UFC 3 -430-11
14 February 2001
7-3.1
Control Configuration. Configure the controls so that the air to fuel ratio
does not have to be manually reset or reprogrammed when switching from one fuel to
another or from one combination of fuels to another. Provide an 8 -point minimum fuel to
air characterization curve for each fuel or combinati on of fuels. Provide controls so that
the air to fuel ratio is automatically adjusted to the proper proportions for all of the
following:
Load change (between minimum firing and MCR)
Fuel change (either change in fuels or same fuel but different
specifications)
Fuel ratio change (more than one fuel firing)
7-3.2
Fuel Changeover. Accomplish the switch over from one fuel to another
without shut ting down the boiler. Configure the controls so that the changeover from
one fuel to another can be made by either a selector switch, pushbuttons, or by entering
commands on a keyboard.
7-3.3
Simultaneous Fuel Firing. Provide automatic controls for maintaining the
proper fuel ratio and fuel to air ratio to compensate for load changes when firing several
fuels at the same time. Do not allow multiple attempts to switch fuels without a full post-
purge and pre-purge after each unsuccessful fuel switch attempt.
7-3.4
Alarms. Furnish alarms to announce the approach of unsafe conditions.
Provide shutdowns to shut the equipment down under unsafe conditions.
7-4
BOILER DRUM LEVEL. Provide controls to always maintain the boiler drum
level within the boiler manufacturer's specifications under all operating conditions.
Three types of control systems used in b oiler plants are single element, two element
and three element. These are shown in Figures 7 -7, 7 -8 and 7 -9 respectively.
7-4.1
Single Element Control. A single element control system utilizes just a level
transmitter to maintain control of the boiler d rum water level. Use a single element
control system only for boilers operating at steady loads.
7-4.2
Two Element Control. A two element control system utilizes a level
transmitter and the amount of steam flow from the boiler to maintain control of the boiler
drum water level. A two element control system provides some compensation for
variable loads. It does not adequately correct for the expansion of water within a boiler
due to the decreased boiler pressure that occurs when a large amount of steam is
required or the contraction of heated water in a boiler due to the addition of cold
feedwater. Do not use two element controls.
7-4.3
Three Element Control. A three element control system utilizes a level
transmitter, the amount of steam flow from the boiler and the amount of water into the
boiler to maintain control of the boiler drum water level. Note that although not shown,
the steam flow from the boiler is usually compensated for pressure and temperature. A
7-2
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