Pipeline Drying / Pigging
So why is pipeline drying necessary? Pressure testing of pipelines, stations, tanks or pressure vessels of any type is critical to prove the integrity of that system. Hydrostatic testing is typically performed after new construction but is also performed to fulfill mandatory integrity requirements. Unfortunately, residual water left after hydrostatic testing is not always acceptable in many natural gas application and other systems.
Determining Dryness Criteria
There are many factors that can influence dryness criteria. Type of hydrocarbon, hydrate formation conditions, corrosion studies, pipeline topography, piggability and environmental regulations are considered. During commissioning, pipelines are filled with hydro testing fluids that must be treated and removed from the pipelines prior to commissioning and start-up. The makeup of these fluids must be considered when determining dryness criteria. For natural gas pipelines, the generally accepted moisture limit is around 40 lbs/mmscf or a dew point of –4°F (-20°C).
Achieving the ideal dryness for a gas pipeline is a crucial step not only for commissioning of a pipeline, but also for is subsequent integrity management. Attaining the correct dryness level can help inhibit micro biologically influenced corrosion (MIC), hydrates and other issues. Drying of a pipeline is usually completed in stages that involve one or more of the following techniques: pigging / swabbing & desiccant air drying. Selection of methods for drying is often driven by economics and time restraints, without sufficient consideration given to operability and corrosion issues. Dew point is a major consideration when evaluating pipeline dryness criteria. A dew point below the hydrate formation curve at expected operating temperatures and pressures is desired. A dew point of -4°F (-20°C) is usually attainable and sufficient for most applications. Tables for converting dew point to moisture content are readily available, and are useful tools to assist in determining dryness criteria.
Swabbing / Pigging
The goal of swabbing is to enlarge the evaporation area of present water by spreading it over the pipe wall. This enables a large contact area between the water and the drying media. Several types of pigs are used for this method. In natural gas pipelines, small amounts of water left in the pipeline can cause corrosion issues and create problems in the way of hydrate formation and icing problems that can start to reduce flow rates. Water can also create product quality issues or system upsets. In systems where water is unacceptable, drying can become a critical service. The most common methods for pipeline drying, station drying and tank drying are Air Drying, Nitrogen Drying, and Vacuum Drying. Adroit feel that high volume air drying has historically proven to be both fast and cost effective.
Drying pipelines initially involves pigging with dry air. The dry air will absorb the water left from the dewatering phase. The process of running pigs and dry air through the pipeline is continued until the desired dew point at the outlet is met. The outlet should be an open pig receiver door the disadvantage to this method is that it requires a large air drying spread and may take a long time. Also, air drying suffers from one major flaw, typically the beginning and end of the pipeline is dried quickly and the centre becomes saturated. This is because the pressure is higher in the centre sections of the pipe, therefore the air cannot absorb as much liquid. When the air reaches the outlet, where the pressure drops to near ambient, and the air can absorb more water vapour. Therefore, the centre of the pipeline is not dried effectively. Moisture content measurements at the outlet can sometimes not reflect actual moisture content in the pipeline. For these reasons it is necessary to perform a soak test. To reduce drying times it is often necessary, due to pipeline profiles, to run multiple pigs to remove as much remaining free water as possible. A soak test is performed by shutting in the pipeline for 12 hours, and then an air volume equivalent to the pipeline volume is passed through the pipeline at the lowest possible pressure while monitoring the dew point of the air at the discharge point. If the dew point at the discharge point conforms to the dryness criterion, the drying campaign is complete.
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