Is it Safe?
|Fig. 1. The Doctor’s first true love, an integral-quench furnace|
The Doctor made a house call the other day, only to find his patient in a highly dangerous condition. Special care is needed whenever furnaces with combustible atmospheres are running, especially when operating below 760°C (1400°F). Let’s learn more.
A Word About Safety
Prevention of industrial accidents involving unwanted fires and gas explosions not only requires knowledge of the flammability characteristics (e.g., limits of flammability, ignition requirements, burning rates) of the combustible gases and vapors likely to be encountered under various conditions of use (and misuse) in the heat-treat shop, but also an understanding of the internal and external conditions that may be present when operating a particular piece of equipment or that exist in the environment surrounding that equipment.
|Fig. 2. Internal furnace pressure monitors|
A Word About Terminology
Confusion exists at times between the words flammable (easily set on fire or that will burn readily) and combustible (capable of catching fire and burning).
In simplest terms, a flammable gas (or liquid) is one that is easily ignited and can burn in the presence of air when brought into contact with heat or flame. The flammable (explosive) range is the range of concentration of a gas or vapor that will burn or explode if an ignition source is introduced. Below the explosive or flammable range – the so-called lower explosive limit (LEL) or lower flammability limit (LFL) – the mixture is too lean to burn, and above the upper explosive limit (UEL) or upper flammable limit (UFL) the mixture is too rich to burn.
NFPA 86 (2011 edition) para. 18.104.22.168 defines an inert special atmosphere (purge gas) as “a special atmosphere of nonflammable gases that contains less than 1% oxygen.” In this writer’s view, any furnace containing flammable gases (e.g., carrier gases, enrichment gases, generated or synthetic atmospheres, gas mixtures containing percentages of flammable gases and the like) must be properly monitored, controlled and protected from dangerous internal and external influences.
When the chamber temperature (Fig. 1) is at or above 760°C (1400°F), as pointed out by NFPA 86, uncontrolled air infiltration could create process quality issues. However, it is not anticipated this will create safety issues. That being said, when either the furnace temperature or any area within the furnace drops below 760°C (1400°F), any introduction of air can create an explosive mixture, and special safeguards must be in place.
Furthermore, any practice that causes a furnace chamber (heated or unheated) to lose positive pressure while running a combustible atmosphere is a major cause for concern and a possible call for immediate action. Examples include power failures, air leaks, loss of the heating system, carrier-gas settings lower than normal, unexpected door openings and loss of furnace temperature. These types of conditions can lead to the uncontrolled infiltration of air into furnace chambers, which could rapidly lead to an unsafe condition – faster in many instances than operators might be able to respond.
Transfer of loads in integral-quench furnaces to the quench-tank area and normal burn-in and burnout of furnace atmosphere are conditions that routinely create loss of positive-pressure conditions. Following manufacturer’s instructions and company practices will ensure these situations remain under control despite loss of positive pressure. Other conditions, such as furnace pressure loss during transfers between chambers, should be monitored (Fig. 2), and the burn-off flame recovery observed (and timed) to understand how quickly the furnace positive pressure has been re-established under these conditions.
Processes are often run, either intentionally or unintentionally, below 760°C (1400°F). Process examples include loss of temperature inside a furnace chamber when a large cold load is introduced, nitrocarburizing at 580°C (1075°F) or stress-relief annealing at 730°C (1350°F) that are run below the industry-recognized auto-ignition temperature. In these cases, good practices and certain precautions are necessary. For example, outer doors should never be opened under low-flow conditions or without properly functioning pilots and flame curtains. Another especially critical time in the cycle is during load transfer or introduction of a load into the furnace while another is quenching.
The Importance of a Good Flame Curtain
A robust flame curtain that covers the full width and height of the furnace is mandatory (c.f. “The Flame Curtain – Function, Adjustment and More,” Industrial Heating, March 2008). This massive wall of flame often scares the uninformed. However, it acts as a physical barrier designed to minimize air infiltration and disrupt the furnace atmosphere inside. Furnaces where strong wind currents (e.g., near open trucking bay doors, under downdrafts from ductwork or air conditioning vents, or near poorly positioned floor fans) can extinguish front-door pilots or force flame curtains to lie down horizontally, create significant risk of large volumes of unwanted air entering the furnace vestibules.
Unusual Furnace Sights and Sounds
Any unusual sights – such as the furnace atmosphere pulsating or surging in a sporadic fashion from the burn-off port – or sounds emanating from inside the furnace – such as “wheezing,” “whistling” or “popping” – are calls for immediate investigation and action. Trapped air pockets, leaky packing glands on cylinders, damaged door seals, holes in radiant tubes, leaky or missing access plates and even partially clogged nitrogen/methanol spargers are common sources.
One of the scariest situations the writer personally experienced was the integral-quench furnace pictured in Figure 1 howling like the proverbial banshee – so loud that the sound of air infiltrating into the furnace and partial combusting in the vestibule could be heard a block away. At one point the building was abandoned, and we were standing across the street literally seconds from disaster before going back in to take corrective action. This unsafe condition was created by substituting rich exothermic gas (~8% hydrogen) for endothermic gas (~40% hydrogen) and not recognizing the dangerous condition we created. We failed to anticipate what would happen when we discharged the load to the top cool of the furnace and the dilution of the atmosphere by air infiltration when the top-cool fan automatically came on. The situation was corrected, and the operation returned to normal by turning the top-cool fan off and increasing the flow rate. The lesson learned was that not all combustible atmospheres are created equal, and one must take extraordinary care to understand the differences between combustible gases when operating a furnace.
In another highly dangerous situation, we arrived at work in the morning only to find our integral-quench furnace (left unattended at temperature overnight) flowing endothermic gas at room temperature! Without a nitrogen safety purge, we were forced to remove the atmosphere by first extinguishing all sources of ignition, then opening doors sequentially and allowing chambers to purge by dilution with room air – a dangerous but necessary method.
Moral of the Story
These stories, though scary, had positive outcomes and no one was hurt. As operators, supervisors and managers, we have an obligation to recognize and understand what type of dangerous conditions can (and possibly do) exist, take proper precautions, be well trained and have procedures in place that everyone in the organization is aware of, for every possible safety scenario. IH