Phase Separation in Midstream Natural Gas Systems & Pipelines – Understanding the Risks and Solutions

For both gas treatment plant operators and transmission systems operators (TSOs), incorrect phase separation can result in a significant loss of profits. The installation of process cameras in high-pressure gas systems reveals a much higher level of two-phase flow than expected. In most cases, dewpoints still report dry gas while liquid flows are present. With large uncertainties around hydrocarbon dewpoint systems, underperforming phase separation systems are going unnoticed, leading to a range of severe problems for the natural gas industry.  In this blog, we will delve into the causes of phase separation problems, their impact on the natural gas processing industry, and the available solutions to minimise the risk.

Causes of Two-Phase Flow

It is important that sales quality gas entering transmission systems is single phase and will remain single phase during the expected pressure and temperature reductions in the journey to the point of use. The biggest factor is a multi-phase flow (wet gas) entering the transmission system from the gas treatment plant. In addition, temperature changes, pressure changes, and the presence of impurities can cause liquids to condense out of the gas further down the transmission system. Leakage of lubrication oil from gas compressors and solid material formed by the reaction of gas components with the pipeline (iron sulphide or black powder) can also cause high maintenance costs and increased risk for TSOs. A big problem is that most of the hydrocarbon dewpoint systems calculate the dewpoint, and this method can have very large errors leading operators to believe the gas is dry when it is not.

Impact on the Natural Gas Processing Industry

The consequences of two-phase flow can be severe for the natural gas processing industry. In addition, the presence of liquids in the pipeline can also pose a serious safety risk. Correct phase separation of liquids, gases, and solids is essential for efficient processing downstream of the separator. There are normally two parties that suffer when two-phase flow is present: the gas processing plant, and the TSO.

For the gas processing plant, two-phase flow at the inlet to gas treatment is the major cause (60%) of plant failures. Two-phase flow causes foaming and fouling, resulting in lost production and high maintenance costs. Also, the last process in gas treatment is the recovery of natural gas liquids (NGLs). This is normally achieved with a pressure drop to reduce the gas temperature significantly below the dewpoint. These are the ideal conditions to produce a fine mist flow, the most challenging for any phase separation system. If liquids are not separated to a very high level, the gas processor sends NGLs to the gas line without getting paid for them, as they are not included in gas-phase Btu measurements.

For the TSO, a two-phase flow means that API and ISO standards for measurements can no longer apply. Any measurements are gas phase only, leaving operators in the dark when two-phase flow occurs. The increased uncertainty in flow and calorific value measurements means that reported values are outside of acceptable limits. The gas supply is also outside of tariff agreements for gas quality standards and legal requirements in place for safety issues. When liquids are present, it results in reduced life of dry gas seals in compressors, increased frequency and cost of pigging and disposal of the resulting liquids and solids. Liquids pool at low points in the gas network and there is an increased risk of corrosion. Pipeline fouling can reduce the flow rate or increase costs for compression.

This is why it is important to minimise the risk of two-phase flow in the natural gas pipeline.

Solutions for Minimizing the Risk of Phase Separation

When it comes to minimising the risk of phase separation, process cameras can play a significant role in detecting and addressing any issues quickly and accurately. These cameras are designed to monitor the flow of gas in pipelines and detect any changes in the flow patterns that may indicate a problem, such as phase separation. By monitoring the flow of gas, process cameras can provide valuable insight into the health of the pipeline and help operators identify potential issues before they become major problems.

There are several solutions available to minimise the risk of phase separation in the natural gas pipeline. When monitoring the exit of a phase separation system, process cameras is a highly effective way to understand the performance of the phase separator under different process conditions. This operator feedback allows multiple processing trains to be balanced for the least liquid carryover and highlights any process failures and maintenance that may be required.  When monitoring reveals an unacceptable performance from the phase separator, there are design companies specialising in improving ageing systems with new internal baffles and de-misting pad designs. When a replacement separator is required, there are also now companies specialising in designing effective phase separation and filtration systems that can remove micron and sub-micron solid and liquid impurities from the gas. Process cameras can also optimise pigging operations and, in some circumstances, recover pigging and disposal costs after a liquid event from a gas processor.

By detecting phase separation early on, operators can take steps to address the issue and prevent further problems from occurring.

Additionally, process cameras can provide real-time video streaming, which can be used to make informed, evidence-based decisions on managing a liquid carryover incident. For example, if the camera detects contamination that suggests phase separation, operators can adjust the flow rate or pressure, to minimise the risk.


Two-phase flow is a common issue in midstream natural gas pipelines that, until recently, has gone unnoticed mainly because of calculating a dewpoint from the gas phase provides operators unreliable information with no alarm when two-phase flow is present. It can lead to decreased production, increased maintenance costs, and safety risks. By understanding the causes of phase separation and the solutions that are available, pipeline operators can minimise the risk and ensure the safe and efficient operation of their pipelines. Process cameras can be an effective tool in mitigating the risks associated with phase separation in the midstream natural gas pipeline. By providing real-time data and early detection of potential issues, process cameras can help operators minimise downtime, reduce maintenance costs, and ensure the safe and efficient operation of the pipeline.

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About the author

Paul Stockwell, the managing director of Process Vision, is a renowned authority in moisture measurement with 35 years of experience in the oil and gas industry. He founded International Moisture Analysers (IMA) and played a key role in advancing moisture measurement techniques. Notably, he introduced tunable diode laser absorption spectroscopy for natural gas measurements, revolutionizing the field and establishing it as the industry standard method. Throughout his 20-year tenure as managing director, Paul has gained valuable insights into process optimization, cost reduction, and safety enhancement. His vision for Process Vision encompasses improving process throughput, reducing maintenance costs and CO2 emissions, and nurturing young engineering talent, aiming to make a significant difference in the oil and gas industry.

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