FIRE PROTECTION THAT’S IN THE BAG
FIRE PROTECTION
A unique fire protection measure developed for the nuclear power industry has proven its worth some 20 years later and has implications for fire service personnel involved in prevention and protection activities.
To protect against fire spread in high-voltage electrical cable grouped in solid-bottom and solid-sided trays, personnel at the Oak Ridge (TN) Gaseous Diffusion Plant proposed and tested the use of vermiculite-filled plastic bags laid along cable tray runs. Depending on the location and the presence of power cabling, the bags of granular vermiculite are laid either directly on top of the cables or on steel wire mesh located above the cables and supported by sides of the cable tray. In a fire exposure, the plastic bag melts, releasing the vermiculite, which smothers the fire by excluding oxygen. The method was chosen as an alternative to automatic sprinkler protection for horizontal cable runs. Automatic sprinkler protection was installed to protect cabling where vertical cable runs were involved.
The Oak Ridge facility and two other gaseous diffusion plants in Paducah, Kentucky and Portsmouth, Ohio enrich uranium for use in nuclear reactors. The enrichment plants use massive amounts of electrical power (up to 2,000 megawatts) that is brought into large switchyards by overhead electrical connection at 345,000 volts. Electrical power is dropped down to 13,800 volts by oilfilled transformers and distributed to electrical motors within the gaseous diffusion process.
Cable trays run through cable spreading rooms within the electrical distribution buildings. The cable trays contain large amounts of grouped electrical cable insulated with butyl rubber, polyvinyl chloride (PVC), chloroprene, and nylon. All gaseous diffusion plants operated by private contractors working for the U.S. Department of Energy also are arranged in this manner.
The views expressed in this article are those of the author. They do not in any way represent the official policy of the United States Government.
(Photos by the Oak Ridge Gaseous Diffusion Plant Photographic Services.)
In the late 1960s, after a large fire within a major process building, personnel from the Factory Insurance Association and Factory Mutual Engineering conducted a detailed fire protection evaluation. Several recommendations to upgrade fire protection for the enrichment plants were implemented. The recommendations included the need to protect cable trays for electrical distribution systems; hence, the vermiculite method was developed and implemented. After considerable testing it was chosen to protect cabling at all the diffusion plants within spreading rooms of electrical distribution buildings.
In November 1988, a high-voltage electrical fault and subsequent fire occurred within switch-gear located on top of a distribution switchhouse at the Portsmouth Gaseous Diffusion Plant. The fire spread downward into the switchhouse, involving grouped electrical cable in trays. Final extinguishment to the switch-gear fire was made by the plant fire department and emergency squad. Damage to the switch-gear was considerable. The grouped electrical cabling, located underneath the switch-gear, was successfully controlled and extinguished by the bagged vermiculite.
The success of the bagged vermiculite in preventing fire spread and limiting damage to a small section of cabling was a major contributor in reducing the size and extent of this loss. Had the vermiculite not been effective, propagation of the fire to other areas of the switchhouse would have been a certainty.
This was the second time that the vermiculite proved its worth in actual fire conditions. In July 1986 lightning caused a fault and fire on an electrical buss that involved grouped electrical cable. This fire also was effectively extinguished by bagged vermiculite.
We use No. 3 size, water-repellanttreated, expanded vermiculite (masonry fill insulation) in 0.008-inchthick self-extinguishing PVC bags (which will not sustain combustion after a heat source is removed). Bags that were installed some 15 years ago remain effective today. Problems have been experienced with bags separating at their seams and bags being misplaced by maintenance workers. Minor discoloration of bags laid directly on cables due to possible thermal degradation has been experienced in some situations.
This unique method of fire protection has proven its worth under severe fire exposure conditions. It could be applied to virtually any industrial facility that requires large electrical cable runs similar to those described in this article—telephone switching stations, steel plants, mines, and so forth. The protection is an economical and effective method to assure that fire losses are held to manageable levels and that production interruptions are minimized.
