maintaining boiler steam pressure within plus or minus 2 percent of the set
pressure over the complete range of boiler operation. The system shall
maintain excess air within plus or minus 10 percent of the original control
setting. The efficiency will be maintained without appreciable manual
adjustment. The system shall be capable of maintaining the specified
pressure provided that the load does not exceed a 15 percent per minute
change in capacity at any one time. The master transmitter shall be
connected to the main steam header where the steam pressure is to be
controlled. The signal transmitted from this point to the master
controller shall be a function of steam pressure. On multiple boiler
installations, a means shall be provided to base load on individual boilers
while on automatic, and each boiler unit shall be individually controlled.
Provision shall be made on the control system for adding on other boilers
to the system with only minor wiring or piping changes on the panel. The
fuel-to-air ratio on this system shall be adjustable from one knob that
indicates increase and decrease of air in proportion to fuel. The range of
this adjustment shall be limited to prevent operation below safe combustion
limits.
2.7.2
Metering Type Combustion Control Equipment
**************************************************************************
NOTE: This paragraph will be deleted if controls
with oxygen compensation specified in paragraph
COMBUSTION CONTROLS WITH OXYGEN TRIM are used.
**************************************************************************
2.7.2.1
Fuel-Flow, Air-Flow Type Combustion Control
**************************************************************************
NOTE: This paragraph will be used for spreader
stokers.
**************************************************************************
[Fuel-flow, air-flow type combustion control equipment shall be the
proportioning and reset type, and shall position the fuel feed or air flow
and then adjust one to the other by a ratio controller operating from air
flow and fuel feed. The controls shall include fuel-flow measuring
elements and air-flow measuring elements. Separate fuel feed and air-flow
controllers shall be panel-mounted along with a fuel-to-air ratio
controller. The air-flow index shall be set by a measuring element in the
air stream. Systems controlling fuel and air by line shafting and
mechanical connections will not be acceptable.]
2.7.2.2
Two Element (Steam Pressure, Steam Flow)
**************************************************************************
NOTE: If underfeed, traveling grate, or vibrating
grate stokers are specified, use this paragraph.
**************************************************************************
[Two element (steam pressure, steam flow) combustion control equipment
shall be the proportioning and reset type, and shall control the fuel feed
or air flow either in parallel or series. The controls shall include
measuring elements for steam flow, fuel feed, and air flow. Separate steam
pressure, fuel flow, and air flow controllers shall be panel-mounted along
with a fuel-to-air ratio controller. The actual steam flow shall be
measured by an orifice or other flow measuring device in the steam line.
The air-flow shall be set by a measuring element in the combustion air
SECTION 15559A
Page 46
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