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Electrode Boiler
electrode and drains through the nozzle plate (5) which forms the bottom of the electrode. As the water
falls from the electrode, it strikes the counter electrode (6) and a second current path R2 is
established.
Since both the nozzle stock and the counter electrode are in contact with the boiler shell, they form
the common connection points of a ''Y" connected load.
At maximum power output with proper conductivity of the water all of the nozzles of the nozzle stock
are discharging to the electrode plate at a constant rate. In order to regulate the output to match
changes in system demand, and to maintain constant steam pressure, the regulating shield is
positioned by a hydraulic cylinder. Vertical movement of the regulating shield results in a nearly linear
change in power output (current flow) relative to the number of streams of water allowed to strike the
electrodes. The change is not exactly linear because the flow rate from the nozzles varies according to
the static head at the nozzle inlet. The time for full travel of the regulating shield can be 20 seconds
or longer and varies according to required operating conditions.
Starting and stopping of the boiler is done by starting and stopping of the pump. The electrodes can
remain energized when the pump is off, since no current can flow unless the pump is running. This
mode of operation provides a 'soft start' and 'soft stop' feature.
In some cases, a standby heater is included. The standby heater is an immersion element type used
to keep the boiler just below the minimum operating pressure during periods when steam production
is not required. When the standby heater is used, the boiler can start producing steam in a much
shorter time after a period of inactivity, as the water temperature has been maintained at a higher level.
Keeping the boiler at temperature is also beneficial to the insulators and gasketed joints.
The boiler works in conjunction with other components within the same system, such as a deaerator
or condensate return tank, which serve as a reservoir for the boiler feedwater. Condensate from the
steam system is recovered at these points and chemical additives are added if needed. The treated
makeup water is fed into the boiler by the feedwater pump through a modulating control valve.
Boiler water conductivity is tested during boiler operation by automatically drawing off small samples
and passing them through a conductivity measuring cell. The conductivity signal is connected to the
automatic conductivity controller. If conductivity is low, the chemical feed pump will be activated to
add the necessary chemicals to bring up the conductivity. If conductivity is high, a solenoid bleed valve
will be activated to evacuate the necessary amount of water and replace it with fresh makeup water.
The water level controller and level gauge are mounted on the water column. The gauge visually shows
the level of water in the pressure vessel. The high and low water limits are mounted in the vessel.
These devices are sensors for the automatic control system.
Regulating Steam Production
The electrical control system automatically positions the regulating shield to maintain the steam
pressure of the boiler at the set point by matching steam output to the load on the steam system.
Should the demand for steam exceed the boiler 's rated capacity, the boiler 's steam output is restricted
by a current monitoring system in the electrical controls. Steam production may be automatically
controlled by a pressure control, or manually by selecting the output desired using the 'POWER LIMIT'
function on the boiler control panel overview screen.
The boiler control system operates primarily to regulate the boiler output to maintain constant steam
pressure, but incorporates also a current monitoring system to prevent the boiler electrical demand
from exceeding the design value - i.e. full load. The 'POWER LIMIT' is provided to enable the operating
engineer to manually limit the boiler to less than full rated MW if necessary. The load regulating system
uses the boiler MW as the controlled variable, and the system is therefore insensitive to changes in
conductivity as long as adequate conductivity is maintained.
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750-272
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