EN ISO 10715:2000

SLOVENSKI STANDARD
01-december-2000
=HPHOMVNLSOLQ6PHUQLFH]DY]RUþHQMH,62
Natural gas - Sampling guidelines (ISO 10715:1997)
Erdgas - Probenahmerichtlinien (ISO 10715:1997)
Gaz naturel - Lignes directrices pour l'échantillonnage (ISO 10715:1997)
Ta slovenski standard je istoveten z: EN ISO 10715:2000
ICS:
75.060 Zemeljski plin Natural gas
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

INTERNATIONAL
IS0
STANDARD
First edition
1997-06-01
Natural gas - Sampling guidelines
Gaz na furel - Lignes directrices pour khantillonnage
Reference number
IS0 10715:1997(E)
IS0 10715:1997(E)
Contents
1 Scope . 1
2 Definitions . 1
3 Principles of sampling . 3
4 Safety precautions . 5
5 Technical considerations . 7
6 Materials used in sampling . 10
7 General preparation of equipment . 11
8 Sampling equipment . 12
9 Spot sampling . 18
10 Direct sampling .
11 Incremental sampling .
Annexes
A Use of a block valve in direct sampling . 23
B Cleaning of steel sampling cylinders . 25
C Procedure for low-pressure sampling into glass cylinders . 26
D Procedure for sampling by the fill-and-empty method . 28
E Procedure for sampling by the controlled-rate method . 30
............................................................................. 32
F Procedure for sampling by the evacuated-cylinder method
........................................................................................
G Guidelines for the calculation of the residence time 34
H Student’s f-table . 38
J Bibliography . 39
0 IS0 1997
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 l CH-1211 Gen&e 20 0 Switzerland
Internet: central @? iso.ch
x.400: c=ch; a=400net; p=iso; o=isocs; s=central
Printed in Switzerland
ii
0 IS0
IS0 10715:1997(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide federation of national standards bodies (IS0
member bodies). The work of preparing International Standards is normally carried out through IS0 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. IS0 collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
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.
International Standard IS0 IQ715 was prepared by Technical Committee lSO/TC 193, Natural gas, Subcomittee
SC 1, Analysis of natural gas.
Annexes A to J of this International Standard are for information only.

0 IS0
IS0 10715:1997(E)
Introduction
This International Standard provides guidance on all aspects of the sampling of processed natural gas. Unless
otherwise stated, all pressures up to 15 MPa in this International Standard are given as gauge pressures.
The determination of the composition and the properties of the gas is highly dependent on the sampling technique.
Also of great importance are the design, construction, installation and maintenance of the sampling system as well
as the conditions of sample transfer and transport.
These guidelines cover sampling strategy, details of sampling methods, the choice of sampling method and
sampling equipment.
This document is intended for use in sampling is not described
those cases where as part of the analytical
procedure.
This document concentrates on sampling systems and procedures. Analyses from the samples collected using
these systems and procedures may be utilized in many different ways, including calculations to determine the
calorific value of the gas stream, identification of contaminants contained in the gas stream, and compositional
information to determine whether or not the stream meets contractual specifications.
iv
IS0 10715:1997(E)
INTERNATIONAL STANDARD o IS0
Natural gas - Sampling guidelines
WARNING - The use of this International Standard may involve hazardous materials, operations and
equipment. This standard does not purport to address all of the safety problems associated with its use. It
is the responsibility of the user of this standard to establish appropriate safety and health practices and
determine the applicability or regulatory limitations prior to use.
All sampling activities shall comply with local safety regulations.
1 Scope
The purpose of this document is to provide concise guidelines for the collection, conditioning and handling of
representative samples of processed natural gas streams. It also contains guidelines for sampling strategy, probe
location and the handling and design of sampling equipment.
It considers spot, composite (incremental) and continuous sampling systems.
This document gives consideration to constituents such as oxygen, hydrogen sulfide, air, nitrogen and carbon
dioxide in the gas stream.
This document does not include sampling of liquid streams or streams with multiphase flow.
Traces of liquid, such as glycol and compressor oil, if present, are assumed to be intrusive and not a part of the gas
to be sampled. Their removal is desirable to protect the sampling and analytical equipment from contamination.
This document can be used for custody transfer measurement systems and allocation measurement systems.
2 Definitions
For the purposes of this International Standard, the following definitions apply:
direct sampling:
Sampling in situations where there is a direct connection between the natura ,I gas to be sampled and the analytica I
unit.
2.2 floating-piston cylinder:
A container which has a moving piston separating the sample from a buffer gas. The pressures are in balance on
both sides of the piston.
2.3 flow-proportional incremental sampler:
A sampler which collects gas over a period of time and at a rate that is proportional to the flow rate in the sampled
pipeline.
2.4 - high-pressure natural gas:
Natural gas with a pressure exceeding 0,2 MPa.
NOTE - The maximum for this International Standard is 15 MPa.

0 IS0
IS0 10715:1997(E)
2.5 hydrocarbon dew point:
The temperature, at a given pressure, at which hydrocarbon vapour condensation begins.
2.6 incremental sampler:
A sampler which accumulates a series of spot samples into one composite sample.
2.7 indirect sampling:
Sampling in situations where there is no direct connection between the natural gas to be sampled and the analytical
unit.
2.8 liquid separator:
A unit, in the sample line, used to collect liquid fall-out.
2.9 low-pressure natural gas:
Natural gas having a pressure between 0 and 0,2 MPa.
2.10 purging time:
The period of time during which a sample purges a piece of equipment.
2.11 representative sample:
A sample having the same composition as the natural gas sampled when the latter is considered as a
homogeneous whole.
2.12 residence time:
The time it takes for a sample to flow through a piece of equipment.
2.13 retrograde condensation:
Retrograde behaviour describes the non-ideal phase properties of hydrocarbon gas mixtures, such as natural gas.
Retrograde condensation is the production of a liquid phase of heavy hydrocarbons at a particular pressure and
temperature where, at that same temperature, the gas stays in a single phase at a higher pressure as well as at a
lower pressure.
NOTE - See also 5.2.
2.14 sample container:
A container for collecting the gas sample when indirect sampling is necessary.
2.15 sample line:
A line provided to transfer a sample of the gas to the sampling point. It may include devices which are necessary to
prepare the sample for transportation and analysis.
2.16 sample probe:
A device inserted into the gas line to be sampled and to which a sample line is connected.
2.17 sampling point:
A point in the gas stream where a representative sample can be collected.
2.18 spot sample:
A sample of specified volume taken at a specified place at a specified time from a stream of gas.
2.19 transfer line:
A line provided to carry the sample to be analysed from the sample point to the analytical unit.
2.20 water dew point:
The temperature, at a given pressure, at which water vapour condensation begins
/.
0 IS0 IS0 10715: 1997(E)
3 Principles of sampling
3.1 Sampling methods
The main function of sampling is to take an adequate sample that is representative of the gas.
The main distinction in sampling is between direct and indirect sampling methods.
In the direct sampling method, the sample is drawn from a stream and directly transferred to the analytical unit.
In the indirect sampling method, the sample is stored before it is transferred to the analytical unit.
The main classifications of the indirect sampling method are spot sampling and incremental sampling.
Figure 1 - Survey of direct and indirect sampling methods
The information needed from the analysis of natural gas falls into two basic categories: averaged and limit values.
i
3.1 .I Averaged values
A typical example is the calorific value. Custody transfer requires the time- or flow-averaged calorific value.
Commercial agreements determine the period and method of averaging.
3.1.2 Limit values
Most gas custody transfer contracts contain specification limits on composition or on gas properties. Direct sampling
can be applied, but often the requirements are such that also indirect sampling has to be applied.
3.2 Sampling frequency
This subclause gives guidelines for the establishment of the sampling frequency. Mostly the sampling frequency is a
matter of common sense. Information on the properties of the gas stream in the past and about expected
(systematic) future changes determines the sampling frequency.
Generally, pipeline gas composition will have daily, weekly, monthly, semi-annual and seasonal variations.
Compositional variations will also occur because of gas treatment equipment and reservoir changes. All of these
environmental and operational considerations shall be taken into account when selecting a sampling interval.
The statistical approach in this paragraph is only intended to support the common-sense approach.
In this context, the required sampling frequency is the number of samples to be taken in a certain period of time in
order to obtain meaningful results.
0 IS0
IS0 10715:1997(E)
The formula for calculating the number of samples is:
I
-
n* =,Xj
where
d is the error margin required;
n is the number of samples;
is the standard deviation;
S
f is Student’s f-factor (see table H.1 in annex H).
This equation shall be solved by iteration: an initial value of f is estimated, and used to calculate a revised value of
n, which is used, in turn, to give a new value of t. The error margin, the number of samples and the standard
deviation shall be taken over the same period of time.
3.2.1 Error margin
There are two different cases of error margins. One case is related to the determination of averaged values. In most
custody transfer contracts, these values are given as an indication of the accuracy.
The other is related to the determination of limit values. Custody transfer contracts specify the limits but rarely give
an indication of the accuracy. In these cases, the difference between the last measured value, or the last year’s
average, and the limit value is the error margin.
3.2.2 Number of samples
The number of samples is the number of samples to be taken in a defined period. It is equivalent to the number of
partial samples in incremental sampling.
3.2.3 Student’s f-factor
Student’s f-factor allows for the finite sample size, and is to be found in standard statistical tables. The value
here to be taken as the number of
depends on the claimed certainty (typically 95 %) and the “degrees of freedom”,
measurements minus one (n-1).
EXAMPLE 1
Determination of the monthly average caloric value
d = 0,4 % (error margin required from custody transfer contract for monthly averaged value)
s= 0,6 % (estimated variation over a one-month period)
First estimate, taking n = 7:
f= 2,45 for 6 degrees of freedom and a certainty of 0,975 single-sided (equals 0,95 double-sided)
-
n2 =2,45xL
J
n=14
First iteration, taking n = 14:
0 IS0 IS0 10715:1997(E)
recalculate for
i! = 2,16 for 13 degrees of freedom, and a certainty of 0,975 single-sided (equals 0,95 double-sided)
-
n2 =2,16x-L
n=ll
Second iteration, taking n = 11:
recalculate for
t= 2,23 for IO degrees of freedom, and a certainty of 0,975 single-sided (equals 0,95 double-sided)
-
ni =2,23x’
n=ll
EXAMPLE 2
Total sulfur determination
Last measured concentration 20 mg/m3 and the contract limit value 50 mg/m3.
d = 30 mg/m” (difference between limit value from custody transfer contract and last measured value)
s = IO mg/m” [standard deviation in spot sample results (in the past year)]
f = 4,30 n - 1 taken as 2, level of certainty 95%
n = 2
Three samples are enough. Recalculation indicates that two samples are not enough.
4 Safety precautions
,
4.1 General
Sampling and sample handling shall follow all relevant national and company-related safety regulations
In the case of inadequate regulations, those responsible for sampling shall establish detailed procedures.
Specifications for equipment shall also be established.
Personnel involved shall be properly trained and educated to a level such that they are able to take necessary
responsibility.
4.2 Personnel
The person responsible for the department/section which is to perform the sampling shall be satisfied that the
sampling can be performed within relevant safety regulations.
0 IS0
IS0 10715:1997(E)
Those performing sampling or installing sampling equipment shall have the necessary training and education to
evaluate potential safety hazards in general.
The above personnel shall have the authority to prevent sampling or installation of sampling equipment which is
unsuitable or unsafe.
4.3 Equipment
Equipment if required
used in the sampling of high-pressure natural gas shall be inspected and recertificated
regularly.
Documentation shall be available and up to date.
meet relevant sampling conditions, e.g. pressure mperature, corrosivity, flow,
Equipme nt shall be designed to 9 te
ibility, vibration, thermal expansion and/o thermal contraction.
chemical compat
Glass cylinders shall not be exposed to pressure.
If provided for, end caps shall be installed on cylinders during transportation and storage.
Cylinders shall have volume, working pressure and test pressure permanently stamped.
Cylinders shall have a test pressure of at least I,5 times the working pressure.
Cylinders shall be protected against damage during transportation and storage. Transportation boxes or cartons
designed for the individual type of cylinder shall be available.
Cylinders shall be accompanied by labels or paperwork with relevant information protected against damage.
Cylinders and associated accessories shall be inspected and leak-tested periodically.
Permanent transfer and sampling lines shall be properly secured. Breakable connections shall have easy access for
leak-testing. Outlets shall be equipped with double block and bleed valves. End caps shall be connected to fittings
when the cylinders are not in use.
The use of flexible high-pressure tubing shall be limited and manufacturers’ instructions for safe application shall be
strictly followed. Transfer lines can be blocked by solid or liquid contaminants. Special precautions shall be
employed when trying to “reopen” such lines. Only qualified personnel may do this.
Transfer lines shall have shut-off valves located as close to the source stream as possible. The sampling probe
shall be equipped with a shut-off valve.
Electrical equipment shall be approved for the relevant sampling application.
Equipment which can create static electricity shall be avoided.
Use of equipment or tools which may create sparks shall be avoided.
4.4 Flammability
In order to prevent fire or explosions, the following restrictions shall be followed within areas where flammable
concentrations of gas (about 4% to 16% for natural gas) may be found:
/
No open fire
No smoking
No use of equipment and tools which may create sparks
0 IS0 IS0 10715:1997(E)
No use of equipment which operates at temperatures above the self-ignition temperature of gas mixtures, mostly
above 400 “C (for natural gas)
No use of chemicals which can react with
vigorously
gas
No running spark ignition motors
Ventilation shall be sufficient to prevent the build-up of a flammable atmosphere.
Purging of transfer lines shall be directed to a “safe area” (e.g. flare). Release of gas during manual (spot) sampling
shall be limited to a minimum at the sampling location.
Gas detectors shall be used at strategic locations relative to sampling locations.
Manual and/or automatic firefighting equipment shall be easily available.
Personnel performing sampling shall be trained to react appropriately in the event of a fire.
4.5 Personal protective equipment
Necessary personal protective equipment shall be available. The need for protective equipment will vary from place
to place. The following factors shall, however, be considered:
Toxic or irritant components in the gas (H+, radon, Hg, aromatics, etc.) may require the use of breathing filters, a
fresh-air supply, gloves and monitors for toxic components.
Sampling of high-pressure gas may require the use of goggles or face shields. Pressure indicators (gauges) shatt
be used to indicate the system pressure. Leak detector spray or a portable leak detection device shall be used to
check that the system is leaktight.
For fire protection, personnel shall wear flame-resistant clothing (aprons, coveralls, lab dress). Personal smoke
protection masks shall also be available.
4.6 Transportation
Sample cylinders containing gas under pressure shall be transported in accordance with relevant regulations.
Constant-pressure-type cylinders shall always be protected in some kind of transportation container. Damage to the
cylinder itself and/or to valves, gauges, etc., may otherwise occur.
During transportation, the cylinder shall also be protected against conditions of temperature which could create
overpressure or condensation of sample.
The container shall be properly labelled in accordance with applicable regulations.
5 Technical considerations
5.1 Flow characteristics
Flow in a pipeline may be laminar or turbulent. However, in the sampling system, laminar flow shall be avoided. It
can be single-phase or multiphase. Most gas streams operate with turbulent single-phase flow. Two-phase turbulent
-flow may also be found in gas lines where the fluid is near saturated conditions.
dew point, and a reduction
For example, the flow from a gas/liquid separator will be near the gas in line temperature
will cause some condensation to occur, resulting in two-phase flow.
It can also happen that, after a mixing station, the combined gases are not completely mixed in the pipeline.
If the composition is not completely homogeneous, a static mixer will improve the homogeneity.
IS0 10715:1997(E) 0 IS0
5.1 .I Laminar flow
Laminar flow will not normally occur in a gas line because the gas viscosity is IOW and the flow velocities are high
enough to ensure that this will not happen. However, the design of the sampling system shall be such that laminar
flow is avoided.
5.1.2 Turbulent flow
In general, turbulent flow is advantageous in a sampling system and in the gas line to be sampled because the
turbulence creates a well-mixed fluid.
5.1.3 Two-phase flow
Sampling of two-phase (gas/liquid) mixtures is not covered by these guidelines and shall be avoided if at all
possible.
Current technology of natural-gas sampling is not sufficiently advanced to accomplish this with reasonable
accuracy.
5.2 Condensation and revaporization
The condensation behaviour of natural gas is rather complicated. Figure 2 gives an example of a
pressure/temperature phase boundary diagram for a natural gas. The shape of the curve depends on the
composition of the gas.
As figure 2 shows, the phase boundary is a complex function between the critical point and normal operating
conditions. Retrograde condensation can occur when the phase boundary is encountered in an unexpected manner
while adjusting the pressure or temperature of the gas.
Before starting the analysis, the sample shall be heated to at least IO “C above the source temperature. If the
source temperature is not known, the sample shall be heated to at least 100 “C. To ensure revaporization, this
heating shall be applied for a period of 2 h, or longer if necessary.
-80 -60 -40 -20 0 20 40
Temperature ("U------t
Dewline
Cooling from 35 "C
m-m-
- Cooling from 25 "C
Critical point
+
Isothermal
Figure 2 - Example of a pressure/temperature diagram for natural gas
0 IS0 IS0 10715:1997(E)
52.1 Example of a condensation problem
An example of how this problem can occur is shown in figure 2. The pipeline contains gas at pressure pO= If the initial
temperature is -10 OC, and the gas is expanded (i.e. has its pressure reduced) isothermally, it will follow the vertical
line in the figure as it approaches the pressure at which it can be analysed, p,= The gas is a stable single phase at p0
and continues to be so until it reaches pressure pZ9 which is on the boundary of the two-phase region
Between p2 and the lower pressure p,, both gas and condensed liquid are present. The relative quantities of the gas
and liquid phases, and their compositions, vary continuously over this range. At pressures below p,, and down to
the analysis pressure p,, a single-phase gas exists once more.
Conversely, a cylinder with an initial pressure of g,, filled isothermally to pO, will, as the pressure passes through p,,
contain two phases. These will in theory recombine at p2, but this process is slow, and any gas sampled from the
cylinder while two phases are present will be unrepresentative, and furthermore its removal will alter the
composition remaining in the cylinder.
these problems, in a state
The use of pressurized piston cylinders may be a to avoid keeping the sample where
way
no fall-out will take place.
In fact, as a gas is expanded, its temperature falls due to the Joule-Thompson effect. The gas whose behaviour is
shown in figure 2, starting from a temperature and pressure of 25 “C and IO MPa, will cool to below -10 “C at p,,
and hence suffer condensation. The initial temperature would need to be 35 “C to reach p, without encountering the
two-phase region.
52.2 Condensation after sample has been collected
A gas sample could partially condense in the sample container when it is being transported or is awaiting analysis in
a lab. High-pressure gas sample containers and the lines to an analytical unit shall always be heated prior to
analysis (except for gas that will not pass through a phase boundary). Heating times and temperatures shall be
sufficient to ensure that any condensed hydrocarbons are revaporized before an analysis is started.
5.2.3 Fall-out from the sampling probe
Liquid heavy hydrocarbons and condensation in the sample line which is returning into the main stream may reduce
the measured calorific value of a gas. This will manifest itself in a day/night sine wave effect on the recorder chart,
with the calorific value recording higher in the heat of the day and lower in the cool of the night.
5.2.4 Precautions by applying heating and insulation
In order to avoid condensation problems, the sample handling equipment temperature shall be kept above the gas
dew point at any pressure in the sampling system. Also the gas may be pre-heated, as indicated in figure 2.
5.3 Adsorption and desorption
The process whereby some gas components are adsorbed on to or desorbed from the surfaces of a solid are called
sorption effects. The force of attraction between some gas components and solids is purely physical and depends
on the nature of the participating materials.
Natural gas may contain several components which exhibit strong sorption effects. Special attention shall be given
to this in the case of the determination of trace concentrations of heavy hydrocarbons or impurities.
5.4 Leaks and diffusion
A regular check of the leaktightness of the lines and devices shall be carried out, in order to detect leaks. Minor
leaks or diffusion would affect the composition in the case of trace determinations (water or atmospheric oxygen
may diffuse into the tube or the container, even at high pressure: the partial-pressure difference for a constituent
determines the direction in which it will diffuse). Take special care when hydrogen is present.
Leaks can be detected using detergent solutions, by pressuring the sampling line, or by more sophisticated
methods such as portable leak detection equipment (e.g. mass spectometers).
0 IS0
IS0 10715:1997(E)
5.5 Reactions and chemisorption
Reactive components can combine chemically with the sampling equipment (e.g. by oxidation) or exhibit
chemisorption. Also the materials used in the sampling equipment can catalyse reactions in the samples (e.g. in
mixtures with traces of hydrogen sulfide, water and carbonyl sulfide).
5.6 Precautions using drip pots
Drip pots or gas/liquid separators in a sample line system are intended to remove troublesome intrusive liquids.
Their application shall be considered carefully (see 8.4). Drip pots can accumulate liquid slugs and then
continuously vaporise into the sample stream. There is a danger that their use may change the composition of the
sampled gas. The concentrations of components which equilibrate between the gas and liquid phases are likely to
be altered by removal of the liquid. Sample lines shall slope up from the sampling point, with no low spots that can
accumulate liquids.
6 Materials used in sampling
6.1 General considerations
The suitability of materials used in a sampling system will depend on the gas being sampled. Generally, it is
recommended that stainless steel be used for all surfaces with which the gas will come into contact (see however
6.1 .I). Valve seats and piston seals shall be made of (elastic) material appropriate for the intended service.
Sampling of wet or high-temperature gases, or gases containing hydrogen sulfide or carbon dioxide, presents
additional material problems. These types of gas may require special materials and coatings in the sampling
system. It is recommended that sample cylinders used in sour-gas service shall be either polytetrafluoroethylene
(PTFE) coated or epoxy coated. Reactive components such as hydrogen sulfide and mercury shall be analysed on
site using direct sampling methods when practical since even coated vessels may not eliminate absorption of these
components.
The use of soft metals such as brass, copper and aluminium shall be avoided where corrosion and metal fatigue
problems are likely to occur. Aluminium can, however, be used for sample containers in some applications where
the sample container reactivity is critical.
Generally, materials coming into contact with samples or calibration gases shall have the following characteristics:
- impermeability to all gases;
- minimum sorption;
- chemical inertness to the constituents being transferred.
Because of the possible presence of small amounts of sulfur compounds, mercury, carbon dioxide, etc., in natural
gas, all equipment and fittings shall preferably be made of stainless steel or, for low pressure, glass. However,
possible alternative materials are listed in table 1.
6.1.1 Carbon steel
Carbon steel and other relatively porous materials may retain heavier components and contaminants such as
carbon dioxide and hydrogen sulfide in the natural-gas stream and shall not be used in a sampling system.
material for use in sampling equipment, the user is
Although stainless steel is generally a good recommended to
consult corrosion experts before using it.
Stainless steel is not generally suitable for streams containing water. However, some stainless-steel materials,
4 CrNi 18 IO and 4 CrMo 17 12 2, have proven to be satisfactory.
0 IS0
IS0 10715: 1997(E)
Table 1 - Compatibility of sampling-system materials with gas components
Compatibility” with gas components:
H
Nlaterial cos CH,OH H,S
co 0 RSH H,O He co
2 Hg
CnH m 2
THT
a a a b b a a
Stainless steel b
a a a a a a a
G lass2’ a
b b b a C C b
PTFE3’ C
Polyamide a a b a C a C a
Aluminium a a a b b a C a
Titanium a a a a a a a a
1) a= suitable
b= with reservations
c= not recommended
2) Glass is a highly inert material, but subject to breakage and unsafe for sampling above atmospheric pressure.
3) PTFE is inert but may be adsorptive. It is permeable to e.g. water, He and Hz. PTFE coatings may have imperfections, and
parts of the interior surface may therefore not be protected.
6.1.2 Epoxy coatings
Epoxy (or phenolic) coatings will reduce or eliminate adsorption of sulfur compounds and of other minor
constituents. It is not practical to coat small fittings, valves and other small areas. Losses of gas components from
such unprotected areas may however be detectable and may be measured if concentrations are in the ppb or ppm
range (see also 7.2).
6.1.3 Other polymers
The use of other polymers shall be limited to tubing or connectors joining items of equipment, where there is little or
no direct contact with the sample. Special care shall be taken in the case of water or sulfur-compound analysis.
However, good results may be obtained using polyamide material for short tubing lengths.
In some cases, soft PVC can be used at low pressures.
Before any new polymer material is used in a sampling system, it shall be tested using certified blends at expected
concentrations to verify that it does not cause any change in the sample composition.
6.1.4 Rubbers
Rubber tubing or connections is not recommended, even at low pressure, because of the high reactivity and
permeability of rubber.
Silicon rubbers are known for their high absorbtion and permeability for many components.
6.2 Bimetallic corrosion
Using dissimilar metals in contact with each other in a sample system may cause increased rates of corrosion and
result in sampling errors and/or safety problems.
7 General preparation of equipment
7.1 Surface treatment
The sorption effects exhibited by some materials can be modified and often reduced by surface treatment. A clean,
grease-free surface shows less absorption. Rough surfaces provide a nucleus for gases to adsorb and accumulate.
0 IS0
IS0 10715:1997(E)
Polishing techniques are now available and can be used to minimize sorption effects and reduce the conditioning
time required to bring the sampling equipment to equilibrium.
Other processes are also available to reduce sorption effects. Some materials can be electroplated with an inert
material such as nickel to reduce adsorption.
Passification of aluminium using proprietary techniques is available to inhibit adsorption.
7.2 Cleaning sampling systems
All parts of the sampling and transfer lines in contact with gas shall be free from grease, oil, mould or any polluting
products. Sample containers shall be cleaned and purged prior to each collection of sample, unless they are special
passivated cylinders used to sample streams containing highly reactive components (see also annex B). They shall
be cleaned properly, e.g. with a volatile solvent, and dried to avoid absorption phenomena, particularly those
caused by sulfur compounds and heavy hydrocarbons. Solvents, such as acetone, that do not leave a residue after
drying are generally acceptable for removing heavy-ends contamination, although they may present hazards such
as flammability and toxicity in some cases. Steam cleaning is generally acceptable only if the steam itself is clean
and does not contain corrosion inhibitors, boiler water treating chemicals or other substances that may contaminate
the sample cylinder.
Special care shall be taken in cleaning cylinders that contain deposits.
If analysis of sulfur components is intended, steam shall not be used to clean stainless-steel cylinders. Sulfur
species will be readily absorbed by the cylinders and the analysis will dramatically underestimate sulfur levels.
Samples to be analysed for their sulfur content need to be collected in special lined cylinders or passivated
cylinders dedicated to that purpose. It is important to note that the entire wetted surface of the sample container and
its secondary components shall be coated. Coating the cylinder, but not the valves, fittings, relief devices, etc., may
not be sufficient protection. In certain cases, e.g. H,S-containing gases, PTFE is the recommended coating.
7.3 Conditioning of sampling equipment
This can be achieved by purging the sampling equipment with the sample gas until gas samples taken in sequence
show analytical consistency. Conditioning times may be reduced by the initial evacuation of the equipment prior to
purging with the sample. Several sequences of evacuation and purging may be advantageous in reducing
conditioning time and achieving equilibrium.
The final assessment that equilibrium has been achieved and the sampling equipment conditioned can initially be
determined by analysis using a known standard.
7.4 Pre-charging
Nitrogen, helium, argon and dry, instrument-quality air are good examples of gases that may be used to dry or
purge cylinders which are free of deposits and heavy-ends contamination. In order to avoid interference, the drying
or purging gas used shall not contain any of the constituents to be analysed. Many laboratories leave a blanket of
nitrogen, helium or other gases in sample cylinders in order to protect the cylinder from air contamination. The
blanket gases and gases used to recharge or back-pressure sample cylinders shall be carefully selected so that, if
leakage does occur within the cylinder or the sample is contaminated by these gases, the analytical system will not
interpret the contamination by these gases as being a part of the sample being analysed. For example,
chromatography using helium as a carrier gas will not detect helium left over from the recharge of a single-cavity
cylinder or helium leaking past the piston in a floating-piston cylinder.
8 Sampling equipment
8.1 Sample probes
The design of the probe shall take into account the possibility of resonant vibration being induced in the probe by
high flow velocities in the pipeline. Gas lines with streams free of entrained liquids and at flow conditions well above
their dewpoint temperatures may be sampled with any probe design. However, lines that are operating at or near
0 IS0 IS0 10715:1997(E)
the gas stream dewpoint require a special probe designed to overcome the problems of condensation and liquid
particle entrainment in the gas.
8.1 .I Straight-tube probe
The most basic sample probe design is the straight-tube probe shown in figure 3. The end may be flat or angle-cut.
Figure 3 - Straight-tube probe
8.1.2 Regulated probe
The other type of probe design in common use in the gas industry is the regulated probe. These probes are
commonly used with continuous analyser systems and are designed to deliver the gas to the system at reduced
pressure. The diaphragm and control spring are mounted externally to the pipe wall, and connected by an internal
rod to the point at which the pressure reduction occurs, which is at the lower end of the probe which is inserted into
the gas stream. This lower end is often finned, so that the temperature drop on expansion is compensated for by the
thermal mass of the gas stream. An illustration of a typical regulated probe is shown in figure 4.
8.1.3 Location and installation
The probe shall be located directly in the gas stream in such a way that problems of aerosols and dust are
eliminated.
It is recommended that the probe be located a minimum of 20 pipe diameters downstream from any flow-disturbing
elements such as elbows, headers, valves and tees.
The location of the probe shall be on the top of a horizontal part of the pipe. The inlet shall be located so as to
withdraw gas from the centre one-third of the pipeline diameter.
_ The probe shall be externally equipped with adequate valving. This makes it possible to disconnect the sample line
from the process line. The probe may be of a stationary or removable type depending on location and operating
conditions.
0 IS0
IS0 10715:1997(E)
Adjustable outlet spring
-
- Outlet
Gauge and relief- -
Thermal
ation-
Point of regul
Inlet
Figure 4 - Regulated probe
8.2 Sampling and transfer lines
Generally, sampling lines shall be as short and as small in diameter as possible, but not less than 3 mm in diameter,
to decrease the residence time.
Sample lines venting to the atmosphere shall be minimized. In addition, high-pressure drops may cause cooling and
condensation, which will affect the representative nature of the sample.
The purging time for spot samples shall be at least IO times the residence time. Annex G gives guidelines for the
calculation of the residence time.
All connections between the sample point and the sample container shall be such that sample contamination cannot
occur. Where necessary and allowed, threaded connections shall be made using PTFE tape. Pipe thread sealing
0 IS0 IS0 10715:1997(E)
compounds shall not be used. These products may contaminate the sample and/or absorb components from the
sample, resulting in erroneous results.
8.2.1 Pressure drop in a sample line
Proper operation of a sample line requires a pressure differential from the collection point to the discharge.
This pressure drop may be provided by an orifice plate, regulator or other appropriate device in the flow line.
8.2.2 Dimensions of sampling lines
The flow rate through the sampling line is chosen to ensure a fast response time. However, each application has to
be considered on its own merits.
8.3 Bypass constructions
When using a bypass, closed loops are preferred due to environmental and safety considerations.
8.3.1 Bypass loop
The bypass loop, also known as a “fast loop” or “hot loop”, shall be of the closed configuration; it shall return to the
process line.
3 mm to IO mm stainless-steel tubing should preferably be used. The loop requires a pressure differential, from
collection oint to discharge, to ensure a constant and steady flow rate through the sampling equipment located in
the loop.
8.3.2 Bypass line
Where it is impractical to provide a sufficient pressure differential, thought can be given to the use of an open-ended
bypass line which will ultimately vent to the atmosphere or to a flare.
The flow rate and pressure loss in an open-ended line will need to be controlled to limit any cooling and
condensation which will affect sample integrity.
8.4 Aerosol and/or dust traps
It might sometimes be necessary to control some characteristics of the gas at the outlet of process units (for
hydrogen sulfide content after desulfurization, dewpoint after
example, water content after dehydration,
compression). Some units, because of the nature of the process, may release some contaminants in the form of
liquid, aerosols or froth (glycol, amine, oils, etc.). In that case, it is necessary to protect the pressure reducer and
also the analytical units from contact with any liquid sampled with the gas. If the probe cannot be installed
downstream of a gas/liquid separator in the line, the devices presented in figures 5 and 6 may be used to stop non-
gaseous materials.
8.4.1 Separators
Separators (or “drip pots”) are generally not recommended in sampling systems They may however be used to
ensure that any free liquids that may have been collected by the sample probe do not enter the analyser or
sampling cylinder. Use of this apparatus can create serious inaccuracies if no precautions are taken
...