ISO/TR 12148:2009

Natural gas - Calibration of chilled mirror type instruments for hydrocarbon dewpoint (liquid formation)

ISO/TR 12148:2009 describes the principles of, and general requirements for, the traceable calibration of automatic hydrocarbon-dew-point chilled-mirror instruments using the indirect automatic weighing method (method B) described in ISO 6570:2001 to determine the potential hydrocarbon liquid content of natural gas, or similar gas. The calibration procedure is intended for use by chilled-mirror instruments in downstream applications transferring processed natural gas. If the gas composition is constant, the manual weighing method (method A) described in ISO 6570:2001 is also applicable. The application of this calibration procedure in the upstream area is not excluded a priori, however, currently there is no experience using this procedure in an upstream environment. The usability of data on the potential hydrocarbon liquid content of natural gas for verification, adjustment or calibration of hydrocarbon-dew-point chilled-mirror instruments is based on the condensation behaviour of natural gases. ISO/TR 12148:2009 provides information on the condensation behaviour of natural gases and the various measuring techniques to determine properties, like hydrocarbon dew point and potential hydrocarbon liquid content, related to the condensation behaviour of natural gases.

Gaz naturel — Étalonnage des instruments du type à miroir refroidi pour points de rosée hydrocarbures (formation de liquide)

General Information

• Status: Published
• Publication Date: 17-Feb-2009
• ICS: 75.060 - Natural gas
• Technical Committee: ISO/TC 193/SC 1 - Analysis of natural gas
• Drafting Committee: ISO/TC 193/SC 1 - Analysis of natural gas
• Current Stage: 9093 - International Standard confirmed
• Start Date: 21-Jun-2024
• Completion Date: 13-Dec-2025

ISO/TR 12148:2009 - Overview

ISO/TR 12148:2009 provides guidance for the traceable calibration of chilled‑mirror hydrocarbon‑dew‑point instruments used to assess the potential for hydrocarbon liquid formation in natural gas. The Technical Report links chilled‑mirror analysers to the gravimetric procedures of ISO 6570:2001 (potential hydrocarbon liquid content - PHLC) by describing principles, general requirements and practical considerations for on‑site calibration, primarily for downstream gas transfer applications.

Key topics and technical requirements

• Scope and intent: Calibration of automatic chilled‑mirror instruments using the indirect automatic weighing method (Method B) of ISO 6570:2001. If gas composition is stable, the manual weighing method (Method A) may also apply.
• PHLC definition: PHLC is the condensable liquid mass (mg) per normal cubic metre (0 °C, 101.325 kPa) at a specified pressure and temperature.
• Performance characteristics: Working principles of automatic weighing and chilled‑mirror devices, functional requirements and measurement uncertainty considerations.
• Practical calibration elements:
  • Process gas and sampling system requirements
  • Selection of PHLC reference value and compositional range covered by the calibration
  • Calibration interval and execution (Method A - trip point change; Method B - direct dew‑point adjustment)
• Limitations & error sources: The report documents major systematic error sources for chilled‑mirror devices (e.g., required condensate mass before detection, mirror temperature measurement, cooling rate, flow regime) and quantifies practical limits such as the ISO 6570 detection limit (5 mg/Nm3) and observed biases (estimates up to about −1.5 °C in some conditions).

Applications and users

• Who should use it: Metering engineers, gas quality laboratories, pipeline operators, instrument manufacturers, and calibration laboratories responsible for hydrocarbon‑dew‑point monitoring in gas transmission and distribution.
• Practical use cases:
  • On‑site calibration or verification of automatic chilled‑mirror dew‑point monitors
  • Adjusting instrument settings so reported dew point corresponds to a traceable PHLC value
  • Improving comparability of dew‑point data used for contractual custody transfer, pipeline specification compliance, and operational risk management

Related standards

• ISO 6570:2001 - Natural gas: Determination of potential hydrocarbon liquid content (gravimetric methods) - the normative reference for Methods A and B used as the calibration baseline.

ISO/TR 12148:2009 helps bridge instrument readings and gravimetric PHLC measurements, supporting traceable, gas‑specific calibration of chilled‑mirror hydrocarbon‑dew‑point monitors used in downstream natural gas operations.

Standards Content (Sample)

TECHNICAL ISO/TR
REPORT 12148
First edition
2009-02-15
Natural gas — Calibration of chilled
mirror type instruments for hydrocarbon
dewpoint (liquid formation)
Gaz naturel — Étalonnage des instruments du type à miroir refroidi pour
points de rosée hydrocarbures (formation de liquide)

Reference number
©
ISO 2009
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ii © ISO 2009 – All rights reserved

Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Symbols . 2
5 Performance characteristics of automatic weighing method in accordance with
ISO 6570:2001. 2
5.1 Working principle. 2
5.2 Functional requirements. 4
5.3 Measurement uncertainty . 5
6 Performance requirements for a chilled-mirror-type instrument for hydrocarbon-dew-
point determination . 6
6.1 Working principle. 6
6.2 Precision. 7
7 Requirements/Points of interest to carry out a calibration . 8
7.1 Process gas. 8
7.2 Sampling system . 8
7.3 Selection of the PHLC reference value. 9
7.4 Compositional range of gases covered by the calibration procedure. 10
7.5 Calibration interval . 10
8 Execution of calibration procedure . 10
8.1 General. 10
8.2 Method A — Change in trip point value. 11
8.3 Method B — Direct change of dew point value . 12
8.4 Which method should be used? . 12
Annex A (informative) Condensation behaviour of natural gas . 13
Annex B (informative) Example of a hydrocarbon-dew-point analysis . 16
Annex C (informative) Examples of a calibration. 19
Annex D (informative) Performance of different types of chilled-mirror instruments for
hydrocarbon dew point determination .29
Bibliography . 31

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
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
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
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
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
simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely
informative in nature and does not have to be reviewed until the data it provides are considered to be no
longer valid or useful.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 12148 was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 1, Analysis
of natural gas.
iv © ISO 2009 – All rights reserved

Introduction
Under certain conditions, higher hydrocarbons present in natural gas or similar gases can condense and the
hydrocarbon liquids formed can cause difficulties in the operation of gas transport and distribution systems.
Hydrocarbon dew-point measurements, by condensation on a mirror, can give an indication of the conditions
under which condensation starts. Theoretically, the hydrocarbon dew point is the temperature at which the first
small droplets of liquid are formed at a fixed pressure. In practice, all dew-point-measurement methods are
based on the observation of the formation of a film of hydrocarbon liquids on the surface of an illuminated,
cooled mirror. The observation can be done visually (manual mirror) or by an electronic sensor (automatic
chilled mirror). The cooling can be achieved in two ways: expansion of natural gas, compressed air or carbon
dioxide or by applying a Peltier-cooling device. Either manual or automatic chilled mirrors can be applied to
measure the hydrocarbon dew point of natural gas.
It is not possible to calibrate commercially available hydrocarbon dew point analysers in a traceable way,
because no hydrocarbon dew-point reference material or reference instrument is available. Because of
differences in working principles, analysers from different manufacturers can give different values for the
hydrocarbon dew point for a given gas. In practice, the dew point of an automatic dew point monitor is often
“tuned” to match the value measured by a manual chilled mirror, or “tuned” to the value calculated from a
known gas composition using a thermodynamic model.
Modern automatic hydrocarbon-dew-point chilled-mirror instruments have the possibility of adjusting the value
of the presented hydrocarbon dew point. Sometimes this adjustment is carried out by changing a physical
setting in the instrument itself; in other cases the setting can be changed by entering a new value into the
instrument’s controller. The availability of such an adjustable parameter is a prerequisite for using the
calibration procedure described in this Technical Report.
NOTE 1 Changing the setting of such a parameter can result in a major change in the presented hydrocarbon-dew-
[2]
point value. For example, a study carried out under the auspices of the National Physical Laboratory shows from real
measurements that the hydrocarbon dew point for a natural gas measured at the same pressure varies between −6,28 °C
and 8,54 °C [Gas B, with pressure equal to 3,5 MPa (35 bars), detector sensitivity varying between 110 mV and 275 mV].
[2]
In 2007, the results of a comprehensive study of the measurement of the hydrocarbon dew point of real and
synthetic natural gases was published. In this study, six analytical methods were examined: one automatic
chilled-mirror instrument, one manual chilled-mirror instrument, two laboratory gas chromatographs and two
process gas chromatographs. In this study, it is concluded that the role of accurate synthetic gas mixtures in
the calibration of chilled-mirror instruments is limited. Furthermore, it is stated that a standard composed of n-
butane in nitrogen is indeed a straightforward and inexpensive way to calibrate a chilled-mirror instrument, yet
forms an atypical, rapidly condensing hydrocarbon film. Therefore, such a calibration gas has limited use in
calibrating a hydrocarbon-dew-point instrument.
NOTE 2 Several studies showed the importance of not only the component concentration in a calibration mixture but
[4]
also knowledge of the nature of the components used. For example, a GERG study shows that adding aromatic and/or
cyclic hydrocarbons has a significant influence on the hydrocarbon dew point.
Based on the working principle of chilled-mirror devices, there are five major sources that can be responsible
for significant systematic errors in the measured hydrocarbon dew point and for which no adjustment can be
made because no proper calibration method exists. These five sources are
a) the often significant amount of liquid that it is necessary to form before the instrument is able to detect the
dew point temperature;
b) the cooling rate, which is often too fast to ensure that the temperature measured by a temperature sensor
somewhere in the mirror equals the temperature of the mirror surface and the temperature of the gas in
the measuring cell;
c) the way the gas flow passes through the measurement cell during the actual measuring phase
(continuous versus stop-flow principle);
d) the measurement of the mirror temperature, which doesn’t take place at the mirror surface itself but near
the mirror surface;
e) the hydrocarbon dew point that, when measured at a certain pressure setting, doesn’t necessarily
corresponds to the cricondentherm pressure valid for the actual gas composition.
In this Technical Report, a calibration procedure is presented which allows the adjustment and even the
calibration of a hydrocarbon-dew-point chilled-mirror analyser against the indirect automatic weighing method
according to ISO 6570. By using this procedure, the measured hydrocarbon dew point corresponds
unambiguously to a given value for the potential hydrocarbon liquid content (PHLC) at this measured dew-
point temperature. In this way, a traceable and much more objective measurement of the hydrocarbon dew
point is possible. By doing an on-site comparison/calibration against the indirect automatic weighing method, it
is even possible to correct for the gas-dependent performance of the hydrocarbon-dew-point analyser, which
exists to some extent for different natural gases.
Measurements carried out according to ISO 6570 consist of cooling down a well defined gas flow to a
specified and accurately measured temperature. The gas has enough time to form liquid and to establish a
gas-liquid equilibrium at a certain pressure and temperature. This process is similar to the process that occurs
in practice, when in a pipeline the pressure and/or temperature are reduced, the complete gas flow is cooled
down and depending on the gas quality in the worst case can result in hydrocarbon liquid drop out. By
calibrating a dew point analyser against ISO 6570, it is made sure that a dew point is obtained, which is
related to the process which actually occurs in the pipeline upon pressure reduction, although the
measurement technique is quite different to the process in the pipeline.
According to ISO 6570, the detection limit of the automatic weighing method is 5 mg/Nm . Theoretically it can
be argued that the temperature corresponding to a potential hydrocarbon liquid content of 5 mg/m is not a
correct estimate for the hydrocarbon-dew-point temperature. However, taking into account the measuring
capabilities of the existing hydrocarbon-dew-point measurement devices, this effect can be neglected.
[3]
Based on the geometry of the measurement cell of a chilled mirror instrument, it is estimated by Cowper
that it is necessary to condense approximately 70 mg/Nm onto a mirror surface to register a hydrocarbon-
dew-point temperature. Depending on the gas composition, this effect results in a bias of up to −1,5 °C in the
measured hydrocarbon dew point value.
NOTE 3 EASEE-gas (European Association for the Streamlining of Energy Exchange) recently agreed upon
[1]
harmonized values of the hydrocarbon dew point throughout Europe (Common Business Practice 2005-001/01) . The
required harmonized measuring method, which it is still necessary to identify, can clearly benefit from the proposed
traceable calibration procedure presented in this Technical Report.

vi © ISO 2009 – All rights reserved

TECHNICAL REPORT ISO/TR 12148:2009(E)

Natural gas — Calibration of chilled mirror type instruments
for hydrocarbon dewpoint (liquid formation)
WARNING — The use of this Technical Report can involve hazardous materials, operations and
equipment. This Technical Report does not purport to address all of the safety problems
associated with its use. It is the responsibility of the user of this Technical Report to establish
appropriate safety and health practices and to determine the applicability or regulatory
limitations prior to use.
1 Scope
This Technical Report describes the principles of, and general requirements for, the traceable calibration of
automatic hydrocarbon-dew-point chilled-mirror instruments using the indirect automatic weighing method
(method B) described in ISO 6570:2001 to determine the potential hydrocarbon liquid content of natural gas,
or similar gas. The calibration procedure is intended for use by chilled-mirror instruments in downstream
applications transferring processed natural gas.
If the gas composition is constant, the manual weighing method (method A) described in ISO 6570:2001 is
also applicable.
NOTE 1 Whether or not a gas composition is constant is difficult to establish. A process gas chromatograph (GC)
measuring calorific values within 0,1 % is absolutely no guarantee for a constant hydrocarbon liquid drop-out content.
Information up to C is required to verify that the gas composition is constant.
NOTE 2 The application of this calibration procedure in the upstream area is not excluded a priori, however, currently
there is no experience using this procedure in an upstream environment.
The usability of data on the potential hydrocarbon liquid content of natural gas for verification, adjustment or
calibration of hydrocarbon-dew-point chilled-mirror instruments is based on the condensation behaviour of
natural gases. Information on the condensation behaviour of natural gases and the various measuring
techniques to determine properties, like hydrocarbon dew point and potential hydrocarbon liquid content,
related to the condensation behaviour of natural gases are given in Annex A.
NOTE 3 Unless otherwise specified, gas volumes are in cubic metres at 273,15 K and 101,325 kPa.
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 6570:2001, Natural gas — Determination of potential hydrocarbon liquid content — Gravimetric methods
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
potential hydrocarbon liquid content
PHLC
property of natural gas defined as the amount of the condensable liquid (in milligrams) at the pressure, p, and
temperature, T, of the measurement per unit volume of gas at normal conditions, that is, at a temperature of
0 °C and a pressure of 101,325 kPa, by passing a representative sample of the gas through an apparatus
where it is first brought to the pressure, p, and then cooled to the temperature, T
NOTE It is necessary to take care that only gas, not a two-phase mixture, has been withdrawn from the pipeline.
3.2
hydrocarbon dew point
HCDP
property of natural gas defined as the temperature, T , at which an automatic chilled-mirror instrument
HCDP
detects the first presence of hydrocarbon liquid drop out at the pressure, p
NOTE In chemical thermodynamics, the “true” hydrocarbon dew point is the temperature (at a stated pressure) at
which the enthalpy of the gas and liquid phases is identical. Since measurement of the dew point involves reduction of the
system temperature, this equates to the temperature at which the first appearance of the liquid phase occurs. At this point,
the quantity of liquid phase is infinitesimally small. Since no instrument is able to detect this infinitesimally small amount,
the aforementioned definition of the hydrocarbon dew point is adopted in this Technical Report. However, depending on
the gas composition and the sensitivity of the detection system of the hydrocarbon-dew-point chilled-mirror device, the
measured hydrocarbon dew point can be considerably lower than the “true” hydrocarbon dew point.
3.3
PHLC reference value
parameter in the calibration procedure chosen by the user
NOTE At the PHLC reference value, the hydrocarbon dew point temperature and the temperature of the ISO 6570
measurement are equal.
4 Symbols
p pressure at which PHLC and hydrocarbon dew point measurement are carried out
T temperature at which the PHLC measurement is carried out
5 Performance characteristics of automatic weighing method in accordance with
ISO 6570:2001
WARNING — The instrumentation used for this method shall comply with local legal and safety
regulations, especially with the local regulations for application of the instrumentation in hazardous
areas.
5.1 Working principle
The principle of the indirect automatic weighing method (method B) is described in ISO 6570:2001. An
example of an implementation of this method is shown in Figure 1. ISO 6570 states that the quantity of
hydrocarbon liquids that can be formed at a certain pressure and temperature is determined by passing a
representative sample of the gas through the apparatus where it is first brought to the required pressure and
then cooled to the required temperature. The liquids formed during cooling are separated from the gas flow
and collected by means of a cyclone separator into a measuring tube. The measuring tube is automatically
drained when it is totally filled up with liquid. The liquid from the measuring tube is collected in a hydrocarbon-
liquids drum. In the indirect automatic weighing method, the determination of the amount of hydrocarbon
liquids formed is derived from the differential pressure measured along the measuring tube.
2 © ISO 2009 – All rights reserved

Key
1 relieve valve
2 pressure controller
3 restriction
4 flow controller
5 heat exchanger
6 gas/liquid separator
7 maximum level
8 measuring tube
9 minimum level
10 condensate drum
11 injection point for calibration liquid
12 differential pressure sensor
13 drain valve
Figure 1 — Implementation of indirect automatic weighing method according to ISO 6570
©
1)
(GACOM of N.V. Nederlandse Gasunie)

1) GACOM is an example of a suitable product available commercially. This information is given for the convenience of
users of ISO 12148 and does not constitute an endorsement by ISO of this product.
5.2 Functional requirements
5.2.1 General
To ensure the correct functioning of the indirect automatic weighing method, it is necessary to fulfil the
requirements given in 5.2.2 to 5.2.8.
5.2.2 Gas sampling
[6]
The sampling point at the pipeline should be situated as described in ISO 10715 . The sampling system
should be such that a continuous gas withdrawal at a pressure and temperature considerably above the
cricondentherm is possible. No liquids or aerosols should be withdrawn together with the gas sample. The
sample line should be heat traced up to at least 35 °C. The sampling system should be clean to prevent or at
least to minimize the adsorption of higher hydrocarbons.
NOTE (Excessive) adsorption of higher hydrocarbons in the sampling system results in a systematic error in the
measured hydrocarbon dew point value.
5.2.3 Calibration of the differential pressure sensor
The indirect automatic weighing method requires calibration of the differential pressure sensor. The differential
pressure transmitter is calibrated by introducing known amounts of a calibration liquid, such as n-decane, into
the measuring tube at the temperature and pressure at which the PHLC measurement will be performed.
Such calibration shall be carried out at least at the start of a new measurement or calibration series and after
(large) adjustments of the measuring pressure, p, or measuring temperature, T.
NOTE Frequent calibration of the differential pressure sensor is required to exclude the possible interference of the
static pressure and/or temperature on the raw output of the sensor. The measured differential pressures are small and,
therefore, any small interferences can considerably disturb the measurement results.
5.2.4 Pressure controller
In ISO 6570, the pressure controller is allowed to introduce a maximum variation of 10 kPa in the gas/liquid
cyclone separator. For this particular application, it is advised to use a pressure regulator that introduces
variations in the pressure of less than 10 kPa. Either the pressure controller should be electrically heated or
the sample line and controller should be heat traced up to at least 40 °C.
NOTE Commercially available, high-quality pressure controllers introduce variations in the pressure of less than
10 kPa only for gas flows of approximately 1 Nm /hr.
5.2.5 Temperature control
In accordance with ISO 6570, the cooling bath shall be capable of maintaining the temperature of the cooling
liquid at any point of the bath with a variation of less than 0,25 K.
NOTE For temperatures below −15 °C, the specification in this subclause is difficult to maintain without taking special
precautions. Especially with high environmental temperatures, such as occur during summertime, regular thermostats are
not able to meet the specified criterion.
5.2.6 Measurement of pressure and temperature
The pressure and temperature sensors shall be calibrated at least once a year.
5.2.7 Flow measurement
The device for measuring the gas flow through the instrument, which is normally operated with a gas flow of
1 Nm /hr, shall be checked at regular intervals against a calibrated gas meter. Such verification shall be
carried out at least at the start of a new measurement or calibration series.
4 © ISO 2009 – All rights reserved

The automatic measuring device is operating with a gas flow of 1Nm /hr at a fixed pressure, most often
between 2,7 MPa and 3,0 MPa [(27 and 30 bar) (at the cricondentherm)], and a temperature (e.g. –3 °C) at
which the hydrocarbon liquid content (often 5 mg/Nm ) is specified in some contracts.
5.2.8 PHLC measurement period
Due to the nature of the PHLC measurement, measurement values become available at a certain time interval.
Since a short cycle period results in PHLC measurements with a high uncertainty, it is advised to use a cycle
time of at least 30 min.
NOTE For a proper calibration of a hydrocarbon-dew-point chilled-mirror instrument, it is advisable to compare data
for a period of at least 24 hours or more.
5.3 Measurement uncertainty
The uncertainty in the PHLC value depends on the pressure and temperature set points and the condensation
behaviour of the gas being measured. PHLC-values above 5 mg/m shall be detected unambiguously. At low
3 3
PHLC values (less than 30 mg/Nm ), the uncertainty shall be equal or less than 5 mg/Nm .
As an example, the results of two individual, indirect automatic weighing instruments are presented in Figure 2.
Both devices measure the potential hydrocarbon liquid content of the same gas and are operating at the same
pressure and temperature. These results give an indication of the achievable reproducibility, which is within
± 5 mg/Nm . During other experiments with a stable, type H-gas (high calorific value), an even smaller
random error (2σ) of ± 2mg/Nm was found.

Key
1 GACOM
2 GACOM
X day
Y condensate concentration, expressed in milligrams per normal cubic metre
©
Figure 2 — PHLC measurements using 2 GACOM units for an L-type gas at 2 700 kPa and −3 °C
6 Performance requirements for a chilled-mirror-type instrument for hydrocarbon-
dew-point determination
WARNING — The instrumentation used for this method shall comply with local legal and safety
regulations, especially with the local regulations for application of the instrumentation in hazardous
areas.
6.1 Working principle
Although there are some significant differences in the implementation, the measuring principle of a
hydrocarbon-dew-point chilled-mirror instrument is identical for all instruments. After pressure reduction, the
gas is passed through a measuring cell. During normal operation, the measuring pressure is chosen to be
close to the value at which the dew-point temperature is at its maximum (the cricondentherm). The measuring
cell has an observation window at one side and a mirror surface at the other side. This mirror is mounted on a
cooling body and the cooling down can be accomplished in a controlled way. The cooling body itself is cooled
either electrically (Peltier element) or by the expansion of carbon dioxide or another gas. The temperature of
the mirror is measured continuously. The sample gas may be flowed through the cell continuously, or, having
flowed sufficiently to purge the cell, mirror and pipe work, blocked in without flow while the cooling cycle of the
mirror starts. The mirror surface is observed by reflected light, either visually by an operator in the manual
version or by photocell in the automated instrument. An example of an implementation of an automatic dew-
point chilled-mirror instrument is shown in Figure 3.

Key
1 light source 5 mirror
2 light detector 6 Peltier element
3 temperature sensor 7 heat sink
4 pressure controller 8 flow controller
Figure 3 — Implementation of an automatic chilled-mirror device
for hydrocarbon-dew-point determination
6 © ISO 2009 – All rights reserved

A dew point analyser does not determine the theoretical dew point temperature of a natural gas, but it
measures the temperature at which a certain amount of liquid is condensed on the mirror so that a significant
and reproducible change in light scattering is observed, either by a skilled person or by a light-detecting
sensor. From calculations, it can be shown that the hydrocarbon liquid drop out that is required to get a
3 3
reproducible dew-point observation often corresponds with 20 mg/Nm to 70 mg/Nm hydrocarbon liquid drop
out. In fact, the dew-point meter can be considered as a potential hydrocarbon-liquid-content meter, and the
measured dew point temperature is the equilibrium temperature, for example at 30 mg/Nm hydrocarbon
liquid drop out.
Therefore, it is necessary to realize beforehand what an automatic dew point monitor is actually determining:
certainly not the true thermodynamic dew point, but a temperature corresponding to a predetermined
threshold value of the detector signal. This threshold value is determined by doing a “calibration” against a
manual mirror or a dew-point calculation, often by using a multi-component gas mixture.
As stated before, existing dew-point analysers measure the “dew point” temperature at hydrocarbon liquid
3 3
drop out values varying between 20 mg/Nm to 70 mg/Nm . A reduction of this amount results in measured
values closer to the “true” hydrocarbon dew point. Ideally, the dew-point analyser should determine a dew-
point temperature corresponding to approximately 5 mg/Nm hydrocarbon liquid drop out, this being the value
that can be determined accurately by ISO 6570 equipment. Then, the pipeline operator or end user can be
certain that no significant amounts of liquid are formed upon pressure reduction. Depending on the gas
3 3
composition, the difference in temperature corresponding to, respectively, 30 mg/Nm and < 5 mg/Nm can be
less than 0,5 °C; however, it also can be more than 3 °C. Therefore, it is better to adjust the threshold level
against traceable hydrocarbon liquid drop out measurements than by the methods used nowadays.
The cooling rate of the mirror of a dew-point analyser appears as another important parameter to verify for a
proper dew-point measurement. Measurement of the dew point occurs in a small cell with the bottom surface
containing the mirror assembly. Only this mirror assembly is cooled, resulting in temperature gradients in the
measurement cell and in the gas inside the cell. Theoretically, the cooling rate of the mirror surface should be
sufficiently slow that the gas temperature is always in equilibrium with the mirror temperature and that there is
enough time for the hydrocarbon liquids to drop out on the mirror surface. In practice, the cooling rate is often
much more rapid. A high cooling rate results in a lag in the decrease in gas temperature and the hydrocarbon
liquids drop-out onto the mirror. Also, the temperature sensor, which is mounted somewhere inside the mirror,
records a temperature lower than the actual surface temperature of the mirror. Both effects result in
temperature readings lower than the real dew-point temperature.
For those analysers that operate with a continuous gas flow during the complete measurement cycle, the flow
through the measurement cell is an important parameter. The continuous flow of warm gas through the
measurement cell influences the temperature of the gas in the vicinity of the mirror and, therefore, influences
the measured dew point. In general, the dew point decreases when the gas is allowed to flow through the
measurement cell during the cooling down of the mirror.
Some general requirements for dew point analysers are
⎯ digital outputs of date/time, dew-point temperature and pressure;
⎯ the possibility of changing the detection point criteria;
⎯ storage of the changes in the measurement method, so that after a power shut-down, the proper method
is still available.
6.2 Precision
In order to obtain a proper calibration of the dew-point analyser by applying hydrocarbon liquid drop out
measurements, the repeatability of the hydrocarbon analyser should be within 1 °C (2σ value). The
reproducibility can be determined at a constant gas composition, or during a period of time when hydrocarbon
liquid drop out measurements give nearly constant values. If the gas quality is constantly changing, a plot of
hydrocarbon liquid drop out versus dew point also gives a good indication of the reproducibility of the dew-
[5]
point analyser; see Figure 4 .
Key
X condensate concentration, expressed in milligrams per normal cubic metre
Y dew point, expressed in degrees Celsius
Figure 4 — Relation between hydrocarbon dew point and hydrocarbon liquid drop out,
showing the variation in dew point at fixed PHLC values: 2σ ≈ 0,3 °C
HCDP
7 Requirements/Points of interest to carry out a calibration
7.1 Process gas
For the purpose of this Technical Report, process gas is free of condensable substances, like glycol, water,
compressor oil.
NOTE It is necessary to prevent the occurrence of mist flow in the sampling system.
During the calibration, there should be no very large variation in gas quality (e.g. from Wobbe
3 3
Index 45 MJ/Nm to Wobbe Index 55 MJ/Nm ). Normal variations in gas quality are no problem and even
calibrations at gathering pipelines from offshore fields should give no problems. Normally occurring dew-point
variations of 10 °C to 15 °C should also give no problems during calibration.
7.2 Sampling system
The sample system should be designed as described in ISO 10715 (see also 5.2.2). The following points shall
be considered.
⎯ Changes in the gas composition caused by the sampling system shall be avoided. Therefore, the gas
sampling system shall be designed with a minimum number of components and the analyser shall be
positioned near the sampling point.
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⎯ If the presence of traces of liquid in the natural gas cannot be excluded completely, a sampling probe with
a sintered stainless steel particulate filter is advised.
⎯ If liquid is present in the natural gas, appropriate filtering at the sampling probe itself or immediately
downstream of the probe (e.g. with a membrane filter) is advised to prevent any intrusion of traces of
liquids and to maintain the integrity of the sampling system and the analyser. A check shall be made that
the filter does not cause changes in gas composition that result in deviations in the measured
hydrocarbon dew point.
NOTE Entrained liquids in the form of mist or small droplets present in the gas stream are sometimes
encountered directly downstream of a gas-conditioning plant. Under these circumstances, it can be necessary to use
a sampling probe with a membrane filter to safeguard the sampling system from being flooded with liquids. However,
it is essential to check the sampling system for undesired changes in the gas composition, as this can occur using a
membrane filter.
By applying a by-pass flow (fast loop), changes in the gas composition due to interaction with the filter
material shall be minimized.
⎯ The sampling line from probe to the analyser cabinet shall be heat traced to prevent any condensation in
the sampling line.
The body of the pressure regulator shall be heated up to prevent condensation of liquids from the natural
gas upon pressure reduction. A temperature of approximately 60 °C generally fulfils this requirement.
Variation in pressure shall be within ± 0,01 MPa  0,1 bar).
⎯ The temperature of the analyser cabinet should be maintained at a level such that no condensation
occurs in the sampling lines in the cabinet. A temperature of 30 °C generally fulfils this requirement.
It is advised to mount a sun screen above the analyser in case it is expected that exposure to direct
sunshine can cause excessively high temperatures in the analyser cabinet.
7.3 Selection of the PHLC reference value
The PHLC reference value is an important parameter on which it is necessary to agree before the actual
calibration of an automatic hydrocarbon-dew-point chilled-mirror instrument can be carried out. As stated in
the introduction of this Technical Report, the detection limit of the automatic weighing method stated in
ISO 6570 is 5 mg/Nm , whereas the amount of liquid which it is necessary to condense onto a mirror surface
3 3
to register a hydrocarbon dew point is approximately 5 mg/Nm to 70 mg/Nm .
Depending on the sensitivity of the chilled-mirror detector system, it is possible to carry out a calibration at an
arbitrary level starting at 5 mg/Nm and upwards. Since the setting of the PHLC reference value determines
the measuring behaviour of the hydrocarbon-dew-point chilled-mirror instrument, it is important to specify the
PHLC reference value for use during the calibration procedure and to clearly report the PHLC reference value
on the calibration report.
Although it is possible to choose an arbitrary level for the PHLC reference value, it is advised to limit the
choice for the PHLC reference value to the following three levels:
⎯ 5 mg/Nm , being the most sensitive value, corresponding to the highest value for the hydrocarbon dew
point; using this PHLC reference value, the measured values of the calibrated chilled-mirror instrument fit
perfectly to the regular contract specifications based on potential hydrocarbon liquid content.
⎯ 70 mg/Nm , being the least sensitive value, corresponding to the lowest value for the hydrocarbon dew
point; using this PHLC reference value, the measured values of the calibrated chilled-mirror instrument
correspond to the measuring behaviour of a manual chilled mirror.
⎯ 30 mg/Nm , being a value intermediate between the aforementioned minimum and maximum values; in
general, it has been shown that automatic chilled-mirror instruments are capable of operating reliably and
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with low measurement uncertainty at a level of 30 mg/Nm to 40 mg/Nm .
7.4 Compositional range of gases covered by the calibration procedure
The thermal and mass transfer processes that take place during the cooling of the mirror surface are of a
complex nature and not completely understood. Current experience with the proposed calibration procedure
indicates that the results of a calibration are valid for a wider range of natural gas compositions insofar as the
gases show similar condensation behaviour. In the study carried out under the auspices of the National
[2]
Physical Laboratory , synthetic mixtures of natural gas are categorized into three classes:
⎯ “mid” condensation rate: selected to be representative of the gases found in most UK gas fields; for these
gases, a decrease in temperature of 1 °C results in an increase in the hydrocarbon liquids formation
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ranging from approximately 100 mg/Nm to 300 mg/Nm ;
⎯ “high” condensation rate for these gases, a decrease in temperature of 1 °C results in an increase in the
hydrocarbon liquids formation ranging from 300 mg/Nm and onwards. In the aforementioned study, an
example of a gas is presented in which a decrease of only 0,25 °C results in an increase in the
hydrocarbon liquid drop out of 250 mg/Nm ;
⎯ “low” condensation rate; for these gases, a decrease in temperature of 1 °C results in an increase in the
hydrocarbon liquids formation of only less than 100 mg/Nm . In the aforementioned study, an example of
a gas is presented in which a decrease of 5 °C results in an increase in the hydrocarbon liquid drop out of
only 25 mg/Nm ;
There are indications that the aforementioned classification can be appropriate to classify the natural gases
with respect to calibration of hydrocarbon-dew-point chilled-mirror instruments. However, it is necessary to
gain more experience to confirm this assumption.
CAUTION — No general advice can be given on the validity of the calibration for a given compositional
range of gases. Therefore, it is necessary initially to carry out multiple calibrations with varying gas
compositions to assess the validity of a calibration.
7.5 Calibration interval
It is difficult to make a general remark on the validity of a calibration in time. The validity depends heavily on
various circumstances, such as the following:
⎯ stability of the mechanical and electrical boards in the chilled mirror instrument itself;
⎯ presence of impurities, like solids, glycol etc, in the gas to be sampled and the effectiveness of the
sampling system to cope with such impurities;
⎯ significant changes in the gas composition resulting in a shift in the condensation behaviour of the gas
sampled as can occur in gathering pipeline systems in which gas wells are taken out or in production.
Existing experience show that for a relatively clean gas with a stable gas composition, a calibration can be
valid for a period of at least a year.
CAUTION — No general advice can be given on the calibration interval. Therefore, it is necessary
initially to carry out calibrations at regular intervals to assess the validity of a calibration in time.
8 Execution of calibration procedure
8.1 General
During the calibration of a dew-point analyser in accordance with ISO 6570, it is preferable that both
measurement systems be connected to the same sampling probe.
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It is preferable that the calibration be performed with the natural gas that is monitored later on by the
hydrocarbon-dew-point analyser.
The complete sampling and conditioning system of the dew-point analyser shall be used during the calibration.
At start-up, the time-stamps of both analyser systems should be checked and, when necessary, syn
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