ISO 9200:2024
(Main)Petroleum measurement systems — Metering of viscous and high temperature liquids
Petroleum measurement systems — Metering of viscous and high temperature liquids
This document gives guidance for measuring a quantity of primarily viscous hydrocarbon liquid using flowmeters at ambient or elevated operating temperatures. This document describes the effects of high viscosities and potentially high temperatures, which can induce additional errors in measurement. It also gives guidance on how to overcome or mitigate difficulties.
Systèmes de mesurage des produits pétroliers — Comptage des liquides visqueux et à haute température
General Information
Relations
Standards Content (Sample)
International
Standard
ISO 9200
Second edition
Petroleum measurement systems —
2024-05
Metering of viscous and high
temperature liquids
Systèmes de mesurage des produits pétroliers — Comptage des
liquides visqueux et à haute température
Reference number
© ISO 2024
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Properties of high viscosity liquids . 2
4.1 Viscosity .2
4.2 Reynolds number and flow profile .3
4.3 Density .4
4.4 Effect of temperature on viscosity .4
4.5 Examples of high viscosity liquids and behaviour .5
4.6 Further considerations .6
5 Metering systems . 6
5.1 General .6
5.2 Installation .7
5.3 Heating.8
5.4 System start-up and filling .9
6 Flowmeters .10
6.1 Differential pressure meters . .10
6.2 Displacement meters .10
6.3 Turbine meters . 12
6.4 Coriolis mass flowmeters . 13
6.5 Ultrasonic meters .14
7 Meter proving .16
7.1 General .16
7.2 Pipe provers .16
7.3 Volumetric measures .17
7.4 Gravimetric proving .18
7.5 Master meter proving .18
8 Volumetric corrections .18
8.1 Standard volume .18
8.2 Thermal expansion and temperature effects .19
8.2.1 Overview .19
8.2.2 Temperature effect on Coriolis meters .19
8.2.3 Thermal expansion for ultrasonic meters and differential pressure meters . 20
8.2.4 Thermal expansion for displacement and turbine meters . 20
Bibliography .21
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
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with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
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Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 28, Petroleum and related products, fuels
and lubricants from natural or synthetic sources, Subcommittee SC 2, Measurement of petroleum and related
products.
This second edition cancels and replaces the first edition (ISO 9200:1993), which has been technically
revised.
The main changes are as follows:
— mass and volumetric metering is now covered;
— a description and definition of viscosity has been added along with a clarification of high viscosity and
high temperature;
— the emphasis on positive displacement meters has been replaced by descriptions of other meter types
including ultrasonic, Coriolis and differential pressure devices.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
This document is intended to guide users in the design, installation, operation, and proving of flowmeters,
and their auxiliary equipment, used in the dynamic metering of viscous liquids. It also provides guidance
when elevated operating temperatures are used to reduce viscosity. The document applies to Newtonian,
hydrocarbon and petroleum liquids. Extra consideration should be given when using other liquids and non-
Newtonian liquids.
The objective of this document is to highlight the considerations to be taken into account when metering
high viscosity liquids at normal and elevated temperatures, in addition to the normal application of metering
less viscous liquids at ambient temperatures.
As the viscosity of a liquid increases, the resistance to flow increases. In a fluid transfer system, this generally
means that the maximum flowrate achievable for any given conduit size is reduced to avoid excessive
pressure loss. This generally results in lower velocities within the measurement system than would be found
in lower viscosity applications. As most flow sensors and meters require a minimum velocity to provide
reasonable resolution of measurement, the operational range of the flowmeter chosen can be reduced.
Each flowmeter type and design has different limitations on the viscosity and flow range across which it
operates at an acceptable accuracy. For higher viscosities and low velocities, it is probable that for many
applications the flow regime is laminar rather than turbulent which again affects the performance of
flowmeters.
To provide efficient transport of the fluid within a pipe, viscous liquids are often heated to reduce the
viscosity. Measuring systems and the associated flowmeters are therefore selected and operated to suit
the chosen operating temperature, taking into account changes in temperature and viscosity from ambient
conditions.
The behaviour of the fluid should be considered carefully to recognize the potential for the liquid to solidify
during idle periods and also to manage the potential for air, gas, solids, and wax content from damaging or
affecting the metering system.
This document supplements the guidance documents applicable to different flowmeter designs and proving
methods in the relevant ISO standards referenced in the bibliography.
v
International Standard ISO 9200:2024(en)
Petroleum measurement systems — Metering of viscous and
high temperature liquids
1 Scope
This document gives guidance for measuring a quantity of primarily viscous hydrocarbon liquid using
flowmeters at ambient or elevated operating temperatures.
This document describes the effects of high viscosities and potentially high temperatures, which can induce
additional errors in measurement. It also gives guidance on how to overcome or mitigate difficulties.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
density
mass in a given volume
3.2
performance indicator
derived value which relates the meter output to the quantity measured and can be used to indicate the
performance of the meter
EXAMPLE Error, meter factor, K-factor, and discharge coefficient.
3.3
pipe prover
displacement prover
device where the volume of a fluid is displaced from the calibrated length of a pipe and used to provide a
calibration reference for flowmeters
3.4
pour point
lowest temperature at which a liquid product loses its flow characteristics and, under specified conditions,
ceases to flow
Note 1 to entry: The temperature at which a liquid ceases to flow is based on a standard test.
3.5
Reynolds number
dimensionless number expressing the ratio between the inertia forces and viscous forces within a flowing fluid
Note 1 to entry: The Reynolds number can provide an indication of the flow profile within a pipe. Generally, numbers
below 2 000 indicate a laminar flow, while a number higher than 5 000 indicates a turbulent flow.
Note 2 to entry: The Reynolds number and performance indicator for some flowmeters provides a single relationship
which accounts for variations in viscosity as well as flowrate.
3.6
viscosity
measure of resistance to flow
3.7
dynamic viscosity
viscosity dynamic
viscosity absolute
ratio of shear stress to shear rate within the fluid, hence the force needed to overcome internal friction
Note 1 to entry: The unit of absolute viscosity is Pascal second (Pa s). The unit of centipoise (cP) is commonly used in
−3
practice. 1cP = (1 × 10 Pa s).
3.8
kinematic viscosity
viscosity kinematic
ratio of dynamic viscosity (3.7) to density (3.1)
2 −1
Note 1 to entry: The unit of kinematic viscosity is metre square per second (m s ). The unit centistoke (cSt) is
−6 2 −1 2 −1 2 −1
commonly used in practice. 1cSt = (10 m s ) = (1 mm s ). The unit mm s is used throughout this document
when giving examples of fluid viscosity.
3.9
performance indicator
derived value which may be used to indicate the performance of the meter
EXAMPLE Error, K-factor, meter factor or discharge coefficient.
3.10
volumetric measure
measure used to provide an accurate measurement of volume to provide a reference for other volume
measuring devices
3.11
wax point
cloud point
wax precipitation point
temperature at which wax precipitates from a hydrocarbon liquid
4 Properties of high viscosity liquids
4.1 Viscosity
In this document a viscous liquid is any liquid that requires special treatment or equipment in its handling or
storage because of its resistance to flow at either ambient or operating temperature. The liquid is assumed
to be Newtonian. If the fluid is non-Newtonian, additional influences on metering should be considered.
Viscosity is typically expressed as being either dynamic or kinematic.
Dynamic viscosity (µ) and kinematic viscosity (υ) are related through density (ρ) by Formula (1).
μν= ρ (1)
where
μ is the dynamic viscosity, expressed in Pascal seconds (Pa s);
−3
ρ is the density, expressed in kilograms per cubic metre (kg m );
2 −1
ν is the kinematic viscosity, expressed in metre square per second (m s ).
When applying this conversion, or any other use of a viscosity relationship, consistent and matching units
should be used. Using the relevant SI unit as given in Formula (1) helps to avoid error.
4.2 Reynolds number and flow profile
The Reynolds number provides a scale to represent the turbulence within a flowing fluid, hence providing a
guide to the velocity profile within a pipe. The Reynolds number is calculated from Formula (2).
VDρ VD
Re== (2)
μυ
where
D is the pipe internal diameter, expressed in metres (m);
2 −1
V is the mean pipe velocity, expressed in m s ;
−3
ρ is the density, expressed in kg m ;
μ is the dynamic viscosity, expressed in Pa s;
2 −1
ν is the kinematic viscosity, expressed in m s .
When calculating the Reynolds number, it is important to ensure the units used are consistent. Using the
relevant SI unit as given in Formula (2) helps to avoid error.
Generally, laminar flow is present at Reynolds numbers below 2 000 and turbulent flow at Reynolds numbers
above 5 000. Between 2 000 and 5 000, a transitional condition is found where either laminar or turbulent or
a changing flow condition exists. Within the transitional range, the condition can switch between regimes.
These values are indicative and the Reynolds numbers at which the transition between laminar and
turbulent flow occurs is variable and depends on of a variety of factors. These include, but are not limited to,
pipe configuration, pipe roughness, temperature, orientation and vibration. Typically, turbulent conditions
cannot easily exist below Reynolds number of 2 000, but the transition from laminar to turbulent can easily
extend to Reynolds numbers approaching 10 000 and sometimes above.
Increasing viscosity results in a lower Reynolds number for a given velocity. Although most industrial flows
of liquids are at a velocity leading to a Reynolds number above 5 000, with higher viscosity liquids a low
velocity is often chosen to reduce the pressure losses. Many applications can therefore operate at Reynolds
numbers within the transition or laminar region.
Laminar flow results in a parabolic or peaked velocity profile across the pipe, with the highest velocity being
at the centre and reducing either side towards the wall. Turbulent flow results in a profile which is essentially
flat, or equal, across most of the pipe diameter, and then reduces rapidly to the pipe wall. The flow profile
can also be changed due to the viscosity reducing additives or contaminants within the fluid or on the pipe
wall. The flow regime and velocity profile have a significant effect on various types of flowmeters.
A stable, predictable and fully developed flow profile (turbulent or laminar) is disturbed by bends and
changes in diameter and devices (valves, filters etc.) in the pipe resulting in a swirling or asymmetric flow
profile. It takes time and distance to return to a fully developed profile. While a turbulent flow profile can be
mostly re-established after a distance of approximately 20 to 30 pipe diameters, a laminar flow profile takes
significantly longer to re-establish and in some applications, can take a distance of up to 100 pipe diameters.
In laminar flow regimes, temperature difference across a pipe due to sun or wind can modify the profile.
Many flowmeters are designed to perform most effectively with a relatively high flow velocity, resulting
in a turbulent flow regime and flat velocity profile. These flowmeters are also significantly affected by a
distorted flow profile. Consideration should be given when such flowmeters are used in conditions where
there is a laminar flow profile.
When characterizing a meter’s performance over its operating range, the Reynolds number can provide
an alternative to volumetric flowrate. In some cases, the Reynolds number can be more suitable than the
flowrate in providing a single relationship for the performance indicator across the range of the device.
The Reynolds number is not measured directly, however there are techniques available to predict the
operational Reynolds number by measuring pressure loss in a straight length of pipe upstream of the
flowmeter.
4.3 Density
Density is related to viscosity insofar as high-density hydrocarbon liquids typically also have high viscosity.
This is not a linear relationship and can differ between different liquids.
Density should not be used to predict viscosity except when used as a very rough guide.
4.4 Effect of temperature on viscosity
The density of a hydrocarbon liquid varies with temperature in an approximately linear relationship,
reducing as the temperature increases. The density and hence volumetric expansion of many hydrocarbon
liquids has been defined and the relationship standardized for measurement purposes.
The viscosity of a hydrocarbon liquid also varies with temperature but follows an exponential relationship
with viscosity, reducing as the temperature increases. As temperature decreases, the change in viscosity
becomes increasingly significant at the lower temperatures.
Viscosity is not required to define the quantity of fluid being measured. It does however, play a significant
role in the performance of flowmeters. To specify the viscosity of a liquid, it is common practice to state the
dynamic or kinematic viscosity at one specific temperature. It is noted that this temperature is not usually
the standard temperature chosen to calculate standard volume. Typically, 20 °C is used to specify viscosity
for lower viscosity fluids. For higher viscosity fluids, significantly higher temperatures are used, such as
30 °C, 40 °C or 60 °C, to ensure they are fully liquid when characterized.
There are no common standard formulae relating viscosity to temperature for hydrocarbon liquids. A
number of formulae exist, most of which relate kinematic viscosity to temperature. The commonly used
formulae all take a similar form and the most common is given in Formula (3).
loglog υ+AT≅log (3)
() ()
where
log is logarithm to base 10;
2 −1
υ is kinematic viscosity, expressed in mm s ;
A is a constant derived from the viscosity measured at two temperatures;
T is temperature, expressed in degrees Celsius (°C).
NOTE The constant, A, can be derived from measurements using different units from those shown above. In this
case, it is expected that units used are carefully recorded and that consistency is ensured when using the formula.
The available formulae, including Formula (3), require the viscosity constant to be derived from
measurement. As a minimum, two specific temperatures are used to define the constant A. These two values
can also be used to generate a viscosity index value which can then be used to predict viscosities at other
temperatures.
The temperature at which the fluid or the components within the fluid start to become solid is also
important. The wax point is the temperature below which wax or solid components within the liquid
precipitate both within the flowing liquid and onto surfaces. The pour point is the temperature below which
the liquid effectively no longer flows. If the temperature falls below these points, this has a profound effect
on the system and the flowmeter.
If the temperature rises to a point where a component within the liquid can evaporate, boil, or come out of
solution, this can lead to gas and two-phase mixture within the system.
4.5 Examples of high viscosity liquids and behaviour
Within this document, high viscosity liquids for measurement are considered to have viscosity at the
2 −1 2 −1
operating temperature when they are greater than 100 mm s and below 2 000 mm s . Crude oil transport
2 −1 2 −1
and refining considers the viscosity to be high when it is in the range of 50 mm s to 1 000 mm s .
Operating temperatures are chosen to provide a viscosity within the above ranges, and to allow adequate
flow through the system. Temperatures range from ambient through to 150 °C. In some applications, for
example, the vacuum refining of residual fuel, the temperature can be as high as 200 °C.
Examples of liquid hydrocarbons which are generally considered as viscous are residual fuels with a
2 −1
viscosity greater than 750 mm s (at 50 °C), bitumen, lubricating oils and grease components, and heavy
crude oils.
2 −1
To provide context, water has a viscosity of approximately 1 mm s at 20 °C.
Figure 1 shows a graph of viscosity variation with the temperature for a representative high viscosity
2 −1
crude oil. The viscosity at an ambient temperature of 15 °C is estimated to be 1 500 mm s and reduces to
2 −1 2 −1
800 mm s at 40 °C, with further reduction to 200 mm s at 60 °C, the specified transport temperature.
Key
X temperature (°C)
2 −1
Y viscosity (mm s )
Figure 1 — Representative relationship of temperature to viscosity for a crude oil
4.6 Further considerations
When considering different flowmeter types, the descriptor high viscosity can be significantly lower than
the definition applied in this document. For example, the specification of a fiscal turbine meter can suggest a
2 −1
high viscosity application over 20 mm s .
High viscosity fluids have a number of additional characteristics which should be considered when choosing
a metering system. These considerations are listed below.
— Entrained air or gas does not separate easily or quickly from a high viscosity fluid. This results in gas
separators or eliminators not being as effective as would be expected when used with low viscosity
fluids. Higher pressure losses expected in higher viscosity applications can result in gas formation. Large
separators can be specified, however all precautions should be taken to ensure air or gas is not present.
— Solids do not separate or filter easily from high viscosity fluids. Installation of filters or strainers can
result in high pressure loss within the system. For this reason, coarse mesh filters are often specified.
— Water or other contaminate liquids do not separate readily when the viscosity is high and can exist as
layers, films or emulsions. Emulsions can alter and possibly significantly increase the effective viscosity.
— For some pipeline applications, water or another fluid can be introduced to reduce the friction losses in
a pipeline. The water forms a layer on the inside of the pipe wall reducing friction, and thus the pressure
loss. Other additives such as viscosity reducers can be introduced to reduce viscosity. The presence and
effect of such additives on the flow profile and the flowmeter type should be recognized.
— Cooling below the wax or pour point when the flow is stopped can result in wax formation or solidification
within the system.
5 Metering systems
5.1 General
Owing to the various types of meters available and the wide differences in liquids and measurement
conditions, it is important that the meter manufacturer be given complete information on the proposed
application. The meter manufacturer can make recommendations specifically for the intended operating
conditions to minimize possible problems. The manufacturer and system designer should be provided with
the following information:
— flow rate range at maximum and minimum operating viscosities;
— maximum and minimum operating pressures;
— maximum and minimum operating temperatures;
— expected ambient conditions and anticipated standby (or off-duty) temperature;
— density or specific gravity of fluid at maximum and minimum operating temperatures;
— viscosity of the fluid at ambient temperature and at the maximum and minimum operating temperatures;
— pour point and wax precipitation temperatures;
— nature and amount of any corrosive elements present;
— nature and amount of any abrasive elements present;
— other fluids such as cleaning and flushing fluids to ensure compatibility;
— presence of additives to reduce friction;
— type of proving method and equipment under consideration.
Certain viscous liquids can contain corrosive materials. This corrosive effect can increase as the temperature
increases. Where significant, the metallurgy of the meter, its trim and auxiliary equipment should be capable
of resisting this corrosion. At elevated temperatures, special construction materials can be required.
It is important to specify the viscosity at the operating temperature, or across the range of operating
temperatures. Providing the single viscosity value given in the fluid specification sheet is not sufficient to
adequately design the system.
5.2 Installation
Meters should be installed according to good practice and the manufacturer’s recommendation. Meters
and systems should be securely supported. Where the temperature varies significantly from ambient
temperature, precautions should be taken to consider thermal expansion of the pipework and avoid resultant
stress on the pipework, particularly at the flowmeter.
It is good practice for all meters to allow a minimum distance of five diameters of straight pipe up and
downstream to provide a location for additional support for the meter and to allow installation of pressure
and temperature measurement. This also provides some degree of profile conditioning.
The manufacturer’s guidance should be sought with regard to an additional straight length of the pipe up
and downstream of the chosen flowmeter to provide a suitable flow profile for the meter proposed for
the installation. For meters requiring a fully developed profile, e.g. turbine meters, ultrasonic meters and
differential pressure meters, a longer length of straight pipe is required following a disturbance. Coriolis
meters can require a suitable length of pipe to provide support and stress isolation.
It is recommended to seek guidance from relevant literature and the manufacturers on the effects of
both laminar and turbulent flow regimes. Flow straighteners or conditioners can be used to reduce the
recommended straight lengths for meters which require a fully developed profile, however the installation
of a flow conditioner introduces additional pressure loss and potentially can hold back liquid when draining.
Tube bundle type straighteners do not have a significant effect on removing asymmetric flow profile
commonly found in laminar flow conditions.
Filtering of the fluid prevents damage to the installed meter. Filters or strainers should be installed upstream
of the metering section with a mesh size chosen to protect the meter. Filters introduce pressure loss within a
system, particularly when the viscosity is high. A compromise between mesh size, protection, and pressure
loss is often required. Filters should be insulated and heated if required to avoid blockage.
The selection of a recommended control valve, installed downstream of the meter, should consider the
viscosity of the fluid at operating and ambient conditions. This valve allows control of the flow through the
meter and maintains an adequate pressure within the metering section.
Similarly, the connections, and double block valves for a bypass line and for calibration connections should
to be sized to suit the viscosity range.
Where an installation has a flowmeter that is required to be calibrated or verified in situ, it is recommended
to install connections allowing a prover or other reference device to be connected. It is recommended that
such connections be installed downstream of the flowmeter and upstream of the control valve. A double
block valve in the flow line between the connections ensures all the flow is diverted to the reference prover.
A schematic diagram showing the main features of an installation is given in Figure 2.
Key
1 insulation and trace heating
2 air/gas/water separator and filter
3 flowmeter
4 control valve
5 shut off isolator valve
6 double block valve
7 return line
8 vent lines
9 air sense and flow control signal
10 temperature sensor
11 temperature and pressure sensors
12 connections to prover reference device
Figure 2 — Schematic diagram of an installation
5.3 Heating
Heating the liquid reduces the viscosity, in turn reducing friction, lowering pressure loss and hence allowing
an increase in velocity through the meter.
In a heated system, it is important that the metering section, the flowmeter and upstream and downstream
piping are insulated to provide stability.
If the ambient temperature falls below the wax or pour points, cooling can allow waxing or solidification to
occur within the system. Waxing or solidification can create a blockage in the system which is difficult to
remove, even by re-heating. This can lead to potential damage to the meter or a change in the performance.
It is important to maintain an elevated temperature during idle or no-flow periods.
It is common practice to insulate the whole system. Often trace heating is applied to maintain the required
temperature and viscosity.
If a system containing a hot product is drained after use, it is good practice to flush with a suitable solvent to
prevent residual high viscosity liquid solidifying within a meter body or moving parts.
Accessory equipment such as valves, strainers and air eliminators should also be insulated and heated. This
can include venting mechanisms, control valve pilot ports, drain ports, pressure taps and impulse lines.
For systems where the liquid is taken from a heated storage tank, it is possible to keep the liquid in the
system hot by continuously circulating the liquid and returning it to the storage. This method is of particular
value on tank trucks where auxiliary heating methods are difficult to provide. It is vital to ensure that this
re-circulation path is demonstrably closed and sealed so that no fluid can bypass the metering section
during a measurement transfer. Double block or twin seal valves should be considered in the return line. An
automatic method of controlling circulation and linking to meter registration is suggested.
Trace heating can be applied using steam or hot or warm water flowing through piping or a jacket around
the pipework device or flowmeter. Meters are available with jackets fabricated around the body. It should be
ensured that variation in pressure from the heating fluid does not affect the meter body.
Alternatively, electric trace heating can be installed under insulation. Precautions to avoid overheating
should be put in place for electric trace heating. In smaller installations, the use of electric heating can be
adequate and significantly less expensive. Trace heating with self-regulating properties is often used.
It is important that trace heating provides a stable and consistent temperature to the fluid within the body
of a flowmeter and around pipework to avoid stratification across the pipe and within the meter. The trace
heating should be controlled to avoid variations in temperature resulting in changes in viscosity, hence
meter performance.
The liquid temperature should be held below the point where vaporization of the product, or any of its
components, can occur at any point throughout the entirety of the system. Precautions should be taken
to prevent overheating of the liquid to a point where ignition can occur if exposed to the atmosphere.
Overheating can also cause coking or chemical changes in some liquid hydrocarbons, affecting meter
performance and thus damaging the meter.
In any heated system, it is important to provide pressure relief for any part of the system which can be
isolated. A rise in temperature expands the liquid and hence increases pressure above safe limits. Similarly,
cooling a closed sealed system can lead to a reduction in pressure and potentially cause damage.
5.4 System start-up and filling
Initial filling of the system should be carried out carefully following an established procedure. The entire
metering system should be filled slowly at a reduced rate until all air pockets have been eliminated from the
system. An air pocket flowing through a meter can create a surge pressure resulting in damage if flow is stopped
or started quickly. Heating can be applied before starting to ensure no blockage is present in the system.
It is difficult to separate entrained air or vapour from viscous liquids. As viscosity increases, the time
required for separating fine bubbles of air or vapour from the liquid increases. The removal of entrained
bubbles requires a large air-eliminator to effect separation. In most instances, this approach is uneconomical
from the perspective of cost and required space. As a result, precautions should be taken to prevent gas
from entering the system. A means to purge the system, by circulating fluid back to the storage tank prior
to a delivery, can be considered, in effect using the storage tank as an air eliminator. Circulation should
be maintained long enough to ensure that all air or vapour has been carried back to the storage tank. Gas
extracted from the system should be vented safely.
The liquid in storage tanks can contain air or vapour bubbles, possibly caused by the method of heating or
by pumping liquid into the storage tank at the same time that liquid is being pumped out. Some crude oils
foam when heated and should be allowed to settle before withdrawing the product. A sampling or testing
method can determine when the air or vapour content is at an acceptable level. If it is not practicable to
give sufficient time for separating out the air or vapour from storage, an air eliminator designed for the
particular operating conditions is an alternative option. However, this can require a large air eliminator if
designed for a high viscosity liquid.
Where air or vapour is not adequately removed during a measured transfer of liquid, a downstream sensor
and valve can be used to detect and stop the delivery of liquid, thus avoiding mismeasurement. Detection
can be implemented within an air eliminator of moderate size. Examples of installations where this type of
system can be required are tank truck meter systems and systems for unloading tank cars, barges, tankers
and transport trucks.
Similar precautions are employed to prevent water being drawn through the metering system. Water takes
time to settle from a high viscosity liquid and gathers at the bottom of a storage tank. Liquid should therefore
not be drawn from the base of a tank.
6 Flowmeters
6.1 Differential pressure meters
The principles of differential pressure measurement and the design, standardization and performance
predictions for differential pressure meters are described in the ISO 5167 series. Differential pressure
flowmeters are described in the ISO 5167 series and cover orifice plates; nozzles; Venturi nozzles and tubes;
and cone and wedge meters. The performance predictions are limited to flows where the Reynolds number
3 4 4
is at least 5 × 10 for square edge orifice plates, 7 × 10 for Venturi meters and 8 × 10 for cone meters. This
limitation reduces the number of high viscosity applications deemed suitable for standard square edged
orifice plates, Venturi and cone meters.
Measurable differential pressure is observed for all differential pressure devices and across a wide range of
flowrates and Reynolds numbers. This can extend across turbulent and laminar flow regimes. ISO/TR 15377
provides guidelines for using meters outside the scope of the ISO 5167 series. ISO/TR 15377 covers the
performance and discharge coefficient for some designs which extend down to low Reynolds numbers.
While square edge orifice plates are not suitable for high viscosity applications, conical and quarter circle
orifice plates have predictable performance for low Reynolds number applications. For conical entry plates,
the prediction of discharge coefficient extends down to Reynolds numbers of 80 and for quarter circle plates,
250. It is noted however, that these predictions depend on the diameter ratio selected for the application.
The selection of devices requires careful consideration of viscosity, density, acceptable pressure loss and the
measurement of differential pressure.
Wedge type meters are specified for applications with high viscosity liquids which are well outside the scope
of ISO 5167-6. These applications cover flows where the Reynolds number is as low as 500 and temperatures
up to 200 °C. ISO 5167-6 describes the construction and the predicted discharge coefficient for Reynolds
numbers greater than 1 × 10 . It states that wedge meters can be used outside the stated conditions, but
this would require calibration across the operational Reynolds number range. Calibration establishes the
discharge coefficient as a function of Reynolds number. It is noted that using water as the calibration fluid at
low Reynolds numbers requires measurement of very low differential pressures. It should be ensured that
the calibration and subsequent prediction adequately covers the transition from laminar to turbulent flow
regimes for the product.
Wedge meters are robust and reduce the chance of damage occurring from solids and erosion. The clear
path at the bottom of the pipe allows for good drainage. The manufacturer should be consulted in selecting
an appropriate wedge meter for a particular application.
Installing a differential pressure meter requires an adequate upstream and downstream pipe length to
provide a developed flow profile. Examples and guidance are given in the r
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