a Specification relating to the transmission and distribution of natural gas must have items relating to the integrity of both the pipeline and the flow of gas. The composition
of gas entering the pipeline must be such that the pipeline is not damaged.
Corrosion, stressing and abrasion must be avoided. The composition must also ensure that, under all temperature and pressure conditions of distribution, the flow of gas can occur. Accumulation of liquid in the pipeline will reduce the capacity of the pipeline and interfere with instrumentation and control equipment. Such interference
can lead to erroneous measurements and instrument failure.
The specification has aspects covering:-
 1. Safe use of domestic appliances
 2. Transmission and distribution needs
 3. Statutory requirements, e.g. hydrogen sulphide content
 There is also a requirement that gas delivered shall be free from materials/dust. Solids or liquids might interfere with the operation of lines, metres or regulators.
Such gas shall be free from objectionable odours so that a distinctive “gas odour” can be added to meet the requirements of the Gas Safety (Management) Regulations.
This transmission specification is the primary criterion in considering whether a gas needs processing. If the gas lies outside the specification, an evaluation is made of the possibility of mixing with other supplies, although this can have an effect on the security of supplies.

  Hydrocarbon Dew Point
A temperature that is low enough to ensure that hydrocarbon liquid does not form under all temperature and pressure conditions of distribution must be specified.
The dew point temperature specified will be governed by the lowest ambient temperature that the gas will experience while the gas is at pressures between 24 and
38 bar g (350 and 550 psig). This is the pressure range at which retrograde condensation can occur. This phenomenon must be avoided in the transmission and
distribution system. If too high a hydrocarbon dew point is allowed, a gas that is single phase at high pressure can become two phase (gas and liquid) at lower pressure
even though the gas is not cooled. The extent of the two phase region within temperature and pressure coordinates depends on the gas composition. In particular,
the amounts of heavy hydrocarbons can give rise to retrograde condensation.
The amount of the various heavy hydrocarbons allowed in the gas in inversely proportional to the carbon number of the hydrocarbon, i.e. in the series C6, C7, Cs,
C9, C10, very much less C10 can give rise to retrograde condensation than that of C6. For most natural gas, after separation of the gas and condensate, the gas phase
will contain too much C6+ component. This must be removed by gas processing and a chillers plant is usually employed.
 
  Water Dew Point
The presence of liquid water in the transmission system must be avoided otherwise hydrate formation and pipeline corrosion can occur. Hydrates are a physical
combination of the lower hydrocarbons and water and once formed are stable. A reduction in the pipeline diameter can result causing the flow capacity of the line
to reduce. In the extreme, blockage of the line can occur. Hydrates will also interfere with the correct operation of instrumentation, and it is more likely that blockage
of supply lines to instruments would occur. Hydrates can only occur if free water is present, i.e. the gas is at 100 % relative humidity. Corrosion, however, can
occur at levels below 100 % RH. Protection against corrosion is ensured by operating at not more than 50 % RH. The RH of gas is often conveniently expressed as
a water dew point.
 
  Carbon Dioxide Content
In early UK gas purchase contracts, it was considered necessary to specify a maximum carbon dioxide content of 2 %. This was to ensure that acid gas corrosion of
the 70 bar (1,000 psig) transmission system was avoided. Later, it became apparent that protection against corrosion was being secured with the water dew point
limit. It remains necessary to have gas supplies of low carbon dioxide content due to requirements at the LNG plants.
 
 Oxygen
The figure of 0.1 mol% was adopted rather than zero, which in practice is the oxygen content of natural gas, to overcome measurement difficulties and false readings
which can occur when endeavouring to determine a zero concentration. This 0.1 mol% limit was sufficiently low to cause air ballasting by gas sellers to be
generally uneconomic. However, as the molecular sieve purification units at LNG plants can be damaged by small oxygen concentrations, the current NTS (IOYS)
specification has been altered to quote a preferred limit of 10 ppm oxygen; however, it should be noted that the GS(M)R limit is 0.2 mol%.
 

  Temperature
Too low a delivery temperature may lead to freezing of the soil around a buried pipeline and damage to other pipelines and services can result. Conversely, a temperature too high can be injurious to pipeline wrappings and coatings. The temperature range of the transmission specification reflects these requirements.

  Natural Gas Interchangeability
When formulating a gas specification, it is essential to take account of how the gas will burn on domestic appliances and the compatibility of the gas and the range of appliances. The international gas union (IGU) recognized the need to categorise gases according to their properties . Within each group, gases that have Wobbe numbers within +5 % and −5 % of the reference gas Wobbe number
should be fully interchangeable, i.e. burn safely, cleanly and efficiently without a need for appliance adjustment.
This 5 % guideline holds good while gases have compositions not markedly different from the Reference gas. However, BG has found that for Group H gas the methane content of the gas must not be <80–85 % for the Wobbe index alone to be a reliable guide to combustion characteristics. Also, a normal distribution limit of +3 % of reference Wobbe number was adopted. However, this aspect is now covered by other standards such as the GS(M)R.

 Natural Gas Composition Uncertainties
When a gas company such as British Gas negotiates the purchase of a new gas supply, it is necessary to establish “how much and what it is”, i.e. quantity and quality. Aspects that require clarification include the following:-
 1. Is the composition the same for the entire gas field?
 2.How reliable is the given composition? What variations are expected?
 3. Are any other natural gas supplies possible? If so, when? What is known about the composition?
 4. What processing is the seller intending? In particular hydrocarbon dew point.
 5. What requirements for gas composition does the seller have?
 6. What restriction on gas composition is placed by the seller’s transmission and compression requirements?
 7. Which pressure does the seller intend for the delivery
 8. Can the supply be guaranteed for 365 days/year?
 9. Can maintenance schedules of the seller influence the availability of gas?
 10. What plans does the seller have for supplying gas to other markets? What requirements do these markets place on gas composition?
 11. If hydrogen sulphide content is quoted as zero is the statement reliable? How, and for how long, was hydrogen sulphide tested for?
 12. What is the total sulphur content of the gas?
Answers to these questions give the company confidence that the chemical and physical properties of the gas will conform to the transmission specification and gas quality statutory obligations.

 Classification of Gas Families
Natural gas and liquefied petroleum gases are two examples of fuel gases in large scale use in many countries. Gas industries have been in existence since the early 1800s often based on gases produced (manufactured) from fossil fuels such as coal and later petroleum-based feedstock.
The composition, physical and combustion properties such as relative density and CV of the gases were often very different depending on the feedstock, the process and the process conditions. Similarly, natural gases and liquefied petroleum gases also have a range of Wobbe numbers as a consequence of differences in composition and hence CV and relative density. Wobbe number is an important criterion in the interchange ability of gases and in burner design. The Wobbe reliable guide to combustion characteristics. Also, a normal distribution limit of
+3 % of reference Wobbe number was adopted. However, this aspect is now covered by other standards such as the GS(M)R.