Spot and Composite Sampling for BTU Analysis
Spot and Composite Sampling for BTU Analysis
Determination and Natural Gas Physical Properties
David J. Fish, Welker, Inc.
The need to be able to take a representative sample of a hydrocarbon product is necessary to ensure proper
accounting for transactions and efficient product processing. The various sampling methods that are available
and the options and limitations of these methods are investigated; the most appropriate equipment to use; the
reasons for its use and correct installation of the equipment are also addressed.
The amount of hydrocarbon product that is transported between producer, processor, distributor and user is
significant. To be able to verify the exact composition of the product is important from an economic and product
treatment standpoint. A small percentage savings made by correctly determining composition will quickly recoup
the investment made in the purchase of a system designed to obtain an optimum sample. In addition, if the best
sampling procedures are followed, the potential for disputes between supplier and customer will be greatly
reduced. The importance of properly determining hydrocarbon gas composition benefits all parties involved and
will achieve greater significance as this resource becomes more expensive and plays a larger role in our energy
needs worldwide.
From the Gas Processors Association publication GPA 2166-05, "The objective of the listed sampling procedures
is to obtain a representative sample of the gas phase portion of the flowing stream under investigation. Any
subsequent analysis of the sample regardless of the test, is inaccurate unless a representative sample is
obtained.” And, from ISO-10715, a representative sample is, “A sample having the same composition as the
material sampled, when the latter is considered as a homogeneous whole.” API 14.1 offers a similar statement in
the latest revision, “a representative sample is compositionally identical or as near to identical as possible, to the
sample source stream”, as does ASTM 5287-08. These standards are the most common referenced on Gas
Sampling procedures, along with the AGA Gas Measurement Manual, Part No. 11, Section 11.3.
Proper sampling is fundamental to the correct determination of the product composition. In a majority of cases,
the sample is also the source for the determination of the specific gravity of the gas. This figure is a critical
component of the flow formula, from which we derive the product quantity. An error in sampling effects both
quality and quantity, and ultimately, profitability. Most current Gas Chromatographs boast an accuracy level of ½
of a BTU, but that should not be the comfort zone for the measurement department. A faulty sampling method or
improperly installed and maintained equipment may alter the BTU content of the flowing stream by 25+ BTU.
While the accuracy of the GC may be considered as a given, the properly executed technique for taking the
sample is certainly not a given.
Natural Gas sampling has been performed for years with techniques handed down from generation to generation.
Most of the methods are not sufficient to meet today's requirements of accuracy and repeatability; however,
standards have been developed to reach toward these demands. The most widely known standards are GPA2166-05 and ISO-10715. API has produced a revised API 14.1, which was published in June, 2001. It has been
updated and revised in 2006. This new standard has already generated significant interest in proper sampling
techniques, due to a large volume of data produced during the revision work.
Proper maintenance of all sampling equipment is vital to the operations of all sampling methods. A review of
relative sampling standards and the manufacturer’s operation, installation and maintenance manuals, is an
important step in the total accurate sampling process. Dirty or poorly maintained sampling apparatus will
adversely affect the final results and profitability of the gas company’s operation.
Sampling can be accomplished by primarily three techniques; spot, continuous composite or continuous on-line
sampling systems. The various components of a sampling system deserve individual consideration, before the
various sampling procedures are investigated. All components should be compatible with the fluid being
Regulators - On-line analysis should use regulators to reduce the pressure to the analyzer. They will reduce the
gas volume to the sampler, thus minimizing the time delay between the sample point, via the regulator, to the
analyzer. This will lessen any negative effect on the gas sample by ambient conditions.
Insertion type or probe regulators are preferable as they will be able to reduce the sample pressure in the flowing
stream enabling a minimization of the Joule-Thompson effect created by the pressure drop. The ambient
temperature of the pipeline gas is transmitted via thermal fins, to offset the cooling effect. The design purpose of
the probe regulator is to not create liquids where liquids do not exist. They were designed for rich, dry gas
systems. All aspects of the regulator systems should consider Hydrocarbon Dew Point impact on the process
and be temperature conditioned to prevent those issues as much as possible. Not all installations will require
attention, but the ones which don’t now days, will be few and far between. Wet gas systems present a new
sampling challenge.
Valves - If shut-off/isolation valves present a restriction that causes a pressure drop, it is possible that
condensation could occur. Large orifice valves should be used, as restrictive valve paths can cause
fractionization of the sampled gas. API 14.1 addresses these porting issues for valves used in the sampling
When used with a collection cylinder it is important that there are no leaks from the gland. Light ends will be the
first to leak off, thereby causing the sample to be overrepresented with heavy ends. It is wise to use valves with
soft seals to give a positive shut-off.
Filters - For on-line analyzers, it is sensible to install a filter. Proper selection of the filter flow capacity and the
particle size capacity should be encouraged. A filter that is too small or does not have a sufficient drip pot
capacity for gases that have entrained water, is a recipe for high maintenance and off spec analysis. It is prudent
to invest in a reasonable filter.
Relief valves - Regulators should have a relief valve installed downstream, if the equipment downstream is not
able to withstand full upstream pressure. Regulators will not always give a guaranteed shut-off and their lock-up
pressure will climb to a dangerous level should there be failure to attain a good shut-off such as seal damage,
diaphragm damage or impurity build-up on working parts and sensing lines.
Pipework - Should be as short and as small a diameter as possible. This will assist in minimizing the time delay
from sample point to the analyzer or cylinder. It will also help maintain the sample integrity. When used with online analyzers, sample delivery lines should slope upward from the probe to the analyzer to prevent condensation
and impurities entering the analyzer. Lead lines to continuous samplers should slope back and drain towards the
Heating Elements – There is sufficient evidence to show that heating all components of a sampling system is a
prudent step in having a reliable and accurate sampling system. The hydrocarbon dew point of a natural gas
stream is a critical issue in obtaining a representative gas sample. API 14.1 spends considerable time on this
issue of heating and Hydrocarbon Dew Point, as does GPA 2166-05.
Probes - The correct placement is at the top of the pipe, into the center one third or at least 200 mm (8 inches)
for larger diameter pipes; in an area of minimum turbulence, that is, away from headers, bends, valves, etc.
Turbulence will stir up the contaminates that usually reside at the bottom of the pipeline and are therefore not
normally part of the gas stream. By having the probe at a point of turbulence these contaminates will be taken
into the sample, giving a sample that is not representative. The key is to have the probe in the center of the line in
the correct spot (positive velocity/no turbulence) with a proper valve on the outlet. Field applications have shown
that mounting the probe on the top of the pipeline is the preferred location. Side, lower or bottom mounts can
easily encourage free liquids (if present) to migrate into the sample system.
Sample Pump - These pumps are, of course, needed to extract the sample from the line and transfer the
sample to the analyzer or collection cylinder. They should have the capability to be able to extract the sample
under flowing conditions, maintain a consistent discrete size of sample, take a fresh purged sample every time
and have the ability to be controlled by a timer or proportional-to-flow controller. This forms the heart of the
continuous gas sampling system. If the pump or sampler is unable to perform all these functions, a
representative sample will not be taken and the sampling exercise will be flawed.
Pumps can be either pneumatic or electric. The safety requirements of the electrical components such as motors
and solenoid valves and the environmental protection rating, dictate careful selection and compliance with
applicable codes. The selection options may well be limited if electrical components have requirements which are
incompatible with the use of standard components elsewhere in the system.
Sample Cylinders - Used for the collection of gases and light liquid hydrocarbons, sometimes called "sample
bombs". The cylinders come in three forms; one is a plain single cavity cylinder with a valve at each end, one is a
single cavity, single fill cylinder with a removable inner bladder and valves at each end, and the third is a Constant
Pressure Sample Cylinder, which takes the form of a closed end cylinder with an internal piston. Before using this
cylinder, one side is pressurized forcing the piston to the sample end. When the sample is taken, the product is
then collected against that back pressure and stored at whatever pressure is pre-charged at the back of the
piston. Using the Constant Pressure Cylinder the sample can be collected at a pressure above the vapor
pressure of the light ends. By having the piston at the end of the cylinder, the need for excessive purging is
eliminated. Pulling a vacuum in the sample cylinder (which is often destroyed by technicians) or using the water
outage method is not necessary. It can be guaranteed that the sample taken is composed entirely of the gas
being sampled. The hook-up is simple and straightforward making the operation easier for technicians and
minimizing the possibility of an incorrect sample being taken. The bladder cylinder also allows for starting with a
voided chamber, as the bag is collapsed by a minimal back pressure prior to starting.
Sample cylinders should be constructed with a material that is compatible with the gas. For instance, H2S can be
absorbed into the structure of 316 Stainless Steel. This will necessitate coating the inside of the cylinder. The
resultant sample will not be truly representative otherwise.
Sample cylinders are normally protected with bursting discs. They are less expensive and are lighter weight than
relief valves, though their proper selection and replacement should have more importance than is sometimes
given them.
With all of the notes on the various components should go the comment which is one of the basic rules of
sampling. The materials of construction of the sampling equipment that come into contact with the
sample are to be compatible with the product being sampled. It is normally reasonably safe to use 316
stainless steel and Viton elastomeric components. Kalrez®, PTFE, Duplex Stainless Steel, Inconel, Titanium and
other materials are becoming common. One should look for these materials in selecting equipment, and ask
questions of suppliers about material selections.
An additional major factor in correct sampling procedures is an awareness of the Hydrocarbon Dew Point of the
gas stream being sampled. The importance of knowing the HCDP is related to 1). The ambient temperature; 2).
The temperature of the equipment being used to collect the sample; and 3). The temperature of the flowing
stream. The creation of liquids due to equipment design and equipment temperature must be avoided.
Determination of the HCDP of the gas stream can be done by the chilled mirror method or by the use of a number
of equation of state models for hydrocarbon dew point determination. There are several programs available such
as Peng-Robinson or SRK. The variations of the calculated results between different equations of state are so
wide, that it is strongly recommended to add 20° to 50°F (11° to 28°C) to the answers. This is to assure the
operator that he is designing his sampling system temperature requirements above the actual hydrocarbon dew
While there are several methods for spot sampling natural gas, three common methods in use today are the fill
and purge method detailed in GPA-2166-05 section 9.1, the piston cylinder method detailed in section 9.7, and
the Helium Pop method detailed in section 9.5.
Spot sampling was the primary method of acquiring a sample for analysis until the early 1970’s. This method is
still widely used today. In today's world of growing trends toward therm-measurement and therm-billing, this
method is increasingly expensive in analytical cost and man-hours, as well as a very questionable method of
assessing an accurate heating value to volume sales. It is at best a "spot" sample of what was present at the
moment the sample was taken. Minutes before and minutes after become unknown guesses. While this may be
a reasonable risk if the gas source is known by a long historical data base, most gas being consumed today is a
combined gas from several origins, or is switched from source to source by contractual updates; in some cases
by daily or even hourly arrangements. This author has been on location and witnessed a 62 BTU increase at a
single sample point, within a one-hour time frame. It was mainly attributed to both a substantial increase and
decrease in flow rate as well as well selection changes within the gathering grid. Also, we find typically, that the
older the well and the longer it stays in production, the higher the BTU value will become. Natural gas is an
extremely fragile product and at almost every step in the production, transportation and distribution of natural gas,
will be exposed to a potential adverse effects on its quality. Switching wells, pressure changes, temperature
changes and storage vessels are only a few of the items that can add or subtract BTU values on the gas moving
through measurement stations. Thus, a spot sample may not even represent the correct source in question.
In early years, the spot sampling method was used where by the gas was introduced into the cylinder until it
reached line pressure, and then was transported to the laboratory for calorimeter or chromatograph analysis. As
the known quality of the gas (BTU value) became more important, tests were conducted to determine if the gas
was being altered by the procedure used to fill cylinders. It was determined that contaminates such as air were
being introduced to the collected sample and a new filling method was needed. The fill and purge method was
adopted and after sometime it was determined that the introduction of liquids was occurring by this process and
thus a newer method was created. This newer method is known as the GPA method using a manifold for filling
the Standard Cylinder. This GPA method reduced the negative effects of the "filling only" procedure. The
manifold allows gas to be "trapped" in the cylinder at full pressure, rather than simply "dead ended" into the
cylinder, i.e. zero pressure up to line pressure.
As the quality of gas became a critical part of billing, along with volume (std. cu. m. or std cu. ft.), the industry
again reviewed the Standard Cylinder and its accuracy.
The need for maintaining the gas at full line pressure from beginning to end became evident. Any reduction in
pressure and change in temperature from the line condition at the time of sample was deemed to alter the gas
analysis in almost every case. Only low BTU gas (975 BTU and below) seemed to possibly escape alteration.
It became evident that when the Standard Cylinder was being filled, the heavy ends dropped out as condensate in
the cylinder until higher pressures were reached in the filling process. The GPA method helped eliminate this
problem. But when the cylinder was being bled into the chromatograph, there was no way to keep the pressure
elevated in that cylinder. As the cylinder was opened, the light ends escaped first, thus giving a certain BTU
value. As the analysis continued, the BTU value increased due to the heavy ends remaining in the cylinder, thus
altering the BTU value in a higher direction. As it is normal that more than one test is performed, due to concerns
of accuracy or custody transfer, repeatability was more often than not, impossible. It became clear that the
decrease in pressure was altering the gas composition.
It was in this environment that the Constant Pressure or Sliding Piston Cylinder was designed and created. With
an internal piston with seals, it was possible to pressurize (pre-charge) the cylinder with an inert gas supply (or the
pipeline gas itself), and then turn the cylinder around and fill it slowly from the opposite end. By letting the gas
push against the piston while "slowly" venting the pre-charge gas, the sample was taken at full line pressure from
start to finish. Then, in the laboratory, a gas supply could be connected to the pre-charge side equal to the
pipeline pressure. As the sampled gas is injected into the chromatograph, the piston is being pushed by the precharge gas. While the cylinder is being emptied, full pressure is being maintained and the gas composition is not
being altered as a result of pressure reduction. The cylinder can be stored, or sent to another laboratory for
confirmation, and when the remaining gas is analyzed, it will give repeatable results, because the condition of the
gas is maintained by the constant pressure cylinder.
The cylinder is equipped with valves, safety reliefs and gauges on both ends, and thus the pressure can be
controlled and monitored at all times on both ends. The temperature is maintained just as with Standard
Cylinders i.e. heating blankets, ovens, or water baths.
This procedure has proven to give extreme accuracy in both spot sampling procedures as well as in automatic
sampling systems. The Constant Pressure Cylinder has been tested against the laboratory chromatograph and
on-line chromatographs, and has shown to maintain the integrity of the sample to within a very close value of the
pipeline gas. No other method consistently performs at this level. Also, the richer the gas, the more alteration
occurs with older methods. Because of this high level of preserving the sample, the cylinders are now used as
calibration standard cylinders. That demand of maintaining accuracy is critical and gives the piston cylinder a
solid recommendation for integrity.
The Constant Pressure Cylinder also brings with it, additional safety in handling the sample. No longer do you
have to purge the cylinder and vent large amounts of gas to the atmosphere. A brief purge of the sample line up
to the cylinder is all that is required. The piston is at the sample end of the cylinder when you commence to fill, so
there is no "dead volume" to purge.
Also, because of the design of the cylinder, with seals on the end of caps, it cannot be over pressured to the point
of exploding. If the cylinder is over pressured, the safety reliefs will allow the pressure to escape. In the rare
event that they fail to work the cylinder will swell and the seals will stop sealing, allowing the product to escape
Constant Pressure Cylinders have served the industry for 25 years to provide accurate sampling procedures,
better sampling systems, repeatability, safer handling, accurate analysis and storage of samples as well as
storage of gas and liquid standards for the laboratory. All updated ISO, GPA, ASTM and API standards and
committee reports, address the proper usage of Standard and Constant Pressure Cylinders for the gas and
liquids industry.
A third cylinder design available today, is a single cavity cylinder with an internal, replaceable bladder or liner.
Relatively new to the industry, it is being used in several research programs as a third method and to establish
data on its performance and results. The bag starts at zero pressure and zero volume (a slight back pressure is
introduced to collapse the bag completely), thus eliminating the need to purge the cylinder cavity. After each use,
the internal bag is removed and discarded, eliminating the need to clean the cylinder.
Because of the increasing cost of one BTU, more and more companies are improving their methods, and
departing from older spot sampling practices.
Composite sampling is the proven middle ground between spot sampling and the continuous on-line analytical
gas chromatographs.
Composite or Grab sampling is the collection of the gas by direct introduction into a sample cylinder from a
probe/valve combination or by means of a timed or proportional-to-flow sampler.
A composite gas sampler or gas sampling system consists of a probe, a sample collection pump, an
instrumentation supply system, a timing system and a collection cylinder for sample transportation. Its sole
objective is to collect and store a representative composite sample at line conditions, allowing it to be transported
to the laboratory for repeatable analysis.
This package will mount on a pipeline and collect samples over a desired sample period unattended. For the
sake of illustration, a description of a common system is provided here.
An engineered probe should be installed which extends into the middle 1/3 of the flowing stream. There is relief
in API 14.1 for large diameter piping, but the accepted norm is the center 1/3 of the pipeline. This location should
be chosen to provide a representative sample of the gas stream, thus devoid of stagnant gas, i.e. blow down
stack, and devoid of free liquids and aerosols, i.e. downstream of piping elbows or orifice fittings which cause
turbulent flow. The probe should have a large ported outlet valve to prevent fractionation, resulting in
compositional changes in the gas.
A self-purging sample collection pump designed to operate under line conditions should be located above and as
close to the probe as is practical and possible. Filters, drip pots, screens, regulators and such conditioning
equipment shall not be placed between the probe and the sampler, as this will affect the representative nature of
the sample which is taken. Inlet check valves can also cause the gas to fractionate, due to the restriction it
causes in the line.
The sampler instrumentation source can be from the pipeline itself (the most common installation) or from an
auxiliary instrument supply.
The timing system can be a simple function timer and solenoid, a proportional-to-flow signal conditioner and
solenoid, or simply, a solenoid ready to be connected to field RTU's or other electronic devices capable of
providing the desired signal.
The sample collection cylinder can be either a conventional single cavity sample cylinder, the lined cylinder or
piston style, constant pressure sample cylinder. As these cylinders will be transported, they should meet design
criteria such as ASME Section 8 or carry approvals from recognized agencies such as D.O.T., DNV, Lloyds, etc.
A typical system would include a 500ml cylinder which would be used on a monthly basis to contain 2200+ bites of
.2 cc size during the sample period.
Using the grab sampler, it is possible to obtain a representative sample over a pre-determined period. It is the
only practical method for collecting a continuous sample. The grab sampler will introduce a set volume, taken in
equal amounts to the collection cylinder over a set period and is the preferred method when a representative
sample has to be taken over time.
It has the advantage of being able to measure precisely a predictable amount over a given period when using a
timer, and can also take samples proportional-to-flow when taking a modified signal from a flow meter.
In addition, the sample is taken from the flowing stream at the system pressure and can be fed into the sampler
or sample cylinder at the flowing pressure; thus any change in composition is avoided.
Another feature required of any sampler is that it should not have areas or pockets where residue of previous
samples can accumulate and, must take a fresh grab or bite of gas each time it samples.
This then describes a typical continuous composite sampling system, which has been proven to provide a
representative sample for analysis. Such systems have been tested against continuous on-line gas calorimeters
and gas chromatographs with + 1 BTU accuracy for the total sample period, at considerably less cost and
maintenance than on-line GC's.
And finally, in the realm of gas sampling there are the continuous on-line analytical units, the calorimeter and the
chromatograph. These units have their place in the past, the present and will continue to have an important place
in the future of gas sampling. It is their cost, power requirements and typical up-keep that precludes their use in
1000's of locations. On-line analysis is convenient, although it is dependent on the accuracy of the analyzer, its
correct calibration and the quality of the sample reaching it. It tends to be expensive to install and maintain.
Economics, remote location, and downtime for service dictate the use of spot or composite sampling techniques
at a majority of sample points and installations. It is also important to point out that with on-line units there is no
second or third chance at analysis, and no second opinion option, as is the case with a sample in a sample
On the immediate horizon, a new technology is emerging. Energy meters are soon to be introduced as an online, instant BTU meter. They will not provide analysis in the manner of the existing GC’s, but they will provide
immediate BTU values. This new technology will fill a current void in real-time billing and plant operations. Their
value is in reduced costs compared to on-line GC’s, reduced maintenance and calibration costs, and in providing
real-time information for operations.
The transportation of natural gas samples is a very important issue for both the companies that are involved and
the individual personnel who are transporting the samples. The United States Department of Transportation
(DOT) covers the transportation of samples in CFR-49. Everyone involved in transporting sample cylinders and
other sampling apparatus, both to and from sample collection locations, should be familiar with the rules and
regulations set forth in CFR-49.
As well as the safety issues, markings and forms that are to be filled out for DOT purposes, other considerations
should be addressed as well. Among these are:
Proper tagging of the cylinder for time, date, location of the sample
Pressure and temperature of the pipeline source
Technician who took the sample
Method used to obtain the sample
Plugging of the valves and checking for leaks prior to transport
Protection of the cylinder and sample apparatus during transport, both to and from the sample
Temperature concerns during transport, both to and from the sample location – if necessary or
Other company procedures that will assist in the success of a quality sample being delivered to
the laboratory for an accurate analysis.
The methods, techniques, and designs of today's sampling systems should be considered by every producer,
shipper, buyer and end-user. Regardless of the application or installation, there is a system which meets your
needs, and will affect your company in the profit and loss column. Sampling and metering are the cash register of
your company. Sampling is an art! Examine your methods, procedures and needs closely.
"Proper Sampling of Light Hydrocarbons", O. Broussard, Oil and Gas Journal, September 1977
"Standard Cylinder vs. Constant Pressure Cylinders", D. J. Fish, Gas Industries, January 1994
"Analyzing Heating Value", T. F. Welker, Pipe Line Industry, October 1990
"Natural Gas Sampling", T. F. Welker, Presented at AGA Annual Meeting, Anaheim, California, 1981
"Methods, Equipment & Installation of Composite Hydrocarbon Sampling Systems", D. J. Fish, Presented at
Belgian Institute for Regulation and Automation, Brussels, Belgium, 1993
“Practical Considerations of Gas Sampling and Gas Sampling Systems”, D. J. Fish, Pipeline and Gas Journal,
July 1997
"Selection and Installation of Hydrocarbon Sampling Systems", D. A. Dobbs & D. J. Fish, Presented at Australian
International Oil & Gas Conference, Melbourne, Australia, 1991
“The Importance of Discerning the Impact of New Measurement Technology”, D. J. Fish, Presented as Keynote
Address, 25th Annual North Sea Flow Measurement Workshop, Oslo, Norway, October 2007
Various Standards of AGA, GPA, API, ASTM and ISO
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