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be set at ¾ opening. If there are three valves and one is set ½ open and another ¾ open,
the third one should be about ¼ open. After several days of operation you should be able
to balance the boilers so that they all reach approximately the same test. Under known
conditions, however, you will want to anticipate variations. For example, if a boiler is to
be shut down for several days or months because it is not required, you can turn off the
flow control valve from that boiler. Remember the automatic control should be adjusted
to hold the average settings of the boilers in question and the hand flow control valve
should balance according to the loads that the various boilers carry.
 NOTE
XCM System will maintain consistent solids control for a boiler system having a make-up valve
that closes and opens slowly as the water level rises and falls . A snap action valve should be
avoided if possible – to avoid the possibility of having to frequently readjust the proportional
control valve .
Avoid vibration, erosion and corrosion which are the typical causes of type XC System failures
Avoid Flash Steam In The Blowdown Water
Flash steam destroys the heat exchangers, valves and piping. Even a small amount of flash
steam in the blowdown lines between the boiler and the XC will result in excessive water-
steam velocities. For example, an 11 psig ( 0.7 bar-g) pressure loss in water at 138 psig
(9.5 bar-g) and 360°F (182°C) will generate only 0.73% of flash steam. However, this small
amount of steam will increase the velocity in the blowdown lines by 230%.
Excessive velocity destroys heat exchanger, valves and piping by causing:
a. Erosion failure
b. Vibration which fatigues and ultimately fractures exchanger bundles.
c. Vibration which results in stress corrosion cracking of the exchanger bundles
Prevent flash steam in the blowdown line between boiler and XC System by:
a. Using only wide-open gate or plug isolation valves between the boilers and the XC.
b. Using extra heavy blowdown piping one size larger than the XC inlet connection.
c. Set the flow control valves (for the multiple boiler XCM System) so that the average
valve opening is always at one-half (see page two of SD 1730). Flow control valves
set in this manner will not generate flash steam.
d. Do not insulate the blowdown piping. A small amount of blowdown cooling is
necessary to compensate for the line friction loss.
Sentry XC Boiler Blowdown System
9
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