In the primitive days of America, skilled trappers depended on subtle signals in the earth to show them signs of the wild game they needed to survive. A single footprint often was enough information for a skilled trapper to be successful.
The situation does not differ much when it comes to staying alive while operating your combustion equipment. The equipment will sometimes give you signals you must understand to stay alive. Sometimes, the messages are subtle, and sometimes, they are not, but how discernable they are usually has a lot to do with the skills of the person who is looking and listening. This article is intended to sharpen your hazard-recognition skills in a few key areas so that safety issues related to explosion relief of gas-fired equipment are no longer subtle.
Consider, for example, the case of a paint oven operator at an automotive plant. The operator heard “poofs” and saw the paint oven's explosion-relief door swing open wildly every time he lit off. The operator considered this an annoyance and took care of it by blocking the explosion-relief door closed. Eventually the oven, having an obvious ignition problem and no longer able to relieve itself as designed, exploded and destroyed itself. The explosion led to days of downtime and repair, and the experience cost the company more than $500,000. The company was lucky that no one was seriously injured or worse.
The oven operator misread the poofs, pops and long light-off periods -- all subtle danger messages. The relief door swinging open was not so subtle, but he did not understand that the relief door was his friend and that it needed to be free and unobstructed. Every operator should be aware of and understand the subtle and not-so-subtle oven-related warning signs to maintain personnel safety.
Poofs and Pops Are Never Good with Industrial Combustion Equipment
Poofs and pops are signs of trouble if they occur when lighting off equipment. Although the National Fire Protection Association's Standard for Ovens and Furnaces, NFPA 86, allows up to 15 sec for pilot and main flame trial for ignition, one should be concerned if it takes close to that long to light off. In most cases, light-offs should occur within 5 sec. If the time for ignition begins to move beyond 5 sec, you may be receiving a not-so-subtle message that something is wrong. Poofs or pops could be caused by anything from a defective (worn down, carboned up or improperly adjusted) igniter to an improper purge. If this situation occurs, it is time to stop and investigate your fuel train, purge system, fuel-air ratio control and ignition systems thoroughly.
Explosion-Relief Panels Need to Be Able to Perform
You must ensure that you have explosion relief -- the ability to contain an explosion -- built into your oven design. NFPA 86 (2003 Edition), section 5.3, calls out requirements for explosion-relief panels. The document mandates installation of explosion-relief panels at a ratio of 1 ft2 for every 15 ft3 of oven volume. Also, explosion relief is to be installed as close to ignition sources as possible. Exceptions occur if the oven construction is of 0.1875" (3⁄16") or heavier steel-plate shells reinforced with structural steel beams and buckstays (see also NFPA 86A, 5.3 Appendix).
If explosion-relief panels or doors already exist, check to be sure they are not compromised or rendered inoperable (figure 1). Obstructions such as conduit running over an explosion-relief panel or door will create a restriction and will not allow the panel or door to respond freely as it was intended. In some cases, you may find dangerous “fixes” such as explosion-relief panels that have been permanently affixed to equipment with sheet metal screws or by welding. These “fixes” do not fix a problem but instead create a dangerous one because they prevent the explosion-relief panels from operating properly. If relief panels are on top of the oven, make sure debris is not piled on top of them. Also, make sure that all doors or panels do not have objects wedged into or against them to keep them closed.
Explosion-Relief Hatches and Doors Must Be Maintained
You should pay particular attention to friction latches, which look like latches used on many commercial coolers or freezers (figure 2). Oven and furnace friction latches are precision-designed devices that are there to save your life. These devices are designed to hold only a specific amount of force; if that force is exceeded, they are designed to pop open and relieve. They must be in good condition to do this. Also, the explosion-relief doors to which they are attached must have hinges that are in good condition, and the doors themselves must be easy to move and swing freely once open.
You should regularly inspect for subtle signs of danger such as people having removed the friction latches or having installed pins in their place to hold warped, sagging doors tightly to an oven (figure 3). Removing or compromising these important safety devices puts you and your coworkers at tremendous risk in the case of uncontrolled ignition of flammables.
Also, verify that retaining devices are installed properly on explosion-relief doors. Typically, these are chains that are attached from the oven or furnace to the explosion-relief door. These retaining devices allow the explosion-relief door to open to relieve pressure but prevent them from flying off and becoming a projectile in the event of an oven or furnace explosion.
Effective Safety Labeling: Make Things Not So Subtle
Now that you know more about explosion relief, take the next step and label all of your explosion-relief panels and doors as NFPA 86 suggests. Section 5.3.3 of the standard states: “Warning signs shall be posted on the vents.” Operators should understand that it is not a good practice to stand near explosion-relief doors during light-offs; in fact, these areas should be avoided at all times. Likewise, everyone should understand what friction latches are and why they are there.
Sharing this information may go a long way toward minimizing the knowledge gap many operators and users of combustion equipment seem to have regarding explosion-relief equipment. Remember, it is better to have a place where unexpected, instantaneous explosion forces can go and do as little harm as possible than for shrapnel and flying debris to be created.
It is hoped you understand that what once seemed subtle is really quite blatant if you know how to interpret the signs. Make sure that your oven or furnace has explosion relief and is designed according to current edition of NFPA 86. If you are not familiar with this nationally recognized safety standard for ovens and furnaces, you can get a copy at www.nfpa.org.
Note: This article was originally published with the headline, "Is Your Oven Telling You It's Unsafe," in the April 2004 issue of Process Heating magazine.