Good Practice Guide
1
Introduction
This good practice guide has been prepared to give an overview o the general principles o calibration or ow measurement. It provides a summary o calibration methods available across a variety o locations and applications and is designed to outline all the basic principles which can be applied to high accuracy calibrations in laboratory and feld applications. The guide also addresses addresses less accurate accurate calibrations or ‘verifcations’ ‘verifcations’ in difcult difcult situations. situations. This document document has been produced to address the needs o calibration laboratories and institutes, users o ow meters, and engineers having to establish calibration and verifcation methods. The guide frst covers general principles o calibration when applied to devices or measuring owing uids (ow meters). It then addresses a variety o individual techniques and methods which may be employed as standards and methods o calibration. Many o the principles o calibration apply equally across metrology and this guide re-iterates these principles and puts them in the context o ow calibrations. Vo Vocabulary cabulary is important to the understanding o the principles and the key defnitions o terms have been reproduced rom the ‘International Vocabulary o Metrology — Basic and general concepts and associated terms’ (VIM) 2008, 2008, and this is reely available rom the BIPM web site. site. (BIPM is the international body responsible responsib le or harmonisation o metrology and the SI). Where appropriate appropriate the older defnitions rom the previous version o VIM (1995) have been used in instances where the older version provides a clearer defnition than the latest version. Establishing the correct terminology is important to provide a clear understanding o what is meant when various terms are used in defning a calibration.
2
What is Calibration?
Calibration embraces a number number o operations, systems and concepts. concepts. This is best explained in a series o descriptions. descriptions.
2.1
Calibration
The ormal defnition o ‘calibration’ taken rom the VIM is given below. What has to be done now is to examine how this applies to ow measuremen measurement. t. Calibration is a comparison between the reading o a device and that
Calibration:
o a standard. standard. The process process which establishes this this relationship relationship is a
operation that, under specifed conditions
set o interrelated measurements and operations which provide the
establishes a relation between the quantity
comparison. Flow measurement measurement does does not rely on a single single operation
values (with measurement uncertainties)
and so neither does a ow based calibration.
provided by measurement standards and corresponding correspond ing indications (with associated
Measurement o the quantity o uid depends on establishing the
measurement uncertainties)
basic quantity and and a number o inuence inuence actors. The quantity o uid may be expressed expressed as a volume or a mass. The measurand may be
uses this inormation to establish a relation
the quantity or the ‘rate’ i.e. the quantity per unit time.
or obtaining a measurement result rom an indication. VIM 2008 2008 - (2.39)
2
The calibration o ow meters
2.1
Calibration cont.
The quantity measured by the standard may be dierent rom the quantity passed through the test device due to changes in volume or even mass between the the meter and the standard. Changes are usually usually related to the inuence actors such as temperature, pressure, pressure, viscosity and expansion. This combination o uid, inuence inuence actors, the standard and the device come together to defne a set o operations which to provide the calibration. As the uid and inuence actors all aect the meter perormance, the calibration is carried out ‘under specifed conditions’ and these must be defned. A calibration is not an absolute operation. It is a comparison between the measuring instrument, in this case the device under test or ow meter, and the standard. Through this comparison, a relationship between the quantity measured by the device under test and the measurement o the same quantity derived rom t he standard is established. This is expressed in some some way which gives a meaningul meaningul expectation o how the device will perorm in use.
Calibration: the set o o operations operations that establish, under specifed conditions, the relationship between values o quantities indicated by a measuring instrument or measuring system and the correspon corresponding ding values realised by standards. VIM 1995; (6.11)
The current defnition in the VIM, although being more precise, seems to lack some o the clarity o the older defnition, and so the older alternative has also been given or reerence. Later in this guide the use o ‘proving’ and ‘verifcation’, terms used to describe particular types o calibration will be described. The comparison during a calibration is against a standard. The standard comprises the system o pumps, pipes, uids, instrumentation, quantity reerence measurement, calculations and operators. These all combined provide provide a measure o the quantity quantity o uid passing through the device or ow meter being calibrated. The measurement o uid ow is dynamic and all measurement devices are aected in some way by the conditions conditions o use. It will be impossible to have a standard which ully reproduces the conditions under which the meter will be used in practice. Flow devices are are aected by temperature, viscosity, ow profle, ow rate uctuations
Standard: realization o the defnition o a given quantity, with stated quantity value and associated measurement uncertainty, used as a reerence. VIM 2008 (5.1) Or measuring instrument or measuring system intended to realise or reproduce a unit or one or more values o a quantity to serve as a reerence. VIM 1995 (6.1)
and pulsations. pulsations. They are also aected aected by the external environment, environment, vibration, stress and and temperature etc. Dierent devices are aected in dierent ways. Similarly the standard will also be susceptible to these same inuences. Since a calibration is a comparison between the measurement made by the device under test and that realised by the standard, the resultant relationship will be or the specifed conditions; conditions; thereore a urther assessment o the relevance to the fnal application must must be carried out. Selecting the standard will will be a compromise to best replicate replicate the conditions o use while providing a suitable reerence standard standard measurement. The standard must also be compatible with the perormance and characteristics o the meter to be tested and the result desired.
3
Good Practice Guide
2.1
Calibration cont.
The extent to which a ow measurement device is aected by the conditions o use is most oten a unction o the owrate. It is thereore important that calibration calibration takes place across a range o owrates to establish this this relationship.
2.2
Rate, quantity and time
The mechanism by which a ow measurement measurement device gives a reading o ow is dynamic. The sensor reacts to the ow o uid through it or past it to realise an output related to the owrate or the quantity passing. Measurement o owrate and quantity are related through through the time interval across which the quantity is measured. In practice the end user o the device has dierent dierent expectations or the behaviour and hence hence the calibration. In establishing this relationship it is vital to relate the response response time o the device to the calibration method. method. Again the current and older older generic defnitions o response times are given. The interpretation o response time is reasonably straightorward straightorward or mechanical meters. The mechanical interace interace between the uid and the indicator can be explained and defned in terms o momentum and drag aecting the meter meter when the ow changes. With the advent o electronics this has become become more difcult difcult to establish. For example, a positive displacement meter in liquid responds very quickly to changes in owrate even very abrupt changes; the ow stops, the rotor stops and the register stops. stops. I a pulse generator is ftted, ftted, the generated pulses stop when the ow stops. A requency counter will not reect this until it completes its measurement cycle which may be some seconds later. later. During that time, a totaliser or register will correctly indicate quantity, but the owrate indicator will not be showing the correct (instantaneous) (instantaneous) owrate. I however the mechanism mechanism in the meter has play or is loose, stopping the rotor may allow the output register to ‘run on’ ater the rotor stops hence generating additional
(Step) Response time:duration between the instant when an input quantity value o a measuring instrument or measuring system is subjected to an abrupt change between two specifed constant quantity values and the instant when a corresponding correspond ing indication settles within specifed limits around its fnal steady value: VIM 2008 (4.23) Or time interval between the instant when a stimulus is subjected to a specifc abrupt change and the instant when the response reaches and remains within specifed limits around its steady value. VIM 1995 (5.17)
quantity or pulses. A dierent type o meter may o course not respond to an immediate change in ow. ow. A turbine rotor will have signifcant momentum and, although speeding up quickly, may take time to slow down when subjected subjected to a change in ow, ow, particularly in gas measurement. measurement. Meters based on non-mechanical non-mechanical sensing techniques e.g. electro-magnetic, electro-magnetic, Coriolis or ultrasonic meters, have dierent response characteristics. characteristics. For example, an electromagnetic meter may take some time to establish and measure a change in the generated voltage ater a change in ow, ow, while an ultrasonic meter output is the average o a number o measurement cycles and this averaging may take appreciable time to complete. Most meters based on non mechanical sensing, and some mechanical meters, have a microprocessor which calculates the output quantity rom the sensor sensor signal. For some meter types this is an additional capability capability while others require require this processing to convert the sensor signal and correct or inuence actors beore calculating and generating an output signal. The output signal can be a pulse requency or a current (mA) output generated rom this calculation process.
4
The calibration o ow meters
2.2
Rate, quantity and time cont.
A digital display may be added to show the required output value and many modern meters have digital outputs to transmit the chosen measurement(s) to a remote readout or computer. All these outputs will have a dierent response times delaying the raw sensor response by the signal processing and calculation time. An example o a processed output output signal comes rom a vortex meter with a signal processor designed to smooth any pulses missed by the sensor and to increase the resolution o the output. Such a device may have output response times o many seconds even though the sensor itsel has responded in under a second. Modern ow meter types such as Coriolis, electromagnetic, or ultrasonic meters are t otally dependent on microprocessor microprocessor based output, and the variety o settings to average, damp or cut-o low owrates must be understood and selected to ensure the response response time matches the limitations o a calibration method. method. Matching the response response time o a device to the chosen calibration method method is a vital part o the process. I the device response time does does not match the time within which a calibration test point is taken, poor repeatability repeatability or calibration osets may be observed. observed. This response response time may however be perectly adequate or even advantageous when the meter is in service.
2.3
Repeatability and reproducibility
To obtain confdence in a measurement it is expected that the measurement should be able to be repeated and give the same result. In practice measurements only repeat to within a certain band over a short time and a (probably (probably)) wider band over a long time period or under dierent dierent circumstances. circumstances. It is generally expected that that a calibration should give some indication o the repeatability o an instrument; however it is not likely that one calibration will show the reproducibility. Repeated calibration may o course be carried out perhaps over many years to show this parameter.
Repeatability:: Repeatability measurementt precision under a set o measuremen repeatability conditions o measurement VIM 2008 (2.21) Reproducibility:: Reproducibility measurement precision under reproducibility conditions o measurement VIM 2008 (2.25)
A measurement standard should have determined repeatability and reproducibility fgures which will have been included in the uncertainty determination. The repeatability o a calibration will o course course include the repeatability o the standard, standard, but also the repeatability o the device under under test. A large part o the repeatability will be the resolution resolution o both the device and the standard. It is interesting to note that a device or ow meter may be believed to have better repeatability than that o the standard. This belie is based on history, history, behaviour in other calibrations and service etc. The repeatability cannot however be proved to be better as it can only be demonstrated through repeated calibrations against the standard.
5
Good Practice Guide
2.4
Resolution
Although it may seem obvious, the resolution o the device must be adequate to allow a calibration to match the uncertainty uncertainty required. To achieve this, the standard must be able to measure enough uid to match the resolution o the device. For example i a ow meter has a resolution o 1 litre, the standard must have a volume o signifcantly
Resolution: smallest change in a quantity being measured that causes a perceptible change in the correspond corresponding ing indication. VIM 2003 (4.14)
more than 1,000 litres to achieve an uncertainty o 0.1%. To meet oil industry norms, a volume o 10,000 litres would be expected to ensure an achievable uncertainty o 0.01%. 0.01%. The requirement requirement or resolution to be assessed applies equally equally to meters with pulsed analogue analogue and digital outputs. For analogue meters the eect o sampling and averaging has to be considered along with the resolution o the instruments being used in the standard. For electronic pulsed or digital outputs it is possible to increase the resolution or calibration purposes. purposes. Care has to be taken however that an artifcially high resolution is not applied, by mechanical or electronic increase, which is not representative o the sensor perormance.
3
The Importance o Calibration Fluid and Conditions
3.1
Fluid properties
All ow meters interact in some way with the owing uid. The nature o this interaction is aected by t he properties o the uid or the velocity distribution o the uid passing through the device. Changes in this interaction alter the ability o the device to give an accurate representation o the quantity quantity.. The magnitude o the error is dierent or dierent meter types and uids. For this reason it is desirable to calibrate using the same uid and pipework confguration confguration within which the meter will normally operate. This is clearly not oten possible; the meter has to be installed in a test laboratory, or calibration standard standard has to be installed in the process application. In either case some degree degree o disturbance disturbance to the meter is inevitable. inevitable. The best economic compromise compromise must be established in choosing the calibration. This will be based on the fnal duty o the meter,, the required uncertainty and knowledge meter knowledge o the meter perormance. For some meters, or example orifce plates, the perormance can be related to Reynolds number. number. This allows a calibration to be carried out in a uid dierent to the meter’ss operating uid. The relationship may even allow a liquid calibration to be applied to a meter or a gas duty by meter’ matching Reynolds numbers. For other meter types, such as turbine meters, the choice o calibration uid is particularly particularly important.
Turbine meters are
viscosity sensitive, and the fgure opposite shows some typical calibration results rom a turbine meter using water and three petroleum products. products. Because o this sensitivity sensitivity to viscosity it is important to calibrate these meters using a uid as close to t he viscosity o working uid uid as is practicable. For this reason, among others, fscal meters or oil are oten calibrated on site using a dedicated pipe prover.
6
The calibration o ow meters
3.1
Fluid properties cont.
For gas meters, air is oten used used as the calibration uid uid or saety reasons. reasons.
When used with other gases the perormance perormance
related to Reynolds number provides provides a good relationship or turbine and ultrasonic meters. Coriolis, and positive displacement meters do not not ollow this relationship. relationship. For variable area meters it is important to correct the gas density to a standard condition condition to match the scale. Gas pressure is probably probably the most important important inuence actor as this aects the density and hence many aspects o a meter perormance. Properties o the uid such as density, temperature, conductivity, pressure may also have to be considered when replicating the use o the meter in a calibration.
3.2
Flow prole
When a uid passes through a pipe, the distribution o velocity across the pipe alters to approach a ‘ully developed’ profle which is dependant on the pipe inter internal nal diameter, roughness and uid Reynolds number. number. The presence o any change rom a straight pipe will alter the profle drastically. drastically. Bends, double bends, bends, valves etc. all introduce asymmetry asymmetry to the velocity distribution and some introduce introduce swirl or rotation. As the way the uid interacts with the sensor can be highly dependant dependant on the velocity profle, these these eects must be considered in the the calibration. Most calibration acilities allow adequate adequate straight pipe lengths and the use o ow conditioners to establish predictable and reproducible ow profles close to an ideal profle. It should be noted however however that care has to be taken to ensure pipes upstream upstream o the meter have the same internal diameter as the meter inlet and step changes or misalignment o joints and gaskets do not introduce irregular irregular profles.
3.3
Traceability raceability,, accuracy and uncertainty
Since a calibration is a comparison between the reading taken rom a device under test and that o a standard, it is necessary to consider what properties are required required rom a standard. Firstly and most importantly, importantly, the standard should measure the same quantity quantity as the device. There is little value in comparing a mass meter output output with that o a volume tank without a measure measure o density to allow allow conversion between mass and and volume. For ow measurement, measurement, the standard standard is a system comprising o a measure o quantity and the subsidiary measurements to determine the uid conditions, properties and inuence actors. Another eature o the standard is that there must be confdence
Traceability:
that the measurement taken by the standard is accurat accurate. e. To achieve
property o a measuremen measurementt result whereby
this all the measurements in the system have to show traceability to
the result can be related to a reerence
higher level measurements and ultimately to National and International
through a documented unbroken chain o
standards.
calibrations, each contributing to the measurement uncertainty
The defnition o traceability provided expresses the process by
VIM 2008 (2.41)
which a measurement can be related through an unbroken chain o comparisons to national/international standards. standards. It must be noted that each step o the chain will have an uncertainty becoming smaller at each step. It must be noted that providing or claiming traceability alone makes no statement regarding regarding the quality or uncertainty o the result; this requires an uncertainty value. Traceability must also be through comparisons comparisons to other/better calibrations and NOT TO an accreditation body. 7
Good Practice Guide
3.3
Traceability raceability,, accuracy and uncertainty cont.
It is contentious to use the term ‘accuracy’ in relation to calibration work. Accuracy has not the rigour rigour or precision precision required required to describe describe a scientifc process. In practice however ‘accuracy’, when used correctly, correctly, is the term to which users relate and can useully be used to express expectation and general general specifcation. specifcation. Accuracy is a qualitative qualitative term and thereore the number associated with it should be used or
Accuracy: Closeness o the agreement between the result o a measurement and a true value o the measurand VIM 1995 (3.5)
indicative purposes. To correctly express the ‘accuracy’ o a standard or a calibration the
Uncertainty:-
‘uncertainty’ must be be determined and quoted. quoted. Uncertainty provides provides
non-negative parameter characterizing the
confdence that the determination o the value lies within the stated
dispersion o the quantity values being
value. For ow measurement measurement the confdence in the result result lying within
attributed to a measurand measurand,, based on the
the uncertainty is normally quoted with a ‘coverage actor’ o k=2,
inormation used
which is approximately approximately 95% confdence confdence level. A ull explanation is
VIM 2008 (2.26)
given in the Guide to Measurement o Uncertainty (GUM) or ISO 5168. Note this is slightly dierent wording wording rom Every standard must be assessed or the uncertainty in the
both VIM 1995 and the GUM
determination o its measured quantity, quantity, as indeed must the result o a calibration. The uncertainty quoted or a calibration or a standard will be estimated rom a detailed examination o all t he components within the system, the use o the system and its history. history. It will specifcally state or what parameter the uncertainty applies applies to. This parameter may be the quantity measured measured by the standard or the quantity quantity passed through the the device under test. The latter is the uncertainty which which is needed initially initially.. It is stressed this is not the uncertainty o o the calibration result. result. The resolution o the meter, the inuence actors and fnally the repeatability and linearity o the calibration results must all be included to provide the uncertainty o the calibration. The purpose o a calibration is to estimate the uncertainty associated with measurements measurements rom the meter in its fnal application. It is clear that the calibration will will only provide provide a component component o this fnal measurement measurement uncertainty uncertainty.. A responsibility responsib ility remains with the end user to use the calibration uncertainty along with an understandin understanding g o the use o the meter compared with the calibration conditions to provide this fnal result. All calibration results should have a stated uncertainty and this should be stated clearly on the calibration report or certifcate. The uncertainty statement should should be clear and unambiguous unambiguous as to what is included and which which quantity it reers to. Uncertainty can be expressed expressed on the the certifcate as being the uncertainty o the measured measured quantity (ow (ow,, volume or mass), or uncertainty o the device under test during calibration. An equation ftted to the data, i provided, provided, should also have a stated uncertainty estimated. estimated. The uncertainty will not include the estimate o uncertainty at dierent dierent time or conditions. Any estimate at a dierent time or condition condition is an estimate (or speculation) and could only be advisory and not part o the calibration. It is also worth noting that uncertainty may vary across across the ow range o the meter. meter. The quantity o uid collected collected by the standard may contribute to dierent uncertainties, or the meter perormance may vary. 8
The calibration o ow meters
3.3
Traceability raceability,, accuracy and uncertainty cont.
In speciying the required uncertainty o a standard relative to that o a meter, it is good practice that the standard should have an uncertainty 10 10 times smaller than that o the requirement requirement o device to be calibrated. calibrated. Although this is a good principle, in ow measurement measurement it is oten not possible to achieve due to the high expectations o ow meters and the applications or them. A standard with an uncertainty uncertainty o a actor o three lower than the requirement requirement o the application is oten all that can be achieved. In some situations, especially in feld testing, the uncertainty o the standard may be higher than the expected uncertainty o the meter. This applies when some methods o in-situ calibrations calibrati ons are called or. or. When this occurs the achieved uncertainty o the ‘calibration’ must be larger than that o the standard, hence increasing the uncertainty uncertainty o the fnal measurement. When this situation is encountered, ‘verifcation’ rather than a calibration is oten specifed. The result is used to provide increased confdence that the meter is operating correctly but not used to make corrections to the meter or as the primary assessment o its
Verifcation: provision o objective evidence that a given item ulfls specifed requirements. requirements. VIM 2008 (2.44)
uncertainty. It is to be noted that uncertainty should not be conused with error. Error expresses how ar away rom rom the ‘true’ value the reading is; however, however, this value may be known to have a much smaller uncertainty. uncert ainty. Knowing the error may allow a correction to be made to the reading.
3.4
Accreditation
Accreditation is the process that a calibration laboratory or service provider undergoes to give confdence that t he result provided to a client meets the expectation expectation stated in the scope o the work. It is a process by which the equipment, equipment, technical methods, contractual arrangements, and quality o the results are examined to give confdence to the client in the delivery o the service. A third party, party, or indeed the client, accredits and organisation hence giving confdence confdence in uture works without individual individual inspection. This process ensures that traceability has been established, an uncertainty budget budget produced, and procedures are sound. To avoid multiple client accreditations, and to provide commonality, accreditation is provided by a National accreditation body and subject to meeting international agreement on the standards or inspection. inspection. Most developed countries have their own accreditation accreditatio n body and it is now recommended that only one body should be appointed in each country. In UK the body is the United Kingdom Accreditation Accreditation Service (UKAS).
3.5
Reporting the result – perormance indicators
To display the result o a calibration, the nature o the meter output has to be understood. Flow meters may indicate owrate or quantity in a number o dierent ways. There may be a mechanical or electronic display indicating quantity or owrate, or an electronic output output based on pulses, requency requency or current (mA). The output may be in the orm o a dierential pressure.
9
Good Practice Guide
3.5
Reporting the result – perormance indicators cont.
Where the output or display is based on the ‘rate’ measuremen measurementt (i.e. requency requency,, owrate, dierential pressure or mA), readings normally normally vary a little during a calibration test point. It is normal to average the readings taken at a controlled sample rate across each calibration determination. I the output is based on quantity passed (i.e. total pulses or display o quantity) the reading o the display has to be compared with with a quantity o uid uid measured by the standard. standard. I the display is a visual visual one, clearly the ow ow has to be stopped to read the display unless some orm o photographic reading reading triggered rom the calibration process process is used. I the output is electrical, electronic gating can coincide with a trigger signal rom the t he standard. Where electronic digital outputs outputs such as serial or ‘feld bus’ data transer is used, rate measurements can be sampled or quantity can be read read at the end o the calibration calibration period. This type o output output cannot normally normally be triggered electronically electronically to synchronise with a calibration standard, thereore extreme care must be taken to recognise update and processing times i a ‘dynamic’ calibration method is used. The result o a calibration is normally given in tabular orm listing the measurements rom the standard and the device. Inormation on the inuence actors and the amount o raw data given will vary depending on the calibration specifcation. specifcation. The presentation o meter and standard standard readings is not not the most helpul to interpret the result o the calibration. calibration. It is thereore normal to calculate a perormance indicator. indicator. A perormance indicator can be used to display the result in a way which best displays the the perormance o the meter across the ow range. range. It will also allow the determination o a quantity when the meter is used in practice. A number o dierent perormance indicators are commonly used. K-actor Used or meters with pulsed outputs proportional proportional to quantity passed. K-actor is expressed as pulses per unit quantity (e.g. pulses per m3 or pulses per kg) Meter actor The generic defnition is ‘correction actor’ in the VIM
Correction Factor:
but in the ow meter industry the term ‘meter ‘meter actor’ is used. The
numerical actor by which the uncorrected
meter actor is normally dimensionless and is calculated as the ratio o
result o a measurement is multiplied to
the meter output to value value determined by the the standard. This can be
compensate or (systematic) error
computed rom rate measurements measurements or quantity measurements. measurements. Units
VIM1995 (3.16)
should be the same. e.g. Where F is the meter actor; Q is owrate; V is volume; i is indicated by the device and s is the measured value rom the standard. As with the K-actor K-actor,, this is the number which the output is multiplied to give the ‘true’ ‘t rue’ reading.
10
The calibration o ow meters
3.5
Reporting the result – perormance indicators cont.
Error : Error is the dierence between the indicated value and the value determined by the standard. Relative error is the error divided by the value determined by the standard and is normally expressed as a percentage.
It is important to always defne this equation in a calibration report as
Error:
some industries use a dierent dierent convention. This is best described as
measured quantity value minus a reerence
the inverse or negative error and this is based on the standard minus minus
quantity value.
the indicated value.
VIM 2008 (2.16)
Error can also be defned or meters with electrical outputs o pulses,
Relative error:
requency,, volts or mA. In this case the indicated value is calculated requency
error divided by a true value o the t he
rom the output reading and the predetermined relationship (normally
measurand
linear) between the output value and the equivalent quantity or
VIM 1995 (3.12)
owrate. An example is i a meter is confgured to provide provide 20 mA = 10 l/s and 4 mA = 0 l/s. The value o Qind would be the owrate calculated rom the measured current and the linear relationship derived rom the maximum and minimum settings. Discharge coefcient: For dierential-pressure dierential-pressure meters, such as orifce plates and nozzles, the perormance indicator used is the discharge coefcient coefcient (C). This is eectively an expression expression o the ratio o the actual ow to the theoretical ow. ow. The theoretical ow is however however,, defned in terms o the diameter o the throat o the device. C is given by the ormula:
Where: d is throat diameter (m) β is ratio o throat diameter to pipe diameter - d/D D being the internal pipe diameter) ( D Δp is measured dierential pressure (bar) ρ is density (kg/m3) qm is the mass ow rate (kg/s) є is the expansibility [expansion] actor.
The coefcient є is used to take into account the compressibility o the uid. For incompr incompressible essible uids (liquids) є = 1; or compressible compressib le uids (gas) the value or є is calculated rom ormulae based on the properties o the uids. The numerical value o C will commonly vary depending on the device. Typical orifce plates have a C value o around 0.6 and nozzles between 0.9 0.9 and 1. Discharge coefcient coefcient is relatively constant or any particular device, device, only varying slightly over the working ow ow range o that device. For these devices the ow range is most most useully expressed in terms o Reynolds number. Values o C derived rom a liquid calibration calibrati on may be used in gas applications with appropriate application o є . 11
Good Practice Guide
3.5
Reporting the result – perormance indicators cont.
It is to be noted that when calibrating a device, the values chosen or the diameters must be recorded and quoted in the calibration certifcate. I these values are subsequently subsequently used to determine owrate rom rom the derived C, any inaccuracy or uncertainty in the diameter values does not not contribute to the uncertainty o owrate. Changes in the diameter due to wear,, ouling or indeed temperature will o course make a dierence. Accurate dimensions are only required wear required where the value o C is derived rom theoretical equations rather than by calibration. Flowrate: To express the overall perormance o a device, its perormance across its ow range has to be determined. The owrate is normally expressed expressed in terms o quantity per unit time with the units chosen chosen to suit the application. Alternatively a more complex ow based parameter may be used such as Reynolds number which can add urther dimensions to the perormance curve by accounting or viscosity and density.
3.6
Calibration requency or how oten should a fow meter be calibrated?’
There is no correct answer to the question. In some applications an answer is apparently easy. easy. An industry standard or third party (regulator or trading partner) dictates the calibration requency. requency. In this case the meter is calibrated whether it irequires it or not and is oten assumed accurate between calibrations. calibrations. For most applications however however, it is the user who must defne the calibration interval and the policy to determine determine when to calibrate. The calibration interval should should be chosen to minimise minimise the risk o an incorrect meter reading making a signifcant impact on the process. For example, high owrates owrates o oil attract huge tax liabilities. The product value is high, high, the risk o meter damage is high and so perhaps perhaps weekly in-situ calibrations calibrations o the meter, meter, in the actual product, product, will be specifed. specifed. Alternatively metering waste water with a Venturi may only require annual inspections, inspections, irregular verifcation, and no ow calibration. calibration. The dierential pressure pressure measurement device will however however be calibrated regularly. regularly. The risk o the pressure transducer being in error is reasonably high, the risk o the Venturi changing is low, and the product value is low. Other actors aecting the decision are the history o the meter, meter, when the process is closed or maintenance, or what checking and diagnostics are monitoring the meter. It is always good practice to keep calibration graphs, and control charts o the meter perormance. This will assist in selecting intervals and also show changes in perormance indicating degradation o meter perormance.
4
Calibration Methods or Liquids
A number o quite specifc methods and systems are recognised recognised or the calibration o ow devices. Methods or both gas and liquid ollow the same principles although major dierences in implementation exist between the two uids. There are two main dierences between gas and liquid ow methods. methods. The frst is that liquids will remain in an open container while gasses gasses need to be contained. contained. Also gasses are are highly compressible compressible while while liquids, or most most practical purposes, may be assumed incompressible incompressible (except or some small corrections); resulting in undamental dierences in approach. This is o course a generalisation as volatile liquids may require to be contained and many liquids e.g. Liquid Petroleum Gas (LPG), have a high compressibility in addition to being gas at ambient conditions. conditions. Liquid methods are discussed in this section, gas is covered in Section 5. 12
The calibration o ow meters
4.1
Liquid collection methods
Unless a liquid is volatile or hazardous hazardous it can usually be contained contained in an open vessel. As a result, calibration standards standards are usually classifed as being ‘bucket and stopwatch’ systems. The ‘ bucket’ is a container which is weighed or has a known volume. The ‘stopwatch’ is a method o measuring measuring the time to fll the the bucket. Static methods o calibration calibration are based on collecting uid in the bucket and determining its quantity by a static measurement. Although dynamic methods methods are available, they are generally less accurate.
4.1.1 Standing start and stop method This method is generally preerred or meters measuring exact quantities o liquid, especially meters or batch measurement. ‘Standing start and stop’ is the simplest method available and can be used or both high and low accuracy calibrations. The ow system is flled, all air purged and the required owrate established. The ow is then stopped using a ast acting valve. When the container is empty, empty, the drain valve is closed, the ow started and the container flled and when the container is ull the ow is stopped. The quantity collected is measured and compared with the meter reading; the time to fll gives the owrate. In order to have an eective standing start start and stop calibration system, a number number o criteria have to be met. Firstly Firstly,, the pump and circuit supplying the ow has to be designed and arranged to allow the ow through the meter to be stopped without damage to the pump pump or pipework; a pump bypass bypass is usually ftted. Secondly no air should be be let trapped in dead ends or T pieces as this will provide a spring eect causing the ow to oscillate when stopped suddenly, resulting in incorrect meter readings. readings. The ow has to be started and stopped stopped as quickly as practical to minimise the rise and all time errors. Too ast a stop will create pressure uctuations uctuat ions and ‘water hammer’ and must be reduced by slowing down the valve until an acceptable perormance is ound. The stop valve should have an equal opening and closing time. The meter being calibrated has to have a ast response time to match the start and stop o the ow. ow. The test time has to be sufciently long in comparison with the acceleration and deceleration periods as to give insignifcant insignifcant error. error. This is illustrated diagrammatically diagrammatically or dierent meters. A slow response response meter is one where the response response time means the output starts ater the ow ow,, is started and continues or a short time ater the ow stops; this may result in error. error. A delayed output meter shows the eect o microprocessor microprocessor or electronically enhanced meter where the sensor responds quickly. quickly. The internal totalising totalisin g o quantity, and indeed the output, starts quickly, but is slow to provide the fnal result. Many meters o course respond respond quickly and closely ollow ollow the ow curve. 13
Good Practice Guide
4.1.1 Standing start and stop method cont. The level established in the vertical pipe leading to the weighing vessel has an indeterminate level when the ow is stopped quickly.. This would be worse quickly worse i the pipe was not vertical and the pipe allowed the draining draining back o the test uid to the meter.. A vertical pipe allows the system to remain ull. meter ull. The weir and overow allows liquid liquid above the weir at the end o a test to drain back to a precise level, hence minimising the the variability o the system. This point corresponding corresponding to the weir is the ‘transer point’ and this is the point at which uid transers transers rom the meter to the standard. It is unlikely to achieve high accuracy with test times less than 60 seconds. seconds. With a slow operating valve, longer times may be required. required. Switches on the stop valve may be used to time the tank flling and start and stop pulse counter gates taking care all pulses are collected, even i delayed. What is discussed above above is a standing start and fnish method based based on a gravimetric (weighing) (weighing) method. I a volume tank is used, flling is usually rom the bottom, and the valve closing time is slow or stepped at the last stages to ensure the level stops within the the measuring neck. The ‘transer point’, (the level level at which the volume volume starts and fnishes) fnishes) will be established established in the lower neck o the tank.
4.1.2 Flying start and nish This is sometimes called the diverter method where the ow through the meter is not stopped but continues uninterrupted. The ow is physically diverted between a return path to the liquid supply tank and the collection container. A switch on the diverter mechanism starts and stops a timer and a pulse totaliser The key to accurate measurement is a clean separation between uid entering the container and uid returning to the supply. This should be accomplished without any change o owrate through the device. For this reason the ow into the diverter is normally conditioned by creating a long thin jet impinging on a splitter plate. This will be open to atmosphere ensuring ensuring no change o pressure occurs when diverting and hence removing the potential or a change in owrate during during a test. The diverter mechanism is operated as quickly quickly as possible to reduce reduce ‘timing errors’ to a minimum. With a well designed diverter, test flling times down to 30 seconds can be achieved. The main source o uncertainty lies in the timing error, shown diagrammatically.. The ‘hydraulic centre’ o the diverter is ound by diagrammatically calibrating a high quality reerence reerence meter at a constant owrate. owrate. The calibration is repeated using alternatively long and short diversion times. The dierence between them defnes the timing error. The diverter sensor is moved until the dierence between calibrations is minimised. This is repeated at dierent dierent owrates owrates and a best compromise comprom ise position ound. ound. The residual scatter and dierence dierence between long and short diversion calibrations gives the uncertainty due to timing error. 14
The calibration o ow meters
4.1.2 Flying start and nish cont. Flying start and fnish methods are used primarily or meters with slow response times and where owrate is the primary measurement rather than quantity passed. Meters with visual displays cannot be calibrated by this method. A three port valve can be used as the diverter. Most valves, even those designed to maintain ow during changeover, will not have equal port area sizes during during the change. The pipe resistance may well well be dierent or the two paths resulting resulting in dierent owrates being produced. produced. These dierences can result in pressure/ow surge during changeover changeover and dierent owrates in the two conditions. It is also difcult to fnd valves which operate quickly quickl y enough to provide a ast changeover.
4.1.3 Dynamic collection methods Dynamic methods are techniques where not only is the ow continuous through the device under test but also through the standard while it is measuring. The static methods outlined above can be modifed to utilise a dynamic measurement measurement technique. The changing weight weight (dynamic weighing) or volume (dynamic level gauging or switching) can be detected and used to trigger the test point measurements o time and meter output. output. Flow is established established into a tank with the drain open. To initiate a test, the drain is closed and the rising level, or increasing weight is detected and used to initiate a test point by starting a timer and reading the meter. meter. When the tank is ull, and ater a signal has been sent to stop collection o o data, the tank drain opened and the tank emptied. The drain has to be large enough to allow the the ull ow to pass through through when open. The techniques are generally generally only used or low accuracy calibrations. calibrations. Repeatability is normally poor poor (0.5 to 1 per cent) is especially on larger systems. There is lack o resolution and response time (on the level instruments and dynamic weighing weighing systems) while being flled which limit the operation. Some careully designed and engineered engineered systems can achieve much better perormance than is suggested suggested here. The method is sometimes sometimes employed using very large tanks in the feld feld where long test times can give reasonable accuracy or in-situ calibration or verifcation o a meter.
4.2
Measurement methods
Two principles are used to measure the quantity o liquid in the container, mass or volume.
4.2.1 Gravimetric calibrators A ow meter can be calibrated gravimetrically by weighing weighing the quantity o liquid liquid collected in a vessel. The vessel is weighed empty, empty, then ull and the dierence calculated. This gives the weight o the uid collected. Since the quantity has to be mass (and probably converted to volume) the weight collected needs to be corrected or the eect o air buoyancy. buoyancy. A weighing machine machine is calibrated using weights weights with a conventional density density o 8,000 kg/m3. The uid collected will have a signifcantly dierent dierent density rom the weights and hence will be subject to signifcantly dierent dierent up-thrust rom the air. air. This dierence is signifcant and amounts to around 0.1 per cent or water. The ormula to calculate the mass is given below.
15
Good Practice Guide
4.2.1 Gravimetric calibrators cont. Where M W
is the mass (kg), is the measured weight (kg)
ρair is density o air (kg/m3), ρ f
is the density o o the uid uid (kg/m3),
ρw
is the density o the calibration weights (8,000 kg/m3)
The term in the large brackets is called the ‘buoyancy correction actor’ which, In some applications, applications, can be pre-determined and applied as a constant. To determine the volume, the mass collected is divided by density at the ow meter. meter. This allows the calculation calculati on o the volume rom the mass measured measured by the standard assuming assuming conservation conservation o mass through the the system. Density can be measured using using a densitometer on-line or sampled sampled at a dierent location. The density at the meter is then calculated rom the density measurement, the expansion actor o the uid and the temperature and pressure at the meter. The weighing machines must must be calibrated using recognised standards standards o mass. Normal platorm machines ftted with steelyards provide provide measurements o weight to high accuracy providing providing they are careully maintained. Electronic ‘orce balance’ or load cell machines provide provide a better perormance and the added addition addition o electronic output. output. Simple strain gauge load cell weighing techniques may be used, but generally will not provide the uncertainty capability much better than 0.1%. Modern shear orce and compensated compensated strain gauge cells are however however now available to rival or even exceed the capability o the orce balance cell. Gyroscopi Gyroscopicc weighing gives the ultimate resolution but probably probably exceeds the requirements o ow measurement. Combined with the other uncertainties (density etc.) uncertainties o 0.05% or better in some acilities, can be achieved or volume passed through the test device.
4.2.2 Volumetric calibrators The measurement o the quantity o liquid collected may be carried out volumetrically by collecting a known volume o liquid in a container. container. In the volumetric method the standard vessel takes the orm o a container with a calibrated volume. Normally this will be a vessel with conical conical ends to acilitate drainage and to reduce the risk o air entrapment. The neck o the vessel is normally ftted with a sight glass and and a scale marked in volumetric units. A typical volumetric tank is shown below. Various shapes o vessel are used. Inclined cylindrical vessels with the necks at opposite ends are one design, as are simple ‘cans’ with no bottom bottom drain and the level being established established at the top ‘brim’. This latter type is used or the calibration o uel dispensers. The tank volume must be determined by by calibration o the vessel. This is can be carried out by weighing the the water contained in the vessel, or, or larger vessels, carried out using smaller volumetric measures which are themselves traceable to national standards by weighing methods. methods. Calibration is usually by flling the vessel with a measured weight o water, water, or by emptying the vessel into a weighing tank.
16
The calibration o ow meters
4.2.2 Volumetric calibrators cont. Volumetric Vol umetric systems are normally used with standing start and fnish methods due to the difculty o diverting ow into the tank and controlling the the fnish o the fll. The technique gives a very high level o repeatability. repeatability. Drainage time (ater the tank is empty) empty) is vitally important. Liquid clingage to the wall can account account or a signifcant part o the volume and takes appreciable time to drain down. It is normal practice thereore to calibrate the tank (including drainage pipework) and establish a consistent consistent drainage time time or the calibration. Each tank has this drain time defned and marked on the calibration plate and certifcate. For this reason higher higher viscosity liquids (above 10 cSt) start to give problems o both accuracy and repeatability due to the unpredictable unpredictab le quantity o liquid let attached to the walls o the tank. For all volumetric methods, a number o corrections and conventions have to be observed due to the expansion and contraction o both the standard, and the device being calibrated. The expansion and contraction o the uid between the standard and the ow meter has to be recognised. Expansion due due to temperature is the most important, but expansion expansion in a pressurised system must also be accounted or. Reerence volume tanks, and pipe provers, have their volume defned at a stated reerence temperature (and pressure). Normal reerence temperatures are 15 °C or 20 °C. Other reerences can be defned or special purposes to minimise the size o corrections. Similarly reerence pressure is normally atmospheric pressure (1.01325 bar(a)). The volume contained in the standard during use is the base volume corrected or the t he expansion, or contraction o the vessel i the temperature is dierent rom the base temperature. As the container makes up a volume, it is the cubical expansion expansion o the material used (three times the linear expansion expansion o the material is assumed). The equation is
Where V S is the volume contained, αS is the linear expansion o the material o construction o the standard (prover or tank), t s is the temperature or the standard and t R is the defned reerence (base) temperature. To defne the volume o uid which has passed through the ow meter into the standard, the expansion o the uid due to the temperature dierence has to be calculated.
Where V = volume passed through the meter, α is the cubical expansion o the uid, t S is the temperature o the standard and t M is the temperature o the meter.
17
Good Practice Guide
4.2.2 Volumetric calibrators cont. Similar corrections have to be applied due to pressure (compressibility (compressibility o the uid) i it changes rom meter to the vessel. It is sometimes ound more practical to reduce all volumes to base conditions rather than correcting to actual conditions and then calculating the error or K-act K-actor or.. Both approaches should give the same answer. answer. In the oil industry these corrections are calculated individually in a ormulaic manner and are given ‘Correction actor’ nomenclature: Ctsp = temperature expansion correction or the steel o the prover Cpsp = pressure expansion correction or the steel o the prover Ctlp = temperature expansion correction o o the liquid or the prover to standard conditions. Cplp = pressure expansion correction o the liquid or the prover to standard conditions. Corrections may be calculated by reerring everything to a reerence condition rather than the dierence in conditions. Tables and algorithms are available to provide these corrections or hydrocarbon liquids. The correction o the ow meter to a reerence condition condition is contentious. The difculty is defning the expansion expansion coefcientss (temperature and pressure) or a ow meter. coefcient meter. Flow meters are complex devices and as such a simple volume correction actor will not be accurate. For this reason it is not normally advised advised to apply corrections to the ow ow meter during calibration, but quote the result at actual conditions. Some industry practice does however demand these corrections are made in which case the assumptions must be stated on the certifcate. Note:
Dierent conventions conventions are are used as the base or reerence conditions conditions o o temperature and and pressure, pressure, e.g. or
temperature, 20 °C, 15 °C, 60 °F are all commonly used. The reerence conditions used must be stated on any report or certifcate.
4.3
Pipe provers
Pipe provers provide provide probably the best calibration calibration devices or truly dynamic calibration. calibration. They are used in a sealed system and provide high accuracy. accuracy. Provers, can be used in-situ as travelling standards, be part o a metering system or be used as the reerence in calibration laboratories. The pipe prover principle principle is illustrated diagrammatically. diagrammatically. A length o pipe is ftted with switches and the volume between the switches is known. I a displacer is introduced to the ow, ow, the time it takes to travel between the switches switches will give a measure measure o the owrate. owrate. I the switches are used to gate a pulse counter, totalising pulses rom a ow meter, meter, a measure o the meter actor (pulse per litre) can be ound.
Proving: The term ‘proving’ is used extensively in the oil industry or a calibration which has the additional operation o demonstrating (or proving) the accuracy and ftness or purpose o a device, normally to comply with standard acceptance criteria.
18
The calibration o ow meters
4.3
Pipe provers cont.
The technique illustrates the ingenuity brought to bear on a calibration problem. The frst prover was a mile long pipe pipe linking two oil refneries in dispute over the accuracy accuracy o the transer ow meters. With no ability to independently independen tly calibrate the meters, the length and diameter o the pipe were estimated and the time or a cleaning ‘pig’ to travel the distance provided an adequate measurement measurement o volume to veriy the transer meters. This concept has been refned to allow the design and manuacture o standardised measuring devices called ‘pipe provers’. provers’. These devices are used extensively to measure all types o high value uid rom LPG to high viscosity crude oil. They are produced in all sizes rom 50 mm to 1250 mm (2-48 inches). With a pipe prover, the uid remains contained and sealed in the system. The calibration uid can be a clean reerence reerence uid or the actual product. A calibration can take place without interrupting the process and i the product is used the conditions o use can be maintained. maintained. Pipe provers are oten installed as an integral part o high value custody transer and fscal metering stations where they are dedicated dedicated to a particular set o ow meters and duty. duty. Pipe provers can also be mobile mobile and taken to dierent metering stations. Four main classifcations o liquid pipe provers are recognised in documentary standards.
4.3.1 Uni-directional sphere prover As the name implies, a unidirectional prover has a displacer which only travels in one direction along along the pipe. The displacer consists consists o an elastomer (neoprene, ( neoprene, viton, polyurethane, polyurethane, etc) sphere which is hollow. The centre is flled with liquid and pressurised to inate the sphere until until it is larger than the pipe bore. bore. A typical ination is around 2 per cent larger than the pipe bore. bore. When the sphere is inserted into the pipe it takes up an elliptical shape and makes a good seal to the pipe wall. The pipe itsel is a long length o steel pipe with a smooth smooth bore. The internal surace is usually coated with Phenolic Phenolic or epoxy resin to provide provide a smooth low riction lining lining and to protect against corrosion. corrosion. As the pipe can be extremely long, it is usually constructed in a series o loops. loops. The radius o the bends is chosen chosen to allow the sphere to pass without without either sticking or leakage. At each end o the calibrated length o pipe a detector switch is located through through the pipe wall. wall. This usually takes the orm o a plunger triggering a switch when the sphere passes under it.
19
Good Practice Guide
4.3.1 Uni-directional sphere prover cont. Located at the end o the prover is the sphere handling valve arrangement, designed to hold the sphere. sphere. At the start o a test the sphere is launched into the ow and carried round the loop captured and returned to the launch position ready or another run. The design o the valve is critical, and must not only only be leak-tight but must have mechanisms to prove it is leak-tight. The valve also normally allows a means to remove the sphere or inspection.
4.3.2 Bi-directional sphere prover Because o the complexities o sphere handling and to reduce the turn round time o the sphere, the bi-directional bi-directional prover was developed. developed. Similar in layout to the unidirectional type, the main dierence is that ow can circulate around the loop in both directions. A our-way our-way valve o high integrity changes the ow path without breaking the ow. The sphere is held in special end chambers which are designed to launch the the sphere and absorb the shock shock o capture. One chamber also provides a means o o removing the sphere. sphere. Note that two switches are provided at each end. This provides better integrity o the measuremen measurementt by giving redundancy and allowing our separate volumes to be defned.
4.3.3 Piston provers For difcult uids which may damage a lining material, or leak past the conventional sphere displacer, displacer, a piston displacer may be used. Since a piston is unable to pass round round bends a piston prover prover is straight and hence these devices devices tend to be quite long. The pipe is normally normally a smooth honed honed bore pipe pipe o stainless or or plated carbon steel. The displacer is a piston piston with multiple seals. Switches can be conventional conventional plungers or high integrity integrity,, non-contacting types. types. By their nature, they must be bi-directional and the our-way changeover valve is normally located midway along the pipe length to equalise the inlet and outlet pipework. This type t ype o prover is not so common however fnds a particular application with LPG, Liquifed Natural Gas (LNG) and other difcult high value products.
4.3.4 Small volume provers A ‘conventional’ prover is defned in this guide as one which counts 10,000, or more, pulses rom the meter being calibrated calibra ted during one pass o the displacer. A ‘small volume’ prover is defned as one where less than 10,000 pulses are counted. As this defnition is strongly strongly dependant dependant on the characteristics o the meter being calibrated, the term ‘small volume prover’ can be applied to two concept designs. Note, 10,000 pulses are chosen in the oil industry to ensure the uncertainty introduced by pulse resolution is less than 0.01 % and is hence insignifcant. Concept 1: This is any prover ollowing ollowing the general design characteristics specifed in standards and generally designed to allow the collection o 10,000 pulses rom a multi-bladed turbine ow meter. meter. In other respects it will be o the t he same design as a ‘conventional’ prover but ‘pulse interpolation’ will be employed to improve meter resolution when required or a particular application. 20
The calibration o ow meters
4.3.4 Small volume provers cont. The prover will be designed to the normal standards but the use with low resolution meters, e.g. twin bladed turbines, will require the use o pulse interpolation. Concept 2: A custom designed or commercially available pipe prover with a volume about one-tenth o a conventional design or the same duty. duty. They are normally piston provers with trade names such as ‘compact prover’ and one company has adopted the generic term ‘small volume prover’. The design below shows one commercially available small volume prover which best illustrates the design criteria required. Other commercial designs are available with specifc specifc design eatures. Chain or mechanically constrained pistons, pistons, ree pistons and even ball type displacers displacers are available. The internal valve described is proprietary and external external by-pass valves are used in other designs. Other orms o high resolution switches or continuous continuous linear position indicators are also part o dierent designs. For this illustrative design, a piston has been chosen as the displacer operating in honed bore cylinder to minimise leakage and pressure loss. To allow a short length, and retain accuracy accura cy,, optical detectors detector s are mounted external to the pipe which can resolve to ractions o a millimetre. A rod fxed to the piston extends extends out o the end o the cylinder and carries ‘ags’ which pass through optical optical detectors. These detectors give very precise control and start and stop signals across the measured volume. For this device the ow is maintained at a constant rate through the use o an internal valve located in the piston and operated through hydraulic control. control. The piston has an integral ‘poppet’ ‘poppet’ valve which allows the ow to pass through through it when held open. open. An external rod allows the piston to be pulled, using hydraulic pressure to the upstream end o the cylinder cylinder while holding the valve open. open. Releasing the hydraulic pressure allows the valve to be shut by a combination o a spring, gas pressure on the end o the external rod and the orce o the ow. ow. The ow then carries the piston down the pipe. When the piston reaches the downstream end, the hydraulic pressure is restored, the valve opens and the piston is returned to the start position. A small volume will not correspon correspond d to a large enough number o pulses rom the ow meter to give adequate resolution, thereore a technique called pulse interpolation is used to increase the resolution o the pulse counting.
4.3.5 Pulse interpolation This technique eectively increases the resolution resolution o a pulsed output by estimating the raction o a pulse missed at the beginning and the end o a test. This can be achieved electronically using requency requency multipliers working rom the input signal, or by pulse timing using one o three available techniques. By ar the most common timing method is double chronometry.
21
Good Practice Guide
4.3.5 Pulse interpolation cont. To estimate the raction o a pulse lost or gained at the start and fnish o a pass, the whole number o pulses are frst counted. This total is multiplied by the interpolation interpolation actor. actor. This is the ratio o the time between the switches, and the time between the frst pulse ater the start switch and the frst pulse ater the stop switch.
The ‘interpolated’ pulse count is thereore no longer an integer number and is expressed as a decimal number o pulses.
The technique works well when when the pulses have a constant requency requency or period. I the period o the individual pulses pulses varies by more than 5-10% lack lack o repeatability is ound. Guidance is given in ISO 7278 7278 Part 3. As a technique t echnique this is provided as a standard option in many prover calculators and ow computers, particularly those designed to accept small volume provers. This allows low resolution meters to be calibrated against large provers in addition to controlling small small volume provers. The technique can be applied elsewhere or gas provers and or other dynamic calibration methods.
4.3.6 Operation and calibration o a prover When using pipe provers the process conditions need to remain stable throughout the calibration and even more care has to be taken to ensure stability when calibrating the prover itsel. To use a prover, the ow is directed through the prover and the meter with no leakage paths between the two devices. The prover may be located up or downstream o the meter. When the system has been flled, all gas removed and the ow established, conditions conditions are allowed to stabilise or owrate and temperature. The displacer is launched into the ow and when the frst detector detector is actuated, a counter and and timer are started. When the second second detector is actuated the the timer and counter are stopped. stopped. From the known known volume between between the detectors, the pulses counted counted and the time, volumetric volumetric ow rate and K-actor are derived. This process is repeated repeated a number o times and at the required number number o owrates. Earlier oil industry standards stated that a successul ‘prove’ was achieved i a fxed number (3 or 5) successive results were within a specifed range (0.04 % or 0.05 % being common). The current standard rom the American Petroleum Industry (API) specifes that tests should be repeated an indeterminate number o times until the standard deviation o the results alls with a specifed criteria based on on standard deviation times times the ‘students t’ statistic. Both methods rely rely on an experienced operator knowing that there is a problem with the calibration and not continuing until ‘chance’ provides a acceptable result.
22
The calibration o ow meters
4.3.6 Operation and calibration o a prover cont. The volume o the prover between the switches is determined by calibration. This is called the ‘base volume’ and is quoted as being the volume at a specifed reerence temperature and pressure (15 °C or 60 °F and 1.01325 bar are the common conventions). The volume is determined by displacing displacing water into a volume (or mass) standard. standard. This may be be achieved by by flling the reerence vessel repeatedly as the displacer passes through through the calibrated volume. A solenoid valve connected to the prover switches ensures ensures the ow is stopped at the correct correct point. For larger provers this is impractical and a reerence reerence meter is used to measure the volume. volume. The reerence meter is itsel calibrated, as part o the same operation, operation, against a volume or mass standard which may be a volume measure, small pipe prover or small volume prover. prover. This is carried out immediately prior and subsequent to use. A prover is a volumetric calibrator and the calculations have to take into account the corrections or temperature and pressure o the prover and liquid. Codes o Practice governing the design, calibration and use o pipe provers, including the small volume versions are available rom ISO, The Energy Institute (EI) and the API.
5
Calibration Methods or Gas Flow Meters
The choice o calibration method or any particular ow meter is governed by the meter type, the ranges o ow and ow conditions, pressure, pressure, temperature and the accuracy o calibration required. required. In general all the methods have analogies with the liquid methods. methods. The main dierence dierence between the calibration o a gas ow meter and a liquid device device is the compressibility compressib ility o the uid. When calibrating a gas ow meter the temperature, pressure and hence volume o gas measured by the standard will be dierent rom that at the device under test; corrections to common conditions must thereore be made. It is oten best to convert to mass ow at the calibrator then back to volume volume using the conditions conditions at the device under test or, i this is the requirement, at ‘standard’ conditions. conditions. The use o ‘standard’ conditions’ is common when considering specifcations specifcations or gas meter calibrations. Note these conditions may be defned as ‘standard’, ‘normal’, or ‘reerence’ conditions and have to be defned. It is vital that the reerence conditions conditions are defned and never assumed.
5.1
Displacement methods
A number o proprietary standard devices are used or gas calibration based on the principles o the pipe prover. prover. The biggest drawback o any prover system or gas is the riction generated by the displacer seal. This riction will require the gas to compress until the pressure dierence dierence overcomes the riction. Variatio Variations ns in the riction can prevent the displacer moving smoothly, causing sticky or juddering movement hence giving poor results. Some piston pipe provers have been produced produced or gas service and are in operation in standards institutes. These are generally specialised devices and used or higher pressures where the gas density is high or the piston is driven (or assisted in some manner). One commercially available available device is available or small meters operating at low pressures pressures and works through a piston driven externally rather than by the ow ow..
23
Good Practice Guide
5.1.1 Mercury seal prover For low ows, mercury seal provers use a very light displacer in a vertical glass tube. The piston travels upwards upwards in the tube and the seal is a mercury ring ormed in a recess recess in the piston. piston. The use o a vertical piston piston and mercury seal reduces reduces riction to a minimum. The weight o the piston has to be counterbalanced by an exter nal weight (not shown on the schematic drawing).
5.1.2 Soap lm burrette Soap flm burettes are again a orm o pipe prover used or both calibration and measurement. In this case a glass tube is vertically mounted with with a reservoir o soap dissolved in water below the the gas inlet. As the gas enters the burette a soap flm is generated and travels up the tube at the same velocity as the gas. By measuring the time o traverse o the soap flm between graduations at either end o this accurately calibrated burette, the rate o ow o the gas may be obtained. What is created is a pipe prover with the displacer ormed by the soap flm. This method is usually used to measure gas ows within the range 10 -7 to 10-4 m3 /s at close to ambient conditions. conditions. Under very careully controlled controlled conditions conditions reerence ows can be determined to within ±0.25 per cent using soap flm burettes.
5.1.3 Bell provers The ‘Bell prover’ is the standard or calibrating low ow gas meters such as domestic meters. A cylinder (or bell), open open at the bottom and and closed at the top, is lowered lowered into a liquid bath. bath. The weight o the cylinder cylinder is supported supported by a wire, string or chain and counter counter balanced by weights. A smaller counterbalance on a shaped cam arrangement (not shown) is added to compensate or the changing changing buoyancy as the cylinder cylinder is submerged. All pulleys are ftted with low riction bearings. bearings. By altering the counterbalance counterbalance weight a pressure can be generated in the cylinder. cylinder. A pipe passing through the liquid is open to the trapped volume and, and, as the cylinder is lowered, gas is displaced rom rom the cylinder to the the meter on test. By timing the all o o the cylinder and knowing the volume/length relationship or the cylinder, the volume ow o gas through the meter may be determined and compared with the meter reading. 24
The calibration o ow meters
5.1.3 Bell provers cont. By closing a valve leading to the meter and opening a valve ( not shown) rom a gas supply, supply, the cylinder can be returned to the start position. Originally the liquid used used would be water. water. As the water evaporates, the humidity changes, changes, and hence the density o the air and this has to be corrected corrected or. or. The water has to be topped topped up when when the bell is in use. use. Most bell provers are now flled low vapour pressure/low viscosity oil. In order to minimise expansion or contraction o the gas the liquid, gas and external air temperatures should should not dier by more than 1 °C. Errors can also arise due to incorrect incorrect compensation or change change in buoyancy o the bell as it is immersed and the gas is not ully saturated. At present, or ows ows up to some 10 10-2 m3 /s, bell provers can can be used to measure measure ows to within ±0.2 per per cent i strict precautions are taken to minimise the errors. errors. Bell provers rom 500 mm to 3 m diameter are available. I required, the bell prover can be used in reverse where gas rom the meter under test is used to raise the bell during a calibration with gas venting to return to the start position.
5.2
Gravimetric and volumetric approaches
For gravimetric systems the vessel is weighed beore and ater flling. Generally Generally,, the vessel has to be disconnected to prevent the connection pipes interering with the weighing and gas vented during disconnection has to be accounted or. For small systems dynamic weighing has been employed both as a collection system and also by a delivery system. The vessel is pressurised and then discharged through the device under test and the eect o flling hoses accounted or in the weighing. For volumetric systems the vessel must must be very stable and o known volume. volume. The pressure pressure and temperature in the vessel is measured beore beore and ater the fll which allows the mass o gas to be calculated. calculated. Filling and emptying the the vessel will create signifcant temperature eects due to the changes in pressur pressure. e. In the t he most accurate systems the vessel is submerged in a water bath to maintain the wall temperature as stable as possible hence minimising temperature corrections to the vessel. For both methods, as gas enters the vessel the pressure rises the owrate will reduce as equalisation o pressure takes place between the test line and the tank. To get round this, these systems are usually usually used in conjunction with a sonic or or critical ow device to ‘de-couple’ the pressure. As will be explained in Section 5.4, the mass ow through through a critical nozzle is dependent on the upstream pressure and independent on the downstream (vessel) pressure. pressure. This allows the mass ow through an upstream device device to remain constant while the tank flls, as long as the sonic device is ‘choked’. ‘choked’. The calibration o devices such as mass meters upstream o the choke point point can be calibrated at a constant mass ow. ow. Alternatively i an accurate sonic nozzle is used, the nozzle can be used as a transer device to calibrate a lower pressure meter downstream with the nozzle itsel being calibrated against the vessel. Any restrictive device (e.g. a valve) may be used to de-couple the system however using a nozzle allows superior control and the simultaneous calibration o the nozzle as a secondary device.
25
Good Practice Guide
5.2
Gravimetric and volumetric approaches cont. Due to gas compress compressibility ibility eects, it is more difcult to maintain a closely controlled owrate owrate than it is or liquid ow systems. As pressure pressure changes, potentially large changes in temperature can occur making it difcult to obtain stable conditions conditions..
5.3
Gas displacement method
For low pressure pressure systems, gas can be displaced rom rom a closed vessel by flling a transer vessel with liquid. liquid. This can be likened to flling the bath o a bell prover rather than allowing allowing the bell to move. The liquid transerred can be weighed weighed or measured in some other way, way, and the volume calculated. The volume o liquid can be equated to the volume o gas displaced, with suitable pressure pressure and temperature corrections. corrections. In one design the liquid is allowed to ow rom a weighing vessel which is located on a rising platorm which maintains a constant pressure on the system as the liquid runs out.
5.4
Critical fow Venturi-nozzle (sonic nozzle)
Sonic nozzles are eectively reerence ow meters which can be used to calibrate other devices. As explained previously previously they can also be a more undamental undamental part o a primary standard. They are mentioned specifcally specifcally due to their extensive use as the calibration reerence standard in many applications applications and laboratories. Although not a primary method o calibration, sonic nozzles can orm part o a system when combined with primary methods. Sonic nozzles provide provide the reerence system or many calibration acilities where their stability requires inrequent calibration o the nozzle or simply reliance on the perormance outlined in documentary standards. I the pressure drop between the inlet and the throat o a nozzle or restriction is increased, the owrate rises until sonic velocity is reached at the throat. At this point the nozzle is ‘choked’ and rom rom this point, the mass owrate owrate through the nozzle will be constant or any given upstream pressure. The expression or the mass owrate o the gas is:
Where Cd is discharge coefcient, C* is critical ow actor, Ao is area o the nozzle throat. P o and T o are the absolute values o upstream pressure pressure and temperature.
26
The calibration o ow meters
5.4
Critical fow Venturi-nozzle (sonic nozzle) cont. At this condition the mass owrate is dependent on the geometry o the nozzle, the properties o the gas, and the upstream upstream pressure pressure and temperature. This makes the device particularly suitable or calibrating meters which can introduce pressure pulsations into the ow. ow. A standard toroidal throat sonic Venturi as specifed in the ISO standard is shown. shown. Other designs based on conical conical entries, or parallel throat orifce plates can be used but these provide a larger pressure loss and hence a narrower operating range.
One disadvantage o the critical ow Venturi-nozzle Venturi-nozzle is the large pressure loss which is normally much greater than that or subsonic nozzles nozzles or other ow metering devices. Moreover Moreover,, an accurate knowledge o the thermodynamic properties o the gas is required. This may cause difculties in gases such as natural gas where the composition may be complex and variable. The device is however particularly suitable suitable or calibrating ow meters in high high pressure gas ows ows at owrates where the throat Reynolds number exceeds 105 and uncertainties o 0.2 per cent may required. required. The large pressure pressure drop can in some situations lead to extended extended test times to establish stable temperature conditions conditions at the test meter. meter. Nozzles are requently installed as parallel arrays, with the nozzles sized to provide a set range o mass owrates by using dierent dierent combinationss o nozzles in parallel. Sonic nozzles in this way are oten used to calibrate low pressure meters where the combination meter is working at ambient pressure upstream and a vacuum is created downstream o the nozzle to allow one unique mass ow or each nozzle. By establishing a ‘choked’ ow where the gas velocity is at the speed o sound at the nozzle throat, variations o pressure pressure downstream cannot be reected to conditions upstream and hence the mass ow becomes independent o downstream variations. The nozzle thereore eectively de-couples the reerence mass ow rom variations in downstr downstream eam pressure when a calibration is being carried out. This condition also allows a downstream vessel to be flled and the mass ow maintained when otherwise the ow would reduce as the pressure equalise.
6
Other Calibration and Verication Methods
6.1
Reerence meters
Any reliable, stable and predictable ow meter type can be used, either in a laboratory or in the feld, as a calibration reerence. Reerence meters are used when when a ‘primary’ method is restricted due due to lack o resolution, capacity or inadequate response response time. A single meter may be be used as the reerence reerence in series with the meter to be calibrated. Equally Equally,, multiple meters in parallel can be assembled to achieve a owrate range in excess o what may be economically managed through a primary system. system. In this way calibration acilities have been designed designed to double their owrate owrate capacity by using two meters in parallel. Maniolds o six, eight or more reerence meters in parallel can be assembled to test and calibrate very large ow meters. The use o reerence meters will add uncertainty uncertainty to the quantity measured when when compared with with that o a primary system. However there is no signifcant signifcant reduction in uncertainty uncertainty using multiple multiple meters as against a single meter or a primary system. Improvementt in the uncertainty o the calibration o a test meter may be lower when using a reerence meter as compared Improvemen with a primary system calibration o the same device; through reduction o uncertainties due to longer test times, reduced resolution uncertainty and removal o response time issues. 27
Good Practice Guide
6.1
Reerence meters cont.
The correct selection and installation installation o a reerence meter is vital. Predictable behaviour behaviour with changes o uid properties properties must be assured and the installation eects on both the reerence meter and the device under test t est should be understood. The installation must ensure that the reerence meter does not interere with the test device by generating pulsations, electronic, vibration or acoustic intererence, or ow disturbance.
6.2
On-site verication methods
Meters can be calibrated on-site using any o the methods previously described such as mobile pipe provers, volume tanks and reerence ow meters. meters. Care has to be taken during installation installation to ensure the inuence inuence actors such as weather and temperature etc. does not add uncertainty. uncertainty. Establishing a steady owrate is also recommended. recommended. Three methods, suitable suitable or veriying (rather than calibrating) ow meters on site, site, are described. Generally these methods are used to calibrate meters where standard laboratory methods or portable calibration standards are not suitable or installation in the feld. This may be due to access, the product product in use, the meter size, or an inability to stop the process. Generally they are methods which do not give the best uncertainty, and may be in some cases poorer than the expected uncertainty o the meter. meter. I doubt exists however these may be the methods methods which can be employed employed to veriy meter perormance.
6.2.1 Tracer methods Tracer methods are used in situations where a ow meter or calibration device cannot be inserted in a process ow and the installed meter cannot be removed removed and calibrated elsewhere. elsewhere. This may be due to size, availability o time to withdraw the meter, meter, or the meter is operating in a ‘difcult’ uid. Tracer methods would normally be used to veriy that a meter is operating within an acceptable acceptable tolerance rather than providing providing a traditional calibration. These methods involve involve the injection o a uid which can be detected in the ow stream and using this to measure the owrate within the pipe. Two recognised techniques are available. Transit time (velocity) (vel ocity) methods: For this method pulses o tracer uid are injected into the main ow stream, and the time taken or the tracer to pass between two detection points is noted. noted. I the volume o pipe between the detectors is known the volumetric ow can be determined. At present, tracers used used are usually usually radioactive isotopes, which can be generated on site, with very short short hal lives. This reduces any potential contamination risk to well below saety limits. The tracer is injected into the ow and the ‘pulse’ o tracer is detected by external sensors on the pipe pipe wall. The distance between the detectors is known as the pipe diameter and when combined with the time between pulses the volumetric owrate can be calculated. The main source o error error is the determination o the internal area o the pipe due to inaccuracy in the pipe wall measurement measurement or internal corrosion/deposition. corrosion/deposition. Also the time is measured rom the the ‘peak’ o the pulse passing the detectors. The ow should have a reasonable profle profle and avoid swirl etc which would cause the ‘pulse’ to be ill defned.
28
The calibration o ow meters
6.2.1 Tracer methods cont. It is claimed that, by incorporating recently developed radioactive techniques, an experienced team can determine the owrate under the most avourable conditions to within 0.5 per cent; however 1 per cent is probably the lowest uncertainty which can be assumed achievable. The second technique is the dilution method; where a tracer uid which can be detected in low concentrations, is injected into the ow at a known rate. I this injected owrate is known accurately accurately and a sample o the mainstream ow ow is taken downstream, downstream, the owrate o the main ow can be deduced rom rom the concentration o dilutant. dilutant. A sample o the main ow is taken, as single samples or continuously, continuously, and the concentration o dilutant is measured. The main line owrate can be derived rom: rom:
Conc, is the concentration o the dilutant q
is the owrate o the dilutant.
The dilutant has to be sampled ater mixing and thereore the ow is sampled at a distance downstream o the injection point, to allow homogeneous homogeneo us mixing to take place. The method is used or both gas and liquid ows. In dilution methods the main source o error occurs in obtaining accurate determination o the tracer concentration particularly i on-line determination is being used.
6.2.2 Insertion meters Using an insertion ow meter, the owrate is estimated by measuring the velocity at a single point location in the duct and rom that estimating the volumetric volumetric ow. ow. The device used to measure the point point velocities may be a pitot tube, insertion turbine or an insertion electromagnetic meter and other types are available. To use this method, the installation and the positioning o the sensor has to be carried out accurately. accurately. The cross sectional sectional area o the duct has to be known, and an ideal ow profle present to allow the calculation calculat ion o volume ow rom point velocit velocityy. Inserti Insertion on meters can be used by inserting them into an existing duct or pipe and this can be done without stopping stopping the process. process. This will allow an installed meter to be verifed in-situ. The main disadvantage o these methods is fnding a location to install the meter where there is a good undisturbed ow profle which approximates approximates an ideal profle. The determination o the area o the pipe at the measurement location also has to be known. Calculation standards standards allow the mean velocity and thereore thereore the volumetric owrate owrate to be derived rom the point point velocity measurement. measurement. Corrections or or ‘blockage actor have to be be applied. The technique is also highly dependent on correct installation, depth and alignment and thereore a skilled operator is required.
29
Good Practice Guide
6.2.2 Insertion meters cont. For gas velocities in the range 0.3 to 3.0 m/s uncertainties o 4 per cent are attainable using vane anemometers and or velocities in the range 6-120 m/s uncertainties uncertainties o within 2 per cent can be achieved using using pitot tubes. Measurement in the accuracy range 1 to 5 per cent can be anticipated in liquids. The technique is used extensively extensively to characterise ow calibration test acilities. For this purpose the probe probe is traversed across the pipe and across dierent diameters to ensure the acility has an acceptable ow profle.
6.2.3 Clamp-on ultrasonic meters Clamp-on ultrasonic ultrasonic meters operate by measuring the mean velocity velocity o the ow across the diameter o a pipe. These devices usually use ‘time o ight’ ultrasonic measurement measurement methods. The transducers are clamped on to the outside wall o a pipe, diametrically opposite each other. other. The area o the pipe is known, and thereore thereore rom the velocity and area the owrate computed. computed. The pipe has to be o suitable construction construction and the transducers ‘coupled’ ‘coupled’ to the wall to allow the signal to penetrate. As a mean velocity is measured across a diameter, diameter, the ow profle has to be known (or assumed); uncertainties between 2 and 5 per cent the norm when there is a good installation. A clamp on ultrasonic ow meter is however an efcient way to veriy in-situ meters when there is no method o installing a reerence and as an alternative to breaking into the pipe pipe to install a insertion probe. Liquid meters have been available available or many years and gas clamp on meters have now entered the market.
6.3
Unusual volume methods
Volumetric Vol umetric methods can be used or calibration and verifcation or specifc and difcult situations. situations. I a reerence volume is available and a means to transer the uid to the meter under test established a calibration method is in place. One method in use employs employs a very large diameter and very tall tower. tower. The tower is flled with water and then discharged discharged through the device device under test. The volume o the tower can be determined with with a reasonable degree degree o accuracy and the level detected as is alls during a calibration. Passage o the water surace (level) across the detectors defnes defnes the start and fnish o the calibration test with the detectors used to start and stop readings rom the test meter. meter. Vo Volume lume and the time can then be used to defne the reerence values. The engineering o such a system is quite quite challenging. Level switches have to be ast and accurate enough enough to detect the alling level. Flow profle profle has to be assured assured as the water turns rom the vertical tower to the horizontal horizontal test section. It is also extremely challenging challenging to maintain a constant owrate owrate as the head reduces as the level drops. drops. The example device known uses a tower some 40 m high and 4 m diameter and is used to calibrate large water meters. Based on this example device other designs and concepts can be considered, however a very careul uncertainty budget has to be created to account or all sources on uncertainty.
30
The calibration o ow meters
7
Expectations or a Calibration
The calibration and o a meter applies to that meter only, only, operating under the conditions with which it was calibrated. I in service these conditions are changed the calibration may not apply. apply. What then are the real orders o uncertainty which might be reasonably obtained rom calibrated meters? First, the meter cannot be calibrated to an uncertainty level better than its repeatability and the uncertainty o the standard. Systematic uncertainties can only be estimated rom knowledge o the calibration system and its method o traceability and transer through to the fnal duty with the addition o inuence actors and historical perormance being added. Liquid ow meter calibration acilities should be able to measure owrates to uncertainty levels between 0.05 to 0.5 per cent depending upon upon the complexity o the system and its design. design. It is noted that volumetric and gravimetric systems systems have similar uncertainty however volumetric systems appear to be able to deliver lower repeatability fgures. Calibration systems or gas ow meters should be able to measure owrate to uncertainty levels o between 0.2 and 0.5 per cent.
31
Good Practice Guide
8
Bibliography
•
Interna Int ernatio tional nalvo vocab cabula ulary ryof ofmet metro rolo logy gy—B —Basi asica cand ndge gener neral alcon concep cepts tsand andas assoc sociat iated edterm terms( s(VIM VIM) )BIP BIPM2 M2008 008 {www.BIPM.org)
•
Interna Int ernatio tional nalV Voca ocabul bulary aryof ofBa Basic sican andG dGene eneral ralT Term ermsi sinM nMetr etrolo ology gy(VI (VIM). M).BI BIPM PM19 1995 95(B (BSP SPD6 D646 461, 1,19 1995 95) )
•
Guide Gui deto tothe theEx Expr press ession ionof ofUn Uncer certai tainty ntyin inMe Measu asurem rement ent19 1995 95(G (GUM) UM).B .BIPM IPM/IS /ISO/O O/OIML IML/IE /IEC/I C/IFCC FCC/IU /IUP PAC/ AC/IUP IUPAP AP. {www.BIPM.org}
•
ISO IS O51 5168 68:1 :199 998. 8.M Mea easu sure reme ment nto of fFl Flui uid dFl Flow ow--E -Eva valu luat atio ion nof ofU Unc ncer erta tain inti ties es..
•
ISO41 ISO 4185 85:19 :1980. 80.Me Measu asurem rement entof ofLi Liqui quidF dFlow lowin inCl Clos osed edCon Condu duits its– –We Weigh ighing ingMe Metho thod. d.
•
ISO93 ISO 9368 68-1: -1:199 1990. 0.Mea Measur sureme ement ntof ofLiq Liquid uidFl Flow owIn InClo Closed sedCo Condu nduits itsBy ByTh TheW eWeig eighin hingM gMeth ethod od-- -- Proceduress For Checking Installations -- Part 1: Static Weighing Systems. Procedure
•
ISO83 ISO 8316 16:19 :1987. 87.Me Measu asurem rement entof ofLi Liqui quidF dFlow lowIn InCl Clos osed edCon Condu duits its– –Met Method hodBy ByCo Colle llecti ction onof of The Liquid In A Volumetric Tank.
•
ISO12 ISO 1291 916. 6.Liq Liquid uidHy Hydr droca ocarbo rbons ns-D -Dyn ynami amicM cMeas easur ureme ement nt-V -Volu olumet metric ricPr Provi oving ngT Tank anksO sOrM rMeas easur ures. es.
•
ISO82 ISO 8222 22.P .Petr etrole oleum umMe Measu asurem rement entSy Syste stems ms-Ca -Calib librat ration ion– –T Temp empera eratur tureC eCorr orrect ection ionsF sFor orUse Use When Calibrating Volumetric Proving Tanks.
•
ISO72 ISO 7278 78-2. -2.Li Liqu quid idHyd Hydro rocar carbo bons ns-D -Dyna ynamic micMe Measu asurem rement ent- -Pro Provin vingS gSyst ystems emsFo ForV rVolu olumet metric ricMe Meters ters- - Methods For Design, Installation and Calibration o Pipe Provers.
•
ISO72 ISO 7278 78-3. -3.Li Liqu quid idHyd Hydro rocar carbo bons ns-D -Dyna ynamic micMe Measu asurem rement ent- -Pro Provin vingS gSyst ystems emsfo forV rVol olume umetri tricM cMete eters rs- - Pulse Interpolation Techniques.
•
ISO72 ISO 7278 78-4. -4.Li Liqu quid idHyd Hydro rocar carbo bons ns-D -Dyna ynamic micMe Measu asurem rement ent– –Pro Provin vingS gSyst ystems emsFo ForV rVolu olumet metric ricMe Meter terssGuide For Operators o Pipe Provers.
•
Americ Ame rican anPet Petro roleu leumI mInst nstitu itute te(AP (API) I)-M -Manu anual alof ofPet Petro roleu leumS mStan tandar dards dsCh Chapt apter er4- 4-Pr Provi oving ngsy syste stems. ms. (This is a set o relevant documents covering many aspects o calibration in the petroleum industry including pipe provers, proving tanks and reerence meter methods.)
•
Energy Ene rgyIn Insti stitut tute, e,Lon Londo don; n;Pe Petro troleu leumM mMeas easur ureme ement ntMan Manual ual.P .Part artX: X:Met Meter erPro Provin ving. g. (This is a set o relevant documents covering many aspects o calibration in the petroleum industry including pipe provers, proving tanks and reerence meter methods. The documents have recently been given a new reerence numbering system under the Petroleum management title.)
•
ISO93 ISO 9300 00.M .Meth ethod odof ofMe Measu asurem rement entof ofGa GasF sFlow lowBy ByMe Means ansof ofCr Criti itical calV Vent enturi uriNo Nozzl zzles. es.
•
ISO IS O91 91-1 -1. .an and dOI OIML MLR R93 93: :S Sch ched edul ule efo forP rPet etro role leum umM Mea easu sure reme ment ntT Tab able les. s. (ISO 91-1, and OIML R63 are currently based on the original paper tables ASTM D1250 1980 and will be brought in line with ASTM D1250 in due course.)
•
ASTM AS TMD D1 125 250 0P Pet etro role leum umm mea easu sure reme ment ntt tab able les; s;2 200 004. 4. (These are also available as API petroleum measurement measurement manual; Chapter Chapter 11. The 2004 version is based on the 1980 tables however the standards is now a computer implementation rather than the older paper tables.)
32
The calibration o ow meters
Addendum Client calibration check list 1.
Type o meter Turbine/DP/Coriolis/Ultrasonic/etc
2.
Make/Model
3.
Size o meter (Length, diameter,) (weight) (other sizes)
4.
Type Fluid to be Calibrated Water/Oil/Gas (air?) / Multiphase (get details ie what is the viscosity o the oil etc)
5.
Flowrate/Flowrange (remember to note Units!)
6.
Operating Pressure (especially or gas)
7.
Operating Temperature
8.
Signal Output: Pulsed/mAmps Pulsed = max requency Resolution: is it a scale, pulses/unit What’ss the electrical characteristics What’ c haracteristics (volts etc)
9.
K-actor (Check pulses required or calibration)
10. Is pipework included. Are all electronics included 11. What uncertainty is required 12. What Flanges: screw etc Are the anges raised or RTJ Some standard fttings are ANSI 150 PN 10 BSP ANSI 600 PN 16 BSP(T) ANSI 300 NPT 13. Measured Points required e.g. (3 @ 5 owrates) (1 at 10 ows) etc. 14. Timescale required 15. Have you had the meter calibrated beore beore Contact Name: Company: Address:
Tel: E mail:
For urther inormation, contact: TUV NEL, East Kilbride, GLASGOW, G75 0QF, UK Tel: + 44 (0) 1355 220222
Email: in
[email protected]
www.tuvnel.com
