Drying a pipeline is a critical process, especially after hydrostatic testing, to remove residual water and prevent issues like corrosion. Pipeline drying is typically achieved using various techniques involving the circulation of dry substances or creating conditions that promote evaporation.
Effective pipeline drying is essential to maintain the integrity of the pipeline, prevent internal corrosion, ensure the quality of the product being transported, and prepare the pipeline for subsequent operations or commissioning.
Common Pipeline Drying Methods
Several methods are employed depending on factors like pipeline length, diameter, location, the required dryness level, and cost considerations. Here are some primary techniques:
- Swabbing or Pigging: This involves propelling specialized tools called "pigs" or "swabs" through the pipeline using compressed air or nitrogen. The pigs are designed to scrape and push out bulk water, often using cups or discs that fit snugly against the pipe walls. Multiple passes with different types of pigs are usually performed.
- Air Drying: This method uses compressed air to push water out and then circulate dry air to evaporate remaining moisture.
- Standard Compressed Air Drying: Uses dehumidified or ambient compressed air. Heating the air can increase its moisture-carrying capacity, speeding up evaporation.
- Chilled Air Drying: Another effective technique for eliminating moisture from pipelines following hydrostatic testing is the use of very dry chilled air. This method leverages the principle that cold air holds less moisture. Highly powerful ventilators can be used to drive a stream of very dry cold air through a pipeline to eliminate residual water. This process can efficiently reduce the dew point within the pipeline.
- Nitrogen Drying: Similar to air drying, but uses nitrogen gas instead of air. Nitrogen is an inert gas and is typically supplied in a very dry state. Using nitrogen eliminates the risk of internal oxidation (rusting) during the drying process, making it suitable for pipelines transporting sensitive products or those requiring very low dew points.
- Vacuum Drying: This method involves sealing the pipeline and creating a vacuum within it. Lowering the pressure reduces the boiling point of water, causing it to evaporate at lower temperatures. This vapor is then pulled out by vacuum pumps. It is particularly effective for achieving very low dew points.
- Methanol Swabbing: For pipelines transporting natural gas, a methanol slug propelled by a pig can be used. Methanol is miscible with water and lowers its freezing point, forming a methanol-water mixture that can be more easily pushed out. However, methanol requires careful handling and disposal.
Choosing the appropriate drying method depends on the specific project requirements, including the target dew point (a measure of dryness), pipeline specifications, timeline, and environmental considerations. Often, a combination of these methods is used for optimal results, such as swabbing to remove bulk water followed by dry air or nitrogen circulation.
