I followed the manager of the facility around and listened to the same old story. “It just keeps tripping out!” he said. I have heard that quote -- and related complaints such as “We just can’t keep all the burners lit!” and “The temperature! It just won’t stay consistent!” -- many times. Some people with heat processing equipment think they need to watch the calendar for full moons and plan their production schedules around them because they just cannot understand, or ever seem to fix, their reliability issues for long. However, you can achieve reliable, consistent operation of your fuel-fired equipment with a little understanding and action.
Most combustion systems have only one device that you can assume is providing “normal” operation: the fan or blower system. It is rare that fan and blower mechanical issues such as bearings become a problem that affects combustion system reliability. Instead, by far, reliability issues seem to relate to ambient conditions in which the safety interlock components are asked to operate. The two main culprits are dirt and heat. Dirt and heat can impact your reliability, but there are things you can do about it.
Fuel Delivery Systems. Reliable fuel-delivery systems are all about making sure you have no dirt or other contaminants and that you have proper pressures.
Oil. If you’re operating on oil, you need to ensure the fuel and everything it touches is always very clean. Dirt and contamination issues are the No. 1 problem with oil. To avoid these problems, take steps to avoid dirt and contamination. Possible steps may include adding biocides to oil that gets stored and used infrequently; scheduling regular cleaning of lines and strainers; installing duplex strainers to aid cleaning; and cleaning nozzle tips immediately before use.
Gas. Gas systems also can have their share of contamination. Therefore, when opening piping systems, be careful -- just the act of opening piping can move sediments around. Make sure that all fuel trains are protected with drip or dirt legs (which are the same thing), and strainers. Drip legs should be installed with sufficient room to allow removal of the pipe cap for cleaning. The drip leg should be emptied during routine maintenance scheduled based on past experience with contamination in the fuel train. Strainers must be oriented correctly so the screen can effectively capture particles. I typically recommend strainer screens that have 100 mesh (150 micron) or higher.
Combustion Air Fan Dirt. One of the most troubling of all combustion equipment reliability issues is the accumulation of dirt from combustion air fans in air switch lines and burner orifices, and even pilot lines. There is no easy way around this. Some designs can incorporate filters for fans, but you need to be very careful when applying filters to fans on combustion systems. You could easily disrupt fuel-air ratios and make for unstable flames, and these conditions could lead to an explosion. If installing filters, it is recommended that they be oriented vertically to prevent the dirt buildup when the blower is not operating.
If you operate in a very dirty environment, consider trying to reposition the air intake or duct so that it draws in cleaner air -- possibly from outside. Also, make sure that regular flame observation and/or flue gas analysis occur to verify that fuel air conditions are still correct.
Regulator Vent Terminations. Regulator vent line terminations also need to be kept free of debris and plugging. I have seen many cases where insects build nests in the ends. If this happens, the regulator cannot breathe or reference itself to atmospheric pressure properly. Eventually, this will lead to downtime. Safety codes for combustion equipment require vent termination protection to keep insects out. Make sure that all of your vent lines have a protective device attached to the end and that the end termination is protected from rain.
Ambient Temperature Issues. Fuel-train pressure switches, flame detectors and burner management systems need to be protected from high temperatures, corrosive conditions, high humidity and excessive vibration. Operating outside of the ranges identified by the manufacturers will reduce useful life and can lead to reliability issues.
Excessive temperatures for flame detectors are a common problem in many plants due to their location relative to the burner. If your fuel-train components and or panels are chronically hot, consider adding cooling air for optical detectors, electrical panel coolers, radiation shields, or simply relocate the devices when possible.
'Tune' Your Industrial Combustion System During Design and Commissioning
Reliability issues should never be something that you “discover” as time goes on. Instead, these issues should be looked for and found in the design or commissioning process. By definition, commissioning is the process of making sure that the system is set up, tuned and tested properly from “day one.” It also means that prior to accepting new equipment, you make sure that it operates reliably and does everything you need it to do under a number of test conditions.
Before the equipment supplier leaves the equipment in your hands, make sure you have all of the setpoints for all high and low gas pressure switches; air switches; operating temperature limits; high temperature limits; and the purge times. The manufacturer also should leave a complete manual that identifies preventive maintenance tasks and frequencies as well as cut sheets describing each individual fuel-train component. It should describe how your system’s burners are tuned and include documentation identifying each burner’s fuel/air ratio (or at least the proper fuel delivery pressure). Optical detectors should be sighted and pilot spark pick up and pilot turn down tests should be run to make sure that the sightings are correct.
Of course, nothing substitutes for real-life “six months down the road” conditions. If during the first few months of operation, you’ve had reliability issues, reaching the six-month milestone might be a good time to re-examine the design and make your systems more robust.
Remember that no combustion system is meant to operate trouble-free without any attention. Besides regular flame observation identified above (which could be daily), you also will need to perform other routine tasks. At least once per year, you must test high and low gas pressure switches, flame detectors, air switches, and automatic fuel shutoff valves (for internal leaks and proper function) as well as checking burner fuel/air mixtures. All of these practices, along with keeping things cool and clean, will go a long way to keeping your equipment trouble-free for years to come.
Note: This article was originally published with the headline, "Beating Dirt & Heat," in the August 2007 issue of Process Heating magazine.