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  • Gas Quality

  • The HSE has issued the Gas Safety (Management) Regulations 1996 (GS(M)R) in which it places a requirement on a Gas Transporter (National Grid) to provide a Safety Case, for the industry to have a Network Emergency Co-ordinator (NEC) and to ensure the content and characteristics of gas conveyed in the system are within certain limits. These limits are monitored closely at associated system entry points and at specified blending points.
    National Grid has the ability to request the curtailment or cessation of flows of gas with characteristics outside of GS(M)R specification. This specification is designed to ensure gas burns cleanly and consistently and the supply pipework is not exposed to contaminants or corrosives.
    Natural gas is made up of several component gases and is therefore subject to natural variation. This inconsistency affects the energy contained within a given volume of gas; the measure used is the Calorific Value (CV) of the gas. The Wobbe Number or Wobbe Index, is another important characteristic which describes the way in which the gas burns and is calculated as a factored ratio of CV and Specific Gravity (SG otherwise known as Relative Density). The GS(M)R specified ranges are as follows:
     
    Content or Characteristic Value
    Hydrogen sulphide (H2S)content ≤5 mg/m3
    Total sulphur content (including H2S) ≤50 mg/m3
    Hydrogen content ≤0.1% (molar)
    Oxygen content ≤0.2% (molar)
    Impurities shall not contain solid or liquid material which may interfere with the integrity or operation of pipes or any gas appliance (within the meaning of regulation 2(1) of the 1994 Regulations) which a consumer could reasonably be expected to operate
    Hydrocarbon Dewpoint and Water Dewpoint shall be at such levels that they do not interfere with the integrity or operation of pipes or any gas appliance (within the meaning of regulation 2(1) of the 1994 Regulations) which a consumer could reasonably be expected to operate
    Wobbe Number (WN)

     (i) ≤51.41 MJ/m3, and  (ii)  ≥47.20 MJ/m3 

    Incomplete Combustion Factor (ICF) ≤0.48
    Sooting Index (SI)  ≤0.60

    Source: Gas Safety (Management) Regulations 1996, Schedule 3, Part I

     
    Elements where specific values are not specified in GS(M)R  but are important to the safe operation of the NTS, for example Hydrocarbon, Water Dewpoint and Temperature, are included in National Grid’s Ten Year Statement (please see below).
     
     
    The following related links may be of use:
    Gas Safety (Management) Regulations (GS(M)R) regulations
    Uniform Network Code (UNC) Transportation Principal Document (TPD)
    National Grid Ten Year Statement  
     
    If you have any feedback or questions regarding this web page please get in contact with us at:
     sysop.centre.reporting@nationalgrid.com