POWER PLANT APPLICATIONS Burner Tilt Systems
System Overview In a Tangentially Fired boiler, a burner tilt mechanism
Windbox #2
Windbox #3
is one of the systems used to control main steam and reheat outlet temperatures in boiler. A burner tilt actuator is used to tilt a burner or group of burners.
Primary Air, Coal and Secondary Air
A fossil-fueled fossil-fueled tangentially tangentially red furnace has burners burners in the corners, not in the walls (see gure 1). The burners aim at points tangent to an imaginary circle at the center of the furnace for intensive fuel/ air mixing. Flame body spreads out due to vortex motion, lling out the furnace area. A furnace furnace can have four or eight corners. Imaginary Firing Circle
Burners are vertically arranged in each corner in a “windbox arrangement.” arrangement.” The windbox arranges burners alternating with air nozzles, ame scanners and ignitors. Windboxes span many furnace elevations. Each furnace typically typically has a unique windbox arrangement. Each burner nozzle nozzle connects
Windbox #1
Figure 1 Tangentially Fired Boiler
Windbox #4
to a tilt mechanism mechanism inside the windbox. windbox. Several
to raise or lower the reball in the furnace and often
nozzles can be grouped together on one internal tilt
have different rotation than than the tilt lever. lever. Due to
mechanism. The internal tilt mechanism mechanism connects
the geometry of internal tilt mechanism, external tilt
to shafts that pass through the windbox wall.
levers usually rotate 78 degrees, or ±39 degrees.
Each shaft connects to external tilt levers. levers. Often
There is often a tilt travel scale marked “+30 UP
a windbox has two or more external tilt levers. levers. A
to −30 Down” by external levers in order to show
connecting rod ties external tilt levers together
nozzle rotation, not lever rotation. One tilt actuator actuator
vertically verticall y in each corner. Typically, there is one tilt
moves all nozzles in the corner at the same time.
actuator per corner mounted near and connected
Sometimes, the customer “splits the tilts”, using
to one external tilt lever, lever, called the drive lever. lever. The
two or more tilt actuators in a corner in order to
burner nozzles commonly tilt inside the furnace ± 30
individually tilt a nozzle or group of nozzles for more
degrees from horizontal (60 degrees total rotation)
customized control of combustion.
PWR-BT 04/13
POWER PLANT APPLICATIONS The Problem The nozzles can become coated with combustion by-products causing them to jam or stick in position due to build up of slag or molten ash. If the burner tilt actuator moves the external drive lever responding to the control system but the nozzle is stuck, damage to the tilt mechanism may occur. To protect the tilt mechanism, in this scenario, the external drive lever is really a two-lever assembly where a shear pin connects the inner and outer lever. The actuator connects to the inner, longer lever. The outer, smaller lever connects to the tilt shaft and to the internal tilt mechanism. If (or when) the shear pin breaks and disconnects the two external levers, the actuator continues moving the longer lever. The actuator position feedback may match signal, but the actuator is no longer connected to the smaller lever or nozzles. The nozzles may continue to be stuck in position, or the nozzles may drop due to weight of parts. The end result of a sheared pin is loss of control of the nozzle position. For emphasis, note two things that happen if a pin shears: (1) actuator position feedback still matches instrument signal, (2) there is a drastic reduction in
temperatures can be greater than 185 °F (85 °C).
required torque.
Electric actuators are subject to overheating motors, overheating grease and lubricant, and wiring
If the plant senses the loss of tilt control via other
degradation. All these events ultimately result in
signals, they can lock the tilt or drive lever in position
more frequent actuator repair or complete replace-
or take reactive measures while running. Some
ment. Sometimes, an electric actuator still under
customers install a limit switch across both levers
warranty will need repair. However, some manufac-
to trip when levers spread apart. In extreme cases,
turers may deny warranty coverage due to exposure
customers say buildup on nozzles causes clogging/
greater than 185 °F, maximum allowable ambient
plugging of air ow at a nozzle which changes “ame
temperature. Air operated actuators with smart
sculpture”—causing damage requiring entire nozzle
digital “current to pressure” (I/P) converter position-
replacement.
ers or analog I/P positioners also suffer due to high ambient temperatures. Removing a positioner for
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An additional problem is that burner tilt actuators
replacement, high up on the windbox in a hot inside
are typically installed in hot areas where ambient
corner while a furnace res, is not safe or easy.
Burner Tilts The REXA Solution
Built into all X2 modules as standard equipment are pressure limiting devices. These devices pro-
REXA linear actuators and drives are widely applied
tect the cylinder from excessive hydraulic pressure
on burner tilts. Required tilt torque changes due to
which protects the tilt lever (and any shear pin) from
build up on nozzles, causing the actuator to increase
excessive output torque. When something restricts
its output to overcome the additional load and make
the rotation of the drive lever, the power module
position match the control signal. Using REXA Dif-
will continue to build hydraulic pressure. In order to
ferential Pressure (DP) cylinder output allows the
protect from the higher pressure, the pressure limit-
customer’s Distibuted Control Systems (DCS) to
ing devices should be set to a DP cylinder pressure
calculate the actual output and directly monitors the
where actuator output is less than shear pin torque.
effects of deposit build-up; thus allowing the cus-
At lift pressure the devices open so oil is harmlessly
tomer to take corrective action. In addition, monitor-
diverted away from the cylinder and into the thermal
ing DP increase can infer build-up on nozzles and
expansion chamber in the power module. REXA’s
resultant nozzle clogging; thereby facilitating the
pressure limiting devices reseat when pressure
customer in taking proactive steps to help prevent
drops 500 psig below lift pressure; therefore normal
nozzle damage due to changing ame sculpture.
operating pressure needs to be less than lift pres-
REXA actuators have two additional features which
sure – 500 psig.
can help protect the burner tilt mechanism: (1) motor shutoff in stall condition, (2) hydraulic pressure limiting devices to prevent excess pressure build up. The “stall” parameter monitors time required for actuator to travel 1% of calibrated stroke. If the actuator fails to move 1% of stroke within the dened stall time, the motor stops and retries ve times. If, after ve attempts, the stall condition still exists, the motor turns off, and REXA sends both an alarm and warning via relays #3 and #4. If so equipped, REXA can report a stall via HART or Foundation Fieldbus.
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POWER PLANT APPLICATIONS / Burner Tilts The REXA Advantage REXA’s Electraulic™ actuators offer accuracy, repeatability, high temperature capability (for use up to 250 °F [121 °C]), continuous-duty, and rigidity required for precise control of burner position. Required torque for burner tilt actuators varies according to windbox dimensions and arrangement (i.e.; number, size, and weight of nozzles). High-temperature capability– For installation up to 250 °F (121 °C), REXA’s high temperature construction survives where other actuators often fail. Typically hot installation is an inside corner of a windbox. High output with low weight– REXA’s output to weight ratio is best in class. Lighter actuator means faster easier installation. Real-time monitoring– Optional pressure transducers and one of REXA’s three optional output boards provide real-time output of differential pressure across cylinder. Customers can use DP cylinder output to calculate real-time actuator torque or thrust to monitor required tilt torque and associated condition of burner and tilt mechanism.
REXA 4 Manley Street W. Bridgewater, MA 02379 (P) 508.584.1199 (F) 508.584.2525 www.rexa.com REXA Electraulic™ Actuators & Drives
