Pipeline Drying Procedures and Requirements for Commissioning
Pipeline construction and pipeline repairs are major projects that require detailed steps before a pipeline can be put into service. One step that can be overlooked until late in the project is pipeline drying. Why is this process so important and what options are available for effective pipeline drying?
Why Is Pipeline Drying Important?
Moisture is not a friend to pipelines. Not only does water need to be cleared from pipelines as part of the commissioning process, removing moisture helps to inhibit corrosion once the pipeline is in service.
Unwanted moisture can accumulate for several reasons. Water and other contaminants can collect in the pipeline during the construction process. Pipeline drying after a hydrotest is particularly challenging since there will be additional water to remove before the pipeline is ready for commissioning.
There are specific problems that can be created by pipeline moisture. For medium-pressure gas pipelines, water can make pressure regulators susceptible to freezing in the winter, and high-pressure gas pipelines can be clogged by hydrates that can form within the pipeline. Finally, in addition to moisture-speeding normal corrosion, the presence of water can make gas pipelines subject to internal stress corrosion cracking (SCC) which can cause damage inside pipelines in as little as a few years.
Common Pipeline Drying Methods
There are several options available for drying pipelines. Effectiveness and cost are normally the primary considerations when selecting a drying method.
Nitrogen Purge Pipeline Drying
Nitrogen is becoming increasingly popular for pipeline drying for several reasons. Nitrogen is very dry, unlike the ambient air. Nitrogen is also inert so it is appropriate for preparing and testing pipelines for a range of purposes, even pipelines that will be used to transport flammable gases.
An on-site nitrogen skid can be transported to pipelines, even in remote locations, in order to fill the pipelines with nitrogen. If nitrogen will be used for pneumatic pipeline pressure testing prior to drying, the nitrogen generator can be used for both processes.
Another advantage to using nitrogen for the drying process is the nitrogen doesn’t have to be purged after drying is complete, unlike air drying. In addition, the low dew point of nitrogen speeds the drying process. Unlike hot air pipeline drying, nitrogen drying doesn’t experience the same issues with moisture remaining in the center of the pipeline.
Hot Air Pipeline Drying
Before nitrogen drying became more popular, hot air drying was the most common method for pipeline drying. In hot air drying, the pipeline is filled with heated air at one end. The water within the pipeline absorbs the heat energy from the air to speed the drying process. Water vapor is pushed out of the other end of the pipeline by the air flow. As hot air is forced in through one end the of the pipeline, compressed air dew point measurements are taken at both ends of the pipe in order to determine when the pipeline is sufficiently dry.
The hot air drying process can take a significant amount of time to complete. One reason for this is because the hot air drying process is most effective at the ends of the pipeline. This is due to the higher pressure created within the center sections of the pipeline as hot air is forced into the line, limiting the amount of liquid the air can absorb. Near the ends of the pipe the pressure is lower allowing more water vapor to be absorbed into the air. This dynamic can make it difficult to adequately dry the center of the pipe. To reduce this problem, it is often necessary to perform pipeline pigging to remove as much water from the pipeline as possible prior to the hot air drying process.
Another problem created by the uneven drying provided by the hot air method is that it can be more difficult to measure when the drying process has been completed. To check for excess moisture in the center sections of the pipeline, a soak test can be performed. This test is done by sealing off the pipeline for 12 hours, then the dew point is monitored again while air is forced through the pipeline at the lowest pressure possible.
Vacuum Pipeline Drying
Another option for pipeline drying is the use of a vacuum. In this approach, a high-power vacuum system is used to remove air from the pipeline. This will gradually lower the humidity level within the pipeline as the air is removed.
As the residual water is evaporated and removed, the dew point within the pipeline will eventually stabilize and lower until the water has been extracted. A sensor attached to the pipeline is used to measure the dew point to determine when the pipeline drying process is complete.
Use a Nitrogen Generator for Nitrogen Purging for Cost-Savings
There are several advantages to the use of nitrogen in pipeline drying. Having a nitrogen generator onsite can allow the nitrogen to be used for pneumatic pipeline pressure testing. Then the same nitrogen generator can be used to purge the line and dry it in preparation for commissioning. After the dewatering has been completed, nitrogen can be used to propel a pig train through the line.
If the pipeline will be used for hydrocarbons, the nitrogen can pack the line to ensure that the system has been purged of any oxygen since it is vital that the hydrocarbons be kept separate from anything flammable. Using nitrogen allows a single piece of pipeline drying equipment to be used for multiple steps in the process to prepare the pipeline for commissioning. That can simplify the process while reducing the manpower and costs associated with coordinating the acquisition, delivery, and operation of process equipment.