Manual of Petroleum Measurement Standards Chapter 5 8 Measurement of Liquid Hydrocarbons by Ultrasonic Flow Meters SECOND EDITION, NOVEMBER 2011 Manual of Petroleum Measurement Standards Chapter 5 8 M[.]
General
This document outlines the criteria for applying Ultrasonic Flow Meters (UFMs) and highlights key considerations for the liquids being measured It also covers the installation, operation, and maintenance of UFMs specifically in liquid hydrocarbon services.
Field of Application
This standard applies to the dynamic measurement of liquid hydrocarbons, specifically for custody transfer measurement However, it can also be utilized for allocation measurement, check meter measurement, and leak detection measurement The focus of this document is on spool type, multi-path ultrasonic flow meters equipped with permanently affixed acoustic transducer assemblies.
The referenced documents are essential for the application of this document For dated references, only the specified edition is applicable, while for undated references, the most recent edition, including any amendments, is relevant.
API MPMS Chapter 4.5, Master-Meter Provers
API MPMS Chapter 4.8, Operation of Proving Systems
API MPMS Chapter 6.1, Lease Automatic Custody Transfer (LACT) Systems
API MPMS Chapter 6.2, Loading Rack Metering Systems
API MPMS Chapter 6.4, Metering Systems for Aviation Fueling Facilities
API MPMS Chapter 6.5, Metering Systems for Loading and Unloading Marine Bulk Carriers
API MPMS Chapter 6.6, Pipeline Metering Systems
API MPMS Chapter 6.7, Metering Viscous Hydrocarbons
API MPMS Chapter 13.1-1985, Statistical Concepts and Procedures in Measurement
API MPMS Chapter 21.2, Electronic Liquid Volume Measurement Using Positive Displacement and Turbine Meters
For the purposes of this document, the following definitions apply.
The path that the acoustic signals follow as they propagate through the measurement section between the acoustic transducer elements.
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A component that produces either an acoustic output in response to an electric stimulus and/or an electric output in response to an acoustic stimulus.
The component of liquid flow velocity at a point in the measurement section that is parallel to the measurement section’s axis and in the direction of the flow being measured.
A device for reducing swirl and velocity distortions
K-factor pulses per unit volume.
The section of piping which includes the upstream flow-conditioning section, the flow meter and the downstream flow section.
Scaling performed in the SPU so that the meter produces a set number of pulses proportional to volume
The electronic system comprises power supplies, microcomputers, signal processing components, and circuits for exciting ultrasonic acoustic transducers These components can be contained within one or more enclosures, which may be located either close to or far from the meter.
Measurement of the time interval associated with transmission and reception of an acoustic signal between acoustic transducers.
4.1 The design of an Ultrasonic Flow Meter (UFM) run shall take into account the following considerations.
The design of the meter run must take into account the user's minimum and maximum flow rates, Reynolds number, temperatures, and pressures It is also essential to consider physical properties such as viscosity, relative density, vapor pressure, and corrosiveness Ideally, the operation should remain within the linear flow range of the Ultrasonic Flow Meter (UFM) tailored to the specific application.
Temperature devices, thermowells for temperature testing, and pressure and density sensing devices must be installed to accurately reflect the true metering conditions, with the ideal placement being immediately downstream of the meter run (refer to Figure 1).
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Transit time Ultrasonic Flow Meters (UFMs) generally do not need strainers due to their lack of mechanical moving parts that could be damaged by debris However, strainers may be necessary to safeguard related equipment, such as meter provers or pumps, and to ensure that flow conditioners remain clear of debris.
4.1.4 If air or vapor is present in the flowing stream, eliminators shall be provided to minimize measurement error (see Figure 1)
The design of the meter run must guarantee that each meter remains liquid-filled under all operating conditions, avoiding placement at high points in the system While Ultrasonic Flow Meters (UFMs) can be installed in any position or plane, it is crucial to position the acoustic transducers away from the top or bottom of the pipe to reduce the impact of air or sediment Installation orientation should adhere to the manufacturer's recommendations.
4.1.6 For multiple meter runs, see MPMS Chapter 6 (all sections) for meter system design considerations.
To enhance the accuracy of Ultrasonic Flow Meters (UFMs), it is essential to minimize the water content in the measured fluid The flow regime, acoustic properties of the oil, and the size and distribution of water droplets can affect meter performance, potentially rendering some measurement paths inoperable Due to the variability of these factors, a specific percentage limit for water content cannot be established It is advisable to consult the UFM manufacturer for specific guidance on acceptable water levels Additionally, meter diagnostics can provide valuable insights into the meter's performance.
4.1.8 The design shall comply with all applicable regulations and codes.
To ensure meters are safeguarded against excessive pressure, it is essential to utilize appropriate pressure relief devices This protection may involve the installation of surge tanks, expansion chambers, pressure-limiting valves, and pressure relief valves, among other protective mechanisms.
4.1.10 The operating pressure in the meter run shall be maintained sufficiently above vapor pressure
Figure 1—Typical Elements of a Single UFM Installation
3 strainer and/or air eliminator a
11 double block and vent valve
Notes a Element may not be required. b See Section 7.1, Flow Conditioning.
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4.2 The following equation may be used to calculate the minimum backpressure.
Consult the meter manufacturer for maximum pressure drop to determine the total backpressure required in Equation 1.
Pb is the minimum backpressure, pounds per square in gauge (PSIG);
∆p is the pressure drop across the meter run; p e is the equilibrium vapor pressure at operating conditions, pounds per square in absolute scale (PSI).
For liquids with vapor pressure greater than atmospheric pressure, a backpressure greater than 20 psi above the liquid vapor pressure at operating conditions is sufficient.
5.1 If the meter is utilized in bi-directional flow:
— both inlets of the meter shall conform to the upstream requirements;
— a meter factor shall be determined for each direction.
When a meter is used more frequently in one flow direction, it is essential to position temperature, pressure, and density instrumentation downstream of the meter run in that direction.
6 Selecting a Meter and Accessory Equipment
6.1 Consideration shall be given to the following items.
6.1.1 The class and type of piping connections and materials and the dimensions of the equipment to be used.
6.1.2 The minimum and maximum operating flow rates.
6.1.3 The minimum and maximum operating viscosity of the liquids to be measured
6.1.4 The minimum and maximum operating Reynolds number See Annex D.
6.1.5 The space required for the installation of the metering and proving system
The acceptable pressure drop across the meter run is crucial, as most Ultrasonic Flow Meters (UFMs) exhibit minimal or no pressure drop However, it is important to account for the pressure drop caused by the flow conditioning element.
6.1.7 Metallurgy, elastomers, coatings and other components are compatible with the process fluid.
Erosive and corrosive contaminants can significantly impact the performance of meters, affecting their accuracy and longevity Additionally, the presence of foreign matter, such as abrasive particles, in the liquid stream can further compromise measurement quality and equipment integrity.
6.1.9 The minimum and maximum ambient and process temperatures.
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6.1.10 Possible depositions such as wax, asphaltenes, or other precipitants that may affect the performance of the UFM.
6.1.11 The size and type of prover and method of proving Unidirectional piston pipe provers with external detector switches (small volume prover) require special consideration to achieve repeatability See Annex B
6.1.12 Maintenance, costs, and spare parts that are needed.
6.1.13 Requirements and suitability for security sealing, auditing, and/or reporting.
6.1.14 Interface requirements for communicating meter pulses, diagnostics and alarms to other electronic devices as needed
General
Applicable industry standards and manufacturer’s recommendations shall be followed when installing the meter run components.
Flow Conditioning
To ensure accurate flow measurement, it is essential to incorporate flow-conditioning elements that minimize swirl and velocity profile distortion The design should provide adequate flow conditioning both upstream and downstream of the meter, typically requiring straight pipe lengths of 10 pipe diameters upstream and 5 pipe diameters downstream However, adjustments may be necessary based on the meter manufacturer’s recommendations and flow research findings Special care must be taken with the upstream flow conditioning section, especially in the absence of a conditioning element, as headers and out-of-plane 90-degree turns can induce swirl that negatively affects the performance of the Ultrasonic Flow Meter (UFM).
The meter run piping's internal diameter must match the meter's inlet and outlet Welds should be ground smooth on the inside, and gaskets must be installed without protruding into the pipe It is advisable to use methods that ensure proper internal alignment.
The effects of different piping configurations or flow-conditioning elements on the flow-conditioning installation requirements has not been fully evaluated; therefore, consult the manufacturer for design considerations.
The repeatability of meter proving and the derived meter factor can be influenced by the design of flow-conditioning elements, including their type and placement It is crucial to maintain the original rotational position of these elements when the meter cannot be proved right after servicing, as this can significantly impact meter performance and accuracy.
Valves
7.3.1 Valves require special consideration since their location and performance can affect measurement accuracy.
The optimal placement of flow or pressure-control valves is downstream of the meter run and prover takeoff valves It is essential for these valves to operate smoothly to avoid shocks and surges in the system.
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Valves located between the meter and prover, such as stream diversion valves, drains, and vents, must ensure leak-proof shutoff This can be achieved using a double block-and-bleed valve equipped with a telltale bleed feature.
Electronics
The Signal Processing Unit (SPU) of the UFMs electronics system encompasses power supplies, microcomputers, signal processing components, and ultrasonic acoustic transducer excitation circuits It can be housed in one or more enclosures, either locally or remotely from the meter, and must be designed and installed to comply with relevant hazardous area classifications Additionally, the SPU is required to function effectively across all specified environmental conditions while meeting the performance requirements of the meter.
Electrical
7.5.1 The electrical systems shall be designed and installed to meet the applicable hazardous area classifications
7.5.2 The pulsed data transmission systems shall be designed to provide appropriate fidelity and security See API
UFMs and their interconnecting cables are vulnerable to Electromagnetic Interference (EMI) due to their low power electrical signals To mitigate interference from nearby electrical equipment, UFMs utilize various shielding materials and methods Additionally, cable jackets and other exposed components are designed to resist ultraviolet light, oil, and grease.
7.5.4 Poorly designed cathodic protection and grounding systems can be sources of potential interference with the UFM signals
7.5.5 An un-interruptible regulated power supply (UPS) shall be provided for continuous meter operation.
The meter factor is established by verifying the meter under stable operating conditions, which include consistent flow rate, density, viscosity, temperature, and pressure Guidance on this process can be found in API MPMS Chapter 4.8 Users generally set the acceptable limits for deviations in these operating conditions.
Proving is mainly driven by regulatory and contractual obligations, along with standard operating procedures It is crucial that proving conditions closely resemble actual metering conditions to effectively verify the meter's performance under normal operating circumstances Additionally, most Ultrasonic Flow Meters (UFMs) offer a method to compensate for geometric changes in the meter due to variations in body temperature.
Utilizing this feature can minimize or eliminate the need for reproofing due to temperature fluctuations It is important to understand the distinctions between calibrating a meter in a laboratory setting and in the field, as these two environments can yield varying results Interchanging these proving locations may lead to measurement errors.
In-situ proving is typically favored as it confirms the accuracy of a meter under real operating conditions These conditions can significantly influence a meter's accuracy and repeatability By conducting in-situ proving under stable operating conditions, variations in performance due to factors such as flow rate, viscosity, density, temperature, pressure, flow conditions, piping configurations, and contaminants are effectively addressed.
Laboratory proving is generally not favored due to the inability of laboratory conditions to accurately replicate the actual piping and operating environments Although laboratory proving involves greater measurement uncertainties, it can still be the most viable option under specific circumstances.
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Delays in manufactured flow pulses from a UFM can impact accuracy, resulting in bias errors in the calculated meter factor The extent of this error is influenced by the magnitude of flow rate changes during the proving run and the duration of the prove run For a comprehensive understanding of this issue, please refer to Annex C.
Master meter proving of an ultrasonic meter can be conducted in accordance with API MPMS Chapter 4.5, as long as the additional uncertainty commonly linked to this method is deemed acceptable by all parties involved.
9.2 Proving run repeatability is used as an indication of whether the proving results are valid.
9.2.1 Some UFMs may produce a non-uniform pulse output, which can exhibit a wide span of repeatability when proved See Annex B for detailed explanation.
Proving run repeatability may not adhere to the standard of five runs with a 0.05% span; however, it is essential that proving runs comply with the guidelines set forth in API MPMS Chapter 4.8.
System Setup
UFM configurations are tailored for specific applications, so it's crucial to install the meter in the appropriate location and measurement context before it becomes operational Additionally, confirm that the configuration parameters and settings align with the manufacturer's documentation.
Hardware
When installing multiple meters, it is crucial to pair the appropriate SPU with its corresponding meter body, as the SPU holds unique factory information for each meter, including dimensional data, acoustic transducer path lengths, and performance metrics.
Operation of Metering Systems
UFMs shall be operated within the manufacturers’ specified flow range, operating conditions and fluid properties.
Setting the UFM Response Time
To reduce meter factor bias errors during proving, it is crucial for the flow pulse from the SPU to quickly respond to changes in flow rate Manufacturers often provide configuration settings to enhance the responsiveness of Ultrasonic Flow Meters (UFMs) to flow rate variations These settings generally include three main categories: the sample interval, which is the duration between ultrasonic flow rate samples; the number of samples, indicating how many ultrasonic samples are processed for each flow measurement update; and pulse output adjustment, which refers to the level of damping or filtering applied to the flow measurements that generate the pulse output signal.
NOTE Not all manufacturers provide items a), b), and c).
For optimal performance, it is advised to adjust item a and/or item b to the manufacturer's minimum settings Additionally, item c should be configured to zero or the minimum level as specified by the manufacturer.
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Any modifications that impact the UFM's response speed to flow rate changes, such as adjustments to the sample rate, sample time period, pulse output filtering, or damping, necessitate a re-proofing of the UFM.
Pulse Scaling
The SPU calculates flow rates and configures pulse output rates to accurately represent these rates Pulse scaling connects the output pulses to the measured volume, allowing for the correlation of pulse frequency with flow rates or defining the number of pulses output by the UFM per volume measured It is crucial to ensure that the pulse frequency output does not exceed 90% of the maximum allowable input frequency of the receiving accessory equipment.
Methods of Controlling Correction Factor
To accurately reflect the measured quantity, various methods can be employed to apply the meter factor, also known as the K-factor Adjustments from indicated to actual quantities can be achieved by modifying this factor, which can be integrated into the UFM SPU, accessory equipment, or applied manually Utilizing the meter factor within accessory equipment is preferred due to its ability to maintain an audit trail Consistency in the chosen method is crucial for reliable measurements.
A UFM is calibrated by the manufacturer to establish calibration coefficients that are input into the UFM SPU, which should remain unchanged despite being adjustable Any modifications that could impact the measured quantities must be documented in the audit trail In scenarios where the flow rate fluctuates during normal operation, it is beneficial to determine meter factors across a range of flow rates to linearize the UFM's output Additionally, if the meter measures bidirectional flow, a specific meter factor should be created for each direction.
If the pulse scaling is changed the meter shall be reproved
SPU output signals meter body transducer
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Zeroing the Meter
Zeroing a UFM requires verifying the output when the meter is blocked-in If the meter does not show zero flow under these conditions, the manufacturer's re-zeroing procedure must be implemented Additionally, any time the meter is re-zeroed, it must undergo reproving.
A UFM typically does not need manual zeroing; however, any changes or replacements of acoustic transducers, electronics, or transducer cables necessitate checking the meter zero If needed, zeroing procedures must be followed Additionally, any modifications or replacements in these components require the UFM to be reproved.
General
API MPMS Ch 21.2 fully addresses the auditing and reporting requirements of a generic Electronic Liquid
The audit requirements for an ELM system utilizing a UFM are largely consistent, with the exception of the need to include specific configuration and setup parameters from the SPU, which must be both auditable and secure.
UFM Configuration Parameters and Settings
The UFM manufacturer shall provide the ability to identify primary UFM components and document the meter’s configuration parameters and settings that affect the flow meter’s output
During the initial installation, it is essential to document and monitor the meter's firmware and software configuration for validation purposes Any alteration in the checksum may signify a change in the meter's configuration, potentially affecting its performance.
A UFM shall provide an audit trail of the meter’s configuration parameters and settings that affect the meter’s output(s).
Alarm, Event Logs
There are no special requirements for the alarm, event and error logs for ELM systems using a UFM other than those specified in API MPMS Ch 21.2.
Monitoring specific parameters is essential for various applications, particularly within the UFM, where diagnostic alarms are triggered when these parameters exceed preset limits Effective troubleshooting often necessitates evaluating multiple parameters simultaneously Utilizing trending plots can provide insights into the UFM's performance over time, allowing for comparisons with the original factory setup and onsite startup conditions A comprehensive troubleshooting process involves assessing both diagnostic parameters and process operating conditions together The parameters listed below are typically accessible through a serial data interface or other methods.
Gain measures the amplification needed to reach the desired signal amplitude for processing A high gain often signifies increased signal attenuation, which can result from factors such as the presence of solids or gas in the liquid, high viscosity, water/oil mixtures, or a weakening acoustic transducer.
The API Manual of Petroleum Measurement Standards, Chapter 5, states that when the gain reaches maximum amplification (saturation), it indicates that no signal is being transmitted, triggering a diagnostic alarm.
— Signal to Noise Ratio (SNR)
The signal-to-noise ratio (SNR) observed by each acoustic transducer in a UFM is crucial for accurate measurements, with a high SNR indicating reliable data Conversely, a low SNR may signal issues with the transducer or the process conditions This low SNR can arise from normal signal strength coupled with high noise due to factors like improper grounding or electrical interference Additionally, it may reflect low signal strength caused by the presence of solids or gas in the liquid, high viscosity, or mixtures of water and oil.
The ratio of an individual's path velocity to the average axial flow velocity allows for the assessment of changes in the flow profile By analyzing these path velocity ratios, one can identify asymmetric profiles that may indicate flow disturbances, such as swirl in the flow stream These disturbances can arise from factors like debris in the strainer or flow conditioner.
— Velocity of Sound Along Each Acoustic Path
The distance between acoustic transducers, known as the path length, allows for the measurement of sound velocity by timing how long the signal takes to traverse this distance Below are the approximate sound velocity ranges for various liquids.
— Brine = 5740 ft/sec through 6000 ft/sec;
— Refined Products = 2000 ft/sec through 4600 ft/sec (includes NGLs);
— Crude Oil = 4400 ft/sec through 5000 ft/sec;
— Propane = 2000 ft/sec through 2500 ft/sec;
— Iso Butane = 2200 ft/sec through 2800 ft/sec;
— Normal Butane = 3100 ft/sec through 3700 ft/sec.
Understanding the velocity of sound in relation to the meter is crucial, as consistent sound velocity across different paths suggests a uniform product stream Conversely, variations in sound velocity may signal changes in the product within the stream Additionally, differences in sound velocity from the top to the bottom of the meter can indicate the presence of a density gradient.
— Flow Stream Turbulence (standard deviation of path velocities)
The stability of flow velocity along each path reflects the turbulence intensity in the flow stream While some turbulence, typically between 2% and 4%, is normal due to friction effects or boundary layers, elevated turbulence levels may signal issues such as partial blockages in upstream flow conditioners, alterations in pipe wall roughness, or gasket protrusion.
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— Percentage of Accepted Measurements for each acoustic path
Each acoustic path undergoes a series of checks to assess its suitability for transit time measurement, and the report includes the number of accepted signals utilized in a batch of samples.
Depending on the UFM manufacturer, additional parameters may be available such as:
Comparing diagnostic parameters determined in a lab with those measured in the field can reveal the impact of field installation and other changes Additionally, it is advisable to periodically assess these parameters throughout the operation of the meter.
To ensure the integrity of configuration parameters and settings, it is essential to protect them from tampering and unauthorized changes This can be accomplished through the implementation of passwords and tamper-proof seals or locks, as outlined in API MPMS Chapter 21.2.
Ultrasonic transit time flow meters utilize acoustic transducers to emit and receive high-frequency acoustic signals These transducers are strategically positioned to allow the signals to traverse diagonally across the pipe The transit time method measures the time intervals of acoustic signal transmission in opposite directions, distinguishing it from the Doppler ultrasonic technique, which measures frequency shifts in reflected acoustic energy.
The measurement relies on the principle that acoustic signals moving downstream through a pipe travel faster than those moving upstream This time difference between the two signals is directly proportional to the average flow velocity along the acoustic path.
The acoustic signal that travels in the direction of flow (downstream) crosses the pipe in time (see Figure A-1):
The acoustic signal that travels against the direction of flow (upstream) crosses the pipe in time:
The acoustic path length, denoted as \( L \), is influenced by the velocity of sound in the liquid, represented by \( c \) Additionally, the angle that the acoustic path forms with the pipe axis plays a crucial role, along with the average axial velocity within the pipe.
A>B is the acoustic signal traveling from upstream to downstream;
B>A is the acoustic signal traveling from downstream to upstream.
The average velocity is therefore determined by the following:
Using multiple acoustic transducers enables the creation of various acoustic paths (beams) across the pipe's cross-section, allowing for a comprehensive analysis of the flow velocity distribution, or flow profile The relationship can be expressed as \( A B > L c V + \cos \theta \).
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1) Emission: The SPU sends an electrical signal to the acoustic transducers (piezo electric crystal) that causes the crystal to generate an acoustic signal into the fluid.