The Value of Scheduled Combustion-System Maintenance
My colleagues and I have been in the combustion business for many years and have a wide range of experience with single-burner and multi-burner applications. What follows is some basic information concerning the maintenance and adjustment of combustion systems.
Experience has shown that regularly scheduled inspection and maintenance of combustion systems can provide heating processes with reliable and predictable results. We also see that in some cases lack or absence of regularly scheduled inspections and maintenance can cause poor performance, higher-than-required fuel usage, higher levels of emission, and unexpected component failures that lead to unnecessary and unwanted downtime.
Conversely, regularly scheduled combustion-system inspection and maintenance can provide longer equipment life, reduced downtime, improved product quality, energy savings and reduced emissions. Additionally, NFPA audits should be performed on an annual basis by a qualified company.
Annual NFPA Audits
Regulations change, and some of the changes may be applicable to your combustion system. The first audit purpose is to review the existing system and see how it stacks up against current NFPA 86 guidelines. This involves going through both the mechanical system (i.e., gas piping, air piping, control components and the control setup itself) as well as the electrical controls (i.e., flame safety, high-limit, temperature controls and sequence of operations – purge timing, light-off procedure, etc.). During the audit, you should document the system with a generic air and gas schematic, taking note of component part numbers and settings along the way. This information is then reviewed against the current NFPA 86 guidelines. Any deviances from the current guidelines should be addressed.
The second part of the audit is more of a safety check to make sure that the system components are functioning adequately. Technicians will go through the system and verify that every safety interlock is in safe working order by tripping the limit and verifying that the system shuts down safely. During this time, we are also verifying that the trip points are adequate for the application.
The burners and the burner controls should be inspected on an annual basis at a minimum. Here is a list of items that should be checked.
- Check fittings and piping for possible leaks.
- The inlet combustion air filters should be clean.
- Replace broken or worn auxiliary components such as pressure switches and gauges.
- Check the burner nozzle for dirt or carbon buildup.
- Sample the oil near the burner for a viscosity test if using heavy oil. This a simple sampling process. Viscosity meters will measure the viscosity of the oil. Refer to the burner manufacturer’s technical data for recommended viscosity. Number 2 oil does not require heating in most applications.
- Clean the atomizer to ensure both oil and air passages are clear.
- Keep the indicators on the shut-off valves visible and clean.
- The main and blocking shut-off valves should have a proof-of-closure switch and must be wired to the burner control per NFPA.
- Check operation and setting of all switches: gas pressure, temperature, air flow, etc.
- Check the furnace/firing chamber pressure. The furnace/system manufacturer will recommend the proper firing chamber pressure.
If the fuel and air received at the burner are not properly proportioned, you will not be getting the most economical working system. A deficiency of air will result in the incomplete combustion of the fuel and the subsequent release of pollutants to the atmosphere. If too much air is used (excess air), additional fuel will be required to heat this air. This will result in higher fuel usage.
Upgraded equipment such as burner components and fuel manifold components will utilize newer safety features that will automatically shut down systems if or when a safety limit is compromised. These safety features can protect plant personnel from injury and also protect the combustion equipment.
The use of orifice meters helps service technicians to accurately measure the flow of any gas. This, in turn, leads to a precise setting at each burner. A properly tuned burner generates peak efficiency. Orifice meters should be inspected and cleaned for optimal performance.
An LGV (limiting gas valve) is recommended for low-fire adjustment and fine tuning. Because pipe runs vary for different burners, a limiting gas valve can be used for fine tuning.
Pressure switches and shut-off valves perform safety functions in the gas lines. Keeping these components clean and functional are essential to reliable operation.
All the conditions of the safety-limit circuit must be made for the system to move on. Check each one physically and electrically to see if conditions are met.
Other system checks include:
- Output signal from fan
- Rotation and speed
- High/low fuel-pressure switches; verify with gauges or manometers
- Status of high-temperature-limit controller
- Temperature or reset controller if necessary
- System interlocks, such as door or material feed switches; verify physically and electrically
Flame safety is essential to every combustion system. Flame supervision is typically achieved through a flame rod or UV scanner. These items need to periodically be pulled from the burner and cleaned. If the flame rod is found to be black with carbon buildup, it can be cleaned with emery cloth. Note that if excessive carbon buildup is found, it may also be a good idea to pull the burner and clean the surfaces within the burner body.
For a flame rod to work properly, it must ground from the flame rod to the burner body itself through the flame. If the flame rod is found to be drooping or sagging inside the burner, it is time for a replacement. Note that this is a sign that the flame rod is in excessive heat. If this is becoming a problem, investigate changing to a UV or IR scanner.
Keep in mind that UV scanners come with some stipulations regarding adherence to NFPA 86. Be sure to check the latest NFPA 86 guidelines for proper implementation. UV scanners work by line-of-sight to the UV emitted from the flame. These can be pulled from the burner and cleaned with a shop rag or towel. Typically, only the bulb needs to be cleaned, but since they are line-of-sight, any lenses or windows will need to be cleaned as well. If the UV scanner bulb is becoming excessively dirty or overheating, investigate using cooling air and/or ceramic heat blocks to help alleviate the issue.
Again, it is very important to record and maintain the burner air and fuel settings for future reference. Once the combustion system is adjusted for desired operation, these setting records will be extremely helpful if or when the system performance has changed.
I have attempted to provide some basic information concerning the maintenance and adjustment of combustion systems. The applications of these combustion systems are numerous, and your system may have some special conditions to consider. For more information or on-site assistance, I would recommend contacting a reliable combustion equipment and service supplier. Help is only a phone call away.
For more information: Contact Mike Shay, director, Sales and Product Support,
HEAT Combustion Solutions LLC, 4858 Provident Drive Unit C, Cincinnati, Ohio
Assisting with this article were Brian Boehmer, sales engineer; Justin Powell, sales engineer; and Derek Seng, engineer.