Optimizing Duct Burner Reliability and Performance through Regular Maintenance and Inspection

Harold F. Honath Jr. (P.E.), Coen Company, Inc.

TYPICAL DUCT BURNER ARRANGEMENT:

Burner Elements

Most new duct burners have horizontal natural gas elements or runners that span the duct width.

Figure 1, Horizontal Natural Gas Fired Duct Burner Element

Other configurations are also used:


  • Vertical gas runners (Shown in Figure 2 with in line oil atomizers for dual fuel capability)
  • Horizontal oil runners
  • Side fired oil burners, using atomizers in wind boxes that take a slip stream of CT exhaust, along with horizontal gas runners. Another type of dual fuel burner. See Fig. 3.
  • Various alternate fuels – process off gases, hydrogen, biogas, landfill gas, blast furnace gas

Figure 2, Vertical Natural Gas Fired Elements with in line Oil Atomizers

Figure 3, Side Fired Oil Burners with in line Horizontal Natural Gas Fired Elements

Flow Baffles

Flow baffles may be required between the burner elements to increase the CT (combustion turbine) exhaust gas velocity to 4,000 to 6,000 feet per minute across the burner for optimum combustion and emission performance.

Structural Supports

Any duct over 10 feet wide will require some kind of support structure to keep burner elements from sagging under their own weight. These are usually vertical supports from the flow baffles, or some sort of truss attached to the HRSG casing.

Figure 4, Horizontal Natural Gas Fired Element with Simple Vertical Support


Figure 5, Sectional View of a Horizontal Natural Gas Element Supported by Flow Baffles


Duct Burner Design Evolution

Heat Recovery Steam Generators (HRSGs) have gotten bigger, with wider, taller ducts and more heat input. More efficient turbines are firing to higher exhaust gas temperatures with lower oxygen and higher water content. Steam injection for power augmentation and NOx control further reduces oxygen levels and raises water content. Duct burner designs have had to improve to keep meeting lower emissions requirements under these more difficult firing conditions.

Figure 6, Large HRSG Under Construction Showing Horizontal Duct Burner Elements and Flow Baffles


TYPICAL OPERATING CONDITIONS:

It’s Hot In There!

Typically CT exhaust temperatures are 750°F to 1100°F. Downstream firing temperatures are 1200°F to 1800°F.

Materials used to accommodate this are:


  • 304SST for parts “cooled” by incoming CT exhaust.
  • 309/310SST for parts under high radiant heat load of burner flame.
  • Higher-grade alloys can be used when a corrosive atmosphere or extremely high temperatures are present.

When Steel Gets Hot, It Expands

Thermal expansion of stainless steel components is approximately 1 inch of growth over 10 feet of length for every 1000°F temperature increase. Burner elements, baffles, supports, liner plates, etc., must be free to move or grow unconstrained.

Some parts may grow more than others. For example, both flow baffles and burner elements typically span the duct width. Both will grow equally as the CT comes on line and hot exhaust flows through the duct. As the duct burner is brought on line, fuel gas (at ambient temperature) flows into the gas runners and cools them. The elements then shrink relative to flow baffles.

The elements need to be designed to expand thermally with the baffles, then shrink back to some degree when operating.

It’s Not Just the Heat, It’s the Humidity

Moisture content of CT exhaust gas can range from 5% to 24%.

When the duct burner is not operating, CT exhaust enters burners, pilots, and associated piping, cools and water vapor condenses. Over time, water can accumulate in piping and burner and pilot components and corrosion can occur. If no isolation valves are present, the fuel piping to the pilots and burners needs to be regularly drained at low point drains during periods of non-operation of the burner and operation of the CT.

Water accumulation can be minimized or prevented when there are automated isolation valves on pilots and main gas lines. These are supplied when pilots and elements have individual light off and shut down capability.

In some cases manual valves may be present that can be closed to isolate fuel to pilots and elements when the burner will be down for an extended period.

Pilots may also have check valves or be air purged which will prevent water from CT exhaust from entering the fuel piping.

Gale Force Winds

CT exhaust gas velocities are ~3000 feet per minute or 30-40 miles per hour. At the burner, this is increased by 25% to 100%. It’s breezy in there! The straightness of the flow and uniformity of the flow are very important. For optimum results, the velocity profile should be ±25% of average free stream velocity, with velocity vectors perpendicular to the burner plane. Eddies and localized areas of high or low flow can cause flames to impinge on burner parts or sidewalls.

Distribution grids and flow straightening vanes are used in expanding ducts to achieve the required profile. Localized areas of poor exhaust gas flow can sometimes be addressed by reducing fuel input to those portions of the burner by pinning or plugging burner orifices.

Figure 7, Isometric View of Duct Burner with Combination CT Exhaust Flow Straightening /Fresh Air Distribution Vanes


BURNER TROUBLESHOOTING AND MAINTENANCE:

Starting Sequence

The six main steps in the Duct Burner Start Sequence are:


  1. Establish Safety Permissives, including master fuel trip conditions, safety shutoff valves proved closed no flame detected and system stop not activated.
  2. Duct burner purge
  3. Prove fuel flow control valve is in the light off position.
  4. Pilot trial for ignition.
  5. Main burner trial for ignition.
  6. Release of burner to modulate.

The basis of these steps is NFPA 85 – Boiler & Combustion Systems Hazards Code. Chapter 1 (Fundamentals of Boiler Combustion Systems) and Chapter 5 (Heat Recovery Steam Generators) apply to duct burners. NFPA 85 establishes minimum requirements for design, installation, operation, training and maintenance, as they relate to the safety of combustion systems. As such it is an important and invaluable reference and tool.

Master Fuel Trip Device Maintenance

Safety Permissives include all master fuel trip (MFT) signals (see NFPA 85 Section 5.10.4.3 or your job specific sequence of operations). These trip signals can come from pressure switches, transmitters, control system contacts and valve position switches. These devices should be checked at least semiannually for proper generation of trip signals at the required set point and for resetting to normal condition within proper deadband. For critical service, if operations allow more frequent inspection, monthly inspections may be prudent.

Flame Scanner Maintenance

Flame scanners must prove no flame is present in order to proceed beyond steps 1 to 3, and must prove flame is present for steps 4, 5 and 6. These devices must be properly sighted to pick up a strong signal when flame is present. They are typically mounted on an adjustable swivel ball mount. These 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. Vibrations, or being used as a step are possible culprits for a scanner becoming misaligned.

Most scanners have quartz lenses that can pick up dust and dirt over time. This foreign material can absorb UV radiation and cause loss of flame signal strength. A schedule should be set based on experience 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. This can also verify the integrity of wiring back to the DCS. I

Inspection of scanners should include verification that conduit connections are tight to prevent water intrusion.

Safety Shutoff Valve Maintenance

All fuel safety shutoff valves must be proven closed in order for steps 1 to 3 to proceed. These redundant valves are of critical importance to ensure that fuel is not entering the unit when the burner is shutdown. 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 in downstream.

Light Off Position Switch on Flow Control Valve

The fuel flow control valve must be proven at the proper position, usually 25% to 35% open in order for purge to proceed. This set point 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.

Igniter Maintenance

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 too 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 earlier.

Fuel Supply

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 we recommend 100 mesh stainless steel strainer baskets.

Main Burner Component Maintenance

For the most part, duct burner elements, especially horizontal gas elements contain no moving or adjustable parts and require no 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 as described earlier. Baffles and burner wings that direct 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.


SUMMARY:

Maintenance & Inspection

The unique characteristics of each operating environment as well as unit availability demands require that each site setup a regular inspection and maintenance schedule based on plant experience. A conservative starting point might be NFPA 85, Section A.1.7.1.1:

A.1.7.1.1 An example of an inspection and maintenance schedule is a follows:

a) Daily. Flame failure detection system, low water level cutout, and alarm

b) Weekly. Igniter and burner operation

c) 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

d) Semiannually. Burner components, flame failure system components, piping, wiring, and connections of all interlocks and shutoff valves, calibration of instrumentation and combustion control system.

Semiannual inspections of internal components are important for components subject to thermal expansion. Maintenance of shutoff valves, limit switches, interlocks, scanners and pilots will ensure reliable light offs and burner operation.

Burner Longevity

The life of duct burner components will be most affected by:


  • Proper design and installation that allows unconstrained thermal expansion of burner elements, baffles, supports and liners.
  • How straight the CT exhaust gas flow is.
  • How uniform the CT exhaust gas flow is.
  • Not firing the burner beyond its rated capacity.