Tiêu chuẩn iso tr 09464 2008

ISO 2186, Fluid flow in closed conduits — Connections for pressure signal transmissions between primary and secondary elements ISO/TR 3313:1998, Measurement of fluid flow in closed con

Guidance specific to the use of ISO 5167-1:2003

No comments on this clause

5.1.5 Principle of the method of measurement and computation

5.1.5.1 Principle of the method of measurement

No comments on this subclause

Copyright International Organization for Standardization

5.1.5.2 Method of determination of the diameter ratio of the standard primary device

See Annex A of this Technical Report

ISO 5167-1:2003, Clause 5, provides the essential equations for determining the flowrate of a measuring system Some calculation results depend on fixed installation dimensions and need to be computed only once, while others must be repeated for each flow measurement point Annex A offers detailed worked examples of the iterative calculations outlined in ISO 5167-1:2003, Annex A, aiding in accurate and consistent flow measurements.

5.1.5.4 Determination of density, pressure and temperature

For details on density measurement, see 6.4

For details on density computation, see Annex B of this Technical Report

The temperature decrease caused by fluid expansion through the primary device depends on the Joule-Thomson coefficient, which varies with temperature, pressure, and gas composition Accurate calculation of this cooling effect requires understanding the Joule-Thomson coefficient, which can be determined using an appropriate equation of state For detailed procedures, refer to Annex B in the documentation.

The detailed method for analyzing natural gas compositions involves molar composition analysis or applying an approximation suitable for mixtures that are not too rich This approximation is valid when pressure (p) and temperature (T) are within specific ranges, ensuring accuracy Under these conditions, the coefficient used in calculations depends solely on pressure and temperature.

For natural gas with a methane content exceeding 80%, optimal processing occurs within a temperature range of 0°C to 100°C and an absolute static pressure between 100 kPa and 20 MPa (1 bar to 200 bar).

The Joule-Thomson coefficient (JT), expressed in Kelvin per bar (K/bar), describes how the temperature of a fluid changes during a throttling process It is crucial to understand that the JT coefficient depends on the fluid's temperature (t in °C) and the absolute static pressure (p in bar) This relationship helps predict whether a fluid will cool or warm when it undergoes a pressure drop, which is essential in refrigeration and natural gas processing applications.

The uncertainty was determined from the differences between this equation and the Joule-Thomson coefficient of 14 common natural gases and is given by

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= ⎜⎝ − ⎟⎠ ⎢⎢⎣ − ⎜⎝ − ⎟⎠⎥⎥⎦ for p>70 bar(7 MPa) (3) where U is the (expanded) uncertainty in the Joule-Thomson coefficient (K/bar)

An orifice plate with a β ratio of 0.6 measuring a differential pressure (Δp) of 0.5 bar can result in a Joule-Thomson coefficient uncertainty that impacts flow rate measurements by approximately 0.001% to 0.009% This variability depends on factors such as temperature, pressure, and gas composition, highlighting the importance of precise conditions in flow measurement accuracy Proper understanding of these uncertainties is crucial for optimizing flow measurement systems in industrial applications.

5.1.6 General requirements for the measurements

5.1.6.1.1 No comments on this subclause

5.1.6.1.3 Table 1, whilst not exhaustive, lists materials most commonly used for the manufacture of primary devices

Table 1 — Steels commonly used for the manufacture of primary devices

High elastic limit stainless steel 420 420-S37 Z30C13

Table 2 gives the mean linear expansion coefficient, elasticity moduli and yield stresses for the materials of Table 1 according to their AISI designation

Table 2 — Characteristics of commonly used steels

Mean linear expansion coefficient between 0 °C and 100 °C

Elasticity modulus Yield stress AISI designation

The values in Table 2 are influenced by both temperature and the steel's treatment process For accurate calculations, it is advisable to obtain data directly from the manufacturer.

When the primary device operates at a temperature different from the calibration or reference temperature, its thermal expansion or contraction must be corrected To accurately determine the diameter ratio and flow rate, the corrected diameter “d” should be calculated using Equation (4), assuming there are no mounting restraints affecting the device This ensures precise flow measurements under varying temperature conditions.

0[1 d 0) d =d +λ T T− (4) where d is the primary device diameter in flowing conditions; d 0 is the primary device diameter at reference temperature; λ d is the mean linear expansion coefficient of the primary device material;

T is the primary device temperature in flowing conditions;

T 0 is the reference or calibration temperature

When automatic temperature correction is not necessary in the flow computer, the uncertainty associated with "d" should be increased to account for temperature-induced changes, as noted in ISO 5167-1:2003, 8.2.2.4 While initial calculations may suggest that this added uncertainty is minimal, it is important to consider this factor to ensure accurate overall uncertainty assessments.

5.1.6.3.2 If there is a likelihood of such a change of phase, a way of overcoming the problem is to increase the diameter ratio, so that the differential pressure is reduced

The following list of inspection equipment is not exhaustive, but provides a basis for inspection control:

⎯ gauge block, feeler gauge (relative position, absolute standard for checking micrometers);

⎯ three point bore gauge (internal diameter)

Only instruments which may be calibrated to primary standards should be used if optimum accuracy is required

ISO 5167-1, section 5.1.7.1.6 emphasizes the importance of properly positioning drain or vent holes near the primary device, as illustrated in Figure 1 Placing the drain or vent hole in the annular chamber is crucial when such a chamber is used, ensuring accurate measurement and system integrity It is also essential to consider the location of these holes relative to pressure tappings, especially in situations where no annular chamber exists, and the drain or vent hole enters directly into the pipe, to maintain correct operation and measurement accuracy.

Fluid flow within pipes can lead to deposition, corrosion, or erosion of the inner walls, potentially affecting pipeline integrity Proper installation is essential to ensure compliance with ISO 5167-1 standards and prevent flow-related issues Regular internal inspections should be conducted at intervals suited to the specific operating conditions to maintain optimal pipe performance and safety.

3 drain holes and/or vent holes a Flow direction

Figure 1 — Location of drain holes and/or vent holes

This subclause emphasizes the importance of reliable temperature measurement to ensure accurate process control While flowing temperature is not directly used in flowrate calculations, it is a crucial parameter for determining variables such as "d" and "D," which are essential for accurate flow measurement Monitoring flow temperature also enables the calculation of critical process parameters under flowing conditions, contributing to overall system reliability and precision.

5.1.7.2 Minimum upstream and downstream straight lengths

When designing a metering pipe installation, it is essential to determine the required minimum straight lengths based on the maximum expected diameter ratio throughout the system's lifespan This ensures optimal flow accuracy and system performance over time Properly accounting for maximum diameter variations helps maintain measurement precision and reduces potential operational issues.

For diameter ratios not explicitly listed in ISO 5167-2:2003, Table 3; ISO 5167-3:2003, Table 3; or ISO 5167-4:2003, Table 1, but falling within the standard's specified limits, linear interpolation between the nearest two diameter ratio values is considered a reasonable and accurate approach.

For accurate flow measurement in either direction, an orifice meter must adhere to specific installation guidelines The minimum straight pipe lengths upstream and downstream of the orifice plate should meet the specifications outlined in ISO 5167-2:2003, section 6.2 and Table 3, ensuring optimal flow conditions and measurement accuracy Proper compliance with these standards is essential for reliable flow rate readings in both flow directions.

5.1.7.3 General requirement for flow conditions at the primary device

This part of ISO 5157:2003 is concerned solely with orifice plates and their geometry and installation It is necessary to read ISO 5167-2 in conjunction with ISO 5167-1

Orifice plate meters with three arrangements of tappings are described and specified: flange tappings; corner tappings; and D and D/2 tappings

5.2.4 Principles of the method of measurement and computation

The density and viscosity of the fluid can be accurately measured or calculated based on gas composition, with methods detailed in section 6.4 and Annex B Several computer programs are available to facilitate these calculations efficiently For compressible fluids, determining the isentropic exponent under operating conditions is essential for flow analysis, and this value can also be derived from the gas composition.

5.2.5.1.2.1 No comments on this subclause

5.2.5.1.2.3 Referring to Annex C, three factors need to be taken into consideration in designing an orifice plate to avoid excessive deformation

⎯ First, the mounting arrangements should not impose any forces on the orifice plate which would cause the limit of 0,5 % slope given in ISO 5167-2:2003, 5.1.3.1 to be exceeded under the condition of no differential pressure

The plate thickness, E, must be designed so that, considering the material's modulus of elasticity, the differential pressure at maximum flow rate does not cause the slope to exceed 1% When flow decreases to zero, the plate should revert to its original maximum slope of 0.5%.

To ensure safety and durability, it is essential to verify that higher differential pressures beyond the maximum design flowrate do not lead to plastic buckling or permanent deformation of the components.

For the first point, great care is needed in both the design and manufacture of the mounting arrangements

Single and double chamber mounting devices are effective options for installing orifice plates When positioning orifice plates between standard flanges, ensure the flanges are aligned at 90° ± 1° to the pipe axis for proper installation Adequate support for the pipe sections on both sides of the orifice plate is essential to prevent undue strain and maintain accurate flow measurement Proper mounting and support ensure the reliability and longevity of the orifice plate system.

Elastic deformation of an orifice plate can introduce measurement errors in flow readings However, if the deformation remains within 1% of the required slope, the impact on accuracy is minimal, ensuring reliable flow measurement results.

ISO 5167-2:2003, 5.1.2.3, no additional uncertainty will result Theoretical and experimental research (see

Reference [13]) indicates that the maximum change in discharge coefficient for a 1 % slope is 0,2 %

Orifice plates that meet the 0.5% slope specified in ISO 5167-2:2003, 5.1.3.1, can safely deform an additional 0.5% slope, resulting in a 0.1% change in the discharge coefficient, while remaining compliant with the standards Table 3 provides the ratios of plate thickness to support diameter (E/D') for various β values and differential pressures, specifically for orifice plates made from AISI stainless steel 304 or 316, that are simply supported at their rims.

Table 3 — Minimum E/D' ratios for orifice plates manufactured in AISI 304 or AISI 316 stainless steel

Table 3 is based on the use of Equation (5) when 100 ∆q m /q m is not to exceed 0,1 in magnitude and

E* is the modulus of elasticity of plate material;

D' is the plate support diameter (this may differ from pipe bore D);

The maximum differential pressure applicable to the metering section must be determined by the designer, especially since it can exceed the values listed in Table 3 This situation may arise when the metering section is isolated and vented to atmospheric pressure for orifice plate inspection or during pre-commissioning pressurization before entering service Proper assessment ensures safe and effective operation of the measurement system.

To avoid plastic deformation (buckling), the orifice plate thickness should be such that:

∆p is the maximum differential pressure determined by the designer, in pascals (Pa); σ y is the yield stress of the orifice plate material, in pascals (Pa)

NOTE 1 For stainless steel, σ y = 300 MPa, but it is advisable to use a value of 100 MPa for design purposes

When selecting the thickness of the orifice plate, it should be the greater value determined by Equations (5) and (6), ensuring compliance with ISO 5167-2:2003, 5.1.5.3, which limits the maximum thickness to 0.05D If calculations reveal that the required thickness exceeds 0.05D, designers should consider reducing the pressure difference (∆p) or utilizing a stronger material to meet safety and design standards.

∆p = 50 kPa (0,5 bar) gives E/D′ > 0,013 from Equation (5) or Table 3

⎯ Equation (6): β = 0,2 σy = 300 MPa for stainless steel, but for design purposes it is advisable to use σy = 100 MPa

∆p = 100 kPa (1 bar) (see NOTE 2) gives E/D′ > 0,023

Consequently, E/D′ should be at least 0,023

NOTE 2 100 kPa (1 bar) is the maximum anticipated differential pressure

5.2.5.1.3.1 Table 4 gives values of deflection of the inner edge of the orifice corresponding to the 0,5 % slope for various pipe diameters and diameter ratios, β, assuming the deformation is rectilinear

Table 4 — Plate flatness tolerances Nominal diameter of the measuring pipe in millimetres

Maximum deflection h in millimetres for 0,5 % slope

The roughness criterion outlined in this subsection may not sufficiently ensure that the edge sharpness requirements specified in ISO 5167-2:2003, section 5.1.7.2, are met To achieve optimal results, it is recommended to use a roughness value of Ra 10^−5 d Additionally, the roughness of the orifice bore should adhere to the same criterion to maintain consistent quality and performance.

Ensuring the bevelled side of the plate is positioned downstream is crucial for accurate flow measurements Installing the plate with the bevel upstream can result in a flow rate underestimation of up to 20% To prevent installation errors, it is recommended to mark the plate to clearly indicate the upstream face, making correct placement easily identifiable during installation.

To accurately identify the upstream face where the orifice plate is installed between flanges, a common method is to install a paddle plate This paddle plate features critical details engraved on its handle, which extends from the flange joint, ensuring clear and easy identification for installation and maintenance purposes.

In no circumstances should the upstream face of the orifice plate within diameter “D” be indented by any marking

The final paragraph of this subclause states that the edge radius must be measured if there is any doubt about its compliance with ISO 5167-2:2003, sections 5.1.7.1 and 5.1.7.2 In exceptional cases, several suitable measurement techniques are recommended, including the casting method described in Reference [8].

A replica of the edge is created through a two-stage casting process, starting with a coloured cold-forming plastic that molds the negative form of the orifice plate edge, followed by backing with semi-transparent epoxy resin to replicate the orifice plate The finished casting is precisely cut into two halves, revealing the replica of the orifice plate edge, which is then polished and photographed under magnification for detailed examination This method allows accurate measurement of the edge condition Additionally, the lead foil impression method (see Reference [8]) offers an alternative technique for assessing edge condition with high precision.

An impression of the edge is created by pressing a 0.1 mm thick lead foil onto the orifice plate edge using a micrometer-controlled inspection gauge, producing a 0.12 mm deep indentation The indentation is examined under a projection microscope or similar magnifying equipment, allowing for detailed tracing of the outline and precise measurement of the edge condition Additionally, the paper-recording roughness method provides an alternative technique for assessing edge smoothness and quality.

Modern low-ratio nozzles with a roughness average (Ra) of approximately 10⁻⁵ d exhibit discharge coefficients that closely align with the standard value (C) specified by ISO 5167-3:2003, section 5.2.6.2, rather than the higher values predicted by section 5.2.7.1 This indicates that such nozzles achieve more accurate flow measurements consistent with ISO guidelines (See Reference [13] for detailed data.)

5.3.5.3.3.3 There should be equal angles between the centre-lines of adjacent tapping points.

For high Re D , even within the criteria in ISO 5167-4:2003, and for very accurate measurement, calibration is advisable

6 Information of a general nature relevant to the application of ISO 5167:2003

Secondary instrumentation

The definition of primary/secondary devices is stated in the Introduction to ISO 5167-1:2003

6.1.2 General requirements concerning installation of secondary instruments

Proper installation of orifice-plate measuring systems is essential for accurate flow measurement, requiring adherence to manufacturer’s specifications The instrumentation must be installed without imparting mechanical stress, avoiding errors caused by improper mounting or impulse pipe connections Ensuring the setup is free from mechanical vibrations within the acceptable limits prevents measurement inaccuracies Additionally, the pressure signal lines should be designed to avoid resonant frequencies within the pipeline noise band, as specified in ISO 5167-1:2003 To maintain measurement reliability, instruments should be housed in temperature-controlled enclosures if environmental conditions are highly variable, preventing significant errors in secondary instrumentation.

10 meter tube per international standard a Flow direction b Sample flow

Figure 10 — Typical metering device installation

Measurement of pressure and differential pressure

For a complete treatment of the subject of pressure-signal transmission, reference should be made to ISO 2186 However, some of the problems that demand special care are briefly mentioned below

6.2.2 Connections for pressure signal transmissions between primary and secondary elements 6.2.2.1 General

To ensure accurate pressure measurement, impulse lines (pressure pipes) connecting the primary device tappings to the manometer or pressure difference meter must be arranged to prevent back pressure or false pressure readings Key considerations include avoiding temperature differences between the two pressure pipes, preventing gas bubbles, liquid droplets, or solid deposits from accumulating in either pipe, and preventing the congealing or freezing of liquids within the pressure lines Proper arrangement of the impulse lines is essential for maintaining measurement integrity and avoiding erroneous readings.

These requirements are met by the following:

⎯ attending to the location of the meter and the size and run of the pressure pipes;

⎯ providing gas vents and liquid catchpots or water seals;

Using a sealing liquid with appropriate properties is essential to transmit pressure from the fluid within the pipe to the measuring liquid in the manometer or instrument Although this method is less commonly used today, it remains a valid technique for pressure measurement, as depicted in Figures 11 and 12.

⎯ Suitable isolating valves should be provided in the pressure pipes The choice and location of the valves is the responsibility of the designer

⎯ A ball valve should be used for fluids liable to form a sediment

8 equalizing valve a To differential pressure transmitter

Figure 11 — Sealing chambers — Metered fluid heavier than sealing fluid

2 level-determining connection 6 equalizing valve

4 sealing liquid a To differential pressure transmitter

Figure 12 — Sealing chambers — Metered fluid lighter than sealing fluid

For specific fluids and conditions, such as steam, special connection arrangements, condensation chambers, etc may be required See ISO 2186 for details

Accurate measurement of the differential pressure across a primary element is essential for precise flow rate calculation in circular conduits Devices such as orifice plates, nozzles, and Venturi tubes rely on this differential pressure to determine flow rate effectively Ensuring reliable pressure measurements is critical for maintaining system efficiency and operational accuracy in various industrial applications.

For accurate gas flow measurement or when higher precision is needed for liquids, it is essential to determine the absolute static pressure at the upstream pressure tapings of orifice plates This static pressure is crucial for calculating the expansibility factor and for applying downstream-upstream corrections related to process parameters like temperature and measured density, ensuring reliable and precise flow measurement results.

Accurate measurement of static pressure is crucial when calculating density using an equation of state, as the sensitivity of static pressure measurements significantly impacts the results Gauge pressure transmitters are commonly used to measure fluid pressure at upstream tappings, but for flowrate and reference calculations, the absolute static pressure is necessary and can be derived from gauge and ambient pressure readings Typically, the standard reference pressure of 101,325 kPa (1.01325 bar) is added to gauge pressure to obtain absolute pressure, avoiding the need to measure ambient barometric pressure directly However, when atmospheric pressure fluctuations cause a 0.1% change in mass flow, it is advisable to switch to absolute pressure instruments for more precise and reliable readings.

The differential pressure across a primary device is commonly measured using electronic transducers connected via impulse lines to upstream and downstream pressure tappings When installed as part of a metering device, the connection to the upstream tapping can also serve both differential and static pressure transducers, as shown in Figure 10 Selecting the appropriate pressure transducer depends on several factors, including measurement accuracy requirements, whether the measurement is continuous or intermittent, fluid characteristics, the data acquisition system, and the desired mounting and location of the transducer. -**Sponsor**Need help refining your article and ensuring it's SEO-friendly? As a content creator, I understand the importance of clear, coherent paragraphs [editorr](https://pollinations.ai/redirect-nexad/z5mxCQQo) offers on-demand proofreading and editing services to polish your writing They'll help you rewrite sentences, ensuring each paragraph's meaning is impactful and optimized for search engines, ultimately transforming your text into a compelling narrative.

Mechanical pressure transducers, although less common with the rise of flow computers, remain valuable in many process applications They operate using an elastic element that converts pressure energy into a mechanical displacement, providing reliable pressure measurement These devices are essential in industries requiring accurate, robust pressure sensing, especially where electronic components may be unsuitable.

The more commonly used electronic pressure transducers incorporate an electric element which converts the pressure to an electrical signal which can be easily amplified, corrected, transmitted and measured

EXAMPLE Examples of some electronic pressure transducers are

Electronic pressure transmitters exhibit significant variations in declared accuracy and operating characteristics depending on their type However, the introduction of smart transmitters, which operate in digital mode, has helped to reduce uncertainties and enhance measurement reliability.

< 0,1 % of the upper range value are claimed Typical characteristics of electronic pressure transducers are given in Table 6

NOTE Table 6 should be regarded as a simple guide Quoted values are orders of magnitude

It should be noted that differential-pressure transducers may be sensitive to changes in both static pressure and ambient temperature, unless automatic compensation arrangements are included within these units

Table 6 — Characteristics of electrical pressure transducers

Type Parameter Variable reluctance Capacitive Bonded strain gauge Thin film strain gauge Piston gauge

% of full range < 1 < 0,2 0,5 0,25 0,1 % of measured value

Full scale output (V) 0,1 1 V/200 Ω < 0,03 < 0,03 10 4 pts digital

Temperature range (°C) −20 to 100 −25 to 90 −35 to 90 −50 to 120 10 to 30

Regular calibration of pressure transducers, including differential and static pressure sensors, is essential to maintain optimal accuracy in secondary instrumentation The choice of calibration devices depends on the specific application and the types of transducers in use, ensuring reliable measurement performance across various metering systems.

Commonly available pressure measurement devices include pressure balances, manometers, piezo-resistive sensors, and precision Bourdon gauges While these instruments are widely used, pressure calibrators based on the pressure balance principle can be challenging to operate accurately in unstable environments Table 7 highlights the performance specifications of some of the most frequently used calibration devices, providing essential information for selecting the appropriate tool for precise pressure measurement.

Table 7 — Characteristics of precision pressure-measuring devices

Pressure balance (deadweight tester) 0,05 to 50 000

0,01kPa to 0,05 % of reading (0,1 mbar to 0,05 % of reading)

Corresponding to 0,025 mm of liquid column height

To reduce the effects of ambient temperature changes to a minimum, it is recommended that the differential and static pressure transmitters be installed in temperature-controlled enclosures

Static pressure transducers are usually calibrated in situ against an appropriate pressure calibrator selected for the specific function

Differential pressure transmitters are typically calibrated at atmospheric pressure using a suitable calibrator to ensure accuracy For the most precise measurements, it is recommended to calibrate transmitters at their actual operating pressure A high-static deadweight tester is commonly employed for this purpose, providing reliable calibration results essential for optimal transmitter performance.

High-static calibration may be challenging in environments with unfavorable conditions such as background vibrations, making it difficult to achieve accurate results In such cases, applying a correction for static pressure shift—either through mathematical adjustments or interim calibration methods like footprinting—is essential to ensure measurement accuracy.

The "footprinting" method involves calibrating the transducer offline in a controlled environment to create an atmospheric "footprint." This footprint serves as a reference point at the worksite for regular calibration checks It allows for accurate verification of the transducer's performance using test equipment that is less sensitive to environmental variations than high-static deadweight testers.

See Annex B of ISO/TR 3313:1998.

Measurement of temperature

Measuring the upstream pressure tapping temperature is essential for accurately determining the fluid's density and viscosity This temperature data allows for precise thermal expansion corrections of both the device and the piping system, ensuring reliable and accurate measurements Proper temperature measurement is crucial for maintaining optimal flow calculations and equipment performance in industrial processes.

The temperature of the fluid should preferably be measured downstream of the primary device

6.3.2 Fundamentals of measuring the temperature of a moving fluid

Ensuring accurate temperature measurements with immersion probes depends on capturing a representative fluid temperature, as probes only measure their immediate surroundings Heat transfer mechanisms such as conduction, convection, and radiation influence the temperature uniformity within the fluid, affecting the reliability of the reading Proper probe placement and understanding of these heat transfer modes are essential for obtaining precise and meaningful temperature data.

Except for great temperature differences, most of the heat is transferred from the fluid to the temperature probe by conduction and convection

When inserting a probe into moving fluid, the boundary layer resists heat transfer to the probe, while heat is simultaneously lost to the surroundings To minimize this heat loss, using thin wire leads and applying thermal insulation around the probe are effective strategies.

Mounting the thermometer probe in a thermowell is essential to protect it from corrosion, vibrations, and excessive pressures, ensuring accurate and durable temperature measurements Additionally, thermowells help insulate the probe from electrically conductive liquids, preventing electrical interference Using thermowells also provides easy access for maintenance and calibration of the probe unit See Figure 13 for an illustration of the setup.

Temperature measurement in gases is more difficult than in liquids because of

⎯ the relativity poor heat transfer between the gas and the probe, as compared with the transfer of heat between the probe and its surroundings, and

⎯ the possibility of rapid fluctuations in temperature within the gas

For practical applications where inserting a thermometer probe into a thermowell is not feasible, a wall-mounted sensing device can be used, provided there is effective heat transfer from the gas to the pipe wall However, this method is not recommended for high-accuracy temperature measurements, as it may compromise precision.

Key a As this length is important, it is essential that it is checked b Forging c Radius carefully designed to minimize stresses

Figure 13 — Example of thermowell design 6.3.3 Sensor installation

6.3.4.1 Sensor position and installation configuration

For accurate temperature measurement, the sensor or thermometer should be mounted perpendicular to the pipe wall, as shown in Figure 14a Improper installation can cause severe vibration of the probe due to fluid flow around it, affecting measurement accuracy Proper probe placement is essential to ensure reliable readings and prevent damage from flow-induced vibrations.

The sensor's insertion depth (N in Figure 14), measured from the inner wall, should be adjusted to position the sensor within the middle third of the pipe for optimal performance Achieving this precise placement can be challenging in pipes that are significantly smaller or larger in diameter, but maintaining this optimal position is crucial for accurate measurements and reliable sensor operation.

To minimize the impact of thermal radiation, it is effective to design piping arrangements that position the thermowell outside the line of sight of the radiating source Using a highly polished thermowell helps reflect maximum radiant energy, further reducing thermal interference.

6.3.4.3 Electrical isolation of the temperature transducer

Electrical isolation enhances measurement reliability by preventing disturbances caused by variations in insulation resistance These variations often result from high temperatures affecting thermocouples or moisture and impurities infiltrating junction boxes in resistance bulbs The decision to use an isolated or non-isolated transducer depends on operating conditions; isolation is recommended for high reliability and accuracy, while non-isolated transducers may suffice for stable, less critical measurement points, offering cost savings.

When installing thermowells, it is essential to avoid in-line arrangements to prevent high stress from vortex shedding and vibrations, especially when multiple thermowells are close together; radial spacing around the pipe can minimize these issues The immersion length of the well should be at least ten times its diameter to reduce conduction errors, and specific positioning guidelines apply depending on pipe size and thermowell length, with strength and vibration calculations necessary for large pipes handling high-density fluids Air gaps between the sensing element and the well can cause measurement errors and increased response time; filling these gaps with heat transfer greases or liquids can improve thermal conductivity, but these materials can cause threading issues and need careful tolerance control Using spring-loaded contacts can further enhance heat transfer efficiency It is advisable to avoid excessive protrusion of the thermowell outside the pipe and to insulate the exposed parts if the process fluid temperature differs significantly from ambient air, following standards like ISO 5167-1:2003 Properly sealing the well mouth minimizes heat loss via convection, particularly at high temperatures, while external temperature conditions, including radiation and ambient influences, should also be considered during installation.

6.3.6 Additional precautions in the case of fluctuating temperatures

To ensure accurate temperature measurements when the fluid temperature is variable, it is essential to minimize heat transfer lag between the fluid and the temperature-sensitive element Taking precautions such as optimizing thermal contact, using proper insulation, and selecting appropriate sensor designs can significantly reduce the response time These measures help improve measurement accuracy by ensuring quick and reliable readings despite fluctuating fluid temperatures.

⎯ the wall of the well should have a moderately high thermal conductivity and the surface in contact with the fluid should be kept clean;

⎯ the temperature-sensitive portion of the thermometer should be small in size and of low mass and of low heat capacity

There are a wide variety of temperature-measuring devices based on different operating principles Among the most common are liquid-in-glass thermometers, thermocouples and resistance thermometers

Selecting the appropriate thermometer depends on understanding the measured medium, temperature range, and desired accuracy and reliability Key features of different thermometers are summarized in Table 8, helping you choose the most suitable device for your specific application.

Note that Table 8 should be considered a simple guide rather than a comprehensive reference; the quoted values are approximate orders of magnitude For detailed specifications and standards, consult IEC 60584 for thermocouples and IEC 60751 for industrial platinum resistance thermometer sensors.

NOTE 2 Temperature sensors tend to be sensitive to mechanical vibration

Table 8 — Main characteristics of different types of temperature sensors

Range Uncertainty Type Materials °C °C Comments

Liquid-in-glass Mercury −39 to 600 0,05 Toxicity; fragile

Liquid-in-glass Alcohol −100 to 50 0,1 Fragile

Thermocouples Pt-Rh/Pt 0 to 1 500 1

Figure 14 — Installation of immersion temperature probe

Determination of density

Fluid density can be determined either through direct measurement or by calculating it using static pressure, temperature, and the fluid's properties with an appropriate equation of state at a specified reference plane.

Liquid density under flowing conditions can be determined through direct measurement or referenced from reliable sources, then corrected for the specific temperature at flow Since density variation with pressure is minimal in most cases, it is often considered negligible unless high precision is required Special caution is essential when handling fluids near their vaporization point to ensure accurate measurements and safe operation.

Gas density varies with temperature, pressure, and composition, and for moist gases, it is additionally influenced by water vapor content Accurate measurement is crucial, as sampling errors can occur when gas composition changes or when temperature falls below saturation, leading to liquid formation and significant inaccuracies Understanding these factors is essential for precise gas analysis in various applications.

The most common techniques used for density measurement are the force balance and vibrating element density meters The fundamental characteristics of different density meters are given in Table 9

Continuous weighing 400 to 2 500 250 (max.) 0,1 % to 0,3 % of measured value Centrifugal (gas only) 1 200 (max.) Variable 0,5 % of span

600 to 1 600 as range 0,1 % of span

For the proper installation of density transducers, it is essential to ensure that the pressure and temperature of the fluid within the density cell closely match those at the metering device Additionally, the sample fluid should be clean, free from particles, and single-phase to ensure accurate readings The installation must also protect the sample cell from ambient temperature fluctuations, solar radiation, and wind influences Sufficient flow must be maintained through the density cell to respond effectively to variations in composition, pressure, and temperature Lastly, providing appropriate facilities for maintenance and calibration is crucial for the long-term accuracy and reliability of the transducer.

The installation of the temperature sensor must balance the goal of accurately measuring the density at the upstream pressure tapping plane with adherence to ISO 5167 installation requirements Ensuring proper sensor placement is crucial for obtaining reliable data while complying with international standards Proper installation practices help prevent measurement errors and maintain the integrity of the pressure tapping system.

For accurate density measurement, it is recommended to install the density cell downstream of the primary device, either as an in-line probe or within a sample bypass If an in-line probe is selected, the installation distance from the primary device must comply with the specifications outlined in ISO 5167-2:2003, Table 3, and ISO 5167-4, Table 1 Proper installation ensures reliable readings and optimal performance of the measurement system.

Venturi nozzle, the density cell should be located in accordance with ISO 5167-3:2003, Table 3 Figure 15 illustrates this type of installation

An alternative installation method involves bypassing or venting the fluid through the density cell, with high-pressure tapping located at least 8D downstream of the primary device to ensure accurate measurements The low-pressure return tapping should be positioned just behind the downstream face of the orifice plate, avoiding interference with flow rate measurements As illustrated in Figure 16, a needle valve (V1) should be adjusted to control flow through the densitometer per manufacturer instructions, while a full flow valve (V2), such as a ball valve, must be fully open to prevent pressure drops between the densitometer and low-pressure return tapping During this bypass operation, the gas density (ρm) is measured at downstream pressure p2 and temperature T3, which facilitates calculating the upstream density using appropriate equations, such as Equation (8).

1 2 1 p T p T p T Z p T Z ρ = ρ (8) where ρ 1 is the upstream density (at p 1 ,T 1 ); ρ m is the measured density (at p 2 ,T 3 ) from the densitometer in bypass mode; p 1 is the upstream pressure; p 2 is the pressure at the downstream pressure tapping;

T 3 is the measured temperature at the downstream recovery point;

Z is the compressibility at p 2 , T 3 Listed below are some advantages and disadvantages of both installations

Advantages of an in-line probe:

1) Temperature and pressure are always at flowing conditions at the point of measurement, but temperature and pressure still need correction

2) The method is suitable for both large and small pipelines

3) The probe can be removed while the line is in service if fitted with an isolating valve

4) The method minimizes contamination by condensates

Disadvantages of an in-line probe:

To ensure safe operation, it's essential to recognize the risk of seal failure, which could cause the probe to be ejected from the line or lead to leakage around the housing Implementing additional safety precautions is crucial to prevent these issues and maintain system integrity.

2) A relatively long response time to changes in gas density occurs at low flowrates or static pressure changes

3) The probe is not easily removed from or inserted into the line when under pressure, if not fitted with an isolating valve

4) Main flow may be affected, causing a change in the orifice discharge coefficient

5) There is no check facility on sample flow through the transducer

Advantages of a sample bypass probe:

1) Filtering or maintaining a filter in the stream if needed is easy

2) The flowrate can be adjusted to comply with the accuracy of the instrumentation needed

3) Access for maintenance and testing is easy

Disadvantages of a sample bypass probe:

1) Large thermal mass may cause poor response time to temperature changes

2) Temperature and pressure could vary from flowing conditions, resulting in measurement error Both side stream and transducers shall be provided with thermal insulation

3) Condensation can occur in the instrumentation and affect the accuracy of the density reading A condensate trap may be necessary

1 sampling system for proving purposes only

Figure 15 — Direct-insertion-type density meter

2 top of pipe a To readout electronics b Flow

Figure 16 — Installation showing density meter installed in sample bypass meter mounted in pocket

6.4.3 Additional method for the determination of the density of gas

The density of the gas can be computed from the real gas equation of state [ISO 5167-1:2003, 5.4.2]

When the composition, temperature, and pressure of a gas are known, the compressibility factor is the key variable that needs to be determined If the compressibility factor is not available in published tables, it must be measured experimentally Common methods for determining this include the expansion method and the weighing method, which can achieve an accuracy of 0.1% to 0.2%.

6.4.4 Special consideration concerning gas density

For high-accuracy measurements, it is essential to introduce a correction to account for the difference between the fluid conditions at the density transducer and the upstream pressure tap This correction adjusts for variations caused by pressure and temperature changes, which influence fluid density Estimating this correction can be achieved using the real gas equation of state and the Joule-Thomson coefficient, ensuring precise density readings.

6.4.5 Special considerations concerning liquid density

For approximate liquid density measurements, pressure effects are typically minimal, allowing for simplified calculations Usually, measuring the fluid's temperature during flow conditions (see section 6.3) and referencing density tables at that temperature provides sufficient accuracy This method is reliable as long as the fluid’s composition remains constant and aligns with the specifications listed in the tables.

When the liquid contains dissolved solids or gases, the specific gravity should be determined if accurate results are required

The pyknometer method can be used for checking continuous density measurements.

Electrical supply and electrical installations

Reference should be made to IEC 60079-0:2007 for general requirements

NOTE It is possible that reference should also be made to other parts of the IEC 60079 series

The instrumentation design engineer determines the cabling specifications, tailored to the type of instrument involved Key guidelines include minimizing signal cable lengths to reduce interference, using shielded cables that are earthed at only one point for effective noise protection, amplifying weak signals prior to transmission to ensure signal integrity, and separating power cables from instrument cables while crossing lines at right angles to prevent electromagnetic interference Additionally, signal lines should be adequately shielded from electrical lines to maintain accurate and reliable measurements.

The installation of electronic equipment should be carried out in accordance with the Code of Practice appropriate for the intended use.

Principles of measurement and computation

When calculating flow measurements, ensure that the dimensions d, D, and β reflect the actual flowing conditions If the flowing temperature varies from the standard measurement temperature of 20°C, these values must be corrected for thermal expansion according to ISO 5167-1:2003, section 5.4.4.1 Proper correction for temperature differences is essential for accurate flow measurement and compliance with industry standards.

An explanation of the symbols used can be found in ISO 5167-1:2003, Clause 4

A.1.2 Formulae common to all devices

Subscript “1” refers to the flow condition at the upstream pressure tapping cross-section

Subscript “R” refers to given conditions of pressure and temperature

A.1.3 Limits of use of primary devices

The formulae given for C and ε in all parts of ISO 5167 for the various primary devices can be applied only when certain quantities lie within given limits

These limits of use are recalled in Table A.1

Four detailed examples are shown below which deal with a compressible fluid and the discharge coefficient depending on β and Re D

As will be seen later, it may be convenient to consider the discharge coefficient C as the sum of two terms,

C = C ∞ + C Re , where C ∞ is the discharge coefficient obtained for an infinite Reynolds number Table A.2 shows the formulae giving C ∞ and C Re for each type of device

Reference should be made to the table of iterative computations in ISO 5167-1:2003, Annex A

For accurate calculations depending on the quantity involved, additional equations derived from Equation (A.1) can be essential Table A.3 outlines the specific equations required for the four common types of problems, along with the necessary known quantities to facilitate precise computations.

In all examples, 10-digit numbers are used, providing a level of precision that exceeds practical requirements but is valuable for verifying the accuracy of computer programs.

The goal in each case is to solve the equation f(X) = X, aiming to find the fixed point of the function By defining the difference as f(X_i) – X_i, where X_i is the ith approximation of the true value δ, the iterative algorithm outlined in Annex A of ISO 5167-1:2003 facilitates convergence towards the solution This process systematically refines approximations to accurately determine the true value, ensuring compliance with international standards for measurement accuracy.

An initial value, X 1 , is required; then X 2 = f(X 1 ); then the above equation can be used for n = 2, …

Equation (A.5) can be rewritten as

Then, given an initial value, X 1 , Equation (A.6) can be used for subsequent iterations with E 1 = 0 and

This table outlines the usage limits for various flow measurement devices, including orifice plates, flanged orifice plates, ISA 1932 nozzles, long-radius nozzles, and venturi tubes, with specific dimensions and roughness criteria For orifice plates with corner tappings, the diameter ranges from D to D/2, with a roughness criteria of uDu up to 1,000 mm, and a relative roughness (uβ) between 0.10 and 0.75 for Reynolds numbers (Re D W) from 5,000 to 16,000, depending on β values, following ISO 5167-2:2003 standards Flanged tappings, ISA 1932 nozzles, and various venturi designs have defined operational ranges based on diameter, roughness, and flow conditions, with specified Reynolds number limits and roughness criteria for optimal accuracy Notably, the devices are suitable for use where the pressure drop ratio (Δp/p1) is less than or equal to 0.25 when used with compressible fluids, ensuring measurement precision across different applications.

Table A.2 — C ∞ and C Re for orifice plates for D > 71,12 mm: C = C ∞ + C Re

Type of device Equations Equation number

L 1 , M 2 ′ and A are as defined in ISO 5167-2:2003, 5.3.2.1

Known parameters Quantities to be computed Equations Equation number f(q m ) = CK q (A.9.1) d D ∆p q m

See Figure A.1 for an example of a flowchart

Assume an orifice plate metering facility using flange taps has to be designed for the following conditions:

⎯ maximum pressure differential: 0,5 × 10 5 Pa (500 mbar)

Use the following typical data:

The exit criterion chosen is 10 −6 (0,000 1 %) The calculation procedure is then:

1) ε, applying equation for expansibility [ISO 5167-2:2003, 5.3.2.2]:

For manual calculations using a calculator with a memory, it is useful to store the value of β 4 , since it is required in a number of the subsequent equations

In each case, p 1 , T 1 , ρ 1 , U 1 , K shall also be known

3) Applying Equation (A.8.1) for corner tappings:

For all the other devices, C ∞ could be readily calculated at this stage

4) The starting value of D is obtained from Equation (A.9.7): f(D) = K D C −0,5

For most practical purposes, the calculation can be stopped at this stage, as the final result will not differ significantly from D1 and will ultimately be rounded up to the next commercially available pipe diameter.

The final result obtained by the complete computation would be:

From the previous calculation of D, the nearest commercially available pipe diameter, D = 0,102 m, would be selected by the designer of the metering station

Refer to Figure A.2 for an example of a flowchart

It is now necessary to calculate the orifice diameter d for the same conditions as in A.2.2, i.e

⎯ maximum pressure differential: 0,5 × 10 5 Pa (500 mbar)

The exit criterion being still 10 −6 , the calculation would be:

3) β being unknown, it is convenient and reasonable to use ε = 0,97 as the starting value, except in the case of incompressible fluids, for which ε = 1

4) β being unknown, it is convenient either

⎯ to use a fixed starting value of C, e.g 0,60 for orifice plates and 0,99 for all types of nozzles, or

⎯ to use as a starting value C = C ∞ (in the case of classical Venturi tubes C is a constant)

When the diameter ratio β (or D for orifice plates with flange tappings) is a known parameter, the second method is preferred In this approach, the discharge coefficient C is calculated iteratively using the equation C = C∞ + CRe, where C∞ has already been determined.

For orifice plates with flange tappings where the diameter D is unknown but the beta ratio (β) is known, the initial value of the discharge coefficient (C) can be approximated as equal to C∞,corner, representing the C∞ value for corner tappings During iterative calculations, the value of C must be continuously refined based on the following computation.

C = C ∞,corner + C ∞,L + C Re where the last two terms have to be recalculated at each step

In most practical cases however, it will be sufficient to assume C = C ∞ and make no iteration

6) Starting value of β from Equation (A.9.3): f(β) = (1 + C 2 ε 2 K β ) −0,25 β 1 = f(β) = 0,603 764 155 8

9) Next value of β from Equation (A.9.3): f(β) = (1 + C 2 ε 2 K β ) −0,25 β = f(β 1 ) = 0,596 831 560 9

No correction being made at the first step (E 1 = 0), the starting value for the second step is: β 2 = f(β 1 ) = 0,596 831 560 9

12) Next value of β from Equation (A.9.3): f(β) = (1 + C 2 ε 2 K β ) −0,25 β = f(β 2 ) = 0,596 879 546 2

13) Deviation in f(β 2 ) is given by: 2 2 2 2

E 2 = 5,526 344 567 × 10 −7 which is less than the exit criterion The iteration is then stopped d = βD d = 0,061 203 169 16 m d 0 from Equation (4): d = d 0 [1 + λ d (T − T 0 )] d 0 = 0,060 736 711 22 m

Assume the metering station is now used to measure a flowrate with a plate of diameter d 0 = 0,061 m in the following conditions:

The exit criterion being 10 −6 , the calculation would be:

7) The starting value for q m from Equation (A.9.1): f(q m ) = CK q q m,1 = f (q m ) = 0,983 561 971 8

9) New estimate of C, from C = C ∞ + C Re

10) New estimate of q m , from Equation (A.9.1): f (q m ) = CK q q m = f (q m,1 ) = 0,991 319 905 8

No correction being made at the first step (E 1 = 0), the starting value for the second step is: q m,2 = f (q m,1 ) = 0,991 319 905 8

11) New Reynolds number from Equation (A.4):

12) New value of C, from C = C ∞ + C Re

13) New value of q m , from Equation (A.9.1): f (q m ) = CK q q m = f (q m,2 ) = 0,991 297 674 7

E 2 = − 6,408 057 577 ×10 −8 which is less than the exit criterion The iteration is then stopped and the result is: q m = 0,991 297 674 7 kg s −1

Assume the pressure differential is required for the maximum flowrate of the same facility if the plate has a diameter of d 0 = 0,050 m

10 −6 being the exit criterion, the calculation would be:

7) ε is taken as equal to 0,97

8) Starting value for ∆p, from Equation (A.9.5): f(∆p) = K ∆p ε −2

10) Next value of ∆p, from Equation (A.9.5): f(∆p) = K ∆p ε −2

No correction being made at the first step (E 1 = 0), the starting value for the second step is:

12) Next value of ∆p, from Equation (A.9.5): f(∆p) = K ∆p ε –2

For manual computation, one would stop here The calculation is carried on to show the effect of the rapid scheme

14) The starting value for step 3 is obtained from: ∆p n+1 = (1 − E n ) f(∆p n )

NOTE If substitution iteration was continued [∆p n+1 = f(∆p n ) for all steps], a total of 5 iteration steps would be necessary to conform to the exit criterion

16) Next value of ∆p, from Equation (A.9.5): f(∆p) = K ∆p ε −2

The iteration is then stopped, the exit criterion being met The result is ∆p = 123 939,141 4 Pa

For compressible fluids, it is essential to know the parameters a, ε, and p₁; however, for incompressible fluids, ε is always equal to 1 In the case of classical Venturi tubes and Venturi nozzles, the discharge coefficient C equals C∞, eliminating the need for loop calculations During the initial step, E₁ is set to zero, but further iterations proceed with the "NO" condition, except when dealing with classical Venturi configurations Additionally, aside from flange tapping or orifice plates, only the product C Re needs to be calculated at this stage, which is then added to the previously determined C∞ coefficient.

Figure A.1 — Flowchart example — Computation of pipe diameter D

Understanding fluid properties is essential: for compressible fluids, key parameters such as ε and p1 must be determined, while for incompressible fluids, ε is always 1 and requires no further calculation In classical Venturi tube applications, the constant C is predefined and does not need to be recalculated; if the fluid is incompressible, this simplifies the process by eliminating the need for iteration Initially, E1 is set to zero, but further steps depend on the specific context, with the exception of Venturi tubes operating with incompressible fluids where different procedures may apply.

Figure A.2 — Flowchart example — Computation of diameter d and diameter ratio β

For compressible fluids, it is essential to know the key parameters ε, a, and p₁ In contrast, for incompressible fluids, ε equals 1 and requires no further calculation Classical Venturi tubes and Venturi nozzles have a constant discharge coefficient (C = C∞), eliminating the need for iterative procedures The initial step sets E₁ to zero, except when dealing with classical Venturi configurations, where different approaches may be applied Additionally, only the Reynolds number (C Re) needs to be calculated at this stage and then added to the previously determined C∞ to obtain the total discharge coefficient.

Figure A.3 — Flowchart example — Computation of flowrate q m

For compressible fluids, it is essential to know key parameters such as ε and p₁ In contrast, for incompressible fluids, the compressibility factor ε equals 1, eliminating the need for looping in calculations Initially, E₁ is set to zero, but this step is generally bypassed unless dealing with classical Venturi tubes or Venturi nozzles, where specific procedures apply.

Figure A.4 — Flowchart example — Computation of pressure ∆p

Computation of compressibility factor for natural gases

The density of a gas may be calculated by means of either of the following equations: u pM ρ = R TZ (B.1)

Z is the compressibility factor; ρ is the density;

M is the molecular weight of the gas;

R u is the universal gas constant

The subscript 0 refers to a reference state of temperature and pressure

Z is a function of the composition of the gas

Modern methods for the computation of Z aim to cover the entire range of transmission metering conditions and gas compositions These are described in ISO 12213 [2]

ISO 12213 has three parts: 1) introduction and guidelines; 2) calculation using molar composition analysis;

3) calculation using physical properties All of these parts were published in 2006 and are based on AGA Report Number 8 [7]

The Z calculation via molar composition analysis, known as the “detailed method,” evaluates up to 21 components and has been extensively tested across various natural gas pipeline conditions, including different temperatures, pressures, and compositions (see Reference [9]) High-precision measurements were conducted on five gravimetrically prepared reference natural gas mixtures by four leading laboratories from Europe and North America These gas samples were chosen collaboratively by pipeline industry representatives and are representative of the diverse natural gas compositions encountered globally.

The “gross method” for calculating Z employs physical properties such as any three from the superior (gross) calorific value, relative density, carbon dioxide content, and nitrogen content, along with pressure and temperature This simplified approach enables accurate predictions of Z within a pressure range of 0 MPa to 12 MPa (0 bar to 120 bar) using limited but essential data By leveraging these key parameters, the method provides reliable calculations across various pressure and temperature conditions while maintaining ease of use.

120 bar) and 265 K to 335 K (−8 °C to 62 °C) with an accuracy of about 0,1 %, about the same as the detailed method

Tiêu đề	Guidelines for the use of ISO 5167:2003
Trường học	International Organization for Standardization
Chuyên ngành	Standards
Thể loại	Technical report
Năm xuất bản	2008
Thành phố	Geneva

Định dạng
Số trang	76
Dung lượng	1,38 MB

Tiêu chuẩn iso tr 09464 2008

Guidance specific to the use of ISO 5167-1:2003

Guidance specific to the use of ISO 5167-2:2003

Guidance specific to the use of ISO 5167-3:2003

Guidance specific to the use of ISO 5167-4:2003

Secondary instrumentation

Measurement of pressure and differential pressure

Measurement of temperature

Determination of density

Electrical supply and electrical installations

Measurement of pressure and differential pressure

Electrical supply and electrical installations