Pipelines are vital in production, distribution, and refining processes, as they deliver energy to various operations where it is turned into useful fuels and products for supply to local communities. There are many products commonly delivered by pipelines, including: crude oil, refined products (such as gasoline and diesel), and natural gas liquids (such as ethane and propane).
Oil and gas pipelines are made from steel tubulars, also known as pipes, that are circular in shape and are usually buried. The products move through the pipelines thanks to pump stations along the system. Many hazardous liquids of various commodities are transported in the same pipeline in ‘batches,’ where each batch of liquid is pushed along at the same speed along the length of pipe.
By Gobind Khiani – Consulting Fellow-Piping/Pipelines
What is Purging and Why it is Required in Pipelines?
Oxidation is a continuous process in pipelines, used specially to weld seams that are exposed to air to prevent oxidation or apply any treatment to pipelines so that the technology can become corrosion resistant, and purging occurs. Purging is done to protect the weld seam until it has cooled to the point that oxidation no longer can occur.
Purging pipelines can prevent oxidation and improve the overall life of systems and associated tools.
Some forms of purge include:
• Inert Purge: This process occurs by changing the contents of a pipe and using an inert gas to displace the original content or to separate the two media being interchanged.
• Dilution: In this process, displacement of one substance by another is achieved.
• Displacement: This happens when the replacement of one substance by another is accomplished.
The purging procedure is necessary to eliminate oxygen, water vapor, and other impurities that are retained within a newly laid pipeline network. Failure to eliminate these substances will alter the quality of fluids being transported through the flow lines.
Purging ensures that a technician has control over which gases are in the gas delivery system, and therefore which gases are being exposed to internal components, sensors, or other equipment. Purging also helps to prevent unwanted reactions from taking place, which can greatly increase the service life of related components.
Common Industry Practices and Methods
Nitrogen is commonly used to remove hazardous gases or dry fluids from a pipeline system. Nitrogen is safe for use and effective in pipelines and is used in many instances, namely: • Prior to commissioning of a new pipeline system.
• When purging a pipeline after a pigging operation.
• Decommissioning of the existing network of pipeline or a system.
• To dry a pipeline after hydrotesting.
Nitrogen is inert, nonreactive, nontoxic, and noncorrosive. It can be shipped as a cryogenic liquid, compressed as a dry gas, or filtered from the atmosphere on-site using a nitrogen membrane unit. Most commonly, nitrogen is delivered in bulk as a cryogenic liquid then converted to a warm gas using a diesel-fired heat exchanger or non-fired flameless nitrogen pumping unit. It can be delivered at temperatures ranging from -150˚C to +400˚C and at pressures from atmospheric to 10,000 psi.
As quoted in Four Quest Energy, “Nitrogen applications can range in size from a two-hour operation involving one pumper and operator to projects lasting months with numerous pumpers and transports. Some projects require very large volumes of nitrogen delivered in a short time, which is normally provided by the conventional fired pumping equipment. A continuous supply of liquid nitrogen can be delivered to site by specially designed nitrogen transports.”
The purge cycles depend on the purity and level of oxygen content requirements in the pipeline system, which is provided by end users.
Calculations that should be considered during nitrogen purging include:
• The volume of product to be displaced.
• The pipeline profile, to determine the maximum change in elevation over the displacement line section.
• The specific gravity of the product to be displaced.
• The volume of nitrogen to be used during the displacement (including a minimum 10% contingency factor).
• Nitrogen injection pressures that must be maintained.
• Displacement rate and anticipated duration.
• Minimum back pressures to be held at terminus, and the consideration being given to elevations along the pipeline.
• Blowdown requirements and duration.
Remaining Safe
Safety is vital in all industries. However, pumping applications can be especially dangerous for on-site workers. That is why safety must always be at the forefront of all industrial processes, especially when handling harmful media like raw nitrogen.
A few precautions to take during the purging procedure include:
• Ensuring all valves that must be operated during the procedure are manually controlled and all valves through which a pig must pass unattended are locked ‘fully’ open.
• Verifying all valves that are to be closed during line displacement are locked closed.
• Ensuring all station by-pass valves are operational.
• Using geophones to track movement of pigs.
• Obtaining necessary access from adjacent landowners and acquiring adequate workspace.
• Notifying landowners and the public authorities in the areas of potential noise, blow down fumes, and additional traffic.
• Acquisitioning ‘specialty’ materials and outside services that may include, but are not limited to: venting (blow-down) connections, separation tanks, batching pigs, and tracking devices.
• Verifying Nitrogen interfaces are delivered to the nearest downstream terminal, while ensuring pumping stations between the injection point and the terminus are by-passed.
• Ensuring a tank is available for displaced nitrogen/liquid mixture.
• Making sure instruments such as thermometers, gauges, and sampling apparatus are calibrated, valid, certified, and installed at appropriate locations.
• Ensuring there is required meters at the injection point for nitrogen and at the terminus for displaced product.
• Verifying that line control monitors flow during displacement runs.
• Ensuring all contingencies are addressed, including: the potential displacement of pigs as they become lodged during the operation, a new injection point is available to continue the displacement, and verifying an alternative blow-down site exists to expedite venting, pressure relief, and locked in requirements due to elevation changes.
Conclusion
Nitrogen is vital to purging and helps in keeping oxygen out of the internal surfaces of a pipeline system, while eliminating sparks and fires. Purging also eliminates the presence of moisture and helps in lowering the dew point. Nitrogen is an inert gas and hence it is non-flammable. This means it is economical and affordable, therefore making it the most-used medium for the purging process in current industry practices.
REFERENCES
Corrosionpedia, JP Services, Four Quest Energy, Compressed Gas Technologies Inc, Trans-Northern Pipelines Inc
About the Author
Gobind N Khiani, a UCalgary alumnus of Masters in Mechanical Engineering is a seasoned change-maker. He has a proven track record in technical and value engineering and holds
a Fellowship in Engineering and an MBA. He is the Chairman of the End User Group at API and Vice Chairman of the Standards Council of Canada. He has done peer review on Emissions Management regulatory documents for ECCA and participated in research and development initiatives. Further, his experience is in the energy sector in the improvement of standards, technical compliance, strategy, governance, digital innovation, engineering management, technology, sustainable development, and operations. He is also skilled in Asset Integrity and Maintenance Management. As a volunteer, he is involved in technical standards (energy, tech, public safety) and has been a mentor/judge at First Robotics Canada. He is also the past chair of the CBEC of APEGA.
