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10.12 Boiler Blowdown:
Blowdown valves and solenoids typically require
120VAC or line voltage. Relays 6-9 are not
covered in this manual for the purpose of boiler
blowdown.
10.12.1
Continuous Boiler Blowdown
Continuous Boiler blowdown is common on larger,
high pressure boilers and routes a continuous low
flow stream through a sample cooler upstream of the
thermally compensated conductivity sensor. (4-wire
probe) The blowdown valve is installed on a parallel
line to the flash tank.
Blowdown control is configured like a cooling tower
bleed and is therefore not detailed in this section of
the manual. See 10.5 Configuring a Blowdown
Relay
10.12.2
Captured Sample Blowdown
Captured Sample is the preferred method for smaller
and medium sized boilers because it does not
require a continuous sample, sample cooler or
cooling water supply. A continuous sample on a
small boiler would drop the conductivity too quickly.
Controlling the conductivity level would be extremely
difficult, wasting water that has been treated with
chemicals, filtered and heated.
IMPORTANTLY, a needle valve or small orifice. The
object of the restriction is to ensure that the flashing,
(high pressure, high temperature water 'flashes'
when subjected to a much lower pressure) is limited
to a point after the motorized valve. The restriction
divides the high and low pressure whereby the
flashing takes place after the restriction, not near the
sensor. In this way, the electronic valve operates in
a constant bath of liquid with no air or gas present.
Air bubbles are the number 1 reason for failure to
correctly read the boiler conductivity!
Therefore, be certain that all valves upstream of the
sensor are 100% open and restrictions minimized.
Flashing must be limited to the needle valve.
The sample routine has 4 steps; Sample, Measure,
Blowdown, and Resample.
Page 79 of 102
Programming the Outputs
The 2-wire conductivity sensor
used in the Captured Sample
method is not thermally
compensated since by design,
boilers maintain a reasonable
constant temperature and
pressure. Therefore, the
Measure sequence requires a
reading from a steady
temperature sensor. Flashing is
disruptive to this method, so
plumbing is critical to a proper
reading.
Refer to Figure 62
A sample from the skimmer in
the drum is passed by the
sensor, a motorized valve or
solenoid, then MOST
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