When the hydrocarbon dew point temperature is not measured, • use a constant pressure spot sampling method while maintaining the sample gas temperature at or above the flowing gas temper
Initial Sampling of a Gas Stream of Unknown Hydrocarbon Dew Point and Composition
To effectively implement the methods outlined in this standard, it is essential to begin with an accurate hydrocarbon dew point value for the sampled stream.
For initial sampling of a gas stream of unknown composition, the following techniques are recommended in order of preference:
To ensure accurate measurements, it is essential to determine the hydrocarbon dew point temperature and keep the sample gas temperature above this level According to ASTM D 1142, which outlines the standard test method for assessing water vapor content in gaseous fuels, the sample should be maintained at least 3ºF (1.7ºC) above the dew point Adhering to these guidelines is crucial for reliable results.
When the hydrocarbon dew point temperature is not measured,
Utilize a constant pressure spot sampling method, ensuring that the sample gas temperature is maintained at or above the flowing gas temperature Conduct an extended analysis to accurately calculate the hydrocarbon dew point temperature.
To accurately determine the hydrocarbon dew point temperature, it is essential to utilize a pressure-reducing sampling method and conduct an extended analysis When employing this sampling technique, ensure that adequate heat is supplied at or before the pressure reduction point to counteract the Joule-Thomson effect.
• use historical information, such as analyses or dew point measurements from a similar gas source, or
To determine the hydrocarbon dew point temperature, collect a spot sample at line pressure and heat the gas sample to a minimum of 30ºF above the flowing temperature at the time of collection Conduct an extended analysis following this procedure, while adhering to the cautions outlined in section 6.6 regarding equipment heating.
To ensure accurate measurements, it is essential to keep the sample gas temperature at least 30ºF (17ºC) above the calculated hydrocarbon dew point temperature, as outlined in section 6.6, General Discussion of Heating.
If there are changes in the stream's composition, pressure, or temperature, or if the hydrocarbon dew point is suspected to have altered, it is essential to repeat the process to confirm the hydrocarbon dew point Additionally, ensure that the gas temperature in the sampling system is kept in accordance with section 6.6, which discusses heating.
6 General Considerations for the Design of a Natural Gas Sampling System
The main consideration in the design of a natural gas sampling system is to deliver a representative sample of the gas from the sample source to an analytical device.
When designing a sampling system, it is crucial to consider several key factors, including the expected gas quality, phase-change characteristics, and the type of sample and analysis required Additionally, the materials used for sample delivery, the extremes of ambient conditions, and the need for cleanliness must be addressed Other important considerations include the availability of power, the flow rate, and the transport time of the samples.
Inert compounds like carbon dioxide (CO₂) and nitrogen (N₂) significantly influence the heating value and density of gas Their presence can also affect the choice of carrier gas for gas chromatography and determine if the gas stream complies with contractual specifications.
The Components of Typical Sampling Systems
Spot and composite sampling methods necessitate the use of sample containers for transporting samples from the field to the laboratory For instance, Figure 1a illustrates a spot sampling apparatus that utilizes the purging-fill and empty method, while Figure 1b demonstrates the application of a composite sampler.
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Figure 2 shows the components in a continuous sampling system or mobile sampling system
For specific details regarding the design of these systems, see Sections 7, Sample Probes, 8, Sample Loops/Lines, 9, Sample Con- tainers, and 13, Automatic Sampling.
For details regarding the heating and insulating of sampling systems, see 6.6, General Discussion of Heating.
Flow Characteristics
Piping elements like valves and orifices can generate re-circulation regions, or eddies, which may lead to significant differences in gas composition compared to the bulk flow Downstream of a gas-liquid separator, the gas is typically close to its hydrocarbon dew point, and a decrease in line temperature can result in condensation Additionally, pipelines that consistently contain both gas and liquid present challenges for obtaining accurate gas samples, which are beyond the scope of this standard For further insights, refer to Appendix C, Lessons Learned.
During Sampling in Hydrocarbon Saturated and 2-Phase Natural Gas Streams Also see Appendix B, Fluid Mechanical Consid- erations in Gas Sampling for a detailed discussion of fluid mechanics.
Users are cautioned not to interpret liquids condensed by the metering or sampling system as free liquids flowing through the pipeline.
Single-phase flow refers to natural gas that is above the hydrocarbon dew point and free from contaminants such as compressor oil and water It is ideal for the gas in pipelines to be in a turbulent flow regime, as this turbulence ensures a well-mixed and representative fluid.
Causes of Gas Sample Distortion
Natural gas, composed of both organic and inorganic gases, can lose its integrity if any of its components are altered, such as through air contamination or hydrocarbon desorption This section explores the key mechanisms that lead to sample distortion, emphasizing that a thorough understanding of these processes can help prevent the design of ineffective sampling systems.
This section focuses solely on the mechanisms that can distort sample collection Additionally, inadequate gas analysis techniques may also lead to inaccuracies in the indicated composition, as discussed in Section 16, Guidelines for Analysis.
Condensation and revaporization of hydrocarbons in the sampling system can lead to substantial distortions in gas samples These distortions may arise in both flowing and non-flowing conditions, whether in a flowing sample delivery system or within sample containers Additionally, contact with sampling equipment that is below the hydrocarbon dew point can exacerbate these issues For a comprehensive understanding of the thermodynamics involved in these phase changes, refer to Appendix A, The Phase Diagram.
Accurate sampling from gas streams at hydrocarbon dew point temperatures is more challenging than from those above this threshold Retrograde condensation may occur if pipeline pressure is reduced, depending on the conditions Some gas mixtures can have a hydrocarbon dew point high enough to induce condensation at typical ambient temperatures If condensation happens and only the gaseous phase of the sample is analyzed, the heating value and density will be inaccurate, leading to a sample that is no longer representative of its source.
To maintain the integrity of gas samples and calibration standards containing condensed components, it is essential to revaporize the condensed liquids without any prior release of gas or liquid from the sample container.
Revaporization, and Appendix A, The Phase Diagram.
A gas sample in a sampling system can undergo changes in temperature and pressure These reductions in pressure and temperature happen as the gas accelerates through the tubing components of the system, particularly when the gas is close to hydrocarbon levels.
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Figure 1a—Typical Spot Sampling System Figure 1b—Typical Composite Sampling System
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Figure 2—Typical Continuous (On-Line) Sampling System/Mobile Sampling System
The gas dew point is crucial as condensation can lead to gas sample distortion It is essential to consider the fittings and components in a flowing sampling system, especially those that result in a significant pressure drop.
When natural gas flows through a restriction, such as a partially closed valve or regulator, condensation may occur if the pressure drops from 1500 psia to 900 psia To prevent this condensation, it is crucial to maintain the sample gas temperature above the hydrocarbon dew point This requires the gas temperature to be sufficiently high to counteract the temperature decrease that accompanies the pressure reduction.
A, The Phase Diagram, for further information
When a non-flowing gas sample is exposed to temperatures below the hydrocarbon dew point, condensation occurs, leading to sample distortion.
A gas sample may condense in the sample cylinder during transport or while waiting for laboratory analysis For instance, in the case of natural gas, as illustrated in Figure 3, if the cylinder is subjected to ambient temperatures that fall below the hydrocarbon dew point temperature, the sample can undergo condensation along path 4–5.
When transferring a small representative sample from a large accumulator, it is crucial to ensure that any condensation has been revaporized before and during the transfer, and that the sample is thoroughly mixed This practice is generally discouraged, as obtaining a representative sample from one cylinder to another is challenging when the gas is at or near the hydrocarbon dew point.
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Figure 3—Examples of Thermodynamic Processes Associated with Sampling System Design and Sampling
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To ensure accurate analysis, gas sample containers and connecting lines must be heated before testing Adequate heating times and temperatures are essential to revaporize any condensed hydrocarbons prior to analysis For more details, refer to Section 6.6 on Heating and Appendix A on the Phase Diagram.
When the sample stream interacts with sampling equipment at a temperature lower than the hydrocarbon dew point, condensation may occur, leading to sample distortion For instance, in the natural gas mixture illustrated in Figure 3, condensation can happen during constant pressure sampling if the equipment's temperature is below the hydrocarbon dew point (path 6–7) For more details, refer to section 6.6 on Heating and Appendix A on the Phase Diagram.
There are three important surface effects that have been identified for gas sampling.
1 Clean, solid surfaces are subject to adsorption (sticking) and de-sorption (unsticking) of gas molecules.
2 Some liquids may dissolve gas molecules, or they may yield certain gas molecules if the liquid already contains significant amounts of dissolved gas molecules.
3 Porous surfaces can cause gas sample distortions.
The equilibrium of the first two effects relies on pressure and temperature Changes in the temperature or pressure of the sampling system can lead to the removal or release of hydrocarbons in the sample stream, resulting in inaccurate measurements of heating value and density.
To avoid surface effect errors, sampling systems must utilize clean, inert, and non-porous materials, along with effective temperature control to ensure conditions remain above the hydrocarbon dew point.
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See 6.5, Cleanliness, 8.3, Tubing Materials, 6.6, General Discussion of Heating, Section 10, Materials for Sweet and Sour Gas
Service, and Appendix A, The Phase Diagram, for further information.
Adsorption occurs both chemically and physically Chemical adsorption is due to a reaction between the gas molecules and the solid surface molecules.
Revaporization
Section 6.3.1, Phase Changes, identifies a phase change as one cause of gas sample distortion This type of sample distortion will have a significant impact on the integrity of the gas sample.
Analyzing the gas phase of a partially condensed sample in a container can lead to biased heating values and density measurements, making the remaining sample unrepresentative of its original source.
If a gas chromatograph's calibration gas standard undergoes a phase change during calibration, resulting in the gas being in two phases, all subsequent analyses will be biased Additionally, this phase change alters the composition of the calibration gas standard.
To maintain the integrity of the sample and calibration standard, it is essential to heat the containers without withdrawing any fluid before revaporization When analyzing a gas sample in a laboratory, it is crucial to ensure that all condensed gases are fully revaporized.
Section 6.6, General Discussion of Heating, provides guidelines for heating samples and calibration standards, and for revaporiz- ing condensed samples and calibration standards.
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Cleanliness
Section 6.3.2.2 addresses the effects of liquid hydrocarbon residues at liquid/gas interfaces It is essential to implement effective cleaning procedures to eliminate liquid hydrocarbon residues and other contaminants, such as water or glycol residues, that may be present.
Sample systems should be designed so that they may be thoroughly and easily cleaned A procedure for cleaning sample systems and sample containers is essential for good gas sampling practices.
Before collecting samples, it is essential to purge and clean sample containers, except for specialized passivated cylinders designed for highly reactive components The most effective method for cleaning is wet steam, which is acceptable only if it is free from corrosion inhibitors, boiler water treatment chemicals, or any other contaminants that could compromise the integrity of the sample cylinder.
Solvents like acetone and liquid propane are effective for removing heavy ends contamination without leaving residue, but they can pose flammability and toxicity risks Decon Contrad ® 70 washing yields comparable results to these solvents Other chemically equivalent solvents to Decon Contrad ® 70 may also be effective, but their performance should be tested before use.
Supercritical carbon dioxide cleaning produces acceptable results, in spite of the fact that some dirty hydrocarbon spot trails may be left in the cylinder after it is cleaned.
Using supercritical carbon dioxide for cleaning cylinders poses significant hazards due to its unique physical properties, as well as the high pressures and low temperatures required Therefore, it is essential to exercise extreme caution when utilizing this method.
After wet cleaning procedures, sample containers should be thoroughly dried and purged to remove residual liquid Achieving a vacuum of 1 millimeter of mercury absolute (133 Paabs) or lower is effective for this purpose Gases such as nitrogen, helium, and dry instrument quality air are ideal for drying or purging cylinders that are free from deposits and heavy hydrocarbon contamination.
To prevent air contamination, laboratories often use a blanket of gases such as nitrogen or helium in sample cylinders It is crucial to select these gases carefully to ensure that any potential leakage or contamination does not interfere with the chromatograph's analysis For instance, when helium is used as a carrier gas, the chromatograph will not register helium that remains from the pre-charging of a single cavity cylinder or from leaks in a constant pressure cylinder.
To ensure accurate sulfur content analysis, it is crucial to collect samples in clean, dry, and specially-lined or passivated cylinders designed for this purpose General-purpose sample cylinders can absorb various sulfur species, leading to an underestimation of sulfur levels Additionally, all wetted surfaces of the sample container and its components, such as valves and fittings, must be non-reactive to sulfur or sulfur-containing compounds.
To ensure accurate sulfur content measurement, it is crucial to note that interaction with water and other components in the sample gas can degrade sulfur levels, regardless of the cylinder material Therefore, prior to sample collection, the sample loop and any separators in the system must be thoroughly purged to eliminate contaminants and accumulated liquids.
General Discussion of Heating
Condensation can occur in various sampling systems, including composite, spot, mobile, or on-line setups If the sampling process causes the sample temperature to drop below the hydrocarbon dew point, it can lead to biased analytical results and non-representative samples To prevent this issue, it is essential to maintain the sample gas temperature above the hydrocarbon dew point during sampling This can be achieved by heating sample probes, heat tracing lines, regulators, and sample cylinders, or by using alternative methods to deliver heat to the fluid in the sampling system.
To ensure accurate measurement of the hydrocarbon dew point, it is advisable to maintain the gas sample at least 30ºF (17ºC) above the expected dew point throughout the sampling system This temperature margin can be adjusted for specific gas compositions where data indicates that the difference between calculated and measured dew points is less than the recommended 30ºF If ambient temperatures exceed the hydrocarbon dew point, heating may not be necessary during the sampling process.
6This term is used only as an example only, and does not constitute an endorsement of this product by API.
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`,,```,,,,````-`-`,,`,,`,`,,` - tion, provide sufficient heat at or prior to, the point of pressure reduction to offset the Joule-Thomson effect (approximately 7ºF (3.9ºC) per 100 psi (690 kPa) of pressure reduction).
Heat tracing methods most commonly used are steam, hot water and electrical.
Electrical heat tracing is commonly employed in remote areas or where steam or hot water usage is impractical It is essential for electrical heat tape to be self-limiting or regulated by temperature-limiting devices to ensure safety and efficiency.
Electrical heat tracing and all associated electrical equipment must comply with the relevant electrical codes for their designated service area Adhering to these standards is crucial to prevent overheating of heating elements in the event of electrical component failures Overheating can lead to serious hazards, including injuries or explosions, particularly in natural gas applications.
To prevent liquid condensation in sampling systems on residue gas lines at gas plant outlets during operational disturbances, it is essential to implement heat tracing Accumulated liquids can lead to delays of several days in obtaining representative samples once normal operations resume, and purging the entire system may become necessary.
A catalytic heater generates heat via an exothermic reaction involving combustible gases like natural gas and oxygen, along with a catalyst The heat output can be controlled by adjusting the gas flow rate to the catalyst, ensuring that the heat produced remains significantly below the ignition temperature of natural gas.
Electrical heat tracing and all associated electrical equipment must comply with the relevant electrical codes for their designated service areas Adhering to these standards is crucial to prevent overheating of heating elements in the event of electrical component failures Such overheating poses significant risks, including potential injuries or explosions, particularly in natural gas applications.
Insulation is essential for safeguarding the flowing stream from cold external temperatures and for covering heat tracing on the external parts of the probe assembly, regulator, and sample line This insulation ensures that the sampled stream stays above the hydrocarbon dew point throughout the sampling process.
To ensure accurate sampling, the cylinder temperature must remain above the hydrocarbon dew point If the sample cylinder is subjected to temperatures below this point post-collection, it can be restored by heating to at least 30ºF (17ºC) above the flowing temperature during sampling and maintaining that temperature for a minimum of 2 hours before analysis Additionally, if the sample is collected at a pressure different from the line pressure, it should be heated to at least 30ºF (17ºC) above the calculated hydrocarbon dew point at the conditions present in the sample cylinder at the time of collection For further details, refer to section 6.6 regarding the operating temperature margin.
Sample containers can be effectively heated using controlled methods such as water baths, heating blankets, heat tape, or heated chambers It is important to avoid using heat lamps and similar devices due to the difficulty in maintaining a consistent temperature Additionally, all heating methods must comply with relevant codes and regulations.
When heating a sample cylinder assembly above 125ºF, it is crucial to consider the impact of heat on seals and materials Always check the label to ensure that the container's contents will not cause overpressure in the sample cylinder when heated Additionally, take precautions to avoid heating cylinders filled with liquids, as this may lead to overpressure.
6.6.5 Pressure Regulators and Regulating Probes
To ensure optimal performance, the gas temperature should be kept sufficiently high to counteract the temperature drop caused by pressure regulation, which is approximately 7ºF (3.9ºC) for every 100 psi (690 kPa) of pressure reduction It may be necessary to heat the regulator, regulating probe, and any exposed tubing to maintain the gas temperature at least 30ºF (17ºC) above the calculated hydrocarbon dew point temperature, as outlined in section 6.6 on heating.
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To ensure that the composite sample accurately reflects the flowing gas stream, it is essential to maintain sampling systems, including the sample container, above the hydrocarbon dew point Field tests have demonstrated that composite sampling systems often fail to deliver representative samples when subjected to ambient temperatures that fall below the hydrocarbon dew point of the sample gas.
Note: No tests were conducted with sample containers below the hydrocarbon dew point and all other components of the composite sampling system above the hydrocarbon dew point.
To ensure accurate calibration, the standard must be heated for a minimum of 4 hours after the cylinder's skin temperature reaches at least 30ºF (17ºC) above the calculated hydrocarbon dew point This requirement guarantees that the core temperature of the calibration standard is sufficiently elevated to facilitate the rapid vaporization of heavy components It is typically unnecessary to exceed 50ºF (27.8ºC) above the dew point, and prolonged heating will not harm the standard Additionally, the sample lines and any regulators in the system should also be kept at a temperature at least 30ºF (17ºC) above the calculated hydrocarbon dew point.
For details regarding condensation and vaporization of natural gas, see 6.3, Causes of Gas Sample Distortion, and Appendix A,
General Design Considerations
Sample probes are essential for directing a representative portion of natural gas from the pipeline to the sampling system, ensuring that the sample is free from contaminants on the pipe wall These probes can be fixed or insertable and retractable, and must be designed to avoid restricting sample outlet flow A well-installed sample probe is crucial for an effective sampling system Various designs are available, with considerations for potential resonant vibrations caused by high flow velocities Probes can be used for gas lines free of entrained liquids and above dew point temperatures, while special designs are necessary for lines near the dew point to prevent condensation issues Selecting the appropriate sample probe begins with assessing the sampling conditions For more details, refer to Section 6, General Considerations for the Design of a Natural Gas Sampling System.
Application
Sample probes and other components of sampling systems must be designed to deliver a representative sample of the sample source.
Types
The straight tube probe is the simplest design for sampling, which can be attached to either a fixed or removable coupling assembly It is essential that all fittings in the sampling system are non-restrictive and rated for the specific line pressure and temperature The materials used must be compatible with the product, its contaminants, and the surrounding conditions Numerous modifications can enhance this basic design, such as adding filters and screens to the probe's collecting end to minimize the entry of small liquid particles, while ensuring that the gas composition remains unchanged However, certain filters may inadvertently create liquids that are not present in the original stream, particularly in gas streams near the hydrocarbon dew point For optimal performance, the collection end of the probe should be straight-cut rather than beveled.
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Figure 4— Straight Tube Sample Probe Figure 5—Typical Regulated Sample Probe
Regulating probes are essential in continuous sampling systems that deliver gas at reduced pressure, as illustrated in Figure 5 These probes typically consist of a straight tube with an integrated regulating mechanism However, if retrograde condensation occurs, it can lead to non-representative gas samples To mitigate this risk, fins may be added to enhance heat transfer and prevent condensation It is also crucial to minimize thermal coupling between the probe and the sample source stream For more details, refer to sections 6.6 on heating, 6.3 on gas sample distortion, and Appendix A on the phase diagram.
A Pitot probe is effective when the gas velocity in the pipeline is adequate to maintain flow throughout the loop Alternatively, installing two straight tube probes at a differential pressure point can ensure consistent flow across the system.
Note: Adequate data is not available to make recommendations regarding the performance of Pitot probes.
Probe Installation
For effective sampling techniques, it is essential to utilize a probe configuration as outlined in section 7.3, Types Industry standards recommend positioning the collection end of the probe within the central one-third of the pipe's cross-section This placement helps avoid areas prone to migrating liquids near the pipe wall Additionally, it is important to limit the probe length to prevent potential failure caused by resonant vibration.
Resonant vibration occurs when the frequency of vortex shedding from a probe in a flowing fluid matches or exceeds the probe's natural resonant frequency Table 1 outlines the maximum recommended probe lengths for various diameters, assuming a maximum natural gas velocity of 100 ft/sec (30.48 m/sec).
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Table 1—Maximum Recommended Probe Lengths
Probe Outer Diameter inches (cm)
Recommended Max Probe Length inches (cm) 0.25 (0.64) 2.00 (5.08) 0.375 (0.95) 3.25 (8.26) 0.50 (1.27) 4.25 (10.80) 0.75 (1.91) 6.50 (16.51)
Calculations utilized a maximum recommended probe length with a Strouhal Number of 0.4, a wall thickness of 0.035 inches (0.089 cm), and construction from 316 stainless steel For different conditions, the maximum recommended probe length can be determined using the formula provided below.
The probe length refers to the distance from the probe tip to its attachment point, while the probe depth measures the distance from the probe tip to the inner wall of the pipe.
Equation for Maximum Recommended Probe Length
L = [[(F m x 4.38 x OD x 10)/(S x V)] x [(E/ρ) x (OD 2 + ID 2 ) ] ẵ ] ẵ (Reference 16) where
The virtual mass factor (\$F_m\$) is a constant that accounts for the additional mass of a cylinder caused by the surrounding fluid that vibrates with it For gases, the virtual mass factor is \$F_m = 1.0\$, while for water and other liquids, it is \$F_m = 0.9\$.
OD = OD of Probe (mm)
ID = ID of Probe (mm)
S = Strouhal number = dependent on the Reynolds No & shape of the cylinder, but can be taken as 0.4 for worst case or 0.2 as suggested by API Chapter 8.
E = Modulus of Elasticity of probe material (kg/cm 2 ) ρ = Density of probe material (kg/m 3 )
L = [[(F m x 1.194 x OD)/(S x V)] x [(E/ρ) x (OD 2 + ID 2 )] ẵ ] ẵ (Reference 16) where
Fm = Virtual mass factor – For a gas, F m = 1.0 and for water and other liquids, F m = 0.9
OD = OD of probe (in.)
ID = ID of probe (in.)
S = Strouhal Number = Use 0.4 as worst case
V = Velocity of fluid (ft/sec)
E = Modulus of elasticity of probe material (psi) ρ = Density of probe material (g/cc)
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Probes must be installed vertically at the top of a straight section of horizontal pipe In gas streams that are not close to the hydrocarbon dew point, any probe location is generally acceptable, provided it does not disrupt the operation of a primary metering element, as outlined in relevant reference standards.
For streams near their hydrocarbon dew point, it is essential to position the probe at least 5 diameters downstream of any major disturbances, such as orifice plates, elbows, tees, and valves, using the inside pipe diameter at the disturbance as the reference This guideline aims to prevent the formation of liquid droplets or liquids caused by the interaction of the flowing stream with the disturbing element Additionally, similar precautions should be observed for streams containing liquid contaminants like glycols, amines, compressor oils, and corrosion inhibitors.
Probes must not be placed in "dead-end" sections of piping, as these areas lack continuous gas flow and may experience recirculation or "eddies." For further details, refer to Section 6.2 on Flow Characteristics and Appendix B, which discusses Fluid Mechanical Considerations in Gas Sampling.
C, Lessons Learned During Sampling in Hydrocarbon Saturated and 2-Phase Streams, for more information.
General Design Considerations
The sample loop, also known as the slip stream, is a crucial component of the sampling system that transports gas from the sample probe to the sampler or analysis device, ultimately reaching a lower pressure point It is essential for sample loops to be designed to provide a representative sample of the gas flowing in the pipeline The gas velocity in the sample line and the sample system's volume dictate the frequency of obtaining new representative samples To reduce liquid accumulation in the loop, it is advisable for the sample line to be inclined upward from the sample probe to the extraction point.
To obtain a representative gas sample, the design of the sample loop must ensure complete replacement of the gas volume between samples, necessitating a high flow rate and a small loop volume However, excessively high flow rates can lead to the inclusion of liquid particles from the pipeline into the sample probe Additionally, sample loops that vent to the atmosphere can result in significant gas waste, potentially breaching environmental regulations Furthermore, substantial pressure loss within the sample loop may lead to cooling and condensation, adversely impacting sample accuracy.
Figure 6—Probe Dimensions Used to Determine Maximum Recommended Probe Length
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Pressure Drop in a Sample Loop
To ensure the proper functioning of a sample loop, a pressure differential must be established between the collection and discharge ends, which can be achieved using an orifice plate, regulator, or pump By positioning the sample loop across a flow restriction, the resulting pressure differential becomes proportional to the square of the flow rate, allowing for a flow in the sample loop that mirrors the flow through the orifice However, custody transfer meters are unsuitable for this setup, as the flow through the sample loop will circumvent the meter, leading to inaccurate flow rate measurements.
Tubing Materials
For most applications, 304 or 316 stainless steel tubing is the preferred choice for the sample loop However, if the sampled product negatively impacts or is influenced by stainless steel, alternative materials like Nylon 11 or similar options may be utilized.
Stainless steel tubing sizes of 1/16, 1/8, and 1/4 inches (1.59, 5.08, and 6.35 mm) have small volumes per unit length, facilitating the efficient removal of free liquids from the loop For added mechanical security, this tubing can be encased in larger tubing or pipe.
Before installation, all sample loop tubing must undergo steam cleaning and drying to eliminate any residual oil from the manufacturing process This oil can trap or release heavier hydrocarbon components, with the extent of retention or release influenced by factors such as the concentration of these components in the sample gas, the temperature of the sample loop, the flowing pressure, and the surface area of the tubing that is coated with oil.
Nylon 11 tubing is suitable for applications that do not involve excessive heat and is especially effective for detecting free liquids It is essential to clean the sample loop if there is any suspicion of contamination by free liquids.
Pressure Regulators
Pressure regulators are essential for reducing gas pressure from pipeline levels to a suitable pressure for sample containers or analysis devices Specialized regulators can be inserted into the pipeline to utilize the flowing gas temperature, while insertion type regulators with integral filtration are designed to eliminate free liquids It is crucial for pressure regulators to have a pressure rating that surpasses the maximum expected line pressure of the gas sampling system, and they should be made from materials that are non-reactive with the sampled gas.
When using any type of gas regulator, it is crucial to prevent gas condensation, as retrograde condensation can happen even at pipeline temperatures If the gas undergoes a phase change and condensation occurs, it will result in an unrepresentative sample.
To minimize heavy end condensation in pressure regulators, utilizing a heated regulator is effective, as it provides sufficient heat during pressure reduction to prevent condensation Additionally, heat tracing can further reduce condensation risks The necessary heat energy to counteract the pressure drop effects varies based on factors such as gas composition, pressure, temperature, pressure drop, and hydrocarbon dew point It is crucial to maintain the gas temperature high enough to counterbalance the temperature decrease that occurs during pressure reduction.
See 6.3, Causes of Gas Sample Distortion, 6.6, General Discussion of Heating, and Appendix A, The Phase Diagram, for further information.
Pumps
Pumps are essential for ensuring adequate gas flow through a sample loop, and they should be installed to maintain a steady flow without pulsations or interruptions For optimal performance, the pump should be positioned downstream of the sample container or analysis device It is crucial to match the pump and sample loop line sizes to prevent potential pump damage A centrifugal pump is the preferred choice for this application.
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Filters
Particulate filters are essential for eliminating solid, abrasive particles from the sample source stream, primarily to safeguard analytical equipment These filters typically range in size from 2 to 7 μm (0.08 to 0.28 mils) and have a minimal impact on analytical results, usually within ± 0.25%.
Be sure to change filters regularly and insure condensation does not occur within them.
Multiple streams, with different heating values, flowing through the same filter, may cause sample distortion and should be avoided.
Separators
The GPA Standard 2166 advises against using the specified separator for sampling single-phase streams, as temperature fluctuations can lead to condensation or vaporization when the gas temperature falls below or rises above the hydrocarbon dew point.
In streams not covered by this standard, GPA separators can be utilized to block free liquids from entering the downstream sampling system For detailed guidance on the use of separators, please consult GPA Standard 2166.
General Design Considerations
A sample container is essential for securely storing gas samples without altering their composition It is crucial that the materials, valves, seals, and lubricants used in the container are carefully selected to prevent any impact on the gas sample Additionally, thorough cleaning and proper handling of the container before each use are vital to avoid contamination For more details, refer to section 6.5 on cleanliness.
Sample containers must be clearly labeled with an identification number and their maximum working pressure Additionally, the date of the last physical inspection should be included on the cylinder label or maintained in easily accessible records For transportation, containers must comply with U.S DOT (CFR 49) specifications.
Types of Sample Containers
A general description of sample containers may be found in the latest revision of GPA Standard 2166.
9.2.1 Single- and Double-Valve Standard Cylinders
These cylinders are also known as constant volume cylinders, single cavity cylinders, and “spun” cylinders An example is shown in Figure 7
When selecting sample containers, it is crucial to choose those specifically designed to withstand the expected operating conditions and resist corrosion from the sampled product Stainless steel containers are preferred to reduce issues related to the absorption or adsorption of heavy components, such as hexanes, and to prevent reactions between contaminants and the container material Additionally, if the container requires transportation, it must comply with DOT specifications and be appropriately labeled according to DOT hazardous materials regulations and relevant state laws For more details, refer to Section 15, which covers safety, labeling, handling, and transportation of cylinders.
Sample containers can be either one-valve or two-valve types, depending on the chosen sampling procedure It is essential that these containers and their valves have a working pressure that meets or exceeds the maximum pressure expected during sampling, storage, or transportation Soft-seated valves are recommended over metal-to-metal seats for better performance All valves and safety devices must comply with the necessary material and pressure standards for safe operation Pressure relief valves, which can be spring or rupture-disc types, are crucial for managing thermal expansion or overpressurization, as their activation may compromise the sample, necessitating its disposal The container size should be determined by the volume needed for laboratory tests and the requirements for obtaining a representative sample from composite sampling systems, with smaller containers being easier to handle.
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Figure 7—Typical Double Valve Sample Cylinder
These cylinders are also known as constant pressure cylinders An example is shown in Figure 8.
A floating piston cylinder container is made from metal tubing with a polished interior and features removable end caps for easy access to the piston These end caps are equipped with drilled and tapped openings for valves, gauges, and relief valves The entire assembly is engineered to endure maximum pressure and temperature during sampling, transportation, and analysis, while remaining non-reactive to the sampled materials, pressurizing fluid, cleaning solvents, and potential corrosives The cylinder's volume is determined by the sample size required for laboratory analysis Additionally, if the container is intended for transport, it must comply with DOT specifications and be properly labeled according to federal and state hazardous materials regulations.
Labeling, Handling, and Transportation of Cylinders.
The cylinder features a moving piston that utilizes O-rings, PTFE (Teflon ®) rings, or similar devices to create a leak-free seal between the sample and the inert back-pressure gas, while allowing for unrestricted piston movement To facilitate smooth travel, the use of guide rings is advised Additionally, both the piston and sealing device must be nonreactive to the sample, back-pressure gas, cleaning solvents, and any anticipated corrosive components in the gas.
All valves and safety devices must adhere to specific material and pressure standards to ensure safe design Pressure relief valves, which can be either spring or rupture-disc types, facilitate the controlled release of contents in cases of thermal expansion or over-pressurization If relieving occurs, the integrity of the sample is likely compromised, necessitating its disposal.
Piston-type cylinders are often made from nonmagnetic materials like 300 Series Stainless Steel, with the piston itself also crafted from stainless steel and equipped with magnets on its precharge side As the piston travels within the cylinder, the magnetic field produced by these magnets activates a series of bicolored flags, effectively signaling the position of the piston and the volume of the sample contained in the cylinder.
Piston-type cylinders often feature a rod connected to the piston, which extends through the end cap of the inert gas back-pressure chamber, equipped with sealing devices to prevent gas leakage This travel rod serves as an indicator of the piston position and the volume of the cylinder occupied by the sample Variations of this design may also be available.
Some types of constant pressure cylinders are equipped with electronic tracking devices to provide for local and/or remote indica- tion of the piston’s position relative to full/empty
Alternative floating piston cylinders exist that lack a visual indicator for directly measuring sample volume These cylinders require a magnet or another locating device to track the piston’s movement.
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Figure 8—Typical Floating Piston Cylinder
10 Materials for Sweet and Sour Gas Service
General Considerations
The types of materials used in a sample system will depend on the gas being sampled Generally, it is recommended that 304 or
For all wetted surfaces, 316 stainless steel is recommended, while valve seats, O-rings, and piston seals should utilize elastomers suitable for the specific service conditions When sampling gases such as H₂S, CO₂, and other wet or high-temperature gases, additional material considerations arise, often necessitating the use of specialized materials and coatings within the sampling system.
For sour and corrosive gas service, it is essential to use specially-lined or coated sample cylinders, such as those made with epoxy While glass or ceramic-lined cylinders can be used, they may exhibit absorptive or adsorptive properties under specific conditions Alternative coating materials and passivation techniques may also be suitable To ensure accurate analysis, highly reactive components like hydrogen sulfide (H2S) should ideally be analyzed on-site, as even coated containers may not fully prevent absorption or reaction with contaminants.
To ensure the integrity of a sample system, it is crucial to avoid using soft metals like brass, copper, and aluminum (excluding hard anodized variants) due to their high corrosion rates and associated metallurgical issues It is essential to evaluate corrosion rates and the risk of sulfide stress corrosion cracking for each sampling system, adjusting the equipment's service life accordingly Adhering to NACE standards or other relevant material guidelines is recommended for the containers and sampling systems.
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Carbon Steel
Carbon steel and other porous materials should be avoided in natural gas sampling systems due to their tendency to retain contaminants like CO₂, N₂, and H₂S The interaction between carbon steel and these gas components can lead to significant errors in gas analysis Additionally, sample valves and cylinders made from carbon steel exhibit high corrosion rates, especially in wet sour gas sampling, which can result in particulate contamination of valves, filters, and analysis equipment.
Dissimilar Materials
Utilizing different materials in a sampling system can lead to higher corrosion rates and potential sampling errors For accurate comparative analysis of gas samples, it is essential to use cylinders made from the same material This practice minimizes the risk of varied reactions between the gas and each sample cylinder, although it does not ensure that the samples will remain undistorted.
Timers
Timers are used on time-proportional sampling systems to actuate the sample system and collect a sample at the desired intervals See 14.2, Composite Sample Intervals.
Flow Computers
Flow computers and flow indicators are essential components in sample systems, offering crucial insights into flow rates for flow-proportional sampling It is imperative that these devices comply with all relevant electrical standards applicable to their designated environments.
Power Supplies
Systems requiring a power supply must meet appropriate electrical standards and should have a backup power source available in case the primary power source fails.
Pressure Gauges
Gauges should be calibrated or compared to a certified pressure standard on a regular schedule to ensure accuracy This is partic- ularly important in reduced pressure sampling methods.
General
The latest revision of GPA Standard 2166 outlines the standard procedures for spot sampling, explicitly excluding composite, on-line, and mobile gas sampling It presents eight accepted methods, with specific comments and cautions acknowledged by API It is crucial to ensure that sample cylinders are clean before using any of these methods; for further details, refer to section 6.5 on cleanliness.
The article outlines various methods for gas extraction, including the evacuated container method, reduced pressure method, helium pop method, floating piston cylinder method, water displacement method, glycol displacement method, and two purging techniques: the fill and empty method and the controlled rate method.
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The evacuated container method, reduced pressure method, and helium pop method necessitate reducing the sample container's pressure to 1 mm Hg (1/2 in H2O or 3.39 kPaabs) or lower For detailed information, refer to sections 2.2 and 2.3.
Pressure Method; and 2.4, Helium Pop Method.
The floating piston cylinder method, glycol displacement method, and water displacement method are effective constant pressure techniques that ensure the pressure in the sample cylinder remains equal to the stream pressure throughout the sampling process For detailed information, refer to sections 2.5, 2.6, and 2.7, which cover the Floating Piston Cylinder Method, Water Displacement Method, and Glycol Displacement Method, respectively.
When the temperature of the source gas stream or any part of the sampling system falls below the hydrocarbon dew point temperature, users should anticipate a decrease in accuracy with any sampling method For further insights, refer to Appendix C, which discusses lessons learned during the sampling of hydrocarbon-saturated and two-phase natural gas streams.
Evacuated Container Method
The evacuated container method necessitates maintaining an absolute pressure of 1 mm Hg (1/2 in H₂O or 3.39 kPaabs) or lower It is crucial that the valves and fittings on the sample cylinder are in optimal condition, ensuring there are no leaks in both the evacuated and pressurized states.
This method achieves results within ± 0.14% accuracy of the reference gas mixture's heating value (HV) and density, provided that the sample gas temperature and all components of the sampling system adhere to the guidelines outlined in section 6.6, General Discussion of Heating.
Reduced Pressure Method
The reduced pressure method resembles the evacuated container technique; however, instead of allowing the cylinder to reach full line pressure, it is gradually filled to about one-third of the line pressure.
This method achieves results within ± 0.12% of the reference gas mixture's heating value and density, provided that the sample gas temperature and all components of the sampling system are regulated as outlined in section 6.6, General Discussion of Heating.
The hydrocarbon dew point temperature of gas varies with pressure, meaning that filling a cylinder to one-third of the line pressure does not ensure the absence of condensation For instance, at a temperature of 20ºF (–7ºC) and a pressure of 1500 psig (10.3 MPa), the reduced pressure method suggests filling the cylinder to about 500 psig (3.4 MPa) However, condensation can still occur at this pressure and temperature.
Helium Pop Method
The helium pop method involves preparing the sample container by evacuating it to an absolute pressure of 1 mm Hg (1/2 in H₂O or 3.39 kPa) This technique is akin to the evacuated container method, but it utilizes a helium charge to ensure the container remains free of air before sampling.
The Helium Pop method significantly decreases the un-normalized total percent calculated in gas chromatographic analysis, making it unsuitable as a diagnostic tool in this context.
This method achieves results within ± 0.15% of the reference gas mixture's heating value and density, provided that the sample gas temperature and all components of the sampling system are regulated as outlined in section 6.6, General Discussion of Heating.
Floating Piston Cylinder Method
The constant pressure cylinder is engineered to keep the sample at pipeline pressure To ensure complete vaporization of any liquids within the cylinder, it is essential to reheat the sample in the laboratory to the required temperature, as outlined in section 6.6.4.
Proper cleaning of sample containers is essential before sampling, especially if there is a concern that previous samples may have condensed inside the cylinder or if the cylinder has been in contact with liquid hydrocarbons like slugs or compressor oil.
The piston sealing mechanism, typically utilizing o-rings or lip seals, must ensure complete separation of the sampled natural gas from the pre-charge gas It is crucial that both the sealing material and lubricant do not adsorb or alter any components of the natural gas mixture, as such interactions can lead to leakage and seal failure.
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This method achieves results within ± 0.14% of the reference gas mixture's heating value and density, provided that the sample gas temperature and all components of the sampling system are regulated as outlined in section 6.6, General Discussion of Heating.
Floating piston cylinders must be robust and non-absorptive, with a design pressure rating exceeding the maximum pressure at the facility They should include a piston position indicator to detect precharge gas leakage PTFE (Teflon®) seals or equivalent, which do not require grease for effective sealing and smooth operation, are essential It is important to avoid using silicon grease or other lubricants that could contaminate the sample Additionally, the precharge side of the cylinder should be pressurized to at least the line pressure, with the piston flush against the inlet end cap, ideally using an inert gas that does not interfere with the sampled stream to prevent analytical discrepancies in case of a leak.
Before starting the sampling operation, it is essential to purge the sample inlet piping using the end cap purge valve on the piston cylinder's inlet side If a purge valve is absent, a "purge-valve tee" should be installed to facilitate the purging of the inlet piping and valves It is crucial to prevent condensation and contamination during the purging process.
Water Displacement Method
When using the water displacement method, it's important to note that water can absorb or release gases like CO₂ and H₂S, influenced by water quality and contact time To minimize desorption, distilled water is recommended, although it does not prevent absorption of these gases Additionally, the displacement fluid may contaminate chromatograph sample systems and columns, making this method unsuitable for water content determination or when temperatures fall below 32ºF (0ºC).
This method achieves results within ± 0.13% of the reference gas mixture's heating value and density, provided that the sample gas temperature and all components of the sampling system are regulated as outlined in section 6.6, General Discussion of Heating.
Glycol Displacement Method
Note: A 50/50 mixture of glycol and water is commonly used with this method.
When using the glycol displacement method, it's important to note that water can absorb or desorb CO₂, H₂S, and other components based on its quality and contact time While distilled water can prevent desorption of these components, it does not stop absorption Additionally, the displacement fluid may contaminate chromatograph sample systems and columns, making this method unsuitable for water content determination.
This method achieves results within ± 0.10% of the reference gas mixture's heating value and density, provided that the sample gas temperature and all components of the sampling system are regulated as outlined in section 6.6, General Discussion of Heating.
Purging—Fill and Empty Method
The purging—fill and empty method involves multiple cycles of filling and emptying the sample cylinder to eliminate residual impurities As detailed in Table 2, a specific number of fill and empty cycles is necessary to effectively remove these impurities This technique has achieved results within ± 0.12% of the reference gas mixture's high volume and density values, provided that the sample gas temperature and all components of the sampling system are kept in accordance with section 6.6, General Discussion of Heating.
1 The cylinder is coupled to the sample point as shown in Figure 9.
A "pigtail" with a flow restriction is connected to the outlet of the cylinder, as illustrated in Figure 9 It is crucial that the cooling generated by the Joule-Thomson effect occurs at the end of the "pigtail."
3 The cylinder is purged of residual impurities using the correct number of fill and empty cycles (Table 2).
Purging—Controlled Rate Method
The controlled rate method described in GPA Standard 2166 employs a continuous purge.
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Table 2—Fill and Emply Purge Cycles
Maximum Gas Pressure in Container, psig (kPa) Number of Fill and Empty
This method achieves results within ± 0.18% of the reference gas mixture's heating value and density, provided that the sample gas temperature and all components of the sampling system are regulated as outlined in section 6.6, General Discussion of Heating.
When using the controlled rate purging method, liquid can accumulate in the sample cylinder and coiled extension tubing, leading to an enrichment of the gas sample This results in an overestimation of the gas's heating value and density in the pipeline To prevent liquid accumulation, it is essential to maintain all sampling equipment above the hydrocarbon dew point temperature, as outlined in section 6.6 of the General Discussion of Heating.
Vacuum—Gathering System Method
In rich, low-pressure, or vacuum-gathering systems, it is advisable to use a vacuum pump to draw gas from the sample point and discharge it into the sample system An alternative method involves using a helium-filled sample cylinder, although this was not evaluated in the research program Prior to sample collection, it is crucial to measure the oxygen content and relative density of the stream with a portable oxygen analyzer and gravitometer to ensure the sample is representative and free from air contamination Once the oxygen and gravity values are verified, the sample can be collected using any of the GPA Standard 2166 methods.
To ensure accurate analytical results, it is crucial to maintain the pressure within the sample cylinder at less than 20 psig (13.8 kPa) and to heat samples according to the guidelines in section 6.6, General Discussion of Heating Larger sample cylinders, exceeding 300 cm³ (18 in³), may be necessary to hold sufficient material for analysis Insufficient heat input can lead to increased sample pressure and potential condensation, which can compromise the reliability of the results.
A, The Phase Diagram, for general discussion of phase changes due to pressure increases.
Samples from vacuum-gathering systems, which have a very high HV content, show increased uncertainty in analytical results due to the tendency of heavy ends to condense.
Use of Thermal Isolation and Throttling Devices
Thermal isolation and throttling devices are essential for preventing the cooling of the sample cylinder during the purging process By throttling the sample gas downstream of the cylinder outlet valve, both the pressure drop across the sample cylinder valves and the Joule-Thomson cooling effects from upstream restrictions are minimized Additionally, ensuring thermal isolation of the throttling device from the cylinder effectively prevents any cooling of the cylinder itself.
When conducting the fill-and-empty spot sampling procedure, it is essential to use a 1/4 inch (0.635 cm) ID extension tube, known as a "pigtail," which should be at least 36 inches (91.4 cm) long, connecting the sample cylinder outlet valve to the purge valve/throttling assembly This pigtail serves to ensure thermal isolation between the throttling device and the sample container outlet valve For further details, consult GPA standard 2166.
The throttling device must have a flow coefficient (Cv) between 0.09 and 0.53, corresponding to orifice diameters of 1/16 in to 1/8 in (0.16 cm to 0.32 cm) A standard multi-turn sample valve with a port size of 1/16 in to 1/8 in has proven effective for this application Alternative devices or combinations may also be utilized.
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Figure 9—API Recommended Spot Sampling Apparatus for Fill and Empty Method Close-Coupled and Direct
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Figure 10a—Vacuum Gathering System Model
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`,,```,,,,````-`-`,,`,,`,`,,` - vide temperature isolation and throttling of the sample gas flow equal to or exceeding that of the “pigtail” and throttling devices described above.
Increasing flow restriction can extend purge time, especially in ambient temperatures at or below the hydrocarbon dew point of the flowing stream This extended purge time may result in the ambient air cooling the sample cylinder more than the purging process can warm it Insufficient throttling can lead to excessive cooling of the sample gas at points of restriction upstream, potentially causing the gas sample to drop below its hydrocarbon dew point temperature, which may render it non-representative.