test' or 'Auto-Run' mode, this circuit supplies power
to the circulating pump.
Hydraulic system control circuit
This circuit is controlled by the load control and is directly responsive to system steam demand. When
the signal for more steam is given, the PLC energizes the hydraulic solenoid valve to drive the
regulating shield to a position which allows more water flow to the electrodes.
When steam production and demand are balanced, the hydraulic solenoid valve is de-energized. When
less steam is required, the hydraulic pump is started and the solenoid valve is energized to drive the
regulating shield to a position allowing less water flow to the electrodes. Time delays are used in these
circuits to prevent cycling of the pump motor starter.
14
750-272
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