ISO/TR 11150:2007
(Main)Natural gas - Hydrocarbon dew point and hydrocarbon content
Natural gas - Hydrocarbon dew point and hydrocarbon content
ISO/TR 11150:2007 describes the various means of estimating hydrocarbon dew point and hydrocarbon content of natural gas.
Gaz naturel — Point de rosée d'hydrocarbure et teneur en hydrocarbure
General Information
Standards Content (Sample)
TECHNICAL ISO/TR
REPORT 11150
First edition
2007-12-15
Natural gas — Hydrocarbon dew point
and hydrocarbon content
Gaz naturel — Point de rosée d'hydrocarbure et teneur en hydrocarbure
Reference number
©
ISO 2007
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Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Background . 1
4 Why is hydrocarbon dew point important? . 2
5 What is the definition of hydrocarbon dew point?. 3
6 Specifications. 4
6.1 EASEE-gas European association for the streamlining of energy exchange — Gas . 4
6.2 United Kingdom Health and Safety Executive. 4
7 Hydrocarbon dew point measurement .4
7.1 General. 4
7.2 General sampling. 4
7.3 Hydrocarbon dew point determination/estimation/monitoring. 5
7.4 Physical methods . 5
7.5 Analysis and calculation. 8
7.6 Comparative studies. 10
8 Conclusions . 10
9 Recommendations. 11
Annex A (informative) Dew scope measurements (cold mirror) . 12
Annex B (informative) Chromatographic methods . 16
Annex C (informative) Potential hydrocarbon liquid content . 18
Bibliography . 19
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
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International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
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International Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that
which is normally published as an International Standard (“state of the art”, for example), it may decide by a
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longer valid or useful.
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ISO/TR 11150 was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 1, Analysis
of natural gas.
iv © ISO 2007 – All rights reserved
Introduction
With Resolution 6 at its Prague meeting in 2004, ISO/TC 193/SC 1, Analysis of natural gas, decided to publish
a Technical Report on guidance for various International Standards on hydrocarbon dew point and
hydrocarbon content.
The main purpose of this Technical Report is to explain to the wider gas community the complex issues
behind the natural gas property called hydrocarbon dew point on the application of various International
Standards on these subjects.
TECHNICAL REPORT ISO/TR 11150:2007(E)
Natural gas — Hydrocarbon dew point and hydrocarbon
content
1 Scope
This Technical Report describes the various means of estimating hydrocarbon dew point and hydrocarbon
content of natural gas.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 6327, Gas analysis — Determination of the water dew point of natural gas — Cooled surface
condensation hygrometers
ISO 6570:2001, Natural gas — Determination of potential hydrocarbon liquid content — Gravimetric methods
ISO 6974 (all parts), Natural gas — Determination of composition with defined uncertainty by gas
chromatography
ISO 7504:2001, Gas analysis — Vocabulary
ISO 10715:1997, Natural gas — Sampling guidelines
ISO 14532:2001, Natural gas — Vocabulary
Technical Corrigendum:2002
ISO 23874, Natural gas — Gas chromatographic requirements for hydrocarbon dewpoint calculation
3 Background
Hydrocarbon dew point is often a requirement of gas quality specifications in sales contracts where gas is
traded or crosses international borders. It can also be quoted in health and safety legislation. It is usually
specified as a temperature at a defined pressure or over a range of pressures above which no hydrocarbon
condensation will occur. It may alternatively be expressed as a maximum amount of hydrocarbon liquid which
may condense under specific pressure and temperature conditions.
Under certain conditions, higher hydrocarbons present in natural gas or similar gases may condense and the
condensate formed can cause difficulties in the operation of gas transport and distribution systems. Phase
behaviour in hydrocarbon mixtures such as natural gas is highly non-ideal. More ideal behaviour, such as that
of water in air, or, indeed, in natural gas, gives a dew point temperature which continually increases with
pressure. Retrograde behaviour, which affects hydrocarbon mixtures, produces dew point temperatures which
have a maximum value at an intermediate pressure. Figure 1 shows a typical phase diagram.
Key
X temperature in degrees centigrade
Y pressure in bar
1 dense phase
2 gas only
3 gas + liquid
4 dewline
5 critical point
6 cricondentherm
7 cricondenbar
Figure 1 — Natural gas phase diagram
In Figure 1 the dew line is the phase boundary. To the right of and above this line only single-phase gas exists.
Within the curve of the dew line, both gas and liquid phases are present, in proportions which are governed by
the pressure and temperature. The closer to the line, the smaller is the proportion of liquid. The
cricondentherm is the point of maximum dew point temperature. Above this temperature only single-phase
gas exists, whatever the pressure. Similarly, at pressures above the cricondenbar, only single phase or dense
phase gas exists, whatever the temperature.
A gas with a dew line similar to that in Figure 1 would be stable single-phase at − 5 °C and 70 bar. If its
pressure is reduced isothermally, it would remain single phase to about 55 bar, at which point it would
encounter the phase boundary and condensate would start to separate. As pressure is further reduced,
varying ratios of gas and condensate will be found until about 15 bar, when it returns to single phase for the
remainder of its depressurization. In fact, once condensate has separated, it is unlikely that it will instantly
return to the gas phase, and so liquids may continue to be present at lower pressures. Another consideration
is that pressure reduction without the addition of heat is isenthalpic rather than isothermal, and
Joule-Thomson cooling will cause the two-phase region to be encountered earlier, unless the gas has been
pre-heated so that the cooling curve stays in the single phase region.
4 Why is hydrocarbon dew point important?
Avoidance of condensate formation is important for pipeline operations. The presence of condensate can
cause problems with filters and with measurement and control instrumentation. There is also an issue with gas
turbines in power plants – significant damage can be caused by the presence of condensate in the burners.
A measured dew point temperature which is lower than the specification limit should give assurance that
condensation will not occur. For certain needs, such as plant design, or if it is known that a certain amount of
condensable material can remain after treatment, knowledge of the quantity of condensable hydrocarbons
produced at specified conditions is useful. This can be physically measured using ISO 6570.
2 © ISO 2007 – All rights reserved
Operators require confidence in the measured result. At present, there is little or no information on the
maximum permissible error of measurement, from any type of measurement which is strongly dependent on
the type and nature of the natural gas. The single determined value as measured is accepted as the basis for
decision making. However, for some natural gases, even a small decrease in temperature can result in a large
liquid drop out.
5 What is the definition of hydrocarbon dew point?
The following definitions can be found in various ISO documents, such as in ISO 14532.
2.6.5 Dew points
2.6.5.2 Hydrocarbon dew point
2.6.5.2.1 Hydrocarbon dew point
temperature above which no condensation of hydrocarbons occurs at a specified pressure
NOTE 1 At a given dew point temperature there is a pressure range within which retrograde condensation can
occur.
The cricondentherm defines the maximum temperature at which this condensation can occur.
NOTE 2 The dew point line is the locus of points for pressure and temperature which separates the single phase
gas from the biphasic gas-liquid region.
2.6.5.2.2 Retrograde condensation
phenomenon associated with the non-ideal behaviour of a hydrocarbon mixture in the critical region
wherein, at constant temperature, the vapour phase in contact with the liquid may be condensed by a
decrease in pressure; or at constant pressure, the vapour is condensed by an increase in temperature
NOTE Retrograde condensation of natural gas is the formation of liquid when gas is heated or pressure is
reduced.
2.6.5.2.3 Potential hydrocarbon liquid content (PHLC)
amount of liquid potentially condensable per unit volume of gas at a given temperature and pressure
Or alternatively in ISO 7504.
3.5.2 Dew point
at a specified pressure, the temperature at, or below which, condensation from the gas phase will occur
3.1 Equation of State
mathematical relationship between the state variables (pressure and temperature) of a gas or gas mixture,
and the volume occupied by a given amount of s
...
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