Evaluation of the Corrosiveness of Glycol-Water Mixtures in Dry Gas Transmission Pipelines

Nearly all natural gas transported by pipelines must be processed to remove moisture content in order to meet the pipeline specification for both economic and pipeline integrity reasons. The absorption method, the most commonly used approach to remove water from natural gas, uses liquid desiccant to absorb water vapor. Commonly used liquid desiccants in natural gas processing are ethylene glycols, Monoethylene glycol (MEG), diethylene glycol (DEG), and Triethylene glycol (TEG).

The dry gas stream processed with glycol is saturated with glycol vapor when it leaves the dehydration unit. The glycol vapor in the dry gas stream can condense out and form a liquid when it encounters low temperatures during transmission. Although the quantities of glycol are small per unit volume of gas, high flow rate and long service time can potentially cause significant accumulations in pipelines. The glycol-water hold-up in the dry gas lines can produce an electrolyte in places that are not normally expected.

Pure glycols are considered non-corrosive, but pipeline steel corrodes in glycol-water mixtures. The corrosion rate of pipeline steel in glycol-water mixtures greatly depends on water content, temperature, composition of the gas transported, and also contaminants in the mixture. This project reviews the state-of-the-art knowledge on the entry of glycol-water mixtures into the dry gas transmission pipelines and the corrosiveness of glycol-water mixtures in various gas transmission pipeline environments. It is aimed to provide guidance for operators to evaluate the corrosion consequence associated with glycol-water mixture in dry gas transmission lines.

Key Results

This study provides a review of the state-of-the-art knowledge on glycol-water mixtures carried over to dry gas transmission pipelines, i.e., physical properties, the causes of entry, and the corrosiveness. Findings obtained from literature include:

Glycol-water mixtures can be potentially accumulated in dry gas transmission pipelines. Operation conditions of the dehydration unit should be optimized to control the amount and corrosiveness of glycol-water being carried over into transmission lines.
The glycol-water hold-up in dry gas transmission pipelines contain high glycol content, i.e., > 95%. The accumulated glycol-water requires removal by direct drain from liquid traps, or by pigging either with methanol or water.
For transmission pipelines transporting natural gas containing CO2, the most likely corrosion mechanism associated with glycol-water mixtures is general corrosion. The corrosion rate has been proven to be low, i.e., much lower than 0.1mm/year.
For sour gas transmission pipelines, although general corrosion has been ruled out as a major concern, localized corrosion and HIC can occur in glycol-water mixtures. The rate of localized corrosion and HIC remains undefined.

A list of recommendations to minimize the amount of glycol-water mixtures being carried over to dry gas transmission lines and to reduce the corrosion risks is developed. More research is required to study the potential corrosion mechanisms, i.e., pitting and HIC, of pipeline steels in glycol-water mixtures with high glycol content (> 95%) under sour gas conditions.

PRCI members can download the draft Final Report in PRIME.