An internal seal for repairing natural gas mains

Abstract

Joint leakage from low pressure natural gas distribution mains (typical value: 0.25 ft[superscript 3] at 6 inwg gas pressure) is a persistent source of maintenance problems for utitlites. External encapsulation is the usual choice for repairing leaking joints. It is reliable and expensive, about $1000 per joint (80% of which is for excavation and resurfacing).

Consolidated Edison of New York is sponsoring a project to develop a cheap and reliable joint seal. No current sealing methods were found to be acceptable in Phase I. Adhesion failures between the seal and pipe were a major cause of seal failure. Preliminary Phase II research recommended the development of an internal mechanical seal. Such a seal would minimize excavation and eliminate adhesion failures.

To complete Phase II, an internal mechanical seal was developed. The initial research indicated that cleaning of the pipe interior had to be minimized or eliminated. Testing demonstrated that a sealant, either asphalt or vulcanizing silicone, greatly enhanced the sealing of uncleaned pipes, while permitting low stress levels. This led to the development of a new internal mechanical seal concept for sealing uncleaned gas main joints. Joint tests of 2 injected silicone seal concepts were run. The Type 1 seal, which filled the joint recess, completely sealed an uncleaned pipe joint. The Type 2 seal, which spanned the joint recess with an elastomeric bridge, had a leak rate of 0.000038 ft[superscript 3]/hr at 6 inwg gas pressure from an uncleaned joint. The leakage was caused by permiability of the silicone bridge. These flexible and compliant seals, clamped in place with retaining bands, provide mechanical support for the sealant throughout the seal life, and thus do not rely on adhesion.