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Quality

Here you will find out about how we regulate gas quality in the National Transmission System (NTS) as a safety measure, including details on gas contents and components.

The Health and Safety Executive (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, 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.

We can 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.

How we determine gas quality

Natural gas is made up of several component gases and is therefore subject to natural variation. We use the Wobbe Number/Index to measure the energy of gas within a given volume, rather than calorific value (CV) which is 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 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 our Gas Ten Year Statement.