Prevent burner fuel delivery accidents
Thermal processes alter the physical, and sometimes chemical, properties of a material or coating. Examples of thermal processing are high-temperature operations such as heat-treating furnaces and kilns, and lower-temperature operations such as drying or baking (see Figure 1).
Heat treating involves the use of heating materials, usually metal, to extreme temperatures and then quickly chilling or quenching to modify its physical properties, making it harder or softer (see Figure 2).
Many industries use baking, drying or other lower temperature heating processes to modify aspects of a material or coating. Facilities may also have incinerators for oxidizing pollutants, or air heaters for tempering climate air. Applications for thermal processing are almost endless.
A valve safety train is at the heart of all thermal processes. (see Figure 3). These fuel delivery devices maintain consistent conditions of gasses into furnaces, ovens, dryers and boilers, among others, making them crucial in assuring safe ignition, operation and shutdown. They also keep gas out of the system whenever equipment is cycled or shut off.
A valve safety train isn’t a single piece of equipment. Instead, it has many components including regulators, inline strainers (sediment traps), safety shutoff valves (SSOV), manual valves (MV), pressure switches and test fittings logically linked to a burner management system. Flame-sensing components ensure flames are present when they are supposed to be, and not at the wrong time. Other components may consist of leak-test systems, gauges and pilot gas controls.
At a minimum, there are two crucial gas pressure switches in a valve safety train — one for low pressure and one for high pressure. The low gas pressure switch ensures the minimum gas pressure necessary to operate is present. It will shut off fuel to the burner if the gas pressure is below the setpoint. The high gas pressure switch ensures an excessive pressure is not present. It will shut off the fuel supply if the gas pressure is too high. Both switches must be proven safe to permit operation. There also is an air pressure switch to ensure sufficient airflow is present to support burner operation. Some systems have supplementary pressure switches such as a valve-proving pressure switch. These switches typically are used to enhance safety or provide other safety aspects specific to the application’s needs. A multitude of sensors within the valve safety train — pressure switches, flame detectors, position indicators — and isolation and relief valves work together in concert to prevent accidents.
Valve safety trains must be compliant with all applicable local and national codes, standards and insurance requirements. The most common of these for North America are NFPA, NEMA, CSA, UL and FM. Annual testing and preventive maintenance are not only an NFPA requirement, but also often required by insurance agencies, equipment manufacturers and national standards, including ANSI, ASME and NEC.
Set the trap
The primary function of a valve safety train is to isolate the inlet fuel from the appliance. Safety shutoff valves are designed to do this. To protect these valves, the initial section of a safety train is used to condition the fuel and remove debris that could potentially damage or hinder all downstream safety components.
The first conditioning step is a sediment trap (a.k.a. dirt leg, drip leg). This trap captures large debris, pipe scale and provides a collection well for pipe condensates. The proper orientation of a sediment trap is at the bottom of a vertical feed. This downward flow arrangement promotes the capture of debris and condensate into the trap. A horizontal feed across a sediment trap is an improper application.
The second conditioning step is a flow strainer or filter element. These devices are fine particulate sieves. The removal of fine particulates from the fuel stream further protects the downstream safety devices from particulate erosion and abrasion. Taken together, these conditioning steps remove particulates and condensates that might block, hinder, erode or otherwise compromise the safety features of the downstream devices.
The explosive force of a bomb
Owing to the presence of hazardous vapors and gases, a poorly designed or inadequately maintained safety train can lead to catastrophic accidents, ranging from explosions and fires to employee injuries and death. When this explosive force is unleashed, the shockwave carries equipment, debris, materials, pipes and burning temperatures in all directions with tremendous force.
The following incidents are examples of why it is important to purchase the highest quality valve safety train, and keep it professionally maintained, inspected and tested:
- In 2018, a furnace explosion at a Massachusetts vacuum systems plant killed two men and injured firefighters as a result of fuel malfunction.
- In Japan, an automobile manufacturer lost tens of millions of dollars when it was forced to shut down production for almost a month after a gas-fueled furnace exploded due to flammable fumes building up in the tank.
- In a Wisconsin bakery, an employee was seriously injured when he ignited an oven’s gas and was struck by a door that was blown off. A malfunctioning valve had allowed natural gas to build up inside the oven.
- In 2017, a van-sized boiler exploded at a St. Louis box company, killing three people and injuring four others. The powerful, gas-fueled explosion launched the boiler more than 500 feet into the air.
- In 2016, a boiler explosion in a packaging factory in Bangladesh enveloped the five-story building in flames, killing 23 people.
Two dangers: Valves and vents
Valves are mechanical devices that rely on seats and seals to create mechanical barriers to control flow. Over time, these barriers wear out for a variety of reasons, whether it is age, abrasion, erosion, chemical attack, fatigue or temperature. Increased wear contributes to leaks, and leaks lead to failures and hazards. Defective valves can allow gas to leak into a boiler even when the boiler is not in operation. This could lead to a destructive explosion later when the boiler is turned back on.
Testing a valve’s integrity is an evaluation of current barrier conditions, and it may be used to identify a valve that is wearing out prior to failure. As such, annual valve leakage tests are an important aspect of a safety valve train inspection program. Along with annual testing, valves should also be examined during the initial startup of the burner system, or whenever the valve maintenance is performed. Only trained, experienced combustion technicians should conduct these tests.
Improper venting is another danger. Numerous components in a valve safety train require an atmospheric reference for accurate operation. Many of these devices, however, can fail in modes that permit fuel to escape from these same atmospheric points. Unless these components are listed as “ventless,” vent lines are necessary. Vent lines must be correctly engineered, installed and routed to appropriate and approved locations. Building penetrations also must be sealed, pipes must be supported and the vent terminations must be protected from the elements and insects.
Even when vent lines are properly installed, building pressures can vary sufficiently enough to where they prevent optimal burner performance. Building pressures often vary with seasonal, daily weather and manufacturing needs. Condensate in vent lines can collect and drain to low points or into the devices. Heating, cooling and building exhausters are known to influence building pressures and device responses. However, so can opening and closing of delivery doors for shipping and receiving. This means a burner tuned for optimal operation might not be tuned for the opposite season’s operation.
The smart alternative to traditional vented valve trains is a ventless system that will improve factory safety and enhance burner operation (see Figure 3). Ventless systems reference and experience the same room conditions where the burners are located, resulting in more stable year-round operating conditions, regardless of what is happening outside. They also can save on total installation costs, remove leaky building penetrations, eliminate terminations that could be blocked by insects, snow or ice, improve inspection access and ensure fail-safe emergency response.
Valve safety trains are critical to the operation of combustion systems. Despite being used in thousands of industrial facilities, awareness of their purpose and function may be absent because onsite training is minimal or informal. To many employees on the plant floor, this series of valves, piping, wires and switches is too complex to take the time to understand. What is known can be dangerously misunderstood.
Understanding fuel-fired equipment, especially the valve safety train, is necessary to prevent potential explosions, injuries and property damage. The truth is, although valve safety trains are required to be checked regularly, they are rarely inspected, especially when maintenance budgets are cut. While codes require training, they offer very little in terms of specific directions.
The onus is on the safety professional, who along with their staff, must have a core level of knowledge regarding safe practices of valve safety trains, even if a contractor will be doing the preventive maintenance work. Most accidents and explosions are due to human error and a lack of training when an unknowing employee, for example, attempts to bypass a safety control.
Preventive maintenance is essential to counter equipment deterioration, as is the documentation of annual inspection, recording switch set points, maintaining panel drawings and verifying purge times. Accidents happen when this type of documentation is not available. Don’t wait for a near-miss or accident to upgrade your valve safety train.
Robert Sanderson, P.E. is the director of business development for Combustion Safety at Rockford Systems LLC, Rockford, Illinois.
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