TABLE OF CONTENTS There are many similarities between transmission and distribution systems, but there are also critical differences from a risk standpoint. A transmission pipeline system is normally designed to transport product to large end-users such as a distribution system, which in turn delivers that product to all users in towns and cities (e.g., natural gas for cooking and heating or water for multiple uses is delivered to homes and other buildings by the distribution system within a municipality). The similarities between transmission and distribution systems arise because a pressurized pipeline installed underground will experience threats common to all such systems. Differences arise due to varying material types, pipe connection designs, interconnectivity of components, pressure ranges, leak tolerance, and other factors. Chapters 3 through 7 detail a risk assessment system to measure pipeline risks with a focus on transmission systems. Distribution systems present some different issues for the risk evaluator, as are discussed in this chapter.
For purposes of this chapter, a distribution pipeline system will be considered to be the piping network that delivers product from the transmission pipeline to the final user (i.e., the consumer). This includes the low-pressure segments that operate at pressures close to those of the customers appliances as well as the higher pressure segments that require pressure regulation to control the pressure to the customer. The most common distribution systems transport water and natural gas, although steam and other product systems are also in use.
An easy way to picture a distribution system is...
