Measurement Library

PRCI Publications (2010)

PRCI

L52323 Performance of Dirty or Worn Flow Conditioners
Author(s): Grimley, Hawley
Abstract/Introduction:
A large portion of current natural gas metering technology requires a fully-developed velocity profile for accurate flow measurement results. Two methods can be used to obtain a fully-developed velocity profile in natural gas pipelines: long lengths of straight pipe, or shorter pipe lengths in combination with a flow conditioner. Due to size limitations, long lengths of straight pipe are not always practical and flow conditioners may be the only viable option. Flow conditioners that are based on a distributed pattern of holes through a plate are commonly used in the natural gas industry and are referred to as perforated plate flow conditioners. The ability of a perforated plate flow conditioner to redistribute a disturbed velocity profile is based on multiple geometric characteristics of the plate (e.g., hole pattern, porosity, and plate thickness) and on the hole details (e.g., edge sharpness). Exposure of perforated plate flow conditioners to conditions in natural gas pipelines could result in changes to the geometric characteristics of the flow conditioner due to dirt buildup on the flow conditioner face or wear of the edge sharpness. This effort evaluatesd the ability of dirty or worn plate flow conditioners to effectively redistribute a disturbed velocity profile.
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PR-015-08610-R01 Laboratory Conformation of the Effect of Methanol on Gas Chromatograph Performance
Author(s): George
Abstract/Introduction:
In natural gas production and processing applications, methanol is commonly injected into natural gas streams containing water to prevent the formation of hydrates in gas lines and subsequent equipment damage. However, gas chromatographs (GCs) at field sites are typically not equipped to identify or measure methanol, and unless excess methanol is expected to carry over into a gas stream, samples sent to a laboratory are not likely to be analyzed for methanol. As a result, the potential exists for errors in gas property determination, particularly in heating value and sound speed. A previous PRCI project investigated the potential for GCs to quantify methanol as a hydrocarbon, and estimated the resulting errors on heating value and other properties. This theoretical study used assumptions about where methanol would elute on GC columns, but experimental data on GC performance in streams with methanol was unavailable to verify these assumptions. To verify the estimates of the theoretical study, this project collected experimental data on methanol elution behavior in a series of field and laboratory GCs, and established the errors in computed natural gas properties caused by methanol behavior. Three GCs used by the laboratory of a PRCI member company were nominated for testing: ABB NGC 8206 C7+ field GC, Agilent Model 7890A laboratory GC, configured for extended natural gas analysis, and Daniel Model 575 C6+ field GC. The separation columns, valve configurations, and other design features of these GCs that could influence methanol elution were reviewed. Since each GC was predicted to respond differently to methanol, the nominated units were accepted for testing. A fourth GC, a Varian CP-4900 Quad MicroGC outfitted to quantify methanol, was provided to the lab to serve as a reference unit. Hydrocarbon base gas compositions were chosen to represent production and transmission gases and a gas blender was consulted to identify an effective method of stabilizing the methanol content of the test gases delivered to the GCs. Lab personnel and the gas blender then provided the required hardware and the test and calibration gases, with the gas blend compositions traceable to NIST.
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PR-015-09603-R01 LNG Measurement Uncertainty Analysis
Author(s): George, Hawley, Owston
Abstract/Introduction:
The U.S. natural gas industry is expected to import increasing amounts of liquefied natural gas (LNG) in the near future. When an LNG tanker ship arrives at an LNG terminal, the quantity of LNG transferred to the terminal is found by measuring the changes in static volume within the ships tanks. The LNG volume is inferred from measurements of the liquid height, along with tables of tank characteristics predetermined by a method known as tank strapping. Once transferred, the LNG is then regasified at the terminal before being sent to limited distribution companies (LDCs) or power plants. There is concern that the basis for uncertainty estimates in the energy content of the transferred LNG (typically taken as 0.5% to 0.6%) may underestimate the true magnitude of measurement uncertainties. Dynamic methods of liquid flow measurement, gas flow measurement, product sampling, and composition determination used elsewhere in the energy industry may reduce the measurement uncertainties at the LNG terminal, as they relate to terminal balances. Measurement uncertainties for conventional meters and equipment placed into LNG service may lead to more accurate LNG measurement and reduced lost-andunaccounted for (LAUF) quantities at receipt terminals. This report describes research to evaluate the measurement uncertainties associated with both static and dynamic methods of determining LNG volumes and energy content delivered to, processed by, and shipped from, LNG terminals. This was performed to determine whether dynamic methods are potentially more effective than existing static methods for accurate measurements and LAUF determination at LNG terminals. Another objective of the research was to establish which methods offer the most potential for reducing custody transfer measurement uncertainty and LAUF within LNG receipt terminals.
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PR-015-09605-R01 Extended Low Flow Range Metering
Author(s): George, Hawley
Abstract/Introduction:
Natural gas meters are often used to measure flows below their minimum design flow rate. This can occur because of inaccurate flow projections, widely varying flow rates in the line, a lack of personnel available to change orifice plates, and other causes. The use of meters outside their design ranges can result in significant measurement errors. The objectives of this project were to examine parameters that contribute to measurement error at flow rates below 10% of a meters capacity, determine the expected range of error at these flow rates, and establish methods to reduce measurement error in this range. The project began with a literature search of prior studies of orifice, turbine, and ultrasonic meters for background information on their performance in low flows. Two conditions affecting multiple meter types were identified for study. First, temperature measurement errors in low flows can influence the accuracy of all three meter types, though the effect of a given temperature error can differ among the meter types. Second, thermally stratified flows at low flow rates are known to cause measurement errors in ultrasonic meters that cannot compensate for the resulting flow profiles, and the literature suggested that these flows could also affect orifice plates and turbine meters. Several possible ways to improve temperature measurements in low flows were also identified for further study. Next, an analytical study focused on potential errors due to inaccurate temperature measurements. Numerical tools were used to model a pipeline with different thermowell and RTD geometries. The goals were to estimate temperature measurement errors under different low-flow conditions, and to identify approaches to minimize temperature and flow rate errors. Thermal conduction from the pipe wall to the thermowell caused the largest predicted bias in measured temperature, while stratified temperatures in the flow caused relatively little temperature bias. Thermally isolating the thermowell from the pipe wall, or using a bare RTD, can minimize temperature bias, but are not usually practical approaches. Insulation of the meter run and the use of a finned thermowell design were practical methods predicted to potentially improve measurement accuracy, and were chosen for testing.
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PR-015-10600-R01 Proposed Sampling Methods for Supercritical Natural Gas Streams
Author(s): George
Abstract/Introduction:
Deepwater natural gas production is a non-traditional operation that is very different than conventional shelf or onshore production, due to the extremely high pressures (2,000 psia, 13.8 MPa abs) and rich gases (1,300 Btu/scf, 48.4 MJ/Nm3) involved. Concerns have been raised about methods used to sample deepwater natural gas supplies in this supercritical state. Sampling methods accepted for natural gas at pipeline conditions have been used to sample gas from offshore platforms and supercritical onshore storage facilities. However, the sample analyses have later been found to overestimate the energy content of the gas by as much as 300 Btu/scf (11.2 MJ/Nm3). Analyses of these samples have also been found to incorrectly estimate other properties of the gas, such as sound speed and density. Due to the potential financial impact of such discrepancies, the need exists to understand their causes, and to identify alternative sampling procedures or methods that can minimize them. A literature search was performed to identify sampling methods with the potential to accurately sample natural gas streams in the supercritical region. The search included methods listed in existing natural gas sampling standards, such as API MPMS Chapter 14.1 and GPA 2166-05, variations and suggested improvements on these standard methods, and sampling methods applied in other sectors of the energy industry. No sampling methods were identified that are designed specifically for sampling supercritical natural gas. However, guidelines were found in various references that are useful in tailoring existing sampling methods or designing new sampling methods for supercritical gas service. These guidelines include means to avoid phase changes in the samples, methods of regulating pressure while maintaining sample temperatures, avoiding issues with adsorption and desorption on equipment, and recommendations for designing a sampling method for high-pressure service.
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PR-369-08609-R01 Online Gas Meter Cleaning
Author(s): Gipson,Trahan
Abstract/Introduction:
To ensure proper operation and accuracy of Natural Gas meters, it is important that the meters and associated piping be maintained in a clean condition consistent with both manufacturer production and AGA report provisions. This can best be assured through a comprehensive diagnostic and in section program complete with efficient internal cleaning as required. The objective of this project was to provide a comparison between continuous chemical injection cleaning with the system remaining online and conventional manual disassembly and cleaning techniques with the system offline. Representative samples of fouling materials were collected from two designated meter locations for complete identification analysis and cleaning product screening. The results from these tests are intended to assist in selecting the most effective product for the online cleaning method.
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