Gas Vapor Damage

Dateline: Edmonton—Air pressurization systems protect cables from the harmful effects of water and other contaminants seeping into underground ducts, but a new problem has surfaced that may come as a surprise to some. We spoke with David R. Jones, Outside Plant Engineering Supervisor, of Edmonton Telephone in Alberta, Canada, who was the first to inform us of a contaminant that air pressure systems have difficulty counteracting. Edmonton Telephone discovered that seepage from unleaded gasoline storage tanks had a detrimental effect at more than 100 locations throughout their pressurization system.

The Discovery
The problem was first discovered by one of Edmonton Telephone's maintenance workers who was letting pressure off one of the cables in a manhole. He happened to have a torch lit and when he opened the valve, a flame shot up. Luckily he was not hurt in the incident, but the telco was very interested in finding out why the air from the cable was igniting.

After doing some research, the telco found that the problem stemmed from leaking underground fuel tanks at gas stations. So far, they have found 158 locations where the tanks were leaking onto the cables—six that are serious and need attention soon.

How Widespread?
This is apparently not just a leakage problem in the Edmonton area. British Columbia Telephone Company, also in Canada, has documented tank leakage problems too, and storage tanks leak everywhere—not just in Canada.

Who's paying for the costs of fixing the cables? The gas companies. The good news is litigation against the gas companies has been successful in getting the mess cleaned up, and an unannounced program to replace gasoline storage tanks seems to be going on all over.

The Real Culprit: Gas Vapor
Through in-depth research, Edmonton Telephone's Ken McDonald found that, surprisingly, it wasn't the gas leaking into the cables that caused the damage, but instead it was the gas vapors that were permeating the sheath walls. These vapors were actually passing through the plastic cable sheath and dissolving the underlying mastic layer. In this case of severe contamination, the mastic then permeated the paper insulated conductors, turning them black. Regardless of air pressure inside, the vapors were able to invade the cable.

Edmonton Telephone found the black fiber duct (BFD) had swelled to a half-inch thick, and it was soft and malleable in the affected areas. A section of this conduit was brought to the lab for research, but after two days, they found that it had returned to its normal state—one quarter of an inch thick and as hard as a rock. The gas vapors had permeated out through a double plastic bag that had been used to transport the conduit piece.

Possible Solutions
Edmonton Telephone is looking into several ways to remedy the problem. They could replace the ducts they have with FRE (Fiberglass Reinforced Epoxy) or FRP (Fiberglass Reinforced Plastic) ducts. Both these materials stop the gas vapors from penetrating. A Nylex fabric coating on the cables could be another solution because it is also impervious to gas vapors. Lead-sheathed cables were considered, but replacing all the cables would be neither cost-effective nor environmentally friendly.

Research continues, and the telco hopes to be able to find a simple and relatively inexpensive solution. They feel that the answer may lie in a positive air flow system, which could "flush" large amounts of air through the pipe route.

Let us know if you have encountered the same problem as Edmonton Telephone, or if you have discovered any other unique situations with your air pressurization system. We would be very interested in finding out how many telcos are actually facing the problem of gas contamination in their cables. With your help, we hope to find a solution to this problem.