A Reference number ISO 7278 4 1999(E) INTERNATIONAL STANDARD ISO 7278 4 First edition 1999 04 01 Liquid hydrocarbons — Dynamic measurement — Proving systems for volumetric meters — Part 4 Guide for op[.]
Ways of expressing a meter’s performance
Proving meters with a pipe prover aims to generate highly precise measurements, typically with four or five significant digits, such as 1.0029, 0.9998, or 21,586 These precise measurements are essential for accurately converting meter readings into precise volume calculations, ensuring reliable data for flow measurement and billing purposes Using a pipe prover in meter calibration enhances measurement accuracy, leading to more trustworthy and consistent flow measurement results.
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There are three key forms of the numerical expression of a meter’s performance that are essential for pipe prover operators Understanding these critical measurements is vital for accurate calibration and ensuring optimal performance of measurement systems Proper knowledge of these performance metrics helps operators maintain compliance with industry standards and improve measurement reliability.
Early petroleum meters were displacement type devices with dials displaying volume directly in units like liters or cubic meters, often providing approximate readings To improve accuracy, these readings could be corrected by adjusting the gear ratio or applying a meter factor Since modifying gear ratios can be challenging, using a meter factor is the more common method for ensuring precise measurement.
The meter factor (MF) is a crucial measurement in flow metering, representing the ratio of the actual volume of liquid passing through the meter (V) to the volume indicated by the meter's dial (V_m) This coefficient helps ensure accurate flow measurement by calibrating the meter's readings against the true volume, making it essential for precise fluid management and process control Understanding the meter factor allows engineers and technicians to correct for any discrepancies between indicated and actual flow, optimizing system performance and reliability.
During a proving operation, the volume value (V) is obtained from the prover, while the measured volume (V m) is read directly from the meter When the meter is used to measure throughput, applying the meter factor (MF) to the meter readings provides corrected volume values, ensuring accurate measurement of the delivered volumes.
Meter factor is a non-dimensional quantity, a pure number This means that its value does not vary with a change in units used to measure volume.
Over the past 25 years, turbine meters have become widely adopted in the petroleum industry due to their reliable performance Unlike traditional meters, turbine meters typically do not display volume readings on a dial; instead, they emit a train of electrical pulses These pulses are counted electronically, with the total number of pulses (n) directly proportional to the volume of fluid passing through the meter, ensuring accurate measurement and monitoring of petroleum flow.
The primary goal of testing this meter is to determine the relationship between the number of pulses (n) and the volume (V) This relationship is effectively expressed through the K factor, a key parameter defined as the number of pulses emitted by the meter per one unit of volume delivered Understanding the K factor is essential for accurate measurement and calibration of the meter's performance.
When proving a meter, it is essential to obtain simultaneous readings of the number of pulses (n) from the meter and the volume (V) from the prover This calibration process ensures accurate measurement and verification of the meter's performance During subsequent operation, the meter's volume delivery is determined by dividing the K factor by the number of pulses emitted, enabling precise calculation of the volume delivered Proper meter proving guarantees measurement accuracy and compliance with quality standards.
The K factor is not a pure number; it has the dimensions of reciprocal volume (1/V), making its value dependent on the measurement units used for volume For example, when expressed as pulses per cubic metre, the K factor is a thousand times higher than when expressed as pulses per litre This unit dependence is important to consider when interpreting or comparing K factor values across different measurement systems.
The reciprocal of the K factor, known as the "one-pulse volume" (q), is a practical metric for field use with hand calculations, as it simplifies multiplication over division It indicates the average volume delivered by a meter per pulse and is defined by the equation q = 1/K = V/n, with units of volume per pulse Multiplying this value by the number of pulses emitted by the meter gives the total volume delivered.
3.1.4 Alternative uses of meter factor, K factor and one pulse volume
Modern metering systems have integrated the use of the meter factor (K factor) across different types of meters, blurring traditional distinctions Displacement meters equipped with electrical pulse-generators now function similarly to turbine meters during testing, allowing results to be expressed as K factors or pulse volumes Additionally, contemporary large-scale turbine metering systems often utilize data processing modules, or “scalers,” which convert pulse counts into accurate volume measurements, making the concept of the meter factor still relevant for calibration and system verification.
Detailed instructions for the use of meter factor, K factor and one pulse volume are given in ISO 4267-2.
How meter performance varies
Manufacturers' literature often claims that the K factor of a specific meter is a constant value, but this is only an approximation In reality, the K factor can vary due to several influencing variables These variables are discussed in sections 3.2.1 to 3.2.6, highlighting the factors that can affect its accuracy Understanding these factors is essential for precise measurement and proper meter calibration.
Meters are designed to maintain almost constant factors over a broad range of flowrates, with the "rangeability" or "turndown ratio" defining this flowrate span Typically, turbine and displacement meters used in hydrocarbon measurement have a rangeability of up to ten to one, though some specialized meters may exceed this significantly Within their effective operating range, the K factor remains nearly constant, with minor variations such as ±0.25% or ±0.5%, known as the meter's "linearity." To accurately assess a meter's performance, it must be tested at various flowrates to determine its rangeability and linearity, as performance characteristics outside this range may decline.
K factor is liable to vary so greatly with flowrate that it is no longer practical to use the meter for accurate measurement.
Changes in the viscosity of the liquid being measured can impact meters of all types, with some designs affected more significantly When viscosity varies, it may be necessary to re-prove the meter to ensure accurate readings The necessity of re-proving depends on the specific type and design of the meter, as well as the extent of viscosity changes Proper consideration of these factors is essential for maintaining measurement accuracy in various applications.
the amount by which the viscosity has changed;
the extent to which the K factor of the meter concerned is affected by changes in viscosity;
Temperature variations impact the K factor through two primary mechanisms: thermal expansion of the meter components and changes in liquid viscosity Thermal expansion alters the dimensions and clearances within the meter, although this effect is typically negligible in turbine meters unless large temperature fluctuations occur Conversely, in displacement meters, thermal expansion is more significant due to the use of dissimilar metals in the measuring chamber, which leads to more pronounced changes in clearances Additionally, temperature changes affect the viscosity of the liquid, influencing measurement accuracy as described in section 3.2.2.
Pressure influences the K factor by causing dimensional changes in the meter and altering liquid viscosity, though viscosity effects are typically negligible in most metering applications While some high-pressure meter designs experience minimal dimensional effects, certain meters may be significantly impacted by pressure variations However, pressure-induced changes in the K factor are generally too minor to warrant re-calibration or re-proving in most cases.
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3.2.5 Effect of wear, damage and deposits
As a meter ages, its K factor gradually changes, necessitating regular re-proving to ensure accurate custody transfer measurements Even if re-proving for viscosity and temperature variations isn't required, it remains essential to account for these changes to maintain measurement integrity Additionally, deposits of wax and dirt can impact meter performance, highlighting the importance of routine maintenance and calibration.
Accidental damage to a meter is likely to alter its K factor considerably If a meter is stripped for repairs it should be proved after it has been reassembled.
The frequency of proving meters varies significantly, from multiple times daily to annually or longer, depending on the application Very frequent proving is justified when the total value of the metered liquid is high, such as in crude oil fiscal metering or large pipeline installations, often involving a dedicated, permanently connected large pipe prover Meters can be re-proven easily when flowrate, temperature, or viscosity change sufficiently, or when different crude types or products are introduced Additionally, specific intervals or throughput increments may determine when a meter needs re-verification to ensure measurement accuracy.
For applications where high accuracy is not critical and viscosity and temperature variations are limited, meters can be re-proved at regular intervals—such as monthly or bi-monthly when the system is new, and extending to six or twelve months once reliability is confirmed While master meters and portable proving tanks are still widely used for calibration, the adoption of portable pipe provers has become increasingly common, with ISO 7278 covering their operation alongside stationary pipe provers.
Correction factors
Liquid pipe prover volume varies with pressure and temperature, as does the specific volume of a liquid To ensure accurate measurements, four correction factors are used to account for these changes These corrections can be applied manually by the operator or integrated into the data processor system for automated calculations Implementing these correction factors is essential for precise flow measurement in industrial applications.
3.3.1 Corrections for change in volume of prover
The base prover volume (Vb) is a crucial measurement for every pipe prover, established through a calibration process conducted during manufacturing and at specified intervals thereafter It denotes the volume within the prover's calibrated section under defined conditions, typically at zero gauge pressure and a temperature of 15 °C or 20 °C Proper calibration of Vb ensures accurate flow measurement and maintains the prover's reliability over time.
However, what the prover operator needs to know each time he carries out a proving run is the volume of the prover at the actual gauge pressure and temperature during that run The gauge pressure will almost always be above zero, and this excess pressure will cause the prover to expand slightly The temperature may be higher or lower than the reference temperature, and so its effect will be to cause the prover either to expand or contract.
To obtain the corrected volume of the prover at the appropriate pressure and temperature, the factors C ps (or CPS) [correction for pressure on steel] and C ts (or CTS) [correction for temperature on steel] are used Detailed instructions for the use of these correction factors are given in ISO 4267-2.
3.3.2 Correction changes in specific volume of liquid
Correction factors C pl (or CPL) and C tl (or CTL) are used to adjust the specific volume of liquid for the effects of pressure and temperature, respectively These factors convert the measured oil volume at actual conditions to a standard volume at an absolute pressure of 1 atmosphere (around 101 kPa) and a defined temperature, such as 15 °C or 20 °C Following ISO 4267-2 guidelines ensures accurate application of these correction factors for volume standardization.
Correction factors outlined in sections 3.3.1 and 3.3.2 depend on the liquid type, density, pressure, temperature, and standard conditions It is essential to verify that the numerical values used for these factors are appropriate for the specific conditions at that time; using default values without proper validation can lead to inaccuracies Always check the relevant parameters to ensure the correctness of the correction factors in your calculations.
Pulse-generating meters
Currently only two basic types of pulse-generating meter are commonly used for high-accuracy liquid metering in the petroleum industry.
The turbine meter is a device that measures fluid flow by using a freely spinning propeller, or turbine, mounted inside a pipe As liquid flows through the pipe, the turbine rotates proportionally to the flow rate, generating electrical pulses These pulses are counted electronically to determine the total volume of fluid that passes through the meter For more detailed information, please refer to ISO 2715.
Displacement meters, formerly known as positive displacement ("PD") meters, are devices that measure flow by displacing a fixed volume of liquid with reciprocating or rotary mechanisms like piston, gear, or vane pumps Their revolution count directly correlates to the total volume of liquid passing through, typically displayed on a mechanical counter When equipped with an electrical pulse-generator, displacement meters can provide output signals similar to turbine meters and can be directly calibrated using a pipe prover However, displacement meters without an electrical output cannot be easily proved or calibrated using standard methods.
Sources of error in operating meters
For a pulse-generating meter to give accurate results, the following three requirements shall be met:
it shall be in good condition, both mechanically and electrically;
conditions of the flowing fluid shall be suitable for metering and proving;
the system shall be arranged so that the counter registers the same number of pulses as are generated by the meter – no more, no less.
The first of these is too obvious to need elaboration, but the other two involve some rather subtle difficulties which are explained in 4.2.1 and 4.2.2.
The four main problems involving the flowing liquid are entrained solids, entrained air, cavitation and swirl.
Adequate filtration should be provided upstream of the meter.
Entrained air or gas can significantly impact all types of flow meters, with turbine meters being more prone to severe and unpredictable effects compared to displacement meters Proper venting during system fill-up is crucial to prevent air pockets from remaining in the line, which can later be swept through the meter and distort readings Additionally, when a pump draws liquid from a low surface level, vortex formation can introduce air into the system; installing a gas separator in such cases helps mitigate this risk and maintain accurate measurement.
An air separator or air eliminator is typically installed upstream of the meter to effectively remove air or gas, preventing them from entering and potentially damaging the instrument Additionally, air or gas can enter a system under vacuum conditions, which underscores the importance of proper air removal devices to maintain system efficiency and accuracy.
Cavitation occurs when air or gas bubbles form directly within a liquid due to low-pressure areas, causing dissolved gases to be released This process generates numerous tiny bubbles that are difficult to remove with separators, making cavitation unavoidable once it occurs Preventing cavitation is essential, which can be achieved by ensuring that the downstream pressure of the meter does not fall below the manufacturer's specified minimum for the fluids being measured.
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Cavitation can occur in volatile liquids such as crude oil, natural gas liquids, gasoline, and LPG when the pressure inside a meter temporarily drops to the liquid's vapor pressure, causing it to boil or “flash” within the meter To prevent flashing, it is essential to maintain line pressure immediately downstream of the meter well above the vapor pressure of the liquid Most meter manufacturers recommend specific back pressure margins above the vapor pressure, and ISO 2715 provides general guidelines for back pressure requirements in turbine meters to ensure accurate measurement and prevent cavitation issues.
Swirl often occurs immediately downstream of a partially opened valve, bend, or pipe fitting, causing the liquid to move in a corkscrew motion instead of a straight path While swirl has minimal impact on displacement meters, it significantly hampers the accuracy of turbine meters To prevent swirl from affecting turbine meter performance, a flow straightener is typically installed upstream of the meter.
A counter may undercount if it misses some pulses produced by the meter, leading to a lower reading Conversely, it can overcount by registering pulses that the meter has not generated, resulting in a higher reading Ensuring accurate pulse counting is essential for reliable measurement and data accuracy.
A common cause of measuring too few pulses is improper sensitivity control settings or electrical faults To resolve this issue, adjust the sensitivity control or repair any electrical faults in the system Proper calibration and troubleshooting typically restore accurate pulse counting and ensure optimal device performance.
Counting spurious pulses can pose a significant challenge in signal accuracy, and this issue is thoroughly addressed in ISO 6551 Proper handling of false signals is essential for reliable measurements, and adherence to ISO standards helps mitigate these problems effectively While detailed guidance is provided in ISO 6551, a brief overview highlights the importance of controlling and filtering spurious pulses to ensure precise data collection Following these protocols improves measurement integrity and compliance with industry best practices.
Spurious pulses can originate in two ways:
from surges in the electrical mains supplying the counter;
Supply-borne noise, also known as "noise from electrical supply sources," and airborne noise, which typically originates from equipment like electrical welding devices and radio transmitters, are common sources of electromagnetic interference Manufacturing companies recognize these challenges and design metering systems that are effectively protected against spurious pulses and external noise disruptions, ensuring accurate measurements and reliable performance.
The defences will usually include:
filters on power mains designed to exclude supply-borne noise;
preamplifiers at the meters, which will ensure a high signal-to-noise in the transmission line and thus make it less likely for airborne noise to be picked up;
appropriately screened signal transmission cables earthed at only one point to avoid the occurrence of ground loops.
The route followed by signal transmission cable is of crucial importance It should be kept as far away from
To ensure safety and reliability, AC power cables should be routed carefully, crossing only at right angles to minimize interference Unauthorized modifications to system wiring by operators often lead to spurious pulse counting, as these alterations can inadvertently break proper cable separation rules, causing signal disturbances and system errors.
Metering systems often feature dual pulse generation, transmission, and counting mechanisms to ensure accuracy and detect spurious pulses While not foolproof, this dual system serves as a valuable safeguard, enhancing reliability alongside the operator’s vigilance.
Pulse interpolators
Pulse interpolators are essential electronic devices that enhance measurement accuracy by allowing meters to count pulses to a fraction of a pulse, thereby minimizing rounding-off errors These devices are particularly effective when used with meters that emit pulses at regular intervals, ensuring precise and reliable data collection during short proving runs Incorporating pulse interpolators into your measurement system can significantly improve the accuracy and reliability of pulse counting in various electronic and metering applications.
Pulse interpolation devices are used to achieve high-precision measurement, enabling discrimination of one part in ten thousand even when fewer than 10,000 pulses are generated per displacer pass These systems enhance measurement accuracy by interpolating between discrete pulses, as detailed in ISO 7278-3.
Conventional pipe provers
The pipe prover operates on the principle illustrated in figure 1, using a piston or sphere called a displacer installed inside a specially prepared pipe segment The displacer moves freely along the pipe, maintaining a sliding seal against the inner wall to travel at the same speed as the flowing liquid When connected in series with a flow meter, the volume displaced by the displacer accurately represents the volume passing through the meter, ensuring precise calibration.
In traditional provers, the displacer is commonly a slightly oversized elastomer sphere wedged into the pipe, while some designs use a steel piston with duplicate elastomer seals Ensuring a smooth pipe bore is essential for optimal sealing and low friction, typically achieved through coatings or plating These design features are crucial for accurate pressure measurement and reliable prover performance.
Detectors installed at multiple points along the pipe wall send electrical signals when the displacer reaches them, enabling precise measurement The first detector's signal directs the meter pulses to the prover counter, while the second detector's signal stops the counter, capturing the total pulses emitted during the displacer's transit This pulse count directly corresponds to the measurement of the meter's output over a known volume, which has been calibrated at standard pressure and temperature By comparing the pulses to the calibrated volume, accurate flow measurement of the fluid passing through the pipe is achieved.
4.4.2 Types of conventional pipe prover
There are two primary methods to return the displacer to its starting point at the end of a run: in the unidirectional prover, the displacer travels around a closed loop of pipe, ending near where it began; in the bidirectional prover, flow reversal allows the displacer to retrace its path back to the start These are the most common types of provers, and ISO 7278 focuses on their detailed descriptions in sections 4.4.2.1 and 4.4.2.2.
1 Calibrated length 4 Inner wall of pipe
3 Flow from or to the meter 6 Seal
Figure 1 — Principle of operation of pipe prover
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A unidirectional prover typically features an elastomer sphere as a displacer integrated into a system with a sphere-handling valve, enabling controlled movement of the sphere When the valve is activated, the sphere drops through it into the liquid flow, where it is carried around the pipe loop for measurement purposes After completing its circuit, the sphere returns to a resting position just above the valve, ready for the next measurement cycle This design ensures precise and repeatable flow verification in fluid systems.
To ensure accurate flow measurement, it is crucial that the valve is fully seated before the sphere enters the calibrated length of the prover, preventing short-circuiting of the liquid flow A pre-run length of pipe is necessary between the entry point and the first detector to allow the valve to close and seal properly, ensuring the sphere does not reach the detector prematurely This pre-run length may be insufficient at flows exceeding the prover’s rated capacity, which can compromise accuracy Some provers include mechanical means to hold the sphere near the start until the valve is fully seated, allowing for a shorter pre-run length and more precise measurements.
Less usual designs of the unidirectional prover use two or three spheres instead of using a sphere-handling valve.
1 Block and bleed drain valve 9 Sphere
3 Detector 2 11 Block and bleed drain valve
6 Sphere handling valve 14 Meter on test
7 Double block and bleed valve 15 Flow
Bidirectional provers typically utilize either a sphere or a piston as a displacer, with spheres being the more common choice due to their ability to navigate around bends effectively This makes them ideal for constructing a bidirectional prover in the form of a compact, looped pipework system For example, as illustrated in figure 3, a sphere-based bidirectional prover can be built into a streamlined configuration, enhancing efficiency and ease of use in flow measurement applications.
1 Block and bleed drain valve 8 Flow reversal valve
4 Calibrated volume 11 Block and bleed drain valve
5 Double block and bleed valve 12 Pulse counter
Figure 3 — Arrangement of a conventional sphere type bidirectional prover
This article explains the use of a flow reversal device, such as a four-way valve or a set of four linked on/off valves, to reverse the flow through the prover while maintaining unidirectional flow through the meter The sphere depicted in Figure 3 is positioned at the end of a proving run, and it begins its return journey once the valve reverses flow However, the sphere only reaches full speed after the valve operation is complete, necessitating a sufficiently long pre-run to ensure the valve has fully transitioned before the sphere enters the calibrated length of the prover Proper flow reversal techniques are essential for accurate and reliable flow calibration in liquid measurement systems.
Some bidirectional provers are equipped with a mechanism to hold the sphere in its rest position during the operation of the four-way valve Once the flow reversal is complete, the sphere is released, allowing it to rapidly enter the fluid stream This design enhances measurement accuracy and ensures reliable flow control in various industrial applications.
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In liquids moving at high velocities, such as those in certain types of propellers or flow systems, the liquid already travels at full speed As a result, the sphere or object within the flow is compelled to accelerate to this maximum velocity very quickly This rapid acceleration often leads to shorter pre-run lengths in devices or systems designed to measure or utilize fluid flow, such as flow meters or provers.
Detector operation is inherently asymmetrical, causing the calibrated volume when a sphere moves from detector 1 to detector 2 (V₁,₂) to differ from that when it travels from detector 2 to detector 1 (V₂,₁) To account for this, the standard practice is to sum these volumes, defining the round-trip volume as (V₁,₂ + V₂,₁) Similarly, the total pulse count for bidirectional proton testing is obtained by adding the pulses collected in both directions, (n₁,₂ + n₂,₁), providing a comprehensive round-trip pulse measurement for accurate calibration.
The most common type of detector features a steel plunger with a rounded end that extends approximately one centimeter through the pipe wall When the displacer contacts the plunger, it pushes it outward against a spring until it is flush with the pipe's interior surface At a specific point during its limited travel, the plunger activates a switch, which can be a mechanical microswitch or a magnetic switch Additionally, non-contacting switch types are available for piston provers, offering alternative detection methods.
Provers typically operate effectively with a single detector at each end; however, some are equipped with twin detectors at each end to safeguard against potential detector failures The use of dual detectors for ensured reliability is detailed in annex A of ISO 7278-2:1988, highlighting best practices for accurate and dependable measurement.
Displacers are primarily thick-walled hollow spheres crafted from oil-resistant elastomers like neoprene or polyurethane, designed to be inflated with water or glycol to create an effective seal within the prover bore They feature inflation valves and are calibrated to expand by typically 2% to 4% beyond the pipe bore diameter, ensuring a tight fit without excessive sliding friction In some cases, the manufacturer may specify the required inflation pressure instead of a specific expansion percentage For smaller provers, solid elastomer spheres are sometimes used as an alternative.
Spare spheres should always be stored properly to ensure safety and longevity They must not be kept in an inflated state or placed directly on a flat surface Instead, storage options include suspending them in a net or sling, or placing them on a protective sheet supported by a hollowed-out bed of sand, following manufacturers’ guidelines Proper storage practices help prevent damage and maintain the integrity of the spheres.
Small volume pipe provers
Several innovative designs of pipe provers, known as “compact pipe provers” or “small volume pipe provers,” have emerged in recent years and are gaining widespread adoption These modern pipe provers are notably smaller than traditional models while maintaining the same flow rate, offering enhanced portability and space efficiency for various industrial applications The introduction of these new designs reflects ongoing advancements in flow measurement technology, making accurate calibration more accessible and convenient for diverse industries.
Small volume pipe provers operate on the same fundamental principle as traditional pipe provers, ensuring reliable accuracy in flow measurement Their compact size is achieved through two key advancements: first, the development of precision-bore cylinders coupled with well-fitting pistons, manufactured using advanced electronic detection methods such as optical, magnetic, ultrasonic, inductive, or capacitive sensors These technological innovations enable these provers to maintain high precision, with some achieving measurement accuracies surpassing traditional counterparts.
Electro-mechanical detectors used in modern provers are significantly more sensitive—20 times greater—allowing the distance between detectors to be reduced to approximately one metre, enhancing measurement precision Pulse interpolation, as described in section 4.3, allows for pulse counts to be determined to a small fraction of a pulse, typically to two decimal places; however, the accuracy of these interpolated counts depends on the interpolation method and the regularity of pulse spacing In well-designed turbine meters, pulse intervals vary by no more than about ±2% from the mean, whereas some displacement meters exhibit variations up to ±20% or more, complicating the calibration process with small volume provers The application of pulse interpolation enables the volume base of small volume provers to be reduced to roughly one-twentieth of that of conventional pipe provers with equivalent flow capacity.
Small volume provers feature short acceleration and deceleration lengths at the start and end of piston travel, achieved through rapid-acting valves or mechanical restraints that ensure precise piston movement These provers are categorized into two types: unidirectional and bidirectional, offering flexible solutions for accurate volume measurement in various applications.
4.5.2 Unidirectional small volume pipe provers
The unidirectional small volume prover differs from the conventional unidirectional prover, as it cannot navigate pipe bends and must retrace its path to return to the starting point Unlike the traditional prover that uses a sphere in an endless loop, the piston in a small volume prover moves in a single direction, allowing flow and measurement only during piston travel in that direction This design ensures unidirectional operation, critical for accurate volume measurement in flow verification processes.
To return the piston for another cycle, open the bypass valve and use an externally-powered piston rod to force the piston in the reverse direction Alternatively, reverse pressure differential across the piston can be created to facilitate its return, eliminating the need for additional equipment Properly managing the piston movement is essential for efficient operation and maintenance of the system.
A unidirectional small volume prover features a compact design with an internal bypass valve system integrated within the piston, allowing oil to bypass the piston for enhanced efficiency The piston detectors, mounted externally on the piston rod, utilize high-precision optical sensors that deliver almost instantaneous response This innovative configuration ensures accurate and reliable performance in small volume measurement applications.
Other designs of unidirectional small volume provers utilize external bypass valves and are generally fitted with external detectors on the piston rod.
Unidirectional small volume provers often feature an external pressure vessel encasing an inner flow tube, forming a double-cylinder design This configuration prevents the expansion of the calibrated volume under pressure, ensuring accurate measurements Additionally, it helps maintain a consistent temperature within the inner cylinder, which is crucial for precise calibration of flowmeters.
Many unidirectional small volume provers are equipped with mechanisms to regularly verify the leak-tightness of seals in both the bypass valve and the piston This is essential because undetected leaks in these seals can lead to measurement errors, compromising the accuracy of volume calibration Ensuring seal integrity through routine leak checks helps maintain the provers' reliability and precision in volume measurements.
The swept volume on the piston rod side is always less than on the opposite side for the same travel length Therefore, a unidirectional small volume prover has two slightly different base volumes For accurate calculations, it is essential to use the base volume corresponding to the end of the cylinder directly connected to the test meter.
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1 Piston and poppet valve arrangement
Figure 4 — Example of a unidirectional small volume prover with internal valve 4.5.3 Bidirectional small volume provers
A bidirectional small volume prover is a compact version of a traditional bidirectional prover, featuring a piston without a piston rod Instead of optical detectors, high-precision electrical or ultrasonic detectors are installed inside the cylinder wall, as optical detection is not feasible in this design This innovative configuration ensures accurate measurement and calibration in smaller measurement volumes.
Full bidirectional operation is enabled by using a four-way valve or an assembly of simple valves to reverse the mainline flow through the prover barrel, ensuring precise flow control Incorporating a mechanism for sudden launching after valve rotation enhances measurement accuracy, crucial for calibration processes The prover barrel can be designed as either single-walled or double-walled, with double-walled configurations offering specific advantages such as improved insulation and thermal stability This setup is essential for maintaining reliable and accurate flow measurement in calibration and testing applications.
Double-block-and-bleed facilities are incorporated into four-way valves to ensure leak-tightness, similar to traditional provers Additionally, there must be mechanisms in place to verify that piston seals are also leak-proof, ensuring overall system integrity.
Methods of installing pipe provers
A pipe prover can be permanently integrated into a metering installation via a network of pipes and valves, allowing any designated meter to be tested without the need for additional pipe connections This setup ensures quick and efficient proving of meters, improving accuracy and reducing downtime Such a configuration is referred to as a permanently connected pipe prover, offering reliable and seamless calibration within the measurement system.
“dedicated” to that metering system.
A portable prover is mounted on a truck or trailer, allowing easy transportation to metering installations It can be temporarily connected using hoses or flexible pipework to pipe stubs, which are permanently linked to the metering system via valves This mobility facilitates accurate calibration and verification of metering devices at various locations.
1 Outlet bypass ports 8 Gate marks (2)
Figure 5 — Example of a unidirectional small volume prover with external valve 4.6.3 Central provers
A central prover is strategically installed near a liquid supply, allowing liquids to be admitted by opening valves Meters intended for calibration are temporarily removed from their operational setups, brought to the central prover, and installed within the prover system for accurate verification This process ensures precise measurement calibration, which is essential for maintaining measurement standards and compliance.
Sources of error in operating pipe provers
Entrained air and cavitation can lead to inaccurate readings from a prover, similar to how they affect meters, as detailed in section 4.2.1 These issues typically arise due to inadequate venting of air during initial system filling or the occurrence of cavitation within the system Proper venting and maintenance are essential to ensure precise prover results and reliable system performance.
Maintaining stable temperatures is crucial for obtaining accurate measurements with a pipe prover When oil starts circulating through a prover that has been offline, it takes time for the system to reach temperature equilibrium, which is essential for precise and reliable results.
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A meter’s performance is dependent upon flowrate which shall be maintained as constant as possible during prover operations.
Conventional pipe provers are equipped with highly sensitive detectors that require expert repair and adjustment Any repairs made to these detectors can alter the calibrated volume of the prover, necessitating a recalibration to ensure accurate measurements Proper maintenance of these detectors is essential for maintaining the reliability and precision of the prover’s performance.
Detector maladjustment errors are especially critical in unidirectional provers, as incorrect calibration can lead to significant measurement inaccuracies When a detector is improperly adjusted, it tends to produce positive errors during one directional measurement and negative errors during the reverse, increasing overall measurement uncertainty Unlike bidirectional provers, where these opposing errors can offset each other to improve accuracy, unidirectional provers experience the full impact of detector miscalibration, resulting in more pronounced measurement deviations Ensuring proper detector adjustment is essential to maintain high accuracy in unidirectional proving systems.
Ensuring a complete seal between the displacer and the pipe bore is essential for accurate measurement Any gaps can lead to liquid leakage past the displacer, resulting in a volume different from the calibrated measurement Proper sealing prevents fluid slippage, maintaining precise and reliable volumetric readings.
To prevent issues, the displacer must be periodically removed from the prover for inspection according to manufacturer or operating company guidelines Piston displacers require seal inspection and replacement if damaged or chemically softened, along with a leak test while still in the prover Sphere displacers should undergo visual inspection and diameter measurements using gauges to ensure accurate performance.
Regular inspection of the prover when opening it to check the displacer provides an important opportunity to assess the coating condition While damaged coatings are uncommon, they can occasionally be encountered, and severely damaged coatings may require removal or replacement to ensure accurate measurements and proper functionality Proper coating inspection and maintenance are essential for maintaining the prover's accuracy and longevity.
Before removing a displacer from a prover, note the warning in 5.3.8.
A prover valve that fails to fully shut off flow can lead to significant measurement errors To ensure leak-tightness, a double-block-and-bleed system should always be installed in these valves, allowing for regular testing Some prover systems feature automatic double-block-and-bleed mechanisms, enabling continuous leak checks during each proving run, thereby maintaining measurement accuracy and system reliability.
Prover calibration and recalibration
The initial base volume of the prover is established before its first use following ISO 7278-2 procedures Regular recalibration is necessary at predetermined intervals, as agreed upon by relevant authorities and stakeholders, to ensure measurement accuracy Additionally, recalibration must be performed after any modifications, repairs, or maintenance—such as detector servicing or prover recoating—that may impact the base volume or overall performance of the prover.
Meter installations
Two typical metering installations are shown in figures 6 and 7 These are only given by way of example, since many variations in the design of installations can be encountered.
Installations for portable prover calibration are typically simple, featuring a single meter run as illustrated in figure 6, facilitating straightforward testing In contrast, dedicated prover setups often employ multi-stream configurations, like the one shown in figure 7, allowing multiple meters to operate in parallel and be proved sequentially against a single, centralized prover.
Installing these types of meters offers several key advantages: first, the meter remains in its normal position without any disturbance, ensuring seamless operation; second, there is no interference with the daily functioning of the meter, allowing it to continue totalizing throughput during verification; and third, the meter is calibrated under actual operating conditions, including the same liquid, pressure, temperature, and flow rate, ensuring accurate and reliable measurements.
Installations typically include on-line data-processing capabilities, which can be implemented through dedicated microprocessors or shared access to a larger central computer, or a combination of both, ensuring efficient and reliable data management.
2 Filter/strainer/air eliminator (as required) 9 Flow control valve (as required)
3 Flow conditioner 10 Non-return valve (as required)
5 Main block valve (double block-and-bleed) 12 Flow in
6 Prover isolation valves 13 Flow out
Figure 6 — A simple turbine flowmeter installation with bidirectional prover
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1 Upstream block valves 7 Pipe prover
2 Filters/strainers/air eliminators (as required) 8 Detectors
3 Flow conditioners 9 Flow control valves (as required)
4 Turbine flowmeters 10 Non-return valve (as required)
5 Main block valves (double block-and-bleed) 11 Flow in
6 Prover isolation valves 12 Flow out
Figure 7 — A typical multi-stream metering installation
General
Safe working practices are crucial across all industries, particularly in the petroleum sector where handling flammable and combustible fluids poses significant risks Extensive safety regulations have been developed over years of experience to ensure the proper operation of pipe provers Pipe prover operators must thoroughly understand and adhere to all relevant safety codes and regulations to maintain a safe working environment.
Safety regulations in the petroleum industry are categorized into three key areas First, government-mandated laws establish mandatory safety standards that must be strictly followed Second, industry-specific safe practice codes are published by reputable international and national standards organizations, serving as essential guidelines for safe operations Adhering to these regulations and codes is crucial for ensuring workplace safety and operational compliance in the petroleum sector.
In the petroleum industry, each company typically develops personalized safety procedures tailored to specific equipment A pipe prover operator must consistently consult their company's safety manual to ensure compliance, ultimately internalizing its provisions through regular reference until following safety protocols becomes second nature.
ISO 7278 highlights key features of good safety practices, emphasizing essential safety measures without claiming to be an exhaustive list It is important to note that the responsibilities of operators to adhere to all relevant regulations remain unchanged, ensuring ongoing compliance and safety in the workplace.
Permits
Operation of a pipe prover requires a written permit issued by the owning company, and if applicable, the site owner Work must not commence without obtaining all necessary permits, ensuring compliance with safety and procedural regulations.
In exceptional circumstances where strict safety regulations hinder essential work, it is crucial to consult the local safety officer and secure written permission for specific safety relaxations Such permits are granted only for the designated purpose and limited duration, emphasizing the importance of responsible authorization If the permit expires before completing the task, a new permit must be obtained to ensure ongoing safety compliance.
Mechanical safety
5.3.1 Pressure and temperature rating of pipe prover
Operators must be knowledgeable about the specified pressure and temperature ratings for the pipe prover and avoid exceeding these limits, including connecting hoses for mobile provers To ensure safety and accuracy, it is advisable to operate well below the maximum ratings to provide a safety margin for unforeseen conditions Before using a mobile prover on-site, verify that its pressure and temperature ratings are appropriate for the specific application Proper adherence to these ratings is essential for accurate measurements and safe operation in pipeline testing.
5.3.2 Pressure testing of connecting hoses
Companies operating mobile provers must conduct pressure tests on connecting hoses at least every two years, with more frequent testing—often annually—recommended based on specific circumstances On-site hose testing at the start of each proving visit is also advised to ensure safety Hoses should be tested with water rather than liquefied gases, and if testing with liquid hydrocarbons, pressure should be applied using a hand pump instead of a high-pressure line Operators must maintain records of the last successful hose tests, ready to present to safety officers upon request Special care is necessary when testing stainless steel hoses, as they can be damaged by salt or alkaline water, requiring appropriate precautions.
Before removing blind flanges from connecting stubs in metering installations, ensure there's no pressure behind the flange by verifying that isolating valves are fully closed If an air vent exists between the valve and the blind flange, open it to release pressure and then close it; otherwise, loosen all nuts slightly, bleed out some oil, and then remove the nuts completely to safely detach the flange.
Before connecting a portable prover to a metering installation, thoroughly inspect the hoses for signs of excessive wear or damage If any issues are identified, promptly report them to the safety officer or supervisor and replace the hoses as necessary to ensure safety and accurate measurement.
Ensure that hose connections between a portable prover and the metering system are properly assembled using approved materials suitable for the application Use hoses, jointing materials, and gaskets that meet appropriate standards to guarantee safety and accuracy For flanged connections, always utilize the full required number of bolts and install a properly sized, high-quality gasket to ensure a secure, leak-proof seal.
5.3.4 Opening prover to supply pressure and filling prover
Dedicated and central provers are often exposed to supply pressure due to continuous oil circulation within the system When a prover is closed off from the supply and drained, or when a portable prover is newly connected to a metering system, it is crucial to exercise caution when opening valves To safely admit supply pressure, ensure that all end closures and fittings are securely fastened, and verify that all vent and drain valves on the prover are closed beforehand.
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When connecting a prover, cautiously admit supply pressure by first cracking open the metering system proving connection inlet valve, then the prover inlet valve, ensuring the prover is pre-pressurized with vapor before introducing liquid to prevent freezing Open air vents as specified in section 5.3.5, and allow the prover to fill slowly while monitoring for leaks and bleeding air; wait until the system is fully filled and leak-tight before fully opening the prover to supply pressure Once confirmed, safely open the outlet valves, and after all connection valves are fully open, close the main block valve between the prover connection branches Run the prover a few times without taking readings, recheck the vents, and verify that all double-block-and-bleed valves are secure.
Exercise caution when venting a newly opened prover to prevent releasing pressurized air and oil mixture directly into the environment Use temporary receptacles to collect vented oil when the system lacks proper piping to a reservoir Always wear protective goggles and open vent valves slowly to ensure safety Never vent liquefied gases into the atmosphere; instead, direct them to a flare-stack or appropriate disposal system for safe handling.
Vent valves releasing fluid into the atmosphere must be installed to direct the flow away from the operator, ensuring safe operation Prior to opening the vent valve, it is essential to verify proper placement, and if misaligned, operational adjustments should be made to vent safely and prevent exposure.
5.3.6 De-pressurizing and draining a portable prover
To safely disconnect a mobile prover from an installation, consult with the local operational management regarding fluid disposal and ensure the drainage system can handle the expected volume and surge, especially for high vapor pressure products requiring special disposal Begin by opening the main block valve between the prover connections, then close the inlet and outlet valves in the connecting pipes and on the prover itself Gradually open the drain valves followed by the vent valves to empty the prover, and once it's drained, close these valves Finally, disconnect the hoses and install blind flanges on the stub connections to ensure the system remains secure and leak-free.
NOTE In many countries it is illegal to take a mobile prover on the public highways without first draining it.
5.3.7 Shutting off a dedicated or central prover
When closing off a dedicated or central prover from the main metering or supply system, it is crucial to ensure that the prover is not trapped between closed valves Always verify that the pressure relief valve is capable of discharging before shutting down the prover to prevent pressure buildup and ensure safe operation Proper procedures help maintain system integrity and prevent potential damage or inaccuracies in measurement.
If a dedicated or central prover is to be drained for maintenance, proceed along the lines laid down in 5.3.6 for a mobile prover.
When removing a displacer, it must be sent to the appropriate end-chamber for bidirectional provers or to the sphere-handling valve for unidirectional provers, following proper drainage procedures After draining the prover as specified in section 5.3.6 and removing the access cover, the displacer can be easily removed if correctly positioned—hand removal for small displacers or mechanical tools for larger ones Proper handling ensures safe and efficient displacer removal in calibration processes.
If the sphere has not reached the end of its travel after removing the access cover and is positioned further back in the prover barrel, the correct course of action is to securely replace the cover, refill the prover with liquid, and restart the process.
Warning: Never try to force out the displacer with compressed air or gas, as this can cause it to be expelled violently like a cannonball, risking serious injury to personnel and damage to equipment.
5.3.9 Special precautions when proving with LPG
For optimal safety, LPG meters should be permanently connected to a dedicated prover If a mobile prover must be used, essential precautions must be followed to ensure safety and accuracy Additionally, procedures outlined in sections 5.3.9.1 to 5.3.9.6 are applicable when filling or emptying dedicated provers with LPG, highlighting the importance of strict safety measures during these processes.
Electrical safety
Prover operators must not perform electrical work themselves; instead, they are required to hire qualified electricians for all electrical tasks This requirement applies to all types of electrical work, including the specific jobs outlined in sections 5.4.2 to 5.4.4, ensuring safety and compliance with electrical standards.
5.4.2 Use of equipment of the correct type
Ordinary electrical and electronic equipment can ignite fires in areas with flammable vapours, making their use in such environments strictly prohibited To ensure safety, specialized electrical and electronic equipment, classified as explosion-proof or intrinsically safe, must be used in these hazardous zones Properly selecting and installing certified equipment is essential to prevent fire hazards and protect both personnel and property in explosive atmospheres.
“intrinsically safe”, “explosion proof” and “flame proof” is provided for use in such areas.
Safety precautions in hazardous areas involve dividing these zones based on the level of risk, ensuring appropriate measures are in place Electrical equipment used in such environments must be certified for safety and suitability for specific zones, minimizing potential hazards and ensuring reliable operation in challenging conditions Proper zone classification and certified equipment are essential for maintaining safety standards in hazardous areas.
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Before deploying any new equipment in different zones, operators must verify that it is approved for use in that specific area Using equipment that is not authorized for the zone is strictly prohibited under all circumstances to ensure safety and compliance.
All cabling shall also be of an approved type.
Ensure that the power supply is properly isolated and the isolating switch is tagged before performing any work This is essential when making temporary connections to mobile equipment, adjusting electrical or electronic devices (excluding those designed for safe adjustment while operational), or opening flame-proof or explosion-proof enclosures for maintenance or inspection Proper isolation and tagging help prevent electrical hazards and ensure workplace safety.
Effective earthing of all electrical and electronic equipment connected to a pipe prover is crucial for safety and proper operation Regular testing of permanent earthing connections is necessary, with testing intervals typically defined in company operating procedures Ensuring that the electrical resistance to earth remains below regulatory or policy-established limits helps maintain compliance and safeguards equipment and personnel.
Bonding cables are essential for earth grounding a mobile prover to a metering system during hydraulic connection It is crucial to ensure that the bonding is properly completed prior to making any additional electrical or pipe connections, ensuring safety and accurate measurement.
Periodic testing of bonding cable continuity is essential and should be conducted in accordance with the company’s operating procedures, typically every six months Additionally, bonding connections must be inspected each time a new bonding connection is made to ensure safety and reliability Bonding terminals should always be kept clean to maintain effective electrical connections and prevent corrosion.
Check the continuity of bonding in hoses at specified intervals.
Fire precautions
Portable fire extinguishers should be strategically placed near dedicated provers to ensure immediate accessibility and safety Before operating a prover, verify that fire extinguishers are correctly positioned, of the appropriate type, and that operators are trained to use them effectively It is important to avoid removing fire extinguishers during prover operation unless there is an actual fire emergency.
A mobile prover must have its own fire extinguisher to be operated safely If it does not carry one, it is essential to verify that suitable fire extinguishers are readily accessible nearby Ensuring the availability of appropriate fire extinguishers on-site is a key safety requirement before using the mobile prover This protocol is vital for maintaining fire safety standards and preventing potential hazards during operation.
Portable fire extinguishers must undergo regular inspections at designated intervals, with the inspection date clearly marked on the device It is essential to verify the last inspection date before using a fire extinguisher to ensure its readiness If a fire extinguisher is found to be out of date, it should be reported immediately to a supervisor or safety officer for prompt replacement, ensuring safety compliance.
Many fires are caused by the careless disposal of oily waste and rags, emphasizing the importance of proper storage Always keep oily rags in a closed metal container specifically designated for this purpose to prevent fire hazards Never leave oily rags lying around, as improper disposal can lead to dangerous fires Proper containment and prompt disposal are essential for workplace safety and fire prevention.
Miscellaneous safety precautions
Wearing protective clothing is mandatory whenever it is provided, as its primary purpose is to safeguard operators from potential injuries caused by harmful materials during work Ensuring that protective clothing is worn consistently helps prevent accidents and health hazards, especially when handling hazardous substances Compliance with this safety requirement is essential to maintain a safe working environment and to protect personnel from possible risks associated with exposure to dangerous materials.
Barrier creams are essential for protecting hands from harmful materials that can cause dermatitis or other skin conditions When these protective creams are provided, it is mandatory to ensure they are used consistently to maintain skin health and safety Proper use of barrier creams helps prevent skin irritation and promotes a safer working environment.
Organic lead compounds are toxic, necessitating strict regulations for handling all lead-containing materials, including leaded fuels like gasoline It is essential to be familiar with and adhere to these safety regulations before operating any equipment with leaded fuel, such as a prover, to ensure safety and compliance.
5.6.3 Braking and jacking of portable provers
Regular testing of brakes, stabilizing jacks, and jockey-wheel gear on trailer-mounted provers is essential every three months or more frequently to ensure safety and functionality These components must also be inspected before unhitching a trailer-mounted prover from the towing vehicle Additionally, truck-mounted provers should undergo similar safety precautions and inspections to maintain optimal performance and compliance with safety standards.
Before coupling the prover to the metering system, ensure that brakes are applied, and stabilizing jacks and jockey-wheel gear are correctly set up and locked in position When removing a portable prover from a site, it is essential to retract and lock all stabilizing jacks and jockey-wheel gear to ensure safe and secure handling Proper setup and secure locking of these components are crucial for accurate measurements and personnel safety.
Records
Each pipe prover must have an individual log book, or an alternative record-keeping procedure must be in place to ensure safety and maintenance documentation Prover operators are responsible for recording all safety, operational, and maintenance incidents, including accidents—regardless of whether they involve personal injury—and any abnormal events that could impact future equipment operation.
The record shall include the dates and results of the testing of all equipment which is required under safety regulations or operating procedures, including those tests mentioned above.
The operator shall submit this record book for inspection as procedures require and have it available at all times for inspection on demand.
Setting up a portable prover
Portable provers pose unique challenges due to extensive preparatory work required before safe installation at a new location Proper setup is essential to ensure accurate and reliable operation This process involves a specific sequence of preliminary steps to guarantee the prover's readiness for use Following these preparatory procedures safeguards equipment functionality and measurement accuracy during deployment.
Before heading to the site, verify that the metering station specifications align with the manufacturer’s guidelines for the mobile prover, focusing on flowrate, pressure, temperature, and liquid type to ensure suitability If the prover was previously used with incompatible liquids, it should be thoroughly flushed before use Additionally, confirm that all relevant parties have been notified of the proving process and that all necessary entry and safety permits are secured to ensure a safe and compliant operation.
Upon arriving at the site, the operator must report to the site supervisor to coordinate assistance, identify the meters requiring proving, and establish connections They should also arrange for electrical power if needed, dispose of any liquid not returned to the pipeline, and set up traffic barriers for safety The prover must then be brake and jacked following the procedures outlined in section 5.6.3, connect earth connections as specified in 5.4.4, and link the prover to the branch connections according to the instructions in sections 5.3.3 or 5.3.9.
6.1.4 If the displacer is not already inside the prover, insert it and securely fasten the end closure.
6.1.5 With ordinary liquids, admit line pressure, fill the prover and vent the air, as described in 5.3.4 and 5.3.5. 6.1.6 With liquefied gases, proceed as described in 5.3.9.
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Portable small-volume provers often feature a swiveling mechanism that allows the prover barrel to be positioned upright upon arrival at the site This design is especially beneficial for installations where fluids may contain solid contaminants like rust, scale, sand, or dirt, as it helps prevent particles from settling on the barrel walls and damaging displacer seals To ensure effective removal of contaminants, the prover's outlet should be positioned at the bottom, enabling the fluid to carry particles out of the system efficiently.
Warming up provers
Provers must reach thermal equilibrium before beginning an actual proving run, a process known as "warming up," which may involve either heating or cooling the prover This is achieved by circulating liquid through the prover until its temperature closely matches that of the liquid Performing multiple dummy runs—moving the displacer back and forth without taking measurements—ensures the prover is properly conditioned These runs also provide an opportunity to vent any remaining air or vapor and verify that the detector switches and prover counter function correctly If time permits, additional checks on the double-block-and-bleed valves can be conducted during the warming-up phase to ensure accurate and reliable prover operation.
During the warming-up period, monitor the temperatures of both the meter and the prover to ensure they are nearing equilibrium If their temperatures are closely aligned and the liquid temperature is similar to the ambient air, stable conditions should eventually be established, resulting in nearly identical temperatures at both points It is essential to continue warming until the temperature difference stabilizes at a constant value, then proceed with the proving process without delay to maintain consistent temperature conditions throughout the operation, ensuring accurate measurement results.