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Natural Gas Quality Designation Parameters

When I was working on my second EPC project in Senoro, which was gas processing facility, I read that there are several natural gas quality that shall be complied. In the project, we also had to guarantee that the facilities can process gas from wells so that it met requirement as stated in contract. In this post, I want to share you the quality designation parameters for natural gas. To understand more about this area, please check ISO 13686: Natural Gas – Quality Designation.

Natural gas quality designation parameter
Natural gas quality designation parameter

Gas Composition

Natural gas composed primarily of methane with smaller amounts of higher hydrocarbons and non combustible gases (please check my previous post about liquefied natural gas typical properties).

Major Components

Major components of natural gas are:

  • Methane
  • Ethane
  • Propane
  • Butanes (i-butane and n-butane)
  • Pentanes (i-pentane, n-pentane, or dimethylpropane)
  • Hexanes plus
  • Nitrogen
  • Carbon dioxide

Minor Components

Minor components of natural gas are:

  • Hydrogen
  • Oxygen
  • Carbon monoxide
  • Helium

Trace Constituents

Trace constituents of natural gas:

  • Hydrocarbon sulfide
  • Mercaptan sulfur
  • Dialkyl (di)sulfide
  • Carbonyl sulfide
  • Total sulfur

Gas Properties

Physical Properties

  • Calorific value (molar-basis, mass-basis, volumetric basis)
  • Density
  • Wobbe Index (WI)
    • Indicates the ability of overall fuel handling and injection system to accomodate the fuel composition
    • Correct representation of the heating value of natural gas arriving, from the gas line, at the orifice where a burner is located
    • If the WI varies too far from the design value, changes to fuel system should be made
    • Expressed as BTU/cuft divided by square root of specific gravity (LHV per square root of SG)
    • High WI indicates heavier hydrocarbon in the fuel
    • Low WI indicates significant amount of noncombustible fuel components or significant amount of hydrogen and carbon monoxide
    • Good design is having WI within 10% range. Fuel can be substituted without making adjustment.
  • Water dew point
    • Indicates the temperature at which water vapor in gas start to condense into liquid water
    • It shall be maintained to prevent hydrate formation and corrosion in gas pipeline
  • Water liquid content
  • Hydrocarbon dew point
    • Indicates temperature at which heavy hydrocarbon components begin to condense out of the gaseous phase.
    • It shall be managed to prevent HC condensation in cold spot, such as under rivers and lakes, where liquid collect in low areas and then move as slug through the system. It will lead to pipe overpressure and overpowering liquid handling facilities
    • HCDP is strongly influence by the concentration of heavier hydrocarbon, especially C6+
  • Hydrocarbon liquid content

Other Properties

Natural gas shall be technically free from:

  • Water and hydrocarbon in liquid form
  • Solid particulate substances in amounts deleterious to the materials normally encountered in transportation and utilization
  • Other gases that could adversely affect the transportation or utilization of the gas

References:

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