Reviewing NFPA's safety labeling and venting requirements

Constructed according to the safety standards enforced during the 1960s and 1970s, the side wall of this oven failed before the explosion-reflief doors could open.
The cause of most oven explosions and equipment failures can be traced back to human error. Inadequate operator training and improper equipment application are the greatest contributors. Warning signs and instructions, in addition to training and maintenance, can serve as reminders to operators that the equipment they are comfortable with can be a hazard.

Safety labeling has been part of NFPA 86 since 1950, yet some users have little or no knowledge of the safety standard. The 1999 edition of NFPA 86 Standard for Ovens and Furnaces includes some rewritten information on safety labeling.

Manufacturers continue to be responsible for signage on the equipment they make. Section 1-7.1 of the standard demands that a clearly worded and prominently displayed safety-design data form or manufacturer's nameplate be provided stating the safe operating conditions for which the system was designed. A warning label also must be affixed to the equipment stating that it should be operated and maintained according to instructions.

According to section 1-7.3, safety data for solvent atmosphere ovens must be furnished on the manufacturer's nameplate and must contain all of the following data:

  • Solvent used.

  • Number of gallons of solvent and volatiles entering the oven.

  • Required purge time.

  • Oven operating temperature.

  • Exhaust-blower rating for gallons of solvent per hour or batch at the maximum operating temperature.

Safety labeling on batch ovens is intended to inform the operator about how many gallons of solvent can be loaded into the oven. "Because batch ovens can easily overload, causing an explosion, it would be a risk-management disaster not to have safety labeling on batch ovens," said Fred Jensen at the Industrial Heating Equipment Association's Safety Sem-inar last month. A member of the NFPA 86 standard committee, Jensen is president of Jensen Industries, Farmington Hills, MI.

Because operators no longer are working with the same equipment for 20 or 30 years, and new operators generally are not receiving the amount and quality of training they deserve, safety labeling is vital. It may provide a novice operator with a small but important amount of information. It may remind an operator to clear the explosion-relief doors and panels. It also may keep the equipment manufacturer out of trouble should an explosion occur.

Explosion-Relief Venting

Venting is critical in preventing injuries and reducing the amount of damage an oven sustains should an explosion occur. NFPA 86, Chapter 3, Location and Construction, describes oven explosion-relief requirements. The intent is to reduce the risk of personal injury and limit damage to the oven, the building and adjacent equipment. The oven enclosure should be able to withstand the rapid expansion of gases and allow the work openings, relief panels and doors to immediately relieve pressure. If the oven explodes, blocked relief doors and panels guarantee more severe damage and possibly injuries.

Explosion relief must be designed as a ratio of relief to oven volume. The minimum design required is at least 1 ft2 of relief area for each 15 ft3 of oven volume. Ovens with an explosion ratio of 15:1 with sufficient external and internal support structure will sustain only minor damage during an explosion. After an incident, the oven enclosure will be structurally sound, minor damage can be repaired and the equipment can be put back into service within a few days. A 20:1 ratio will cause moderate equipment damage while a 30:1 ratio will cause severe damage, requiring total replacement of the oven. A 30:1 ratio also may cause structural damage to the building.

Train operators to respect the oven's relief ratio and be aware of the panels and doors. Should your oven need to expand, explode or relieve, whether the panels and doors are able to vent can mean the difference between a tragedy and two days of lost production.