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Ejectors in Flare Gas Recovery Systems

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Ejectors in Flare Gas Recovery Systems

What is Flare Gas?

In Petroleum refineries, Natural gas processing plants and in Oil and gas Production sites, mixture of natural gases is generated from the below sources:

  • Extraction of Natural Gas along with oil from oil wells,
  • Gases formed above the liquid level in storage tanks
  • Flashing of high pressure crude to low pressures.

This gas consists of a mixture of various hydrocarbons and is also known as associated petroleum gas.

The Gas is flammable in nature and its composition varies from plant to plant and will also vary with time over the operating life of the plant/Oil-well.

Traditional Flare Gas handling systems:

Since the commencement of Oil and Gas Production and advent of petroleum refineries, little importance was given to the generated flare gas mixtures. This gas was termed as waste gas and was collected in large diameter headers. The gas was then burned off and the combustion products were let off to the Atmosphere.

Need for Flare Gas Recovery:

  • Flare Gas being a combination of petroleum gases can be used as a source of Energy. An industrial survey has indicated that the amount of Gas burned in the Flare Stacks throughout the world in the year 2011 amounted to almost USD 29.8 billion.
  • Utilizing Flare gases helps the plant to cut down on its Energy costs and also create an in-house Fuel source.
  • Improper burning and incomplete burning leads to generation of toxic and harmful gaseous pollutants. The produced gases are known to create harmful and deadly effects on human life and the surrounding environment.
  • The Flaring of Flare gas is also a considerable source of carbon di-oxide emissions.
  • Hence utilizing flare gases can help an industry to reduce Energy costs, reduce its Carbon footprint and be responsible to the environment.

Challenges in Recovery and Utilization of Flare Gases:

  • Variable composition of constituents in Flare gases during the operating life of the plant. This provides a challenge for design of involved equipment and also for estimating the energy input from these gases.
  • Flare Gas mixtures are produced at low pressure. For utilization they need to compressed to high pressures.
  • Flare Gases have a low molecular weight and hence tend to occupy large volumes. This increases the size of the involved equipment leading to more infrastructure cost.
  • Being a flammable mixture, care in handling is important.
  • Flare gas production is a continuous process and hence the recovery systems also need to work round the clock.

Ejectors in Flare gas Recovery systems:

Ejectors perform a central role in Flare Gas Recovery Systems [subsequently denoted by FGRS] by compressing the low pressure gas mixtures to working pressures. Ejectors utilize the available high pressure gas as energy source to bring about this compression.

Ejectors are preferred over Mechanical type Compressors due to the following reasons:

  • Ejectors satisfy the criteria for very long running times without any downtime which is instrumental for FGRS
  • Ejectors are static devices and require very less maintenance during their operating life.
  • No contamination of gas mixture as the motive fluid is also natural gas and hence the gas mixture at the Ejector Outlet can be injected directly in the Fuel-gas headers.
  • Low Capital costs compared to a Mechanical Compressor.
  • No risk of ignition of gases inside the Ejector.

Criticality of Ejector performance

  • The Ejector has to operate with a range of Motive Pressures and a range of Suction Flows. Ensuring a design that can satisfy a wide array of operating conditions in a Ejector is a critical task as Ejectors are single point devices.
  • In case of reduction of suction flow than the design value the Ejector may create vacuum in the flare header and the involved upstream equipment. The generated vacuum will disrupt the other processes and can also create mechanical damage. Hence in order to maintain the suction pressure a recycle line is used to recirculate flow form the discharge side to the suction side.
  • Matching the characteristics of the Recycle with the Ejector Characteristics is critical.
  • In case of higher than design suction flows the Ejector will not be able to provide the necessary compression, hence the relief valves on the suction side must be appropriately sized and matched with the Ejector operating characteristics.
  • In case of Multi stage ejector system the criticality is further compounded.

Crystal TCS’s expertise in Ejectors for Flare gas Recovery Systems

We at Crystal TCS have taken a theory based approach towards establishing the Ejector performance estimation and have developed design from the basic principles of Thermodynamics and Fluid flow dynamics.
The developed theory and obtained algorithms have further been validated with rigorous performance evaluation test at our Ejector test facility.

As stated above Flare Gas Ejectors have to operate with a variable range of working conditions and compositions over their lifetime. It is not possible to recreate each and every operating condition in the test facility.
Our theory based approach has enabled us the flexibility to simulate each and every design case with the help of our algorithms and arrive at a precise and optimum solution.

We manufacture Flare recovery systems in a wide range of materials ranging from Carbon Steel, Stainless Steels and Hastelloys depending upon the application.