Avoiding Safety Issues with Natural Gas-Fired Steam Boilers
These energy sources drive the process industry, but require attention to safety.
In today’s fast-paced business and manufacturing environment, the safety department is faced with many responsibilities. The safety person in a facility may be in charge of health, safety and the environment, which presents a host of challenges. If you were to have a conversation with most safety professionals or industrial hygienists, they would probably describe the need to recognize, evaluate and control physical, chemical and biological stressors in the workplace.
With advances in system controls, programming, burner controls, analytical instrumentation and other engineering controls, it would be easy to assume that the safety issues may be simplified. However, challenges can show up at any time in the form of system failures and potential incidents due to poor maintenance, improper installation, lack of training or good, old human error.
Because safety hazards vary significantly with the type of energy source and the process they are powering, this article will review only key safety issues involved with natural gas-fired steam boilers. Steam boilers provide steam as part of a heat transfer media. Large-capacity water boilers are of two types: watertube and firetube. Watertube boilers heat the water inside the tubes and are capable of producing higher steam pressures. Firetube boilers have the gas flame heating up the tube, which is surrounded by water. Firetube boilers usually will not generate as high of steam pressures.
These are important considerations: The largest use of natural gas in the industrial sector is 42 percent for process heating, and the second largest use is for boilers at 22 percent. The petroleum refining and chemical industries have the highest demand for process heating.
Driving the System
Energy sources used to drive industrial heating processes rely on carbon-based fuels, including natural gas, coal, refined fuels and biomass. Additional sources include electricity and steam. Though widely used, natural gas has the potential to cause fires, explosion and asphyxiation. In addition, through combustion, it can generate toxic gases such as carbon monoxide, oxide of nitrogen and oxides of sulfur.
Natural gas composition varies from 60 to 95 percent methane. It also will have smaller concentrations of other compounds, including nitrogen, ethane, propane, carbon dioxide, isobutene, butane, pentane and hexane (1 to 5 percent). Hydrogen sulfide and methyl ethyl sulfide are present in parts-per-million quantities.
Modern Industrial Boilers
Fortunately, boiler technology has come a long way since the industrial revolution. What was once a crude, unrefined form of energy production has since become quite safe and energy efficient. Boiler design, production and testing are heavily regulated to prevent repeats of catastrophic events of the past. Today, manufacturers are held to strict safety standards. While no burner/boiler system is foolproof, engineers have learned a lot from boiler failures and explosions. NFPA 85, Boiler and Combustion Systems Hazards Code, and NFPA 86, Standard for Ovens and Furnaces, give guidance to manufacturers on boilers.
While the safety and performance of boilers and their burners has improved remarkably, there are still many boilers in the field that have been in service for some time. As a result, industry has not fully disengaged from major incidents with boilers.
Safety Issues and Hazards
When you consider the various types of process heating technologies, the energy sources used to power them and the array of materials that are treated, it is easy to see the potential for a number of hazards and safety issues. Some of these hazards are highlighted in table 1.
Emissions from Furnaces
Excess air combined with thorough mixing and adequate time in the burner chamber will provide the best chance for complete combustion. The emissions from natural gas-fired boilers and furnaces include oxides of nitrogen (NOX), carbon monoxide (CO), methane (CH4), nitrous oxide (N2O), trace amounts of sulfur dioxide (SO2) and particulate matter (PM).
From a safety point of view, carbon monoxide is considered a silent killer with an OSHA permissible exposure limit (PEL) of 35 parts per million (ppm). The rate of CO emissions from burners depends upon the efficiency of natural gas combustion. Increased CO emissions are produced when burners are operating out of design specifications and have inadequate combustion air. CO also can be increased when NOX control systems are installed. UL 795 standard specifies 400 ppm of CO permitted in the flue gas. It is possible to generate high levels of carbon monoxide during inadequate combustion associated with inadequate air/oxygen.
Natural gas burners generate emissions. Such emissions are regulated and controlled by the U.S. Environmental Protection Agency via the Continuous Emission Monitoring (CEM) standards. The levels of allowable pollutants such as NOX, SO2, CO and particulates are quite low and can vary depending upon the air-management district in which a facility is located. EPA has promulgated national emission standards under the Clean Air Act for hazardous air pollutants from three major source categories: industrial boilers, process heaters and commercial and institutional boilers.
Maintenance, Inspection, Training and Monitoring
Four key aspects of any process heating operation are maintenance, inspection, training and monitoring of the process. No matter how safely engineered the system may be, there is always the potential for hazardous conditions to build up in the system. Utilize gas-detection instrumentation for detecting flammable gases (natural gas) and toxics such as CO and NOX in the process area to protect workers. Be sure workers are trained on how to use and calibrate them. Look at optimizing the burn through the use of low emission air-heating burners. These burners thoroughly mix the air and fuel, resulting in much lower emissions of NOX and CO.
The potential for incident could emanate from human error, improperly maintained equipment, faulty programming and procedures, or lack of training. For many years, safety professionals believed in the Heinrich Safety Pyramid, which essentially said that for every 330 near-miss incidents, 300 would result in no injuries, 29 would result in minor injuries and one would result in a major injury. We now know that in major incidents such as a boiler explosion, it is not that simple. A future incident may be due to missing controls, poor procedures, poor equipment design, failed equipment or any of a number of other process-specific reasons that create a high risk situation with the potential for a catastrophic event. To avoid this from happening to your team, it is imperative that you know and understand your system.