Tiêu chuẩn iso 15169 2003

Microsoft Word C026576e doc Reference number ISO 15169 2003(E) © ISO 2003 INTERNATIONAL STANDARD ISO 15169 First edition 2003 12 01 Petroleum and liquid petroleum products — Determination of volume, d[.]

Safety precautions

When utilizing HTMS equipment, it is essential to adhere to ISO standards and relevant national and local safety regulations regarding material compatibility Following the manufacturer's guidelines for equipment use and installation is crucial, as is complying with all regulations related to entry into hazardous areas.

HTMS sensor connections are essential to the tank structure, ensuring that all HTMS equipment can endure the expected pressure, temperature, and environmental conditions during operation.

All electrical components of a Hazardous Area Telemetry Management System (HTMS) must be suitable for the specific classification of the area and comply with relevant national and international electrical safety standards, such as IEC, CSA, CENELEC, and ISO.

Equipment precautions

4.2.1 The HTMS equipment should be capable of withstanding the pressure, temperature, operating and environmental conditions likely to be encountered in service

4.2.2 All electrical equipment and components should be certified for use in the hazardous area classification appropriate to their installation

4.2.3 Measures should be taken to ensure that all exposed metal parts of the HTMS have the same electrical potential as the tank

All components of the HTMS that come into contact with the product or its vapors must be chemically compatible to prevent contamination and equipment corrosion.

4.2.5 All HTMS equipment and components should be maintained in safe operating condition and the manufacturers' maintenance instructions should be complied with

The design and installation of a Heat Transfer Measurement System (HTMS) or its components typically require approval from the national measurement organization This organization usually grants type approval for the HTMS design based on the specific service it will be used for Type approval is generally issued after the HTMS undergoes a series of rigorous tests and is contingent upon the system being installed according to approved guidelines.

Type approval tests may include the following: visual inspection, performance, vibration, humidity, dry heat, inclination, fluctuations in power supplies, insulation, resistance, electromagnetic compatibility, and high voltage

5 Selection and installation of hybrid tank measurement system equipment

General

A hybrid tank measurement system is composed of four key components: an automatic level gauge (ALG), an automatic tank thermometer (ATT), multiple pressure sensors, and a hybrid processor that stores tank parameters and conducts calculations Detailed requirements for each of these components are outlined in sections 5.2 to 5.6.

The user should specify whether the HTMS is to be used primarily for standard volume or mass measurements and the measurement accuracy required for custody transfer

To ensure optimal performance, users or manufacturers must choose and configure HTMS components according to specific application requirements The accuracy needed for the application directly influences the individual accuracy standards for each HTMS component.

Annex A offers a summary of the HTMS theory and its calculations, while Clause 6 and Annex B present guidance and methods for estimating how the selection of individual components impacts the overall accuracy of HTMS.

Copyright International Organization for Standardization

Automatic level gauge

When selecting an automatic level gauge (ALG) for a Hydrocarbon Transfer Measurement System (HTMS), it is essential to consider the specific application, whether for volume-based or mass-based custody transfer, or both Additionally, the installation of the ALG must ensure that the accuracy meets the requirements of the intended application.

The pressure sensors are designated as P 1, located near the tank bottom, and P 3, situated in the ullage space, following the naming convention established by ISO 11223-1 for hydrostatic tank gauging.

5.2.2 The intrinsic accuracy of the ALG, demonstrated by the factory calibration, and the installed accuracy, demonstrated during field verification, should be as given in Table 1

Table 1 — Maximum permissible error for ALG

Volume-based custody transfer application mm

Mass-based custody transfer application mm

The accuracy of the ALG does not influence the mass calculated above the P 1 level due to the cancelling effect of density and volume errors However, the uncertainty in calculated density from ALG errors impacts the heel mass below P 1 Consequently, the selection of ALG accuracy in Table 1 for mass-based custody transfer aims to reduce errors in heel mass Furthermore, minimizing uncertainty in calculated density enhances the ability to independently monitor the performance of pressure transmitters.

5.2.3 In general, the accuracy of an ALG for an HTMS in a volume-based custody transfer application should comply with ISO 4266-1 for vertical cylindrical tanks.

Pressure sensor(s)

When selecting HTMS pressure sensors, it is essential to consider the uncertainty calculation specific to the application, as outlined in clause 6 and Annex B Installation must adhere to the guidelines provided in ISO 11223-1 The accuracy requirements for the pressure sensors vary based on the intended use of the HTMS, whether for volume-based or mass-based custody transfer applications, or both Maximum permissible errors are detailed in Table 2.

Table 2 — Maximum permissible errors for pressure sensor(s)

Maximum error of pressure sensor

For volume-based custody transfer application

For mass-based custody transfer application

Linearity error 0,1 % of reading 0,07 % of reading

Linearity error 0,5 % of reading 0,2 % of reading a If P 3 is used

The span of pressure sensor P 3 can be much smaller than the span chosen for pressure sensor P 1 because the gauge vapour pressure is typically limited to a maximum of approximately 5 kPa

The HTMS pressure sensors must exhibit stability and precision, strategically positioned on the tank shell or at designated points above the reference datum plate In atmospheric storage tank applications, these sensors should function as gauge pressure transmitters, with one port exposed to the atmosphere.

5.3.3 Use of electronic analogue output or digital output depends upon the overall accuracy requirement of the pressure transmitter for its intended application.

Automatic tank thermometer (ATT)

When selecting an automatic tank thermometer (ATT) for a Hydrocarbon Tank Measurement System (HTMS), it is essential to consider the specific applications, such as volume-based or mass-based custody transfer, or both Additionally, the installation of the ATT must ensure that its accuracy meets the requirements of the intended applications.

5.4.2 The intrinsic accuracy of the ATT, demonstrated by the factory calibration, and the installed accuracy, demonstrated during field verification, should be as shown in Table 3

Table 3 — Maximum permissible errors for ATT

Volume-based custody transfer application

Mass-based custody transfer application

0,25 °C including sensor, converter/transmitter/display b) by components:

5.4.3 In general, the accuracy of an ATT for an HTMS in volume-based custody transfer application should be as given in ISO 4266-4 for vertical cylindrical tanks

The choice of Average Temperature Transmitter (ATT) in HTMS applications depends on the specific requirements for accuracy For systems focused on calculating standard volumes, an averaging ATT with multiple fixed-temperature sensors is recommended In contrast, HTMS systems aimed at measuring mass typically utilize a single spot temperature sensor, such as a resistance thermometer (RTD), which is generally sufficient for this purpose.

The ATT can optionally assist in calculating vapor density when multiple elements are available to independently measure vapor temperature, even if other elements are submerged Alternatively, submerged elements of an ATT can be utilized for estimating vapor temperature in an insulated tank.

Hybrid processor

The hybrid processor can be implemented in multiple configurations, such as a locally mounted microprocessor, a remote computer, or the user's own computer system It can either be dedicated to a single tank or shared across several tanks.

The hybrid processor analyzes data from sensors along with tank and product parameters to calculate key metrics such as observed density, reference density, mass, observed volume, and standard volume inventories for the product stored in the tank.

5.5.3 The hybrid processor may also perform linearization and/or temperature compensation corrections of the various HTMS components

5.5.4 All variables measured and computed by the hybrid processor should be capable of being displayed, printed, or communicated to another processor

NOTE Computations normally performed by the hybrid processor are given in Annex A.

Optional sensors

A middle transmitter (P 2 ) can be used for an alternative density calculation, such as a hydrostatic tank gauge (HTG), for comparison, alarming, or as a backup if the ALG component fails For more details, refer to ISO 11223-1.

5.6.2 Instrumentation for ambient density determination

Ambient air density is a second-order term in the HTMS density calculation This International Standard does not cover methods for determining ambient air density However, for more accurate measurements, ambient temperature and pressure sensors can be utilized.

5.6.2.2 Single measurements of ambient temperature and pressure may be used for all tanks in the same location

6 Accuracy effects of HTMS components

General

The accuracy of each component in the HTMS significantly influences the measured or calculated parameters While some applications may prioritize high accuracy for specific parameters, compromises may be necessary for others For instance, when the HTMS is primarily designed for gross standard volume measurement based on the product's density, it is crucial to select components that ensure the average product density's accuracy does not impact the Volume Correction Factor (VCF) determination.

The impact of component accuracy on measured and calculated parameters is detailed in sections 6.2 to 6.4 Annex B provides equations to help users assess the error magnitudes in static measurements of observed density, mass, and gross standard volume, which arise from uncertainties in the primary measurements of the HTMS system, including level, pressure, and temperature.

Accuracy effects of the ALG

The accuracy of the ALG component and its installation has the most effect on level, observed and reference density, and observed and standard volume

Errors in the measured level have little effect on the computed mass because of error cancellation of product volume and density

The mass error cancellation effect is most significant in vertical cylindrical tanks, while it is somewhat reduced in spherical or horizontal cylindrical tanks The impact of ALG accuracy on mass for different tank geometries can be estimated using the uncertainty equations outlined in section B.3.

When utilizing an HTMS for standard volume in custody transfer, the accuracy of the ALG must comply with the standards outlined in ISO 4266-1 However, if the HTMS is mainly employed for mass or density measurements, the accuracy requirements for the ALG can be less stringent than those specified in ISO 4266-1, as detailed in Table 1, which lists the maximum permissible errors for ALG.

Accuracy effects of the pressure sensor(s)

The precision of pressure sensors P 1 and P 3 significantly influences the observed and reference density, as well as the mass However, inaccuracies in these sensors do not impact the observed volume and have only a minimal effect on the standard volume.

The accuracy of a pressure sensor is influenced by zero and linearity errors, with zero error being an absolute measurement in pressure units (e.g., pascals, in H₂O) and linearity error typically expressed as a percentage of the reading At low pressure levels, zero error significantly impacts uncertainty analysis Manufacturers must clearly specify both zero and linearity errors—zero error in absolute units and span error as a percentage of reading—across the expected operating temperature range This information allows users to assess whether the pressure sensor's error contribution meets the required accuracy for HTMS applications For maximum permissible zero and linearity errors, refer to Table 2.

The total error in pressure units of a pressure sensor can be calculated by the formula:

U P-total = U P-zero + (p applied ⋅ U P-linearity)/100 where

U P-total is the total error of pressure sensor, expressed in pascals;

U P-zero is the zero error of pressure sensor, expressed in pascals; p applied is the pressure as input to the pressure sensor, expressed in pascals;

U P-linearity is the linearity error of pressure sensor, expressed as percent of reading

The applied pressure for pressure sensor P 1 (p 1 applied ) is approximately the sum of the liquid head, the vapour head and the maximum setting of the pressure relief valve (see Annex B)

The P 3 pressure sensor indicates that vapour pressure is independent of liquid level, necessitating the use of the maximum value of the pressure relief valve (p 3 max) for the applied pressure (p 3) Refer to Table 2 for the maximum permissible errors associated with pressure sensors.

Accuracy effects of the ATT

The accuracy of the ATT significantly influences the precision of reference density and standard volume measurements To ensure accurate determination of these values, it is essential to average temperature measurements, as outlined in ISO 4266-4.

The accuracy of ATT does not influence the observed density across various tank geometries, and it only has a minimal impact on the mass measured by an HTMS For HTMS systems focused mainly on mass measurement, utilizing a single-point or spot temperature sensor, such as an RTD, is generally sufficient.

A temperature error can significantly impact the accuracy of calculated volume and mass, especially when a thermal expansion correction is necessary, as the operating temperature of the tank often differs from the calibration reference temperature.

General

As the product level nears the bottom pressure sensor (P 1 ), the uncertainty in the calculated density increases This rise in uncertainty is attributed to the growing inaccuracies in the ALG level measurement relative to the level itself, as well as the heightened uncertainty in the P 1 pressure measurement in relation to the diminishing liquid head pressure It is essential to account for this effect when calculating various parameters at low product levels.

HTMS measurements and calculations can be configured in two modes, Mode 1 and Mode 2, based on the user's primary measurement preference—either standard volume or mass The choice of mode also depends on the product's characteristics, such as whether it is uniform or density stratified.

HTMS Mode 1

HTMS Mode 1 is ideal for situations where maintaining a standard volume is crucial and product density is consistently low When the liquid level exceeds a specified threshold (h min), Mode 1 continuously computes the average density of the tank's contents Conversely, if the level drops below h min, it relies on the last recorded reference density (D ref) obtained during the previous decline to h min.

Alternatively, below h min , D ref may be manually entered if the product is stratified or if new product is introduced into the tank

Table 5 (method A) and Table 6 (method B) specify the HTMS measurements and calculations required for Mode 1 at and above h min , and below h min , respectively

See Figure 1 for additional clarification of how calculation methods A and B apply to HTMS Mode 1 as the level changes.

HTMS Mode 2

HTMS Mode 2 is ideal when the primary focus is on product mass as the output value It is also recommended when standard volume is the main output concern, particularly if the user anticipates that a stored reference density (Mode 1) may not accurately reflect the actual density at low liquid levels, which can occur due to stratification or the introduction of new products.

HTMS Mode 2 operates without an h min or product density storage, calculating the reference density (D ref) at all levels above the P 1 cut-off level This cut-off ensures the pressure sensor remains fully submerged; if the product level is at or below this threshold, the last calculated D ref is maintained Measurements and calculations above this level adhere to method A, while those below follow method B, as illustrated in Figure 1, which clarifies the application of these methods in HTMS Mode 2 as the level varies.

8 Commissioning and initial field calibration

General

All measuring components undergo factory calibration prior to installation The commissioning of the HTMS system, which occurs before it is put into service, includes calibration, configuration, and verification processes.

Initial preparation

The hybrid processor will normally store sufficient data to reproduce the tank capacity table These data should be checked against the tank calibration table

8.2.2 Establishment of the hybrid reference point

The positions of the P 1 transmitter and the ALG must be established in relation to the reference datum outlined in the tank calibration table To facilitate this, a hybrid reference point is utilized, which is defined by the dimension $ h_o $ in relation to the tank datum (refer to Figure A.1).

It is advised that the hybrid reference point be located close to the P 1 pressure transmitter's process connection, and should be clearly and permanently marked on the tank shell

Accurate measurement and recording of the hybrid reference point's position relative to the tank datum plate (h o) are essential for proper data entry into the hybrid processor Following this, the elevation of the pressure sensor's effective center (h b) must be measured The position of the pressure sensor in relation to the tank datum plate can then be calculated using the formula $ z = h o + h b $ and entered into the hybrid processor.

The hybrid reference point serves as a reliable method for verifying or determining the position of the P 1 transmitter after its reinstallation, thereby eliminating the necessity to remeasure its position in relation to the tank datum.

HTMS parameters must be defined and input into the hybrid processor, encompassing essential tank data such as the capacity table, dimensions relative to the hybrid reference point, and the ALG reference height.

P 1 sensor, the HTMS Mode, the value of h min , “P 1 cut off”, ambient data, pressure sensor parameters, ALG and ATT component parameters, and product parameters (See Table 4.)

Initial calibration and verification of HTMS components

Each of the HTMS components should be independently calibrated, e.g the ALG should not be calibrated using measurements derived from the pressure sensors, and vice-versa

The ALG should be field-calibrated in accordance with ISO 4266-1, but using the appropriate tolerance given in Table 1 of this International Standard

8.3.3 Pressure sensor calibration and zero adjustment

HTMS pressure sensors are typically calibrated at the factory, and aside from zero adjustments, field adjustments are generally not feasible It is essential to verify the calibration of installed pressure sensors with precision pressure calibrators that are traceable to national standards If any sensors are discovered to be out of specification, they must be replaced.

Zero adjustments of pressure sensors should be carried out using the procedure given in ISO 11223-1

The ATT should be calibrated in accordance with ISO 4266-4, but using the appropriate tolerance given in Table 3 of this International Standard

Verification of hybrid processor calculations

Hybrid processor calculations should be checked against manual calculations for verification of proper data entry.

Initial field verification of HTMS

The final stage in the commissioning and initial verification of an HTMS system involves validating its measurements against manual checks If these manual assessments reveal that the HTMS measurements are outside the expected tolerances, it is necessary to repeat some or all of the commissioning calibrations and manual verification processes.

8.5.2 Initial field verification of volume-based HTMS applications

For a volume-based Hydrocarbon Transfer Measurement System (HTMS) used in fiscal and custody transfer applications, it is essential to verify key components The Automatic Level Gauge (ALG) must be checked according to the procedures and tolerances specified in ISO 4266-1, while the Automatic Temperature Transmitter (ATT) should be verified following ISO 4266-4 guidelines Additionally, pressure sensors, including any separate transmitters, need to be zeroed and assessed for linearity These verifications should be conducted in situ, necessitating the availability of a means to read the digital pressure values, which can be done through a local display, hand-held terminal, or a separate computer.

1) For zero adjustment, the transmitter should be isolated from the pressure port vented to atmosphere The zero error after this adjustment should be approximately zero

To ensure accurate measurements, linearity must be verified using a high-precision pressure calibration reference that is traceable to national standards This verification should occur at two test pressures, specifically around 50% and 100% of the range The linearity error is calculated by finding the difference between the pressure sensor's indication (after accounting for any zero error) and the pressure reference, which can then be expressed as a fractional error and converted to a percentage It is crucial that the resulting linearity error does not exceed the maximum limits specified in Table 2 for any of the test pressures.

NOTE For high-precision pressure transmitters, it may be difficult or impractical to adjust transmitter linearity under field conditions

Once the sensors and transmitters have been zeroed and their linearity verified, it is essential to conduct a final check to ensure that the zero error stays within the accuracy limits specified in Table 2 Additionally, it is important to document the zero reading and the linearity error as they are left.

The reference product density obtained from the HTMS must be compared with the average product density derived from testing a representative tank sample Sampling should adhere to ISO 3170 standards, while the density should be measured according to either ISO 3675 or ISO 12185.

Density comparisons should be conducted at approximately (4 ± 0.5) m above P1, where HTMS provides real-time density measurements, ensuring the level is above h min The density difference between HTMS readings and tank samples must remain within ± 0.5% of the measurement In cases of homogeneous tank contents, the uncertainty from manual sampling decreases, allowing for a stricter tolerance of less than ± 0.5% This refined tolerance can be determined through statistical quality control methods.

For pure homogeneous products, such as certain petrochemical liquids, the reference density can be precisely determined through physical science This reference density serves as an accurate representation of the product's density, allowing for effective comparison with the density measured by the HTMS.

The ± 0.5% tolerance reflects the estimated uncertainty associated with manual sampling and the repeatability of laboratory analysis This uncertainty can vary considerably in tanks with density stratification, depending on the location of the gauging access point and the specific sampling procedure employed.

The acceptable uncertainty of the HTMS density is assessed by evaluating its effect on the volume correction factor (VCF) and the correction for temperature effects on liquids (CTL).

For non-stratified products, utilizing a recently calibrated on-line densitometer allows for the comparison of the density measured during batch transfers with the mean density obtained from the HTMS This method ensures compliance with the established tolerance levels.

8.5.3 Initial field verification of mass-based HTMS applications

The key components of a volume-based Hydrocarbon Transfer Measurement System (HTMS) for fiscal and custody transfer applications must undergo specific verifications The Automatic Level Gauge (ALG) should be verified according to ISO 4266-1, with a relaxed tolerance of 12 mm The Automatic Temperature Transmitter (ATT) must be verified following ISO 4266-4, allowing for a tolerance of 1 °C Additionally, pressure sensors, including any separate transmitters, significantly impact mass measurement accuracy and should be zeroed and spanned using a calibration reference, such as a hand-held terminal and precision pressure calibrator, traceable to national standards Calibration is essential to ensure that the sensors and transmitters maintain accuracy as specified in Table 2.

NOTE For high-precision pressure transmitters, it may be difficult or impractical to span the transmitter under field conditions Under these circumstances, this procedure cannot be performed

8.5.3.2 Density comparison of the HTMS should be verified by the method given in 8.5.2.2

8.5.3.3 The HTMS mass transfer accuracy should be verified using the method given in ISO 11223-1

NOTE The tolerance given in ISO 11223-1 is for “transfer accuracy” and therefore the verification involves a transfer of liquid into or out of the tank

General

After commissioning and initial field verification, an HTMS in custody transfer service should be regularly verified in the field This subsequent or regular verification is also called “validation”

Post-commissioning HTMS verification and any necessary recalibrations are given in 9.2 to 9.6

NOTE Verification differs from calibration in that it generally does not involve correction of the sensors or the HTMS hybrid processor parameters

Objectives

Regular verification aims to ensure that the performance of HTMS stays within required accuracy limits and facilitates the use of statistical quality control to determine recalibration frequency, contingent upon the agreement of all parties involved in custody transfer.

Adjustment during regular verification

If the verification process reveals that HTMS performance has drifted beyond the established limits, recalibration or readjustment of the HTMS is necessary Conversely, if the performance remains within these limits, no adjustments should be made It is essential that these limits consider the anticipated combined measurement uncertainties of the HTMS, the reference equipment, and the performance requirements of the HTMS.

Subsequent verification of HTMS in volume-based custody transfer application

To ensure the accuracy of a volume-based Hydrostatic Tank Measurement System (HTMS), it is essential to verify its major components The Automatic Level Gauge (ALG) must be checked according to the procedures and tolerances outlined in ISO 4266-1 for upright cylindrical tanks Similarly, the Automatic Tank Gauge (ATT) should be verified following the guidelines specified in ISO 4266-4 for upright cylindrical tanks Additionally, the stability of the pressure sensor or transmitter requires thorough verification to maintain system integrity.

It is essential to verify the transmitter zero in situ, ensuring that the zero reading, or "as found" value, does not surpass the manufacturer's specifications or the maximum recommended zero error value outlined in Table 2.

If the zero reading exceeds the maximum recommended values in Table 2 but remains within the manufacturer's specifications, the transmitter should be zeroed or a software zero correction applied It is essential to document the zero reading values as "as-found" and "as-left." If the readings surpass the manufacturer's specifications, consultation with the manufacturer is advised.

To ensure accurate transmitter performance, it is essential to verify linearity in situ with a high-precision pressure calibrator that is traceable to national standards The linearity error must remain within the manufacturer's specifications or the maximum recommended values outlined in Table 2 If these specifications are surpassed, consultation with the manufacturer is necessary Additionally, it is important to document the linearity readings for both "as-found" and "as-left" conditions.

NOTE For high-precision pressure transmitters, it may be difficult or impractical to adjust the transmitter linearity under field conditions

The density of the product measured by an HTMS must be compared with the density obtained from a representative tank sample and laboratory analysis Sampling should adhere to ISO 3170 standards, while the density determination should follow either ISO 3675 or relevant guidelines.

Density comparisons should be conducted at approximately (4 ± 0.5) m, ensuring that HTMS provides on-line density measurements when the level exceeds h min The acceptable tolerance between the density measured by HTMS and that obtained from tank samples should be within ± 0.5% of the reading In cases where the tank contents are homogeneous, the uncertainty from manual sampling decreases, allowing for a more stringent tolerance of less than ± 0.5% of the reading This refined tolerance can be determined using statistical quality control methods.

The verification frequency for major components of the HTMS is outlined as follows: a) The ALG must be verified according to the subsequent field verification frequency specified in ISO 4266-1 b) The ATT should adhere to the verification frequency detailed in ISO 4266-4, with newly installed or repaired units requiring verification every three months; this can be extended to once per year if performance remains stable c) Pressure sensors and transmitters must have their zero stability and linearity stability verified at least annually after the initial verification.

9.4.2.2 The comparison of product density should be performed, using the procedure given in 9.4.1.2, at least once every 3 months following initial verification

NOTE 1 Use of statistical quality control methods, rather than the above predetermined time, may also determine the frequency of regular verification

Regularly comparing product density can facilitate the early identification of issues in ALG, ATT, or pressure sensor/transmitter components, while also offering essential statistical insights into the HTMS.

Subsequent verification of HTMS in mass-based custody transfer applications

To ensure the accuracy of a mass-based HTMS, it is essential to verify its major components The ALG must be checked following the calibration verification procedure outlined in ISO 4266-1, adhering to the tolerances specified in Table 1 Similarly, the ATT should be verified according to the calibration procedure in ISO 4266-4, meeting the tolerances in Table 3 Additionally, the zero and linearity stability of the pressure sensor/transmitter(s) must be assessed as per the method described in section 9.4.1.1 c), ensuring compliance with the tolerances listed in Table 2.

9.5.1.2 Comparison of product density determined by HTMS and by manual methods is optional in mass- based custody transfer applications This comparison, if required, should be made in accordance with 9.4.1.2

Regular verification of key components in a mass-based custody transfer HTMS is essential Newly installed or repaired Automatic Level Gauges (ALGs) should be verified quarterly, with the option to reduce this to every six months if performance is stable and density comparisons are conducted quarterly Similarly, newly installed or repaired Automatic Temperature Transmitters (ATTs) require the same verification frequency as ALGs The zero and linear stability of pressure sensors/transmitters must be verified quarterly after initial checks, with the possibility of extending linearity verification to every six months if stability is confirmed Density comparisons between HTMS and manual methods are optional, but if used to justify reduced ALG verification frequency, they must occur at least quarterly.

NOTE More frequent comparison of product density will ensure early detection of problems in the ALG, ATT, or pressure sensor/transmitter(s), and will provide valuable statistical data

Handling out-of-tolerance situations during regular verification of HTMS in custody

If a component of the HTMS is discovered to be out of tolerance during routine field verification, it is essential to investigate the cause to decide whether the component needs adjustment, calibration, resetting, or repair.

9.6.2 After adjustment or repair, the component should be reverified using the procedure given in 8.5

Figure 1 — Summary of HTMS calculation methods as they relate to level for Modes 1 and 2

Tank data Tank roof type

Tank roof mass Critical zone height Pin height

Tank wall type Tank wall material Tank capacity table Tank calibration temperature

Fixed or floating or both Floating roofs only Floating roofs only Floating roofs only Insulated or non-insulated Thermal expansion constants Volumes at given levels

Temperature to which the tank capacity table was corrected

Distance between hybrid reference point and datum plate

Vertical distance from datum plate to ALG mounting

Pressure sensor data Sensor configuration

Tank with one or more sensors

At applicable reference point(s) (see Figure A.1)

ATT component data Type of ATT

Element type Number of elements Vertical location of elements

May be programmed in the ALG

Resistance or other, may be programmed in ALG

Product data Coefficients related to liquid expansion Vapour parameters Free water level

See for further information ISO 91-1

Ambient data Local acceleration due to gravity

Obtained from a recognized source Optional

Table 5 — HTMS measurements and overview of calculations – Calculation Method A

Parameter Method of measurement or calculation

Product level (L) Measured by ALG

Average product temperature (t) Measured by ATT

Observed product density (D obs ) Calculated using equation in A.3

Reference density (D ref ) Calculated from D obs and t, by iteration (see note 4)

Volume correction factor (VCF) Calculated as VCF = D obs / D ref

Gross observed volume (GOV) Calculated from L by ALG and tank capacity table (see note 3)

Gross standard volume (GSV) Calculated as GSV = GOV × VCF

Mass (in vacuo) Calculated as m = GOV × D obs

NOTE 1 This table is applicable to Mode 1 at levels at and above h min , only

NOTE 2 This table is applicable to Mode 2 at all levels above P 1 “cut off”

NOTE 3 After deducting for free water (FW), if any, from the total observed volume (TOV) of the liquid in the tank GOV = TOV − FW

NOTE 4 Manual density may be used if the HTMS measured density is not reliable or not available

Table 6 — HTMS Measurements and overview of calculations – Calculation Method B

Parameter Method of measurement or calculation

Product level (L) Measured by ALG

Average product temperature (t) Measured by ATT

Observed product density (D obs ) Calculated as D obs = D ref / VCF

The reference density (D ref) should be set to the most recent calculated value and will remain constant when L is below h min in Mode 1 or below P 1 in Mode 2 The volume correction factor (VCF) is determined using the time measured by ATT and the stored D ref value.

L = h min in Mode 1, or when L is below P 1 in Mode 2

Gross observed volume (GOV) Calculated from L by ALG and tank capacity table (see note 2)

Gross standard volume (GSV) Calculated as GSV = GOV × VCF

Mass (in vacuo) Calculated as m vacuo = GSV × D ref

NOTE 1 This table is applicable to Mode 1 at levels below h min , only

NOTE 2 After deducting for free water (FW), if any, from the total observed volume (TOV) of the liquid in the tank GOV = TOV − FW

NOTE 3 Manual density may be used if the HTMS measured density is not reliable or not available

This Annex outlines the calculations executed by the HTMS hybrid processor to determine the density of tank contents and other related variables It does not cover specific calculations or features that may be unique to individual manufacturers' designs of HTMS, such as pressure sensor linearization formulas.

Table A.1 — Units table for HTMS equations

Constant Units used in the equations

N Gauge pressure Capacity table volume Local acceleration due to gravity Level Observed and standard volume Observed and reference density Mass

In atmospheric tanks, the densities of in-tank vapor and ambient air have minimal impact on the calculated variables, allowing them to be treated as constants or calculated for enhanced accuracy The in-tank vapor density can be determined using the gas equation of state, which incorporates absolute vapor pressure, absolute vapor temperature, and vapor relative density.

Ambient air density is determined using the gas equation of state, which relies on absolute ambient pressure and temperature Variations in ambient air density primarily have a minor impact on the observed density It is crucial that all sensor input data provided to the hybrid processor is synchronized.

GOV = [(TOV − FW) × CTSh] ± FRA where

GOV is the gross observed volume;

TOV is the total observed volume as calculated from ALG and tank capacity table;

FW is the free water quantity (volume);

FRA is the floating roof adjustment, if applicable;

CTSh is tank shell thermal expansion correction

CTSh = 1 + 2 α (∆t) + α 2 (∆t) 2 where α is the linear thermal expansion factor given in Table A.2

∆t = t sh − t B where t sh is the temperature of shell; t B is the base shell temperature (the shell temperature at which the tank capacity was computed);

L is the liquid level by the ALG, referenced to the tank capacity table reference point Effect of linear thermal expansion, if any, has been compensated in L

Table A.2 — Linear thermal expansion factor

316 Stainless steel 0,000 015 9 17-4PH Stainless steel 0,000 010 8

A.3 Observed product density (in vacuo) ( D obs )

The hybrid density calculation (D obs ) is fundamentally based on pressure balance, where the total pressure increments between any two points remain constant, irrespective of the path taken to measure them.

Thus: p 1 − p 3 = (total liquid product head + in-tank vapour head) − ambient air head between P 1 and P 3

Also, head pressure in either liquid or vapour may be approximated by the product of average density and head, thus:

Liquid head pressure = g × (L − Z) × D obs (at P 1 elevation)

In tank vapour head = g × [h t − (L − Z)] × D v (at liquid surface)

Ambient air head = g × h t × D a (at P 1 elevation)

Thus, the value of D obs may be calculated from:

D obs is the observed liquid density in vacuo;

N is the unit constant (see Table A.1);

L is the ALG (innage) level reading;

The equation $ Z = h_b + h_o $ represents the relationship between the heel under the P1 transmitter and the tank datum plate, as illustrated in Figure A.1 Here, $ h_b $ denotes the vertical distance from the center of force on sensor P1 to the hybrid reference point, while $ h_o $ indicates the vertical distance from the tank datum plate to the hybrid reference point Additionally, $ g $ refers to the local acceleration due to gravity, and $ h_t $ signifies the vertical distance of the centers of force on the diaphragms of sensors P1 and P3.

D v is the in-tank vapour density;

D a is the ambient air density

NOTE h o is zero if the hybrid reference point is at the same altitude as the tank datum plate

NOTE 2 ATT is not shown

Figure A.1 — Measurement parameters and variables – Fixed roof tank

A.4 Product mass calculation (in vacuo) ( m ) m = GOV × D obs where

GOV is the gross observed volume as calculated in A.2;

D obs is the observed product density (in vacuo) as calculated in A.3

NOTE For atmospheric storage tanks, m vapour may be assumed to be zero

A.5 Product apparent mass in air ( m a ) m a = m × (1 − D a / D obs ) where m is the total product mass (in vacuo) as calculated in A.4;

D a is the ambient air density;

D obs is the observed liquid density (in vacuo) as calculated in A.3

GOV is the gross observed volume as calculated in A.2;

VCF is the volume correction factor, typically obtained from ISO 91-1

Measurement accuracy and uncertainty analysis

The accuracy of calculated density, mass, and standard volume relies on the proper installation of the ALG and pressure sensors, as well as the combined precision of the pressure sensor(s), ALG sensor, ATT sensor, hybrid reference point measurement, tank capacity table, and local gravitational acceleration.

Local acceleration due to gravity can be estimated with an uncertainty of 0,005 % The uncertainty in the gravitational term is neglected in the accuracy equations in B.2 to B.4

The symbols, terms and units used in the inventory accuracy equations below are as follows:

L ALG innage reading m p 1 Reading of pressure sensor P 1 Pa p 3 Reading of pressure sensor P 3 Pa t Reading of ATT temperature sensor °C

Z Offset of P 1 from tank datum plate (= h o + h b ) m

D v Vapour density kg/m 3 g Local acceleration due to gravity m/s 2

(NOTE For uncertainty calculation purposes, this density is a hypothetical actual density, which is the same as observed density if there are no measurement errors.) kg/m 3

U AE Percent uncertainty of tank capacity table %

U D15 Percent uncertainty in standard density %

U D Percent uncertainty of observed density %

U L Uncertainty in ALG level measurement m

U P1-zero Uncertainty of P 1 when no pressure is applied Pa

U P1-linearity Uncertainty of P 1 related to applied pressure (fraction of reading)

U P1-total Total uncertainty of P 1 (combination of zero error and linearity) Pa

U P3-zero Uncertainty of P 3 when no pressure is applied Pa

U P3-linearity Uncertainty of P 3 , related to applied pressure (fraction of reading)

U P3-total Total uncertainty of P 3 (combination of zero error and linearity) Pa

U t Uncertainty in ATT temperature measurement °C t ref Reference temperature for standard volume °C

K 1 , K 0 Constants of thermal expansion factor defined by ISO 91-1

F Q Tank geometry factor (F Q = 1,0 for vertical cylindrical tanks) (see B.5 for equations)

The uncertainty examples, as given in this Annex, neglect the four following uncertainty sources because of their minor effect: gravity (g); ambient air density D a ; vapour density D v and distance h t

The examples of uncertainty presented in sections B.2 to B.6 illustrate various configurations of HTMS, each comprising multiple cases In these cases, the maximum allowable measurement uncertainty for each parameter contributing to the final measurement is applied.

 Case 1: HTMS configured for both mass- and volume-based custody transfer

 Case 2: HTMS configured for volume-based custody transfer

 Case 3: HTMS configured for mass-based custody transfer

B.2 Accuracy equation for observed density

The accuracy of observed density (in percent) may be estimated from:

Examples of calculations are given in Tables B.1 and B.2

Table B.1 — Example of observed density accuracy – Floating roof tank

Product: Gasoline in floating roof tank

P 1 zero error (U P1-zero ) linearity error (U P1-linearity)

Observed density accuracy [±±±± %%%% of reading]

Table B.2 — Example of observed density accuracy – Fixed roof tank

Product: Diesel (or miscellaneous liquid) in fixed roof atmospheric tanks of various geometries

P 1 zero error (U P1-zero ) linearity error (UP1-linearity) [Pa]

P 3 zero error (U P3-zero ) linearity error (UP3-linearity)

Observed density accuracy [±±±± %%% of reading] % Vertical cylindrical tank

The accuracy of a spot measurement for mass (in percent) may be estimated from:

Examples of calculations are given in Tables B.3 and B.4 These examples apply to static conditions (i.e product level and temperature are constant) and are not to be confused with transfer accuracy

Table B.3 — Example of mass measurement accuracy – Floating roof tank

Mass measurement accuracy [±±±± %%%% of reading]

Table B.4 — Example of mass measurement accuracy

Mass measurement accuracy [±±±± %%%% of reading]

B.4 Accuracy equation for standard volume

The accuracy of standard volume inventory (in percent) may be estimated from:

Examples of calculations are given in Tables B.5 and B.6 These examples apply to static conditions (i.e product level and temperature are constant) and are not to be confused with transfer accuracy

Table B.5 — Example of standard volume measurement accuracy – Floating roof tank

Product: Gasoline in floating roof tank t ref = 15°C t = 25°C

Standard volume measurement accuracy [±±±± %%%% of reading]

Table B.6 — Example of standard volume measurement accuracy – Fixed roof tank

Product: Diesel (or miscellaneous liquid) in fixed roof atmospheric tanks of various geometries t ref = 15 °C t = 25 °C

Standard volume measurement accuracy [±±±± %%%% of reading]

The factor F Q is used to adjust accuracy equations given in B.3 (mass) and B.4 (standard volume) for difference in tank geometry

B.5.2 Spherical tanks (of internal diameter d i )

B.5.3 Horizontal cylindrical tanks (of internal diameter d i )

The minimum liquid level, denoted as \$h_{min}\$, is the threshold below which the accuracy of the measured density falls below the user-defined permissible value The calculation of \$h_{min}\$ can be performed using specific formulas.

 Define the following two constants (A and B) to simplify the equation for h min :

 Calculate h min from the following equation:

Examples of h min calculations are given in Tables B.7 and B.8

Table B.7 — Example of h min calculation – Floating roof tank

Uncertainty density = 0,2 % Uncertainty density = 0,3 % Uncertainty density = 0,5 % Uncertainty density = 1,0 %

Table B.8 — Example of h min calculation – Fixed roof tank

P 1 zero error (U P1-zero ) linearity error (U P1-linearity) [Pa]

U Z [m] 0,003 0,003 0,003 h min (m) (Independent of tank geometry)

B.7 Effect on Volume Correction Factor (VCF) due to uncertainty of density

The impact of uncertainty in observed density ($D_{obs}$) on the calculation of Vapor Control Factor (VCF) for crude oils and refined products is illustrated in the examples provided in Tables B.9 and B.10.

For heavy oils such as crude oil, the change in VCF is less sensitive to the error on product density, as can be seen in Tables B.9 and B.10

Table B.9 — Example – Effect on Volume Correction Factor (VCF) for a crude oil due to uncertainty of density

Basis: Product temperature = 20 °C “True” density is 885 kg/m 3

Table B.10 — Example – Effect on Volume Correction Factor (VCF) for a refined product due to uncertainty of density

Basis: Product temperature = 20 °C “True” density is 745,0 kg/m 3

Tiêu đề	Petroleum And Liquid Petroleum Products — Determination Of Volume, Density And Mass Of The Hydrocarbon Content Of Vertical Cylindrical Tanks By Hybrid Tank Measurement Systems
Trường học	International Organization for Standardization
Chuyên ngành	Petroleum and Liquid Petroleum Products
Thể loại	international standard
Năm xuất bản	2003
Thành phố	Geneva

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