Getting the highest efficiency out of your combustion system includes many factors. Consider these tips about linkageless burner management systems before specifying your next combustion control system.

During the late 1800s and early 1900s, the discovery and use of natural gas stimulated an industrial boom. Numerous manufacturing plants were built utilizing this cheap fuel. Many of today's leading industrial burner manufacturers such as Bloom, Eclipse, Hauck, Maxon and North American began in the United States 75 to 100 years ago. Some of the greatest combustion technological advances were developed from these manufacturers, including burners that provide higher turndowns and lower emissions.

Through the 1990s, burner technology in many ways exceeded combustion-control technology. Essentially, burners could operate at a higher level than the accuracy provided from the typical controls adjusting the air and fuel to the burner. Today, combustion control manufacturers have introduced electronic air-fuel linkageless burner control systems that can improve burner performance, provide energy savings and provide greater process uniformity.

A linkageless burner management system provides electronic fuel/air ratio control. This can improve the accuracy and resolution of the burner system vs. a typical combustion system. Separate direct-connect motors control the air and gas flows without the use of mechanical linkages. Many linkageless systems can control optional motors for a second fuel such as oil or propane, or for flue-gas recirculation. There may be provisions to incorporate the burner control (flame safeguard), an optional PID temperature/pressure control function (load controller) or an integrated configurable gas valve-proving system. All of the burner management system’s major components -- specifically the base unit, display unit and actuators -- communicate via a reliable safety bus.

Should your process require the tight control possible with these electronic combustion controls, consider the following when evaluating the linkageless combustion controls:

How Easily Can Adjustments be Made to the Combustion System? Some of the most important adjustments to the combustion system will be to the air-fuel ratio curve. This may be done via multiple inputs, including a separate LCD display, laptop computer or PLC or touchscreen via a bus system.

A laptop and software program may be required to download and upload configurations. If the program is not downloaded to the laptop or if the controller fails, the entire configuration and commissioning of the base unit may need to be re-entered. Other types of controllers allow a backup of the entire configuration to be stored in the display unit. This allows for quick downloading of the configuration via the display. Having the flexibility to program the system directly from the LCD display may eliminate the requirement for a laptop computer or PLC for programming the burner system.

Many linkageless systems offer various levels of passwords to authorize changes to the system. For example, the end-user password may allow only changes to the temperature setpoint, thus preventing unauthorized personnel from making air/gas ratio changes. Other password levels allow additional parameter changes, from timings to servicing the fuel/air ratio.

What Type of Control Signal Is Sent to the Motors? Stepper motors receiving a digital control signal are more accurate than motors receiving an analog (4 to 20 mA) signal. For example, stepper motors with a resolution of 0.10^o, or 900 repeatable positions within 90^o of rotation, are available. A stepper motor utilizing a digital signal may offer 20, 30 or more times the resolution of a motor receiving an analog signal. Repeatability is ensured by digital feedback from each actuator. Consequently, combustion performance and energy efficiency, especially in high turndown applications, can be maximized.

How Easy Is Installation of the Control Motors? There are vast differences between the types of control motors used on linkageless systems. Some of the aspects that affect motor installation include precalibrated internal potentiometers, direct or reverse operation without rewiring, and parallel or series wiring of motors.

The linkageless system using motors with precalibrated internal potentiometers ease installation and minimize the wiring in the field. Some linkageless systems allow changing the direction of operation of the motor from the display unit without recommissioning the motor. Systems in which the motors are wired in series are each addressed and communicate via a bus system. This requires only one cable from the base controller, thus simplifying electrical installation.

Other types of control motors may require rewiring for changing the direction of rotation. Some motors include potentiometers that may require a special adjustment tool for calibration of the potentiometer. The control motors may include hidden installation and commissioning costs when comparing linkageless systems.

How Easy Is Adjustment of the Air-Fuel Curves? Ease of adjustment of the electronic fuel/air ratio control is an important selection criterion. A typical air-fuel curve having points that can be added, deleted or adjusted while it is running or in standby simplifies startup. Some displays continuously show the process variable, current setpoint, load and flame signal.

How Easy Is the Installation and Wiring in the Control Panel? The typical control panel may include all of the following separately wired to terminal strips: flame safeguard, annunciation package, temperature control and valve-proving system.

Options exist for combustion packages in which the only wires required to the panel is the main power (460 V), while the wire leaving the panel is a bus connection from a safety PLC. Systems such as these may provide more annunciation from the combustion safety circuits while reducing wiring and installation costs.

Does the Linkageless System Allow for the Use of a Variable-Speed Drive? As energy efficiency has become more important, linkageless systems allow for use of a variable- frequency drive (VFD) to control the combustion or process blower. Many times, these blowers are only turned down 50 percent via the VFD in order to achieve the energy efficiency, and the use of an air damper may be recommended for additional turndown.

How the VFD is controlled and the feedback from the motor are important issues when comparing linkageless systems. Some systems retransmit the input signal or calculate the motor speed, which may not represent the actual motor speed. Inaccuracies in speed measurement can result and therefore affect fuel-to-air ratios. A more accurate means of control accepts the speed signal from an inductive sensor mounted to the motor shaft. For safety reasons, the actual motor speed and direction should be monitored.

If Oxygen Trim Is Required, What Type of Oxygen Sensors Should Be Used? Not all manufacturers produce their own oxygen sensors. Some utilize a number of moving parts, including pumps, switches and pinch valves. Other oxygen analyzers have no moving parts and require less maintenance, recalibration and repair. If the sensor is in the flue-gas stream, a much faster response to changes in the flue gas creates more accurate trim control, especially during load changes.

Where Options Can Improve Your Combustion System Operation? Typical options on a burner management system include a leak-proving detection system that can be programmed through the display. Some systems allow the setting of an ignition position independent of the air-fuel curve for reliable lightoff. The ignition position can typically be held for up to approximately 1 min to allow the combustion chamber to warm up before releasing to modulation. For applications running more than 24 hours continuously, the flame detector should be continuous duty or self-check. Some detectors also house the flame amplifier, eliminating reliability issues associated with remote amplifier flame sensor wiring.

Since the first electronic linkageless systems were introduced more than 20 years ago, the functionality and ease of use have increased while the cost has decreased. A linkageless burner management system provides electronic fuel/air ratio control, improving the accuracy and resolution. Electronic air-fuel linkageless burner control systems can improve burner performance, provide energy savings and provide greater process uniformity.

Links

• SCC Inc.