Design flaw in oven range causes gas ignition flashback with resulting injuries

Persona

Technologies

Technologies
Reference number
#II-741521-2018 (#8524)

Incident overview

The homeowner turned the oven on and began to prepare a meal by activating the pre-heat setting of the “bake setting”, followed by activating the switch for the convection fan. The homeowner then placed a meal of nachos into the oven, closing the oven door and leaving the oven alone. Approximately 15 minutes later, the homeowner returned to the oven to check the meal, noticed that no heating occurred, and subsequently opened the oven door to check on the meal. The homeowner described observing a rolling flame and a fireball forming, leading to a flashback.

The homeowner described sustaining facial, hearing, and vision injuries as a result of the flashback.

Regulated industry sector

Gas - Natural gas system

Location

Victoria, BC

Investigation conclusions

Site, system and components

The incident appliance was a BlueStar brand oven range. The BlueStar oven range (model RPB304BV2, also represented as RPB304BSS by the original equipment manufacturer (OEM) and the equipment distributor) is part of a series of household consumer gas-fired oven ranges. This gas range series holds a certification number with INTERTEK and Underwriters Laboratory Canada. The design is also required to meet the code requirements of ANSI Z21.1 and CSA 1.1 and must be installed by a licensed gas contractor.

The gas range from BlueStar contains a propane-natural gas dual-fuel regulator, which will drop natural gas pressure from 14” W.C. to 5” W.C. to burners on the top of the range and in the oven cavity. Each burner has a nozzle that contains a restriction orifice that produces a BTU rating. Each burner is supplied with an electric striker that must provide a continuous spark to ensure ignition is achieved once gas is supplied by the valves.

The oven and broiler burners are equipped with electronic hot-surface igniters. The oven cavity contains a convection fan, gas burner, and an infrared burner:

  • The convection fan is activated by a manual switch located directly beside the gas valves. The BlueStar gas range convection fan will stay on once the switch is activated.
  • The oven range gas burner uses a hot surface ignitor and a gas valve to preheat the oven. The BlueStar gas range will activate the hot-surface ignitor and the gas is delayed until the ignitor is fully heated, which will then allow gas to be introduced for ignition.

For natural gas-fuelled appliances, the ideal (stoichiometric) ratio of air-to-gas is approximately 10 to 1. Achieving this ratio allows for effective combustion.

Failure scenario(s)

The oven and convection fan were activated. The meal was placed in the oven. The placement of the meal and the operation of the convection fan led to a fuel-rich mixture in the oven cavity. The fan created a draft that did not allow the air to mix with gas. Once the cavity door was opened, the build-up of fuel mixed with the rush

of air in the oven made contact with the hot-surface igniter, causing an ignition.

Facts and evidence

Homeowner statement:

The homeowner was the only operator of the appliance. The homeowner described the following sequence of operation before the incident:

  • The gas oven range was turned to “on” position
  • The convection fan was turned to “on” position
  • A plate of nachos was placed into the oven
  • After approximately 15 minutes, the homeowner checked on the food
  • The homeowner reported smelling gas in the kitchen
  • The oven door was opened, and the homeowner watched a flame begin to take shape followed by a large “fireball” and a flashback that led to the homeowner’s injuries

OEM Product Literature

Within the BlueStar operating manual, there are several warnings to not operate the convection fan while broiling or during oven pre-heating. (Figure 1)

Safety Notices and Recalls

The OEM, Health Canada and the US Consumer Product Safety Commission issued safety notices and recalls regarding this appliance on September 11, 2018. The Safety Notices and Recalls indicated that approximately 50 BlueStar RPB series gas ranges experienced flashback incidents in North America. These were a result of a combination of (1) user ability to operate a manual convection fan switch simultaneously during oven pre-heating, and (2) hot surface ignitor unable to ignite due to a fuel-rich environment caused by the convection fan’s draft in the oven which resulted in gas accumulation. Refer to appendices in this report:

  • Appendix A,Safety Recall Notice, Recall Number 18-218by OEM (September 11, 2018)
  • Appendix B,Consumer Product Recall, Identification number RA-67736by Health Canada (September 11, 2018)
  • Appendix C,Recall number 18-218by United States Consumer Product Safety Commission (September 11, 2018)

Conclusion 1: OEM literature and safety notices indicate an operating issue with this appliance. Specifically, a design flaw that can cause a flashback.

Equipment/product nomenclature

It was confirmed that the range design was certified by a certification body.

This gas range contains the nomenclature RPB304BSS. The nomenclature can be described by:

  • RPB – Range Power Burner
  • 304 – 30-inch wide, 4-burners
  • BSS – Stainless Steel Model

Conclusion 2: The range involved in the incident is subject to product recalls and safety notices.

On-site evaluation

Observations made following the incident:

  • Gas can be smelled during the operation of the convection fan and the broiler setting - a lower explosive limit (LEL) detector confirmed a presence of gas
  • Gas could not be smelled during the operation of the convection fan and the oven pre-heating
  • The hot-surface ignitor was in good condition prior to testing. Some heat discolouration of the protective cover was observed (Figure 2)
  • The oven door did not sit flush against the oven surface
  • The simmer gas burner on the top of the range had a larger orifice that was rated for 15,000 BTU
  • The simmer gas burner did not immediately ignite – the electric striker continuously created an electric arc for more than 1 minute before igniting
  • The appliance shut-off valve behind the gas range was in the “on” position and the valve handle was not connected to the stem and was lying beside the valve – to shut off and disconnect the appliance, the handle was placed back on the stem, returning the valve to normal functionality.

Conclusion 3: All gas lines leading to the gas range were checked for distances to validate pressure drop. It was concluded that there was adequate gas pressure to the regulator inlet and there were no issues observed with the installation or operation of the appliance.

Causes and contributing factors

The flashback was caused by a design flaw that allowed a manual switch to activate the convection fan, leading to a poor air-fuel mixture and an accumulation of natural gas.

Operating the range contrary to the OEM’s use and care instructions may have contributed to the incident.

Impact

  • Injury
    • Qty injuries: 1
    • Injury description: The homeowner reported that a gas ignition flashback in a household range caused facial, visual, and hearing injuries.
    • Injury rating: Minor
  • Damage
    • Damage description: None
    • Damage rating: None
    • Incident rating: Minor
Excerpt from BlueStar Use and Care Manual
Figure 1 - Excerpt from BlueStar Use and Care Manual
Oven hot surface ignitor - Good condition
Figure 2 - Oven hot surface ignitor - Good condition
Top of range with one burner plate removed
Figure 3 - Top of range with one burner plate removed
Infrared broiler with hot surface ignitor
Figure 4 - Infrared broiler with hot surface ignitor
Final condition of the gas range prior to leaving site
Figure 5 - Final condition of the gas range prior to leaving site
Attachment Size
8524-741521 r 04-26-2019 final.pdf 1.81 MB