Flowserve URB 2 - Manual (Page 42)

42
Glossary
Continuous boiler blowdown (“top 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 vapor into steam lines and
superheaters. As a consequence, the operational safety is impaired and severe damage to boiler and
tubes may occur.
To keep the TDS concentration within admissible limits, a certain portion of boiler water must be
removed continuously or periodically (by means of a blowdown valve) and fresh make-up water must
be added to the boiler feed to compensate for the water lost through blowdown.
Intermittent boiler blowdown (“bottom blowdown”):
During the evaporation process fine sludge deposits settle 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.
To perform a bottom blowdown the intermittent blowdown valve must be opened abruptly.
The resulting suction effect occurs only at the moment when the valve is being opened, the opening
time should therefore not exceed 2 seconds. Longer blowdown periods will merely waste boiler water.
The timed pulse/interval control of the intermittent blowdown valve optimises sludge removal while
minimising loss of boiler water.
Sensitivity (conductivity level control):
The level electrode NRG 6-42 works only when used in water with a minimum electrical conductivity.
If the electrical conductivity of the boiler water is < 0 μS/cm at 75°C set the sensitivity to 0.5 μS/cm.
Operating position of the continuous blowdown valve:
It is common pratice to use the continuous blowdown valve to remove water from the boiler in order
to keep the TDS level 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 control valve is equipped with a CANopen actuator. For more information see installation manual
URZ 40.
Measuring range (conductivity control):
The measuring range setting establishes the actual value output 4-20 mA of the control equipment
LRR -40.
Proportional band X
p
(conductivity control):
If the controller is to work as a proportional controller, the proportional band can be set between and
50 %. It refers to the adjusted setpoint w. If X
p
= 0 is set, the controller is configured as two-position
controller.
Proportional band Xp (capacitance level control):
Switchpoint 2 marks the upper limit and switchpoint 3 the lower limit of the proportional band. The
difference between switchpoint 2 and switchpoint 3 gives the magnitude of the proportional range X
p
.
Switching hysteresis Hyst (conductivity control):
If X
p
= 0 is set, the controller is configured as 2-position controller, which means that the valve will
open if there is a positive deviation (X > w). The conductivity must then decrease until a new value
that is lower than the setpoint minus the adjusted hysteresis is reached. Once this value is reached the
valve will be motored into the operating position.