The safety of combustion equipment personnel and their physical surroundings begins at burner system startup. To help operators, Coen Co. Inc., Foster City, Calif., prepared and posted a white paper, "Optimizing Duct Burner Reliability and Performance through Regular Maintenance and Inspection," on its web site. Written by Harold F. Honath Jr., P.E., a Coen engineer, the information contains guidelines for safe troubleshooting and maintenance of duct burners.
The startup of a duct burner should follow a specific sequence. Honath writes that the guidelines he spells out are based on NFPA 85, Boiler and Combustion Systems Hazards Code, and on Chapter 4 (Fundamentals of Boiler Combustion Systems) and Chapter 8 (Heat Recovery Steam Generators), both of which apply to duct burners.
Honath says that NFPA 85 is an important and invaluable reference and tool for burner operators and maintenance personnel because it establishes minimum requirements for design, installation, operation, training and maintenance of combustion systems.
The white paper lists six main steps in the start-up sequence for duct burners:
- Establish safety permissives, including those covering master fuel trip (MFT) conditions, prove all safety shutoff valves closed, no flame detected, and no system stops activated.
- Conduct duct burner purge.
- Be sure the fuel-flow control valve is in the light-off position.
- Conduct a pilot trial for ignition.
- Conduct a main burner trial for ignition.
- Release of the burner to modulation by the control system.
Flame scanners must prove that no flame is present in order to proceed beyond Steps 1 through 3, and must prove that a flame is present for Steps 4, 5 and 6. These devices must be properly sighted to pick up a strong signal when a flame is present, and typically are mounted on an adjustable swivel-ball mount. They should be sighted by a qualified start-up technician during commissioning and should only require re-sighting if the swivel mount is accidentally directionally altered. A scanner can be thrown out of alignment from vibrations as well as from personnel using the device as a step or foothold.
Most scanners have quartz lenses that can pick up dust and dirt over time, absorbing ultraviolet radiation and causing loss of flame-signal strength. A schedule should be set, based on operating history, to clean these lenses regularly using a clean soft cloth. Precautions need to be taken when removing a scanner head while the duct is hot.
Scanners can be tested with an open flame from a lighter, which also can verify the integrity of the wiring back to the distributed control system (DCS). Inspection of scanners should include verification that conduit connections are tight to prevent water intrusion.
All fuel safety shutoff valves must be proven closed in order for Steps 1 through 3 to proceed. These redundant valves are of critical importance to ensure that fuel is not entering the unit when the burner is shut down. These valves fail closed and require power to open. If they are kept clean internally by strainers upstream in the fuel line, they will require little maintenance. Proper operation and switch settings should be checked semiannually. Leak testing should be performed monthly. Coen provides a leak-test valve downstream of each safety shutoff valve to facilitate leak testing with the shutoff valve pressured upstream and blocked downstream.
The fuel flow control valve must be proven at the proper position, usually 25% to 35% open, in order for purge to proceed. This setpoint is determined during initial commissioning. The control system will position the valve upon prompting by the burner management system. This switch setting should be checked semiannually.
Igniters (or pilots) have a somewhat checkered past as a source of problems. Improved electrodes and premixing air with fuel have much improved the reliability of pilots, even in difficult applications.
For a pilot to work, you need fuel (and oxygen, of course) and a spark. The pilot fuel regulator must be properly set so a constant regulated pressure is available. Fuel lines must be kept clear of condensate by valves or by draining during non-operation. Pilots have small orifices and passages on the fuel side that can become plugged with debris if strainers are not properly maintained in the fuel lines.
If air is premixed with the fuel, it must be available at the flow and pressure specified by the manufacturer. In addition, spare electrodes should be kept on hand, and scanners must be properly sighted and maintained as described above.
Fuel supplied to the duct burner piping train must be at a constant regulated pressure. Regulators must be sized properly to be able to keep up with changes in fuel demand to the burner. Having a sufficient volume of gas between the last regulator and the safety shutoff valves is important to cushion the large increase in flow on a burner start without tripping the unit on low fuel-supply pressure.
Strainers need to be present and properly maintained to prevent debris from fouling shutoff and flow control valve seats and seals, and burner orifices. For natural gas service, Coen recommends 100-mesh stainless steel strainer baskets.
For the most part, duct burner elements, especially gas-fired elements, do not contain moving or adjustable parts and do not require maintenance other than inspecting during shutdowns to make sure parts are secure and in good condition. Internal inspections should be done at least semiannually to check that elements, baffles, support structures and liners are intact and free to expand thermally, Honath says. Baffles and burner wings that direct combustion turbine (CT) exhaust flow, and pilot covers that shield pilots need to be reasonably intact. Some minor distortion of parts and some oxidation of surface metal can be expected and tolerated without affecting performance.
The unique characteristics of each operating environment as well as unit-availability demands require that each site schedule a regular inspection and maintenance schedule based on plant experience. A conservative starting point is NFPA 85, Section A.18.104.22.168, which gives the following example of an inspection and maintenance schedule:
Daily. Flame-failure detection system, low water-level cutout, and alarm.
Weekly. Igniter and burner operation.
Monthly. Fan and airflow interlocks, fuel safety shutoff valves for leakage, high steam-pressure interlock, fuel-pressure and temperature interlocks for oil, high, and low fuel-pressure interlocks, and gas strainer and drip leg for gas.
- Semiannually. Burner components, flame-failure system components, piping, wiring, and connections of all interlocks and shutoff valves, calibration of instrumentation and combustion control system.
Following all troubleshooting and maintenance guidelines will help ensure that a duct burner system is safe and efficient.