1 CDC 09 (5661) P 2 
 
  
 
 
 
 
 
 
DRAFT TA NZANIA STANDARD 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Specification  for General Purpose Natural Gas        
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TANZANIA BUREAU  OF STANDARDS 
 
 
 
 
 
 
 
1st edition  
 
 
 
 
 
 
© TBS 2018 
 2 CDC 09 (5661) P 2 
 0. Foreword 
This Tanzania Draft Standard was prepared by the Gases P roducts  Technical Committee CDC 9  
under the supervision of Chemical s Standards Division al Committee, and it is in accordance with 
the procedures of the Bureau . It serves as a specification for general  purpose natural gas, with an 
emphasis on the safety of the gas for use in natural gas  appliances and equipment and for use as 
fuel in natural gas vehicles.    
The quality specification provided by this Standard defines the requirements for providing a  natural 
gas suitable for transportation and for general purp ose uses  and provides the range of  gas 
properties consistent with safe operation of the natural gas appliance population.  
Natural gas conforming to this Standard is suitable for transportation through pipelines;  however 
operational problems have been repor ted in transmission systems with sulfur  concentrations in 
natural gas below the 50 mg/m3 specified in this Standard. Research on  this issue is continuing. 
The outcome of the research will be taken into account in  subsequent review of the Standard.  
Certain limits defined in the specification may be temporarily departed from under some  
circumstances. The issue of such departures is not within the scope of this Standard and is  subject 
to and provided for under relevant gas sales contracts, legislation and/or  government guidelines.  
The Standard is not intended to apply to gas supplies where no ‘general purpose’ users are  
connected to the supply system, e.g., a dedicated supply to an industrial user.  
Gas consumers who have specific needs that are not provided for  by the specification would  need 
to nominate in appropriate commercial contracts with gas suppliers or retailers the gas  composition 
suitable for their particular use.  The term ‘informative’ has been used in this Standard and applies 
to appendices that are  for information and guidance only.  
0.1. In the preparation of this standard assistance  has been derived from : 
AS 4564 :2011 Specification for general purpose natural gas , published by Australian Standard 
Institute  
IGS-M-CH-033(1) :2014  Specification for Iranian Natural Gas Quality , published by National 
Iranian Gas Company  
ISO 13686:2005, Natural Gas – Quality Designation published by International Organization for 
Standardization  
NZS 5542:2008, Specification for Reticulated Natural Gas published by New Zealand 
Standards Body   
Natural Gas Composition and Specification based on real and designed data of Gas 
Processing Plant, GASCO a subsidiary of Tanzania Petroleum Development Corporation, and 
Songas Gas Processing Plan t, 2017  
0.2. For the purpose of deciding whether a particular requirement of this standard is complied with, 
the final value observed or calculated expressing the result(s) of a test or analysis shall, be rounded 
off in accordance with TZS 4 (see clause 2). The number of significant places retained in the 
rounded off value should be the same as that of the specified value in this standard.   
 
1. Scope  
1.1. This Draft Tanzania Standard prescribes requirements, methods of sampling and testing, 
calculation  and interchangeability indices of natural gas  for general pur pose use .  
1.2. The specification applies to natural gas  —  
a) from petroleum, landfill, biogas, coal seam and other sources where these sources provide 
gas for direct or blended supply on a commercial basis through supply systems serving 
general purpose customers ; 
b) transported and supplied to users for use in natural gas appliances and equipment 
complying with the relevant Standards. This includes natural gas -powered vehicles, natural 
gas compressors and refuelling  facilities.  
NOTE:  This specification is supported by informative Appendices that provide further information and guidance 
on the derivation of the requirements, additional issues to be considered in contracts and testing methods to 
verify compliance, see Appendices . 3 CDC 09 (5661) P 2 
 2. Normative references  
The f ollowing nor mative references are indispensable for the application of this standard. For dated 
references, only the edition citied applies. For undated references, the latest edition of the normative 
references (including amendments) applies  
CDC 09 (56 63)/ISO 15403 P3, Natural gas — Designation of the quality of natural gas as a 
compressed fuel for vehicles  
CDC 09 (56 75)/ISO 6326 -5 P3, Natural gas -- Determination of sulfur  compounds -- Part 5: 
Lingener combustion method  
CDC 09 (56 76)/ISO 6327 P 3, Gas analysis – Dete rmination of the water dew point of natural gas – 
Cooled surface condensation hygrometers  
CDC 09 (56 77)/ISO 6974 -3 P3, Natural gas  — Determination of composition with defined 
uncertainty by gas chromatography  – Part 3: Determination of hydrogen, helium, ox ygen, nitrogen, 
carbon dioxide and hydrocarbons up to C8 using two packed columns  
CDC 09 (56 78)/ISO 6976  P3, Natural gas  — Calculation of calorific values, density, relative density 
and Wobbe Index from composition  
CDC 09 (5679 )/ISO 10101  P3, Natural gas  — Determination of water by the Karl Fischer method  - 
Part 2: Titration procedure  
CDC 09 (56 80)/ISO 10101 P 3, Natural gas — Determination of water by the Karl Fischer method - 
Part 3: Coulometric procedure  
CDC 09 (5681 )/ISO 10715  P3, Natural Gas – Sampling Guidelines  
CDC 09 (5682)/ISO 23874:2006 P 3, Natural gas -- Gas chromatographic requirements for 
hydrocarbon dewpoint calculation  
CDC 09 (5683 )/ISO 19739:2004 P 3, Natural gas -  Determination of sulfur  compounds using gas 
chromatography   
TZS 4,  Rounding off  numerical values    
3. Terms and definitions  
For the purpose of this Standard the following terms and definitions apply:  
3.1. Heating Values  
3.1.1.  Higher heating value  
amount of energy in MJ/ Sm3 released when one cubic metre of dry gas, at standard  conditions, is 
compl etely burnt in air with the products of combustion brought to standard  conditions, with the water 
produced by combustion condensed to the liquid s tate 
Note : Higher heating value is also known as gross heating value or superior heating value  or gross calori fic 
value .  
3.1.2.  Lowe r heating value  
amount of energy in MJ/ Sm3 of heat which would be released by the complete combustion in air of 
a specified quantity of gas, in such a way that the pressure at which the reaction takes place 
remains constant, and all the pro ducts of combustion are returned to the same specified 
temperature as that of the reactants, all of these products being in the gaseou s state  
Note: Lower heating value is also known as ‘net heating value’ or ‘inferior heating value’  or ‘net calorific 
value ’. The difference between 'higher heating value' and 'lower heating value' is in the latent heat  “thermal 
heat”  of condensation of water vapor produced during combustion.  
 
3.2. Inert gas  
any material that exists predominantly in a gaseous state at standard cond itions and  which does not 
contribute to energy release when the gas burns ; includes but is not  limited to carbon dioxide, 
nitrogen, oxygen and noble gases   4 CDC 09 (5661) P 2 
 3.3. Limit  
value beyond which the specified characteristic or conce ntration of the component shall not be  
permitted to vary  
3.4. Maximum allowable operating pressure (MAOP)  
maximum pressure at w hich a pipeline may be operated  
3.5. Natural gas  
gaseous fuel consisting of a mixture of hydrocarbons of the alkane series , primarily  methane but , 
which may also include ethane,  propane and higher hydrocarbons in much  smaller amount that 
remains in the gaseous state under the  temperature and pressure condi tions normally found in 
service; i t may also include some inert gases plus minor amounts of other  constituents including 
odori zing agents   
3.6. Gas quality  
quality of a natural gas is defined by its composition and the physical  properties, which  are Calorific 
value, Wobbe Index, Density, Compression factor, Relative density  and Dew points  
3.7. Density  
mass of a gas divided by its volume at  the specified pressure and temperature  
3.8. Relative density “specific gravity”  
mass of dry natural gas divided by the mass of an  equal volume of dry air, both at the same the 
specified pressure and temperature (see  ISO 6976)  
3.9. Standard conditions  
temperature “T s” of 15  °C and an absolute pressure “Ps” of 101.325 kPa Standard Pressure  
3.10. Normal conditions  
conditions are referred to as normal conditions a nd denoted by the subscript “n”, where Normal 
Pressure “Pn” is 101.325 kPa and Normal Temperature “ Tn” is 0 ºC = 273.15 K  
3.11. Total sulfur  
sulfur  from all sources i ncluding odo urization of the gas  
3.12. Wobbe Index  
number expressed in MJ/m3 produced when the higher heating value of the gas is divided  by the 
square root of the re lative density of that same gas  
3.13. Compression Facto r “Z-factor”  
quotient of the volume of an arbitrary mass of gas, at a specified pressure and temperature, and that 
of the same gas under the same conditions as calculated from the ideal gas law the terms 
compressibility factor and Z -factor are synonymous w ith compr ession factor (see ISO 12213 -1) 
3.14. Cricondenbar  
highest pressure at which liquid and vapo ur phased can exist at equilibr ium in a multi component 
system  
3.15. Cricondentherm  
highest temperature at which liquid and vapo ur phased can exist at equilibrium in a  multi component 
system  
3.16. Critical temperature  
maximum temperature for a pure component at which the component can exist as a liquid a bove 
which the fluid is a gas and cannot be liquefied regardless of the pressure applied  5 CDC 09 (5661) P 2 
 3.17. Critical pressure  
vapo ur pressure o f a substa nce at its critical temperature  
3.18. Critical density  
density of a substance at its critical te mperature and critical pressure  
3.19. Retrograde region  
area inside phase envelope where condensation of liquid occurs by lowering  pressure or increasing 
temperat ure 
3.20. Retrograde condensation or vapourization  
conde nsation or vapo urization that i s the reverse of usual behaviour;  condensation  caused by a 
decrease in pressure or increase in temperature  while v apourization caused  by an increase in 
pressure or decrease in  temperat ure can only occur in mixtures  
3.21. Dehydration  
act or process of remov ing water from gases or liquids  
3.22. Hydrate  
solid formed by the physical combination of water molecules and  certain of the molecules contained 
in gas  that looks solid like true ice but possess  different characteristics   
3.23. Dew Point  
temperature at any given pressure, or pressure at any given temperature, at  which liquid initiall y 
condenses from a gas or vapour ; it is specifically applied to the  temperature at which water vapour  
starts to co ndense from a gas mixture (water dew  point), or at which hydrocarbons start to c ondense 
(hydrocarbon dew point)  
3.24. Water Dew Point  
temperature above which no condensation of water occurs  at a specified pressure ; for any pressure 
lower than the specified press ure there is no condensation of water  vapo ur at this temperature (see 
ISO 6327)  
3.25. Hydrocarbon Dew Point  
temperature above which no condensation of  hydrocarbons occurs at a specified pressure at a given 
dew point, there is a pressure  range within which conden sation occurs except  
3.26. Total Sulfur  
total amount  of sulfur  found in natural gas  
Note: The total amount of sulfur , both organic and inorganic i.e. hydrogen sulfide , sulfur  oxides, 
Mercaptan , carbonyl sulfide s, …, may be determined by an analytical method not differentiating 
between individual sulfur  compounds combustion methods.  
3.27. Molar composition  
The molar composition of a gas is the term used when the proportion of each  component is 
expressed as a molar (or mole) fraction, or molar (mole) percentage of the wh ole thus the mole 
fraction, xi, of component i is the quotient of the number of moles of component i and the number of 
moles of the whole mixture present in the same arbitrary volume. One mole of any chemical species 
is the amount of substance which has th e relative molecular mass in grams. A table of 
recommended values of relative molecular masses is given in ISO 6976. For an ideal gas, the mole 
fraction (or percentage) is identical to the volume fraction (percentage), but this relationship cannot 
in gener al be assumed to apply to real gas behaviour . 
 6 CDC 09 (5661) P 2 
 3.28. Gas composition  
concentrations of the major and minor components and trace constitu ents in natural gas as analysed  
Note:  
 The major components of a gas are specified as volumetric, molecular or mass proportions  
in percentage. They also determine the allocation of the fuel gases to the gas families.  
 The minor components of a gas can be present as a gas, liquid or solid. The concentration 
is specified in mg/m3, cm3/m3 (also vppm), mg/kg (also ppm) for the majority  of gas 
secondary substances, or a specification can be used which relates to the behaviour  of the 
gas during transport.  
3.29. Interchangeability  
measure of the degree to which the combustion characteristics of one gas  resemble those of 
another gas  or ability of  a distributed natural gas to be substituted by another without the need for 
adjustm ent at the customers' equipment, thus t he appliances will continue to op erate safely and 
satisfactorily  
Note 1:  two gases are said to be interchangeable when one gas may be  substituted for the other 
without affecting the operation of gas burning appliances or equipment;   
Note  2: The criteria to be considered for interchangeability are as follows:  
Thermal input:  Flow of gas through an orifice at constant pressure, a function of Wobbe Index.  
Flash back:  The tendency for the flame to contract towards the port and for the  combustion to take 
place inside the burner.  
Lifting:  Burning surface expands to the point where burning ceases at the port and  burns above it.  
Yellow Tipping:  Incomplete combustion where excess hydrocarbons could, but does  not always, 
results in unacceptable levels of carbon monoxide. It may result in soot  deposition and a continuing 
deterioration of combustion.  
The substituted gas may be deemed to be interchange able when, without the need  for adjustment of 
the appliances, it provides a thermal input comparable with that  provided by the gas previously 
distributed, without the occurrence of flash back,  lifting or yellow tipping.  For the examination of the 
interchan geability there are two routes which can be  followed, namely:  
Wobbe index or gas composition based prediction methods.   
3.30. Odo urization  
addition of  an odo urant to the natural gas fed into the distrib ution system for safety reasons , thus 
permits the detection of the gas by smell at very low concentrations  
Note:  It is a common requirement that natural gas in air be readily detectable by smell at a  concentration of 
20% of the lower flammability limit (LFL). The LFL of natural gas is normally  taken as volume conte nt in air of 
4% to 5%.  Natural gas is normally odourless thus addition of odorant is necessary for detection purpose.  
3.31. Water Content  
mass concentration of the total amount of water  contained in a gas  
Note 1:  Water content is expressed in grams of water per cubic meter  of gas  
Note 2:  For raw gas, this means water in the forms of both liquid and vapour , but for pipeline gas this means 
only water vapour . 
 7 CDC 09 (5661) P 2 
 4. Requirements  
4.1. General requirements  
4.1.1.  The gas shall be technical  free from: 
4.1.1.1.  materials , dust, and other solid or liquid matter, waxes, gums, gum forming constituents, and 
unsaturated or aromatic hydrocarbons to an extent which might cause damage to, or interfere with 
the proper operation of, pipes, meters, regulators, control systems, equipment or appliances;  
Note : Technically free means that there are no visible traces of the components mentioned under actual conditions 
and they are sufficiently removed to ensure the operation of gas appliances and technical gas equipment of standard 
or normal construction.  
4.1.1.2.  water and unsaturated or aromatic hydrocarbons to an extent which causes unacceptable 
sooting;  
4.1.1.3.  other  gases that could adversely affect the transportation or utilization of the gas . 
4.1.1.4.  other substances such as solid particulate substances to the extent that they cau se damage 
to, or problems in operation of, pipelines or appliances or that cause the products of combustion to 
be toxic, or hazardous to health, other than substances that are usually found in natural gas 
combustion products.  Solid particle size of natural  gas should meet equipment manufacturer 
recommendations.   
4.1.2.  Odourizing of natural gas where applicable is provisional , if required by contract terms  or 
regulatory requirements  shall not lead to the  deviation  of the natural gas properties from the  limits 
spec ified in this standard . 
4.2. Specific requirements  
Natural gas shall also comply with specific requirements in Table 1 when tested according to the 
methods prescribed in the Table . 
Table 1 – Specific r equiremen ts for general purpose natural gas  
S/ No.  Characteristic  Requirement  Test method  
(i)  Wobbe Index, MJ/m3. 46.05 - 52.34  
ISO 6976  (ii)  Heating value, MJ/Sm3  
Higher heating value, max.  
Lower heating value, min.   
43.96  
33.95 
(iii)  Relative density  0.55 – 0.70 
(iv)  Oxygen , mol % , max.  0.2 ISO 6974 -3 
(v)  Hydrogen sulfide , mg/m3, max.  5.7 
ISO 6326 – 3 
(vi)  Mercaptan  sulfur  a), ppm.   2 – 6  
(vii)  Total sulfur , mg/m3, max.  50 ISO 6326 – 5  
(viii)  Water content, mg/m3, max.  100.0 ISO 10101  
ISO 6327 b) 
(ix)  Hydrocarbon dew  point  at 7000 kPa gauge , °C, max . -12  ISO 23874  
(x)  Methane Number  c) 115 - 130 ISO 15403  
(xi)  Water  dew point  at 70 00 KPa , °C, max.   -15 ISO 6327  
Note:  
a) The prescribed requirement is intended quality for natural gas conveyed in a distribution pipeline  which can be exempted  in 
transmission pipeline  
b) The method relies on calculation of water content based on the dew point  measurement  
c) Methane  Number is the additional property measure d only when the gas is intended to be used in vehicle; the figure indicates 
the anti -knocking c apacity of natural gas resulting from its application to vehicles, its limits being subject to a comparison with the 
octane rating of gasoline. Anti -knocking power is the capacity of the fuel to resist knocking in vehicles at reigning temperature 
and press ure levels in the engine combustion chamber, caused by compression to which the air/fuel mixture is submitted.  
5. Sampling  
The sampling shall be carried out in accordance with CDC 09 (5681) .  8 CDC 09 (5661) P 2 
 Annex A  
(Informative)  
Derivation of Specification  
A.1 Background  
Any known composition of Tanzanian  natural gas so f ar, can be regarded as ordinary natural gas.  
In order to retain the greatest flexibility of supply no attempt has been made to detail the  
composition of the gas beyond those compounds normally regarded as g as contaminants and  
which have a detrimental effect on the properties of the gas  because the chemical composition of 
natural gas depends on reservoir behavior and controlling processes that are different in gas 
treating plants . Most of the specification li mits in this document are presented in details with studies 
approach and investigations results.  Specification limits  in this standard  are generally consistent 
with the global practice since apart  of having abundant reserve of natural gas in our country fo r 
domestic consumption  also there is huge potential for export market which might also trigger 
importation of the same . Thus, this standard has been prepared taking into consideration the 
country’s natural gas profile while harmonizing with the global prac tices and demand . In this 
respect, the specification is intended to cover only those gases that are  primarily methane. This 
specification does not cover other gases such as LPG/air mixtures  and synthetic natural gas.  
A.2 Limits  
In any natural gas specifica tion, a balance must be achieved between optimum performance  which 
requires the narrowest possible combustion limits, and cost of supply that, if  possible, requires no 
limits at all so that any available gas can be used. In an endeavor to  resolve this issu e the limits are 
set as wide as possible while still maintaining safe  combustion performance in gas burning 
appliances intended  for use in Tanzania . If the specification of gas deviates outside the limits , both 
safety and performance may be  compromised. Th is is dependent on the duration, and extent of the 
excursion.   
Although the rate of change of variables within the limits is not addressed in this specification,  it is a 
matter that may be addressed in gas supply contracts. It is relevant to the performanc e of 
combustion control systems particularly in lean burn and low NOx applications. In  addition, rapid 
changes in Wobbe Index, for example, may lead to customer complaints, as  there may be a 
noticeable change in performance with some appliances.   
A.3 Chara cteristics and Components  
A.3.1 Wobbe Index  
Wobbe Index is a measure of the heat input to gas appliances  that reflects a measure of the energy 
input rate to a burner at constant supply pressure , and also relates to the combustion 
characteristics of the bu rner. A change in the  Wobbe Index of the gas will result in a proportional 
change in the energy output of all gas  appliances and equipment supplied, and in the energy 
carrying capacity of gas pipelines and  distribution networks.  Fuel gases of various  compo sitions 
with the same Wobbe Index produce a lmost the same heat load at the burner under the same 
pressure (flow pressure) , (see ISO 6976) . 
The permissible range of Wobbe Index is the range over which the present Tanzanian  population of 
gas appliances and e quipment can be expected to operate safely.  Appliances adjusted on the 
reference gas, at the normal pressure, and judged to  perform satisfactorily with the limit gases at 
the test pressures, are approved for use  within all natural gas resources. In this ap proach , the 
Wobbe Index is the primary gas  parameter.  
The Wobbe Index of a gas is an indicator of its combustion acceptability for a given  population of 
appliances and is the most significant parameter in terms of gas  combustion safety ; it is  derived 
from the orifice flow equation .  9 CDC 09 (5661) P 2 
 The Wobbe Index is calculated by dividing the higher heating  value (or gross calorific value) by the 
square root of the relative density of the gas as shown in equation below:  
𝑊𝑜𝑏𝑏𝑒  𝑁𝑜= 𝐺𝑟𝑜𝑠𝑠  𝐶𝑎𝑙𝑜𝑟𝑖𝑓𝑖𝑐  𝑉𝑎𝑙𝑢𝑒  𝑖𝑛 𝐵𝑇𝑈 /𝑆𝐶𝐹
√𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐  𝐺𝑟𝑎𝑣𝑖𝑡𝑦  (𝑟𝑒𝑙𝑎𝑡𝑖𝑣𝑒  𝑑𝑒𝑛𝑠𝑖𝑡𝑦 ) 
In all gas appliances the flow of gas is regulated by making it flow through a hole or  orifice of given 
size at a constant pressure.  The higher the Wobbe Index of a gas, the  greater the heating value of 
the quantity of gas that wi ll flow through an orifice of a given size in a given amount of time.  
The Wobbe Index is therefore an indicator of the rate of heat fl ow from the burner.  Too high a 
Wobbe Index can result in the over -heating of appliances, or sooting and  carbon monoxide (CO) 
formation due to incomplete combustion and consequently  present increased risk of deaths by 
poisoning. Too low a Wobbe Index may ca use flame abnormality  that can result in  flame lift, 
becoming unstable or even extinguishing of flames, resulting in the release  of unburned flammable 
gas. With respect to gas -fired power generating plant, Wobbe  Index is a critical factor for generator 
combustion dynamics.  
A.3.1.1 A survey on combustion performance on domestic gas appliances   
Under the UK Gas Quality Programme , during 2005 a major test exercise was performed on behalf 
of the UK Government Department BERR to examine the variation in combusti on performance of a 
range of representative domestic gas appliances working at the extremes of the EASEE -gas 
Wobbe Index limits and beyond. Some 25 appliances were tested representing about 70% of the  
UK gas appliance population.  
The following data was co llected from the  appliances operating under different gas qualities:   
 CO emissions;  
 CO 2 emissions;  
 CO/ CO 2 ratio;  
 NOx emissions;  
 Air Factor from CO 2 and O 2 measurements in flue gas;  
 Surface temperature of appliance at three locations (minimum);  
 Particulate  matter measurements in flue gas;  
 Flue gas temperature;  
 Derive thermal efficiency of appliance (flue loss method);  
 Record any changes in the performance of safety devices;  
 Record any changes in ignition characteristics; and  
 Fuel leakage measurement from ap pliance exhaust.  
Figure  A.1 reveals test results of the study for a wall -hung condensing boiler.  10 CDC 09 (5661) P 2 
  
Figure A.1 Plot of Wobbe Index Versus amount of Carbon monoxide release during 
combustion of the natural gas in the domestic app liances  
The conclusions from this work were:  
 Tests within Wobbe Index 47.2 – 51.4 MJ/m3 (GS(M)R) resulted in acceptable 
performance;  
 Exponential increase in CO emissions are seen at Wobbe Index beyond 53 MJ/m3; and  
 There is a potential for badly maintaine d appliances to produce CO levels that may cause 
risk to consumer safety.  
Generally,  the test gases within the range of Wobbe Numbers from 45 to 52 MJ/m3 resulted in 
acceptable appliance operability compared with current standards for G20  (pure methane) 
reference gas. Increasing the Wobbe Number of the gas used will  almost certainly increase the 
emission level of carbon monoxide. Whilst the levels  produced from the appliances tested in this 
program were on the whole modest,  except for Wobbe Number gases >53  MJ/m3, there is the 
potential for badly  maintained or installed appliances to produce CO levels that may cause an  
increasing overall health risk.  
A.3.2 Heating Value   
A maximum higher heating value of 43.96 MJ/Sm3 (10500 kcal/Sm3) is required by this 
specification. Gases with higher heating values greater than this limit have an unacceptably higher 
propensity for incomplete combustion and sooting, which would not necessarily be detected by 
tests using existing appliance standard limit gases.  
Lower heating  value refers to q uantity of heat released in the form of heat, on complete combustion 
of a defined quantity of gas with air,  at a constant pressure and with all combustion products 
returning to the original temperature of the reagents.  All products, inclu ding the water formed during 
combustion, are in a gaseous state.  
11 CDC 09 (5661) P 2 
 The above -mentioned pressure and temperature must be specified for both higher and lower 
heating values, which differ by the heat of condensation of water formed by combustion, can be 
specifi ed on a molar, mass or volumetric basis. For the volumetric basis, the pressure and 
temperature shall be stated at standard reference conditions calorific values can also be stated as 
dry or wet, depending on the water vapor content of the gas prior to com bustion. Normally, the 
heating value is expressed as the higher heating value, dry value specified on a volumetric basis 
under standard reference conditions.  
 
A.3.3 Relative Density   
Relative density is limited to a maximum of 0.70 in order to control risk  of sooting.  
The basis for selection of this value is as follows:  
 Sooting is associated with the presence of higher hydrocarbons and relative density 
correlates well with higher hydrocarbon content.  
 Studies carried out by Marcogaz working group to correlat e UK parameters with relative 
density indicate that sooting limit considerations are not significant below a relative density 
of 0.70.  
 Since the comportment of the natural gas depends on reservoir behavior, in case the 
country will have some wells with hav ing gases with relative density greater than 0.70 
though will be out of this specification, it is assumed arrangement will be deployed to 
ensure will be consumed locally however the guarantee for cross -border trade  is not certain 
due to the global requirem ents. Moreover, further technical consultation need to be 
encouraged  within the gas industry  with the purpose of confirming such recommended . 
A.3.4 Oxygen   
Oxygen is generally not present in significant quantities in producing gas fields. Its  presence in 
natural gas is usually attributed either to contamination during  transportation, processing, storage 
and distribution, or to air ballasting as a means of  moderating the Wobbe Index. National limits vary 
widely across the world, reflecting  different standard s, custom and practice.  
Oxygen in the presence of water vapo ur can cause corrosion within pipelines, and at  very high 
levels has the potential for forming explosive mixtures.  The contribution made by the presence of 
trace levels of oxygen in the gas supply  to the combustion process is negligible. Rather, the air -fuel 
ratio during combustion,  that determines most of the associated emissions behavio ur, is governed 
by primary  and secondary air entrainment as the gas leaves an appliance injector nozzle. The  
contribution made by oxygen in the gas supply to the fuel efficiency of gas appliances  in the 
combustion process is also believed to be negligible.  Corrosion products from oxidation in steel 
pipelines have the effect of stripping the  injected odorant from gas . 
An oxygen concentration of no more than 0. 2 mol% has been set in Australia while in New Zealand 
0.1 mol% is required as stipulated in the New Zealand  Specification (NZS 5542), for the preferred 
limit, but this is permitted to rise to 1.0%  mole in the abs olute limit provided the gas is not to be 
transported through steel  pipeline at high and intermediate pressure.  
A maximum oxygen concentration of 0.2 mol% is  preferred by this specification . 
A.3.5 Hydrogen sulfide   
Hydrogen sulfide  is a colou rless  flammable gas which has an offensive odour  similar to  rotten eggs. 
It is extremely poisonous. Unfortunately the human system adapts  readily to H 2S odour  so that 
increasing concentrations may not be noticed. High  concentrations of H 2S result in rapid poisoning 
and death. In many cases it is  necessary from a health and safety standpoint.  12 CDC 09 (5661) P 2 
 Hydrogen sulfide  is a corrosive gas and its corrosion effect on pipelines and copper  components in 
gas installations tends to be cumulative. H 2S is also a stress corros ion cracking agent for carbon 
steel pipeline and the likelihood of creating stress cracks is  dependent on the partial pressure of the 
hydrogen sulfide  in as much that at higher  gas pressure, thus, it attracts a lower level of H 2S to be 
specified.  
In the pr esence of water, hydrogen sulfide  can cause hydrogen induced cracking and  sulfide  stress 
cracking in high tensile steels at high pressure. This is of particular  importance for transportation of 
gas in steel pipes and for the use of natural gas in  vehicles where natural gas is typically stored at 
pressure up to 26 MPa. By  dehydrating, corrosion by H 2S is effectively eliminated. Similarly the 
potential for  corrosion acid gas can also be controlled by dehydration.  
Hydrogen sulfide  has also been associated with  chemical attack on copper and its  alloys leading to 
the formation of copper sulfide , which can cause malfunctions in  appliances, including the blockage 
of pilot jets.  
In general the limitation of hydrogen sulfide  in natural gas is important because of the  gas quality 
specifications, the corrosive nature of H 2S on pipeline materials and the  effects of hydrogen sulfide  
on utilization equipment. Hydrogen sulfide  removal is  desirable to reduce its detrimental effects.  
A.3.6 Total sulfur  
Many sources of natural  gas contain varying amounts and types of sulfur  compounds,  which are 
odorous , corrosive to equipment and can inhibit or destroy  catalyst in gas processing. Their  
accurate amount is essential to gas processing,  operation and utilization.   
This parameter is  the quantity of total sulfur  contained in the sulfur  containing compounds  that are 
present in natural gas. The limitation is related to air pollution and indoor air  quality control. 
Furthermore the sulfur  content of gases adversely affects the life of  pipelines and consumer 
appliances, it is therefore limited for all fuel gases.  
The sulfur  content of natural gas, used for fuel proposes contributes to SO X emissions and can lead 
to corrosion in appliances, engine and exhaust systems.  Sulfur  compounds in the gas may also 
lead to the deposition of elemental sulfur  in pipelines, which also have a corrosive effect particularly 
in the presence of water.  The sulfur  compounds may have a very strong and unpleasant smell and 
may lead to  nuisance leak reports. Some pro cess catalysts used in petroleum and chemical  
refining can be poisoned by sulfur  bearing materials in the feed stocks. Additionally,  certain 
industrial processes, e.g. glass manufacturers, may be adversely affected by  high sulfur  content in 
the gas stream.  
A maximum permitted sulfur  concentration of 50 mg/m3 includes sulfur  from all sources  including 
odorization of the gas. Odorants commonly used in worldwide  contribute to the  sulfur  level in the 
gas. With the widespread use of flueless heating and increasi ng awareness of air quality it  is highly 
desirable that the sulfur  content of natural gas be kept as low as possible . Nevertheless, sulfur  
deposition as an operating issue in some transmission systems  has occurred in some systems 
even though sulfur  concent rations was below 50 mg/m3, thus, in future there might be a need for 
further research in the domain . 
A.3.7 Water  
Natural gas and associated condensate are often produced from the reservoir  saturated (in 
equilibrium) with water. Liquid phase water can caus e corrosion and the formation of gas hydrates 
and condensation of free water in transmission systems. In combination with hydrogen  sulfide  and 
carbon dioxide, water can also lead to stress corrosion cracking and  hydrogen embrittlement.   13 CDC 09 (5661) P 2 
 The stated limits a re designed to ensure that hydrate formation and excessive corrosion do  not 
occur in transmission systems.  
For applications such as natural gas vehicles requiring compression to higher pressure than  the 
maximum transmission pressure it may be necessary to use a gas dryer to remove  moisture from 
the gas to prevent liquid water or hydrate formation.  The temperature of the gas stream must be 
maintained at a level sufficiently above  the water dew point temperature to avoid liquid water 
formation in pipelines.  
This specification covers water content by nominating the dew  point at maximum  transmission 
pressures. Additionally, an absolute limit (mg/m3) is stated. Figure A .2 defines  the water content in 
mg/m3 producing a dew  point of 0  °C for a range of transmissio n pressures.  
 
Figure A.2 DEW  POINT AT ZERO DEGREES CELSIUS (Derived from ASTM D1142)  
When  the water content in gas is determined, the water dew point at any pressure is  obtained from 
Figures A.3 and A .4. Also ISO 18453 specifies a method to provide  users with a reliable 
mathematical relationship between water content and water dew  point when one of two is known.  
Dew point is the temperature at which the first drop of liquid occur s when gas undergoes cooling or 
compression. Liquids normally found are water,  hydrocarbonates or glycol, which have distinct dew 
points.  The most important safety requirement in respect to natural gas is the temperature of dew 
point in order to  avoid the formation of liquid. Water in a liquid state is the precursor of the 
formation  of corrosive  compounds by means of the combination of natural gas components, 
specifically CO 2 and H 2S. The  combination of corrosive agents and variable pressure during 
transport of the fuel may result in cracks in the  metal and cause obstructions in the gas system.  
Note:  Above mentioned figures are made available and have been extracted from Gas Conditioning and 
Processing Handbook – Volume 2, Campbell Petroleum Series, 1984.  
14 CDC 09 (5661) P 2 
 
 
 
Figure A.3 (a) Hydrocarbon – Water Behavior, Water content of Lean, Sweet Nat ural Gas  
 15 CDC 09 (5661) P 2 
 
 
 
Figure A.3 (b) Hydrocarbon – Water Behavior, Water content of Lean, Sweet Natural Gas  16 CDC 09 (5661) P 2 
 A.3.8 Hydrocarbon dew  point  
The hydrocarbon dew  point limit controls the level of heavy hydrocarbons (propane and  heavier) in 
the gas. At high pressures, con densation of heavy hydrocarbons may occur at  low temperatures.  
These compounds are of concern because at the high pressure used  in both gas transmission and 
CNG storage condensation of heavy hydrocarbons may  occur at low temperatures  
Additionally, retrogra de condensation can occur in transmission systems at pressures lower  than 
the MAOP. It is this phenomenon that effectively determines the maximum allowable  limits of heavy 
hydrocarbons in natural gas. The pressure and temperature specified are  designed to limit 
retrograde condensation for typical Tanzanian  natural gases (refer to  ISO 13686).   
The liquid hydrocarbon must be removed from natural gas  because of the following reasons:  
 The high amount of liquid hydrocarbons can cause pressure drop and decrease t he flow 
capacity of pipeline.   
 Liquid hydrocarbons can collect and form slugs which can travel along pipelines and 
potentially damage gas flow regulating and measurement equipment. Hydrocarbon liquids 
also cause odorant removal from the gas phase. The pres ence of both hydrocarbons and 
odorant in the liquid phase can cause degradation of the rubber components of pressure 
regulating installations.  
 A high hydrocarbon dew point may mean a low Wobbe Index, which may cause flame 
abnormality. It can also cause inc reased carbon monoxide formation in surface combustion 
burners. Too high of a Wobbe Index may give rise to high levels of carbon monoxide in the 
combustion products of conventional appliances. It can also promote overheating.  
 The amount of the higher satur ated and unsaturated as well as aromatic hydrocarbons in 
gases should be limited relative to the gas distribution and the burning behavior of the 
gases. The concentration permissible for a trouble -free combustion depends not only the 
type of hydrocarbons b ut also on the hydrogen and oxygen content of the gas. Carbon 
dioxide also favors the combustion of unsaturated and aromatic hydrocarbons in 
comparison to nitrogen, particularly in diffusion burners.  
Generally, for a multi component mixture of substances, the phase envelope is  similar to Figure 
A.4. This phase curve is very sensitive to composition, which is the  variable that reflects the 
thickness of diagram. In high pressure pipeline, liquid can be  prevented by removing enough of the 
heavy hydrocarbons an d to change the shape  and position of the phase curve. Consider line I -J, 
and the points I and J represents  inlet and outlet conditions of pipeline. When line I -J crosses the 
dew point curve,  liquid will begin to condense in the line. From that point on, t wo-phase flow exists. 
If the composition is changed enough, then the dashed phase curve would be the  outcome, in 
which case, no liquid will form in the pipeline because the dew point is  not reached. Any liquid 
present would be due to carryover of liquid fr om separator or  having a different composition from 
the one used to construct the phase curve.  
At high pressures, condensation of heavy hydrocarbons may occur at low  temperatures. The 
pressures and temperature specified do not guarantee the total  absence o f retrograde 
condensation. Additionally, retrograde condensation can occur  in transmission systems at pressure 
lower than the MAOP. It is this phenomenon that  effectively determines the maximum allowable 
limits of heavy hydrocarbons in natural  gas. The pre ssure specified approximates the 
cricondentherm pressure which not  typical in Iranian natural gases, and is usually within the range 
of 23 to 51 bar.  17 CDC 09 (5661) P 2 
  
Figure A.4 Phase diagram, a plot of Temperature Versus Pressure  
A.3.9 Total inert gases  
The specificati on for total inert gases is intended, in conjunction with the Wobbe Index  limits, to limit 
the levels of higher hydrocarbons.  
Carbon dioxide (CO 2) is classified as an "inert" along with other inert gases,  principally Nitrogen 
(N2) and to a lesser extent H elium (He) and Argon (Ar) that may  be present in the gas stream. Inert 
gases by themselves do not create a safety  hazard and the specification limit is a method of 
controlling the levels of non -methane  hydrocarbons (ethane, propane, butane, etc.) so as not  to 
exceed the specified  Wobbe Index limits. Manufacturing plants that use natural gas as a feedstock 
may suffer adverse impacts to their processes where there are high carbon dioxide or  nitrogen 
levels.  High levels of CO 2 in particular could  have signific ant implications for some gas consumers 
that have specific needs, however this  is a commercial issue and, where necessary, would be 
specified in contracts.   
A.3.10 Other combustion parameters  
Other combustion parameters e.g., sooting index, flame speed, li ft index were considered  but not 
deemed necessary for inclusion in the specification.  
A.3.11 Solid particles  
Causes contamination, obstruction and erosion of vehicle feed systems and industrial burner 
injector  openings. When natural gas is destined for us e as a turbine fuel, the solid particles can 
cause erosion of the  parts that circulate the hot gas.  
A.3.12 Solid particles  
Cause sharp alterations to flame temperature and in the gas turbine load, drawback of flame from 
pre-mixed  flames and can nuclear th e condensation of heavier natural gas fractions. When the 
presence of liquid is  identified in the gas to be employed in gas turbines, separators are used and 
the flow is heated in order to  vaporize the liquid phase .  
18 CDC 09 (5661) P 2 
 Annex B  
(Informative)  
Specification for  Natural Gas Components  
B.1 The chemical composition of natural gas depends on reservoir behavior and  generally for some 
of the bel ow components there is no specific controlling process in gas refineries. Hence, this 
chemical composition shall be considere d as typical and not exact , though is the actual figure 
obtained in Tanzania from Gas Processing Plant, GASCO a subsidiary of Tanzania Petroleum 
Development Corporation . Some of the methods which can be used for chemical analysis are:   
a) ASTM D 1945:2003, Test Method for Analysis of Natural Gas by Gas Chromatography  
b) GPA 2261:1999, Analysis for Natural Gas and Similar Gaseous Mixture by Gas 
Chromatography  
c) GPA 2286:1995, Tentative Method of Extended Analysis for Natural Gas and Similar 
Gaseous Mixture by Temper ature Programmed Gas Chromatography  
 
Table B.1 – Typical Specification for Natural Gas Component  
SN Components  Units  Composition  
1.  Methane (CH 4) Mole%  99.61922  
2.  Ethane (C 2H6) Mole%  0.00000  
3.  Propane (C 3H8) Mole%  0.13047  
4.  nButane(nC 4) Mole%  0.02107  
5.  iButane(iC 4) Mole%  0.01717  
6.  nPentane(nC 5) Mole%  0.00000  
7.  iPentane(iC 5) Mole%  0.00217  
8.  Hexane(C 6) Mole%  0.01384  
9.  Hexane and Higher(C 6++) Mole%  0.00142  
10.  Neopantane  Mole%  0.00000  
11.  Nitrogen(N 2) Mole%  0.00000  
12.  Carbondioxide(CO 2) Mole%  0.19607  
13.  Density (D)  Kg/m3 0.72279  
14.  Relative Density(RD)  - 0.55904  
15.  Low Heating Value MJ/m3 35.95012  
16.  High Heating Value MJ/m3 39.88833  
17.  Hydrocarbon Dew Point  Deg. C  -18.99  
18.  Water Dew Po int Deg. C  at 70 Bar  -15 
19.  Water Content    
 
Source: GASCO a subsidiary of Tanzania Pet roleum Development Corporation, 2017   19 CDC 09 (5661) P 2 
 Annex C  
(Informative)  
Interchangeability   
C.1 Interchangeability can be defined as the ability of a distributed natural gas to be  substituted by 
another without the need for adjustment at the customer's equipment.  The appliances will continue 
to operate safely and satisfactorily.   
The criteria to be considered for interchangeability are as follows:  
Thermal input: Flow of gas through an orifice at constant pressure, a function of  Wobbe Index.  
Flash back: The tendency for  the flame to contract towards the port and for the  combustion to take 
place inside the burner.  
Lifting: Burning surface expands to the point where burning ceases at the port and  burns above it.  
Yellow Tipping: Incomplete combustion where excess hydrocarbo ns could, but does  not always, 
results  in unacceptable levels of carbon monoxide. This may result in soot  deposition and a 
continuing deterioration of combustion.  
The substituted gas may be deemed to be interchangeable when, without the need  for adjustment  
of the appliances, it provides a thermal input comparable with the  provided by the gas previously 
distributed, without the occurrence of flash back,  lifting or yellow tipping.   
For the examination of the interchangeability there are two approaches: Wobbe Index  or gas 
composition  based prediction methods.  
The A.G.A. Interchangeability program, catalogue No. XH 8810 uses Index Method  based on 
A.G.A. Research Bulletin 36 "Interchangeability of other fuel gases with  natural gas" (1952, 2nd 
edition), the Weaver  Index Method, and the Knoy Constant  techniques to determine compatibility of 
gases. All methods involve an adjustment  gas and a substitute gas. The A.G.A. and Weaver Index 
methods require a complete  analysis of gas components, but the Knoy constant method  uses only 
the higher  heating value and relative density. Only the A.G.A. Index method is covered for this  
explanation.  
C.2 Equations to Determine Interchangeability Indices   
The possibility of interchangeability, to mix or substitute one type of gas with another  gas for any 
purposes (e.g. peak shaving), is determined by interchangeability indices.  The indices and limits for 
natural gases in this standard specification (high methane)  are given below.  
𝐼𝐹: Flash – back Index          𝐼𝐹= 𝑘𝑠 𝑓𝑠 √ℎ𝑠1000⁄
𝑘𝑎 𝑓𝑎 <1.20 
𝐼𝐿: Lifting Index                     𝐼𝐿= 𝑘𝑎
𝑓𝑎 𝑎𝑠
𝑓𝑠 𝑎𝑎 (𝑘𝑠−log 𝑓𝑎
𝑓𝑠 ) <1.60 
𝐼𝑌: Yellow  Index                  𝐼𝑌= 𝑓𝑠 𝑎𝑎 𝑦𝑎
𝑓𝑎 𝑎𝑠 𝑦𝑠 >0.8 
Where:  
A = air theoretically required for complete combustion,  m3 per m3 of gas (cu ft per cu ft of 
gas) 1 
a = 100 A/h – air theoretically required for complete combustion , m3 per 100 MJ of gas (cu ft 
per 100 BTU of gas)  
d = specific gravity of gas (air = 1.0)  20 CDC 09 (5661) P 2 
 E = total inerts in gas mixture, mole percent  
F = lifting constant1  
f = 1000 d 0.5 / h =primary air factor  
h = higher heating value of gas, MJ per m3 (BTU per cu ft)  
K = F/d = lifting limit constant  
O2 = Oxygen in gas mixture, mole percent  
T = yellow tip constant, m3 of air required to eliminate yellow tips per m3 of gas  (cu ft of air 
required to eliminate yellow tips per cu ft of gas)  – See Table C.1 
Y = 100 T/A + 7E – 26.3 O 2 = yellow tip limit, percent primary air  s and a = substitute gas 
and adjustment gas, respectively  
Use of these equations requires knowledge of the chemical composition, heating value and specific 
gravity of the gases.  
Table C.1 – Interchangeabili ty Constants1 
 
*Representative analysis of 3C2H4+1C 6H6. 
**Always negative. Subtract from total.  
Example:  
 
                                                           
1 Interchangeability of other fuel gases with natural gas, American Gas Association Research Bulletin 36, 1946  
21 CDC 09 (5661) P 2 
 Lifting Index I l = 0.941  
Flashback Index I f =1.034  
Yellow tip Index I y = 0.762  
Interchangeability in the A.G.A program is determined by calculating indices for  lifting, flash back, 
and yellow tipping (complete combustion), and by establishing  preferable and objectionable limits 
for each. The equations used for the individual  indices were derived experimentally by the Bulletin 
36 work. An exactly  interchangeable gas, that is, if the adjustment gas and the substitute gas were 
of the  same composition, would yield values of I l, If and I y equal to 1 .0. The preferable  limits are 
pegged at this value. The objectionable limits are values which just provide  satisfactory 
performance. Such Values are determined by testing a variety of  appliances, by setting them up on 
the adjustment gas, and by changing gas mixtures  until the three interchangeability criteria occur, 
namely, lifting, flashback and yellow  tipping.  
The other methods are used for calculating interchangeability Indices that you can  refer to ISO 
13686.  
  22 CDC 09 (5661) P 2 
 Annex D  
(Informative)  
Matters that may be considered in contracts  
The following gas quality matters that are outside the scope o f this gas specification  may be 
considered, if relevant, in gas transportation and supply contracts.  
a. The rate of change of critical parameters.  
b. Frequency of agreed tests for verification of compliance.  
c. Exchange of results of all monitoring/testing undertaken.  
d. Retention period for  records of the tests carried out to verify compliance.  
e. Excursions and their duration outside the absolute limits set out in Table 3.1.  
f. Response actions to be taken relating to excursions, emergencies, etc.  
g. Commissioning activities, such as hydrostatic tes ting and drying of pipelines and  initial odorant 
injection activities.  
NOTE:  This is not an exhaustive list.  
  23 CDC 09 (5661) P 2 
 Bibliography  
Gas Conditioning and Processing Handbook – Volume 2, Campbell Petroleum Series, 1984.  
ASTM D 1826:2003, Test Method for Calorific ( Heating) Value of Gases in Natural Gas Range by 
Continuous Recording Calorimeter  
ASTM D 3588:2003, Practice for Calculating Heat Value, Compressibility Factor, and Relative 
Density of Gaseous Fuels  
ASTM D 5287:2002, Practice for Automatic Sampling of Gaseo us Fuels  
The Commission for Energy Regulation, Northern Ireland Authority for Utility  Regulation 
(CER/09/037)  “Draft Final Report on the Requirements for a Single Natural Gas Quality  Standard 
for Northern Ireland and the Republic of Ireland ” 
CAMPBELL, "Gas  Conditioning and Processing"  Volume 2, Campbell Petroleum Series, 1984  
Katz, "Hand book of Natural gas engineering"  
Oil and Gas Production Handbook , An Introduction to Oil and Gas Production, Havard Devold  
Marcogaz WG “Gas Quality”  First Position Paper on  European Gas Quality Specifications, 2003  
Marcogaz WG “Gas Quality” , Second Position Paper on European Gas, Quality Specification for 
natural gas  Interchangeability, 2003  
Marcogaz WG “Gas Quality” , Third Position Paper on European Gas Quality Specificatio n for 
Natural Gas  Interchangeability, 2006  
Interchangeability and Wobbe Index Used as Quality Parameter for Liquefied  Natural Gas  Rafael 
Rosal 1 and Sabrina Di Scipio  
Natural Gas Composition and Specification based on real and designed data of Gas Processing 
Plant, GASCO a subsidiary of Tanzania Petroleum Development Corporation, 2017  
Interchangeability of Other Fuel Gases with Natural Gases , Research Bulletin Number 36 , 
American Gas Association Testing Laboratories, 1946