ABSTRACT

   Obtaining a representative gas phase sample of natural gas sources containing entrained liquid has caused many problems. A new technology was developed which consists of a technique and hardware for sampling natural gas having entrained liquid. It removes the liquid under pipeline pressure and temperature conditions thereby preventing gas phase composition changes that would otherwise occur. After the liquids are removed the pressure is regulated in a manner which prevents excessive cooling and possible condensation of some gas phase components. The hardware can be inserted/retracted at normal pipeline pressures to facilitate maintenance. The technology applies to sampling for BTU determination, moisture and H2S analysis.

Introduction

   Liquids, entrained in natural gas, are the source of many problems in sample conditioning systems. The primary problem is that the impact of entrained liquid on the gas phase composition is often overlooked. Entrained liquid is generally removed only when it is likely to cause analyzer damage.

   A second problem is that frequently no distinction is made between "entrained" and "condensed" liquid. Whether the liquid was present (entrained) in the source gas or is the result of condensation during the sampling process has a large bearing on the sampling procedure.

   Liquid in a natural gas pipeline can be present in the form of a pool, film, small droplets or aerosol. It is highly probable that the forms are in a constant state of change as a result of internal pipe or vessel geometry and gas velocities. For example, liquid film can be sheared into small droplets or aerosols as it flows across a sharp surface, only to be impinged and coalesced upon a downstream surface, such as a pipe bend. It is highly probable that this constant change in liquid form is a major contributor to the vast differences of opinion on suitable natural gas sampling techniques.

   Yet a third problem is the fact that entrained liquid is not always easy to detect. In some cases entrained liquid is detected not by sight but rather by its impact on analysis of the gas phase. It is the author's opinion that in many cases where the following situations arise, entrained liquid is the culprit:

1. Erratic composition reported by an on- line gas analyzer.
2. Large differences between spot or composite samples and on-line analyzer analysis of gas from the same source.
3. Valve or pressure regulator freezups.

   It must be remembered that a small volume of liquid is equivalent to several hundred times that volume of gas. Therefore, even microscopic amounts of hydrocarbon aerosol droplets, which may be difficult or impossible to detect visually, can have a significant impact on composition and BTU value.

   Gas phase components of a gas sample can condense as a result of temperature and/or pressure changes. It is often difficult to determine if the source of liquid was entrainment or condensation. However, the proper method for prevention or elimination of the liquid will very much depend on its origin.

   This presentation discusses the previously described problems in the context of "current state of the art methods" and newly developed technology.

Physical properties of fluids which influence gas sampling techniques

   A keen understanding of the physical relationships between liquids, gases and vessel surfaces that contain them, is a must for anyone attempting to solve gas sampling problems. Some of the most important of these physical relationships and their impact on composition and BTU value are summarized below:

1. Pressure and temperature changes in a gas containing a mixture of liquids alter the gas phase composition. Pressure increases typically reduce liquid vapor concentrations in the gas phase while decreases of pressure have the opposite effect. Even minor alterations of the liquid vapor content have a significant impact on the gas composition and BTU value.
2. Adsorption and desorption of components by containment surfaces can alter the composition of the gas phase. With a given surface and gas composition, increases of pressure and decreases in temperature increase surface adsorption. Decreases of pressure and increases of temperature have the opposite effect. The gas and surface material compositions determine the degree to which adsorption occurs at any condition of pressure or temperature.
3. Changes of pressure can shift the hydrocarbon dew point of a gas. The effect can be bi-directional. For example an increase of the pressure may reduce the hydrocarbon dew point of one gas composition and increase it in another composition.
4. Reduction of pressure cools a gas due to a phenomenon known as the Joules-Thompson effect. The cooling effect may lower the gas temperature below its dew point. When the temperature of a gas drops below its hydrocarbon dew point condensation occurs. This in turn causes changes in the gas phase composition and BTU value.

Current state of the art for sampling natural gas containing entrained liquid

   There is a lack of suitable technology for extracting a natural gas sample containing any form of liquid in proportion to the liquid load of a source gas. Some of the standard industry practices specifically state that it cannot be achieved. Other standard practices state that liquid should be separated in the source gas, measured and accounted for separately from the gas phase.

   After reviewing the nature of problems associated with hydrocarbon liquids present in natural gas and the physical relationships between liquids, gases, and surfaces, the following conclusions can be drawn:

1. The most practical approach to consistent sampling is to extract a sample representative of the gas phase composition as it exists at the source.
2. If the source contains entrained hydrocarbon liquid, then the liquid should be excluded from the sample gas at the prevailing pressure and temperature of the source. This will prevent temperature and pressure related alteration of the gas phase during sample conditioning and transportation.
3. Pressure reduction, with entrained hydrocarbon liquids present, will alter the gas phase composition. If entrained liquid enters a pressure regulator, whether it is externally mounted or inserted into the pipeline, gas phase composition changes will occur.
4. Heating a gas sample, which does not contain entrained liquid is an acceptable method for preventing condensation.
5. Heating a sample gas when entrained liquid is present will alter the gas phase composition and heating value.
6. Glycol liquid entering a gas sample train will distort the water vapor composition of the sample gas.
7. (g) Amine liquid entering the sample train will distort the acid gas composition of the sample gas.
8. (h) All liquids entering the sample train may either alter sample composition and/or damage on-line analyzers.

New technology is developed

   A technology has been recently developed which makes it possible to extract a representative gas phase sample from a source of gas containing entrained liquid. The new technology also provides a means for preconditioning the sample gas in a manner that reduces the risk of condensation while the sample gas is transported to an analyzer. The hardware developed for utilization of the technology can be employed for composite sampling, spot sampling, portable analyzers, and on-line BTU, moisture and H2S analyzers. It is applicable to gas that contains entrained liquids, as well as gas which is free of liquid.

What is the new preconditioning technology?

   The new technology consists of first removing entrained liquid from sample gas followed by immediate pressure reduction to reduce condensation risk during transportation. This is accomplished inside the pipeline or vessel. The hardware consists of a small cylindrical housing and a probe having a cylindrical, phase-separating membrane and pressure regulator (Fig.1). The housing is inserted through a thread-o-let into the pipeline and the probe is inserted into the housing. Once the housing is installed in the pipeline, the probe can be easily retracted/inserted in order to maintain the membrane and regulator (Fig.2). This can be accomplished quickly and safely at line pressures up to 2000 PSI.

Detailed hardware description

   The housing is constructed of an open tube terminated by a foot valve at its lower end. A sturdy poppet and stem comprise the foot valve. The valve-sealing element is a well-protected "O" ring. A heavy spring holds the poppet in a closed position when the probe is retracted from the housing. Pipeline pressure provides additional force for closing. The probe consists of a phase-separation membrane and pressure regulation valve in its lower end, with a pressure regulator housing at its upper end. The regulator diaphragm is located in the housing and the pressure regulation valve is positioned immediately above the membrane. When the probe is fully inserted into the housing, the membrane and pressure regulation valve are positioned inside the pipeline. The housing protects the membrane from direct exposure to the pipeline gas.

   As the probe is being inserted into the housing, "O" rings form a seal between the housing and probe, prior to the foot valve being opened. Clockwise turning of the insertion nut on the threaded upper end of the housing forces the probe downward. Lowering the probe also pushes the valve stem downward to an "open" position. This allows source gas to flow into the membrane probe.

Operation

   TIn operation, gas from the pipeline enters the housing through the foot valve located on the bottom end of the housing. Pipeline gas flowing across the probe opening creates a turbulent zone in the lower probe housing. This turbulence and the sample flowing upward together provide fresh sample at all times in the housing area immediately surrounding the membrane (Fig.3). Sample gas then flows through the cylindrical membrane. Any entrained liquid present in the sample gas is rejected and coalesced on the membrane surface (Fig.4). The coalesced liquid then returns to the pipeline by gravity-induced flow. Since the membrane is located inside the pipeline, separation and removal of liquid is accomplished under the prevailing pipeline pressure and temperature conditions. These conditions are an absolute must to prevent gas phase composition changes during the removal of entrained liquid. The phase-separation membrane was specifically designed for removing entrained liquids from natural gas samples, without altering gas-phase composition.

   Gas flowing through the membrane continues upward through the pressure regulation valve, probe stem, regulator housing and then enters the external sample transport system. This action regulates the sample gas to the desired pressure. Since the pressure drop during regulation occurs in the pipeline heat is transferred from the source gas to the regulator. This method prevents excessive Joules-Thompson cooling from occurring which could otherwise cause gas phase components to condense.

Applications

   The membrane probe regulator is an ideal "front end conditioner" for on-line BTU, moisture, and H2S analyzers. In those applications it will precondition the sample, prevent sample system contamination, and protect the analyzer against damage by liquids (Fig.5).

   A housing can be installed at each desired sample point along a pipeline. Thereafter, a single membrane probe regulator can be employed to precondition sample gas for a portable analyzer.

   In cases where pressure regulation is not required, a probe, having only a phase-separation membrane, is inserted into the housing (Fig.6). Such may be the case where this technology is employed for spot or composite sampling (Fig.7). Some types of composite samplers may be mounted directly to the upper end of the probe.

Safety

   After the housing is installed into the pipeline it is locked firmly to the top surface of the thread-o-let. This is accomplished by a locking mechanism, and set screw which locks the nut to the housing. Additional set screws lock the nut and housing to the upper surface of the thread-o-let (Fig.1).

   A second safety feature prevents the removal of the probe from the housing so long as the housing is pressurized. Mechanical stops require the housing to be depressurized in order for the probe to be pushed downward and turned manually as required for its retraction.

Conclusion

   Obtaining a representative gas phase sample from gas sources containing entrained liquid has caused many problems. A new technology was developed which consists of a technique and hardware for sampling gas having entrained liquid. It removes the liquid under pipeline pressure and temperature conditions thereby preventing gas phase composition changes that would otherwise occur. After the liquids are removed the pressure is regulated in a manner which prevents excessive cooling and possible condensation of some gas phase components. The hardware can be inserted/retracted at normal process pressures to facilitate maintenance.

   The definitions of words and terms in this glossary were purposefully narrowed for their application to the sampling of gas.

   adsorption - attraction of a thin layer of gas or liquid molecules to a surface
aerosol - a microscopic droplet of liquid suspended in a gas
condense - to change from a gas or vapor to a liquid
desorption - to release from a condition of being absorbed or adsorbed
droplet - a small drop of liquid
entrained liquid - liquid in any form carried along or suspended in a stream of natural gas
fluid - anything that flows in any way, either a liquid or a gas
gas - any substance that has no shape or size of its own and can expand without limit
gas phase - a phase consisting exclusively of gas and/or vapor - Liquid in any form, even though it may be suspended in a gas is not a part of the "gas phase".
liquid - a liquid is composed of molecules that move freely over each other so that it has the shape of its container, like a gas, but, unlike a gas it has a definite volume
liquid phase - a phase consisting of liquid in any form - even microscopic aerosol droplets, suspended in a gas phase, are a part of the liquid phase
liquid vapor - see vapor
membrane - a thin sheet of semipermeable synthetic or natural material
phase - a state of matter such as solid, liquid, gas or vapor
phase-separating membrane - a membrane adapted for separating entrained liquid in any form from gases. Gas passes readily through membrane leaving behind any liquid that may have been entrained.
sample train - see sample system
sample transport system - all associated pipe, tube, fittings and hardware such as filters, rotameters, etc. which transport a gas sample from its source to an intended destination such as an analyzer or sample cylinder
sample system - all components associated with extracting, transporting, and conditioning of a sample
vapor - a substance, which is normally liquid at ambient temperature and atmospheric pressure but becomes a gas at elevated temperature or lower pressures

1. ASTM Designation: D 5503-94, "Standard Practice for Natural Gas Sample-Handling and Conditioning Systems for Pipeline Instrumentation", American Society for Testing and Materials Standards, 1915 Race St., Philadelphia, PA.
2. ASTM Designation: D 4150-94, American Society for Testing and Materials Standards, 1915 Race St., Philadelphia, PA.
3. Upp, E.L., Fluid Flow Measurement: A Practical Guide to Accurate Measurement, Gulf Publishing Company, Houston, Texas, 1993.
4. Hodgman, Charles D., editor, Handbook of Chemistry and Physics, 40th edition, Chemical Rubber Publishing Company, Cleveland, OH, 1959.
5. The World Book Dictionary, 1977 edition, Volume A-K, Doubleday & Company, Chicago, IL, 1977.
6. American Petroleum Institute Manual of Petroleum Measurement Standards, Chapter 14, Natural Gas Fluids Measurement, Section 1, Collecting and Handling of Natural Gas Samples for Custody Transfer, fourth edition, August 1993.
7. Behring, Kendricks A., "Accuracy of Natural Gas Sampling Techniques, and the Impact of Composition Measurement Errors on Flow Rate and Heating Value Determination", Flomeko '98, The 9th International Conference on Flow Measurement, Lund, Sweden, June 1998.
8. Behring, Kendricks A., "Lessons Learned from the API 14.1 Gas Sampling Research Project", American School of Gas Measurement Technology, 1998, Pages 193-203.
9. Houser, E. A., Principles of Sample Handling and Sampling Systems Design for Process Analysis, UMI, Ann Arbor, MI, 1997.
10. Gas Processors Association, Standard 2166, "Obtaining Natural Gas Samples for Analysis by Gas Chromatography".
11. Gas Processors Association, Standard 2261, " Analysis for Natural Gas and Similar Gaseous Mixtures by Gas Chromatography".

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