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79
Glossary
Continuous boiler blowdown
As the boiler water evaporates, the concentration of non-volatile dissolved solids (TDS) left behind in
the boiler increases over time as a function of steam consumption. If the TDS (= total dissolved solids)
concentration exceeds the limit defined by the boiler manufacturer, foaming and priming occurs as
the density of the boiler water increases, resulting in a carry-over of solids with vapour into steam
lines and superheaters. As a result, the operational safety is impaired and the steam boiler and/or
pipelines can be damaged. To keep the TDS concentration within admissible limits, a certain portion of
boiler water must be removed continuously or periodically (by means of a continuous blowdown valve)
and fresh make-up water must be added to the boiler feed to compensate for the water lost through
blowdown.
Intermittent blowdown
During the evaporation process, fine sludge is deposited on heating surfaces and in the lowest part of
the steam boiler. The accumulated sludge sediments form a thermally insulating layer and can damage
the boiler walls due to excessive heat.
Intermittent blowdown is performed by opening the intermittent blowdown valve quickly.
The resulting suction effect occurs only at the moment when the valve is being opened. The open-
ing time should therefore not exceed 2 seconds. The timed pulse/interval control of the intermittent
blowdown valve optimises sludge removal while minimising loss of boiler water.
The interval between the intermittent blowdown pulses can be set between 1 and 120 h
(intermittent
blowdown interval)
. The
duration of the intermittent blowdown
can be set between 1 and 60 s.
For larger boilers it may be necessary to repeat the intermittent blowdown pulses. The
intermittent
blowdown pulses
can be repeated up to 5 times within 5 – 30 seconds
(pulse interval)
.
Sensitivity (conductivity level control)
The level electrode NRG 16-42 only works when used in water with a minimum electrical conductivity.
If the electrical conductivity of the boiler water is < 10 μS/cm at 25°C set the sensitivity to 0.5 μS/cm.
Operating position of the continuous blowdown valve
It is common practise to use the continuous blowdown valve to remove water from the boiler in order
to keep the TDS within certain predefined limits. This means that the valve must be permanently open
so that a steady flow of water is ensured (the valve is in the operating position).
The operating position is adjustable between 0 and 25 %. For the corresponding amount of boiler
blowdown refer to the capacity charts of the continuous blowdown valve.
CANopen actuator
The continuous blowdown valve and/or the control valve is equipped with a CANopen actuator. See
user’s manual EF 1-40 / URZ 40.
Attenuation (conductivity control)
The presence of steam bubbles can lead to greatly fluctuating conductivity readings. An attenuator will
damp the oscillations in the measurements.
Measuring range (conductivity control)
The measuring range setting establishes the actual value output 4-20 mA of the control equipment
LRR 1-40.
Proportional band X
p
(conductivity control)
If the controller is to work as a proportional controller, the proportional band can be set between 1 and
150 %. It refers to the adjusted setpoint w. If X
p
= 0 is set, the controller is configured as a 2-position
controller.
Proportional band X
p
(capacitance level control)
Switchpoint 2 marks the upper limit and switchpoint 3 marks the lower limit of the proportional band.
The difference between switchpoint 2 and switchpoint 3 gives the magnitude of the proportional range
X
p
.