6 - Pipeline Drying

Abstract

This chapter focuses on the process of pipeline drying. Newly constructed pipelines are typically hydrostatically tested, using water as the test medium, after the completion of construction. Older lines already in service may be re-tested either to confirm an earlier test or to qualify the line for a higher operating pressure. Once the hydrostatic testing has been completed, it is necessary to remove the water from the line and place the line in service. Dewatering can be a simple process or, if the procedure is not properly planned, a difficult one. When the pipeline is used to transport materials that must meet a specified dryness requirement, the pipeline has to be dewatered, cleaned, and dried. Pipelines used to transport natural gas will need some drying, depending on the operating pressure and the location of the line, to prevent the formation of hydrates. Other pipelines may require drying to protect the pipe from internal corrosion caused by the formation of corrosive acids, such as carbonic acid in the case of carbon dioxide pipelines. The most common methods for drying pipelines are drying with super dry air, drying with methanol, drying with inert gas such as nitrogen, internal sand blasting, drying with the medium to be transported, and vacuum drying. All of these methods may be applied to pipeline drying depending on the particular line and amount of dryness required. Sometimes, a combination of two or more methods will be used to achieve a dry pipeline at the least cost. The first three methods are probably the most economical and technically feasible for most pipeline drying applications.