Oven Safety Is No Accident
Fatal accidents involving industrial ovens and dryers are relatively rare, fortunately. Incorporating safety features can enhance operator and technician safety when working near industrial ovens and dryers.
In so many factories these days, the focus is “Safety is no accident,” and personnel well-being is prioritized more than ever. The benefits of a safety culture in an industrial setting are many: improved worker happiness and retention, reduced insurance costs and higher productivity. According to OSHA, a robust safety culture is the single greatest driver for workplace accident reduction.
Industrial ovens incorporate high temperatures, moving parts and high voltage to do their job of heat processing. This presents many potential hazards that must be accounted for in the equipment design. There are specific safety features dictated by the standard for industrial ovens and furnaces from the National Fire Protection Association — commonly known as NFPA 86. Among the safety features called out in the standard are the following:
- Sufficient and proper ventilation to remove flammable vapors and natural gas from the oven.
- A high temperature instrument to prevent a runaway temperature situation if the primary control instrument fails.
- Explosion relief to help reduce damage in the event an explosion were to occur.
- Airflow detection for the recirculation and exhaust fans to shut off the heat in the event of a fan failure.
In addition to the risk-abatement equipment and designs specified by NFPA 86, industrial oven manufacturers may offer additional safety features on their ovens to help protect personnel.
Trapped-Personnel Pull Cord. This feature is designed to prevent the horrifying situation where a person is trapped inside an oven that is then inadvertently started up. A cable is routed inside the oven sides (or, sometimes, the rear). It passes through a sealed wall penetration and is connected to a relay on the oven exterior. In the event the cable is pulled, an alarm sounds, and the oven is shut down.
In order to service the interior of an oven that has a vertical-lift door, there must be a device such as a vertical-lift door maintenance latch. This prevents the door from falling onto personnel passing into the oven.
Access Door Interlock. It is common for conveyor ovens to have access doors on the side wall for inspection and maintenance of the conveyor inside the oven. This access is only safe when the conveyor is at rest. To prevent personnel from opening the doors while the conveyor is moving, the doors can be electrically interlocked to stop the conveyor motion upon opening the doors.
Vertical-Lift Door Maintenance Latch. In order to service the interior of an oven that has a vertical-lift door, there must be a device to prevent the door from falling onto personnel passing into the oven. One common design is a large-diameter steel rod with a heavy plate welded to it and a handle on the other end. Before entering the oven, the technician swings the latch into place beneath the oven, where a detent holds it in place. It has a spot to add a padlock for lockout purposes. In the unlikely event the door-lift system fails while the oven is being serviced — due to the chain, cable or door-lift mechanism failing catastrophically — the door will come to rest on the latch rather than fall onto a technician or operator.
Vertical-Lift Door Safety Hook. This device works similarly to the door maintenance latch; however, it does not require operator action and does not need to be manually swung into place. A passively acting hook is positioned at the top of the vertical-lift door travel. When the vertical door opens, the hook automatically engages a bar on the door, holding it in position. In the event of failure of the door-lift system, the safety hook provides redundant support of the door.
A passively acting hook is positioned at the top of the vertical-lift door travel. When the door opens, the vertical-lift door safety hook automatically engages a bar on the door, holding the door in position.
Safety Devices for Material-Handling Systems. Material-handling systems such as conveyors or powered load carts can cause injury if they come in contact with personnel. There are several safety devices used to reduce this risk.
- Motion Stop Buttons. These mushroom-shaped buttons are used to immediately stop motion when pushed. They should be located on the oven control panel and wherever there will be personnel in the vicinity of the potential hazard. When conveyors are loaded and unloaded manually, for example, they should be located at the four corners of a conveyor system (two on each end). In that way, no matter which side the operator is on, they will have a motion stop button within reach.
- Motion Stop Pull Cord. This operates much like the motion stop button. Pull cables are located on both sides of the conveyor or load cart travel rather than the motion stop button. They are used when it would be impractical to put a motion stop buttons within reach of the operator at all times.
- Light Curtains. Light curtains are used to keep operators out of a specific area such as a load/unload mechanism when moving parts are present. They are a passive device, requiring no proactive action on the part of the operator. The light curtains form a “wall” of light. If personnel break the curtain, all motion stops and an alarm sounds.
- Electrical Safety Devices. The danger of arc flash — an electrical short causing injury or death — has been getting more scrutiny in recent years. There are a number of protections available to reduce this risk.
- Segregated High Voltage Controls. In this strategy, the high voltage (480, 380 or 240 V) components are located in a separate electrical cabinet from the control voltage. For routine maintenance involving the control instruments and related devices, the technician only needs to open the low voltage enclosure.
- Viewing Windows in the Door of the Electrical Cabinet. In order to check the status of the flame-safety device, high temperature instrument, variable-frequency drive and other devices inside the enclosure, viewing windows can be located in the control panel door. This allows operators and technicians to view these devices without opening the enclosure. It is advisable to also have a light mounted inside the enclosure for better visibility of interior components.
- 24 VDC Control Wiring. Most instruments and devices in the control panel are available in a 24 VDC version in lieu of 120 V. This makes them inherently safer because 24 V is less hazardous.
- Voltage Indicators. These are located on the front of the control panel and indicate when there is live voltage inside the enclosure. This ensures the power is disconnected prior to opening the control panel door.
Viewing windows can be located in the control panel door. These allow the operators and technicians to view the status of the flame-safety device, high temperature instrument, variable-frequency drive or other devices inside the enclosure without opening it.
Ladder and Railings. When access to the oven roof is required periodically, a ladder can be provided and railings included to reduce fall hazard. When over 20’, the ladder must have a cage in order to meet OSHA requirements. When trip hazards such as structural members or conduit exist on the roof, they must be covered with a walking surface such as expanded metal or bar grating.
Tie-Off Points on the Roof. In lieu of railings on the oven roof, a tie-off point can be provided. This allows the use of fall-protection harnesses for roof access. A roof tie-off consists of a vertical post properly mounted to the oven roof structure.
Local Motor Disconnect Switches. For larger ovens, the motors that power the recirculation, exhaust and combustion fans are sometimes not located without direct line of sight of the control panel. In this case, it is advisable, to provide a local disconnect switch at each of the devices. In order to safely service them, a technician can turn off and lock out the motor at the point of service. For ovens controlled by a PLC, the local disconnect often has a separate safety circuit wired to the PLC, notifying the system of the status of the disconnect switch.
Burning Protection from the Hot Door Interior. When an oven with horizontal swing doors is opened while hot, the interior of the door is exposed to the operator and is hot enough to cause a burn upon contact with the skin. To make matters worse, the doors can have a tendency to slowly swing closed if the oven is not perfectly level. This can be prevented by using high strength magnets to hold the doors in the open position. The magnets are mounted to a bracket on the oven front and make contact with the doors when they open, holding them in place. The other option for avoiding the hot door interior is to utilize a vertical-lift style door instead of horizontal swing.