GESTRA SPECTORcontrol II, Original Installation & Operating Manual

Page: 5 / 32

5
Important notes
Definition of terms
continued
Cutout threshold
If the control point is below the cutout threshold during pump control, the control point = 0.
This prevents the pump from constantly switching on and off.
Continuous blowdown
As the boiler water begins to evaporate, the concentration of non-volatile total dissolved solids (TDS) left
behind in the boiler increases over time as a function of steam consumption. If the TDS 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.
The consequences are adverse effects on reliability and severe damage to steam generating units and
pipes. 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 feedwater
must be added to the boiler feed to compensate for the water lost through blowdown.
To determine the TDS in the boiler water, its electrical conductivity is measured in μS/cm, although
some countries also use ppm (parts per million) as the unit of measurement.
Conversion 1 µS/cm = 0.5 ppm.
Intermittent boiler blowdown
During the evaporation process, fine sludge is deposited on heating surfaces and on the floor of the
steam generating unit. This sludge is caused by adding oxygen-scavenging agents, for example. As the
sludge prevents the transfer of heat, the boiler walls can become dangerously overheated.
Intermittent blowdown is achieved by the abrupt opening of the intermittent blowdown valve.
The resulting suction effect occurs only at the moment the valve is first opened. The opening time
should therefore not exceed about 3 seconds. Longer opening times result in a loss of water.
Time-based pulse/interval actuation of the intermittent blowdown valve optimises the removal of sludge
from the boiler. The interval between intermittent blowdown pulses can be set, as can the blowdown
time
.
For large boilers, repeated intermittent blowdown pulses may be required. In this mode, you can then
set the repetition and interval between pulses.
Operating position of the continuous blowdown valve
To keep the TDS within the desired limits, it is common practice for the continuous blowdown valve to
remove a certain amount of water from the boiler. This means that the valve must be permanently open
during operation, so that this amount of water can be discharged (valve in operating position).
This operating position can be adjusted accordingly. For the corresponding amount of boiler blowdown,
refer to the capacity charts of the continuous blowdown valve.
Compensation for steam bubbles (conductivity control)
The presence of steam bubbles can lead to greatly fluctuating conductivity readings. An attenuator will
damp the oscillations in the measurements.
Switching hysteresis (conductivity control)
If the controller is configured as 2-position controller, the valve will OPEN if there is a positive deviation
(X > w). The conductivity must then decrease until it reaches a new valve that is lower than the setpoint
minus the adjusted hysteresis. After this, the valve is moved into the Closed or Operating position,
depending on your settings.
continued