ISO 2714:2017

Liquid hydrocarbons - Volumetric measurement by displacement meter

ISO 2714:2017 describes and discusses the characteristics of displacement flowmeters. Attention is given to the factors to be considered in the application of positive displacement meters to liquid metering. These include the properties and nature of the liquid to be metered, the correct installation and operation of the meter, environmental effects, and the wide choice of secondary and ancillary equipment. Aspects of meter proving and maintenance are also discussed. ISO 2714:2017 is applicable to the metering of any appropriate liquid. Guidance is given on the use of positive displacement meters in the metering of two-component mixtures of the same phase such as water and oil. It is not applicable to two-phase flow when gases or solids are present under metering conditions (i.e. two-phase flow). It can be applied to the many and varied liquids encountered in industry for liquid metering only. It is not restricted to hydrocarbons. Guidance on the performance expected for fiscal/custody transfer applications for hydrocarbons is outlined. ISO 2714:2017 is not applicable to cryogenic liquids such as liquefied natural gas (LNG) and refrigerated petroleum gas. It does not cover potable water and fuel dispenser applications.

Hydrocarbures liquides — Mesurage volumétrique au moyen de compteurs à chambre mesureuse

General Information

• Status: Published
• Publication Date: 21-Nov-2017
• ICS: 75.180.30 - Volumetric equipment and measurements
• Technical Committee: ISO/TC 28/SC 2 - Measurement of petroleum and related products
• Drafting Committee: ISO/TC 28/SC 2 - Measurement of petroleum and related products
• Current Stage: 9093 - International Standard confirmed
• Start Date: 11-Apr-2023
• Completion Date: 13-Dec-2025

Relations

• Revises: ISO 2714:1980 - Liquid hydrocarbons - Volumetric measurement by displacement meter systems other than dispensing pumps
• Effective Date: 12-Feb-2014

Overview

ISO 2714:2017 - "Liquid hydrocarbons - Volumetric measurement by displacement meter" provides comprehensive guidance on the use of positive displacement (PD) meters for volumetric measurement of liquids. Although its title references hydrocarbons, the 2017 edition extends the scope to cover metering of any appropriate liquid (excluding two-phase flows, cryogenic liquids such as LNG, potable water and fuel-dispensing applications). The standard focuses on practical issues affecting accurate liquid metering: meter design, installation, operation, calibration (proving) and maintenance.

Key topics and technical requirements

ISO 2714:2017 is primarily guidance (not prescriptive rules) and addresses these technical areas:

• Design and operation of displacement meters - basic characteristics and modes of operation, including reciprocating, rotating, intermeshing screw spindle, oscillating and disc-type meters.
• Performance aspects - factors affecting meter accuracy, pressure drop/back pressure, flow profile and general performance characteristics.
• Liquid property effects - influence of viscosity, temperature, pressure, lubricity, cleanliness and behaviour of two-component single-phase mixtures. It excludes two-phase (gas/solid) metering.
• System design and selection - criteria for selecting PD meter types, ancillary and secondary equipment (mechanical accessories, pulse generators, electronic instrumentation) and recommended flow algorithms.
• Installation requirements - piping layout, flow pulsation control, electrical installation and pulse/security considerations that influence volumetric measurement.
• Environmental and safety considerations - electromagnetic interference, humidity and general installation safety.
• Calibration and proving - guidance on proving methods, proving conditions and verification frequency for reliable meter calibration in fiscal/custody-transfer contexts.
• Operation and maintenance - start-up procedures, routine maintenance, diagnostics and use of control charts; includes an informative annex on performance specification.

Practical applications

ISO 2714 is relevant wherever accurate volumetric liquid metering is required:

• Fiscal/custody transfer of hydrocarbons where PD meters are used and performance expectations must be documented.
• Process industries (petrochemical, chemical, lubricants) measuring a wide range of viscosities and liquids.
• Bulk transfer and loading/unloading operations using displacement meters for inventory and accounting.
• Meter system design and integration, including selection of secondary electronics and flow algorithms.

Who should use this standard

• Instrumentation and metering engineers
• Flowmeter manufacturers and calibration houses
• Custody transfer and terminal operators
• Maintenance and reliability teams responsible for metering systems
• Procurement and compliance professionals specifying PD meter systems

Related standards

• ISO/IEC Guide 99 (VIM) - metrology vocabulary
• ISO 4006 - Measurement of fluid flow in closed conduits: vocabulary and symbols

Keywords: ISO 2714, positive displacement meter, displacement flowmeter, volumetric measurement, liquid metering, meter proving, custody transfer, calibration, viscosity, flow profile.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 2714
Second edition
2017-11
Liquid hydrocarbons — Volumetric
measurement by displacement meter
Hydrocarbures liquides — Mesurage volumétrique au moyen de
compteurs à chambre mesureuse
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior
written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of
the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. [phone]
Fax [phone]
[e-mail]
www.iso.org
ii © ISO 2017 – All rights reserved

Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms, definitions, symbols and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Symbols and abbreviated terms. 5
4 Design and operation of positive displacement meters . 5
4.1 Basic characteristics and mode of operation . 5
4.2 Reciprocating displacement types . 7
4.3 Rotating displacement types. 7
4.4 Intermeshing screw spindle type. 8
4.5 Oscillating displacement types . 9
4.6 Disc type meters . 9
5 Performance aspects . 9
5.1 General . 9
5.2 Factors affecting meter performance . 9
5.3 General performance characteristics . 9
5.4 Pressure drop and back pressure considerations .11
5.5 Flow profile .11
6 Liquid property effects .11
6.1 General .11
6.2 Effect of viscosity .11
6.3 Effect of temperature .13
6.4 Effect of pressure .13
6.5 Lubricity and liquid cleanliness .14
6.6 Two-phase flow and air elimination .14
6.7 Two-component operation .14
6.8 Pulsating and fluctuating flow .14
7 System design .15
7.1 Design considerations.15
7.2 Selection of displacement meter type .16
7.3 Ancillary equipment .17
7.3.1 General.17
7.3.2 Mechanical accessories .18
7.3.3 Pulse generators and secondary electronic instrumentation .18
7.4 Flow algorithms .19
8 Installation aspects .20
8.1 General .20
8.2 Installation pipework .20
8.3 Flow pulsation .22
8.4 Electrical installation .22
8.5 Pulse security .23
9 Environmental considerations .23
9.1 General .23
9.2 Electrical interference .23
9.3 Humidity .23
9.4 Safety .23
10 Calibration .24
10.1 Proving and verification .24
10.2 General considerations .24
10.3 Proving conditions .24
10.4 Proving methods .24
10.5 Proving frequency .25
11 Operation and maintenance .25
11.1 General .25
11.2 Initial start-up .25
11.3 Meter maintenance .26
11.4 System diagnostics and control charts .26
Annex A (informative) Specification of performance.28
Bibliography .35
iv © ISO 2017 – All rights reserved

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 has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely 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 documents 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).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: 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, in collaboration with Technical Committee ISO/TC 30, Measurement of fluid flow in
closed conduits.
This second edition cancels and replaces the first edition (ISO 2714:1980), which has been technically
revised.
Introduction
This document gives recommendations on the design, installation, operation and maintenance of
positive displacement meter systems used for liquid measurement. This widens the application scope
from the previous document, which was primarily aimed at hydrocarbon custody transfer applications.
The guidance now applies to all suitable liquids measured across different applications and industry
sectors.
Displacement meters are extensively used in general fluid measurement in addition to fiscal, custody
transfer and legal metrology applications involving hydrocarbon and non-hydrocarbon products. These
can range from the light products such as gasoline, through to higher viscosity fluids.
The document has an extended scope from the first edition to cover applications for a wider range of
liquids and duties and to remove restriction to hydrocarbon liquids. It now provides guidance, rather
than mandatory requirements, on performance to allow meters to be specified and verified to meet
relevant regulatory, fiscal and custody transfer specifications. The document also now includes
additional meter designs. This revision has been achieved through the participation of ISO/TC 30 in the
preparation, hence, providing a single standard for the measurement of flowing liquids using positive
displacement flowmeters.
vi © ISO 2017 – All rights reserved

INTERNATIONAL STANDARD ISO 2714:2017(E)
Liquid hydrocarbons — Volumetric measurement by
displacement meter
WARNING — The use of this document might involve hazardous materials, operations and
equipment. This document does not purport to address all of the safety problems associated
with its use. It is the responsibility of the user of this document to establish appropriate safety
and health practices.
1 Scope
This document describes and discusses the characteristics of displacement flowmeters. Attention
is given to the factors to be considered in the application of positive displacement meters to liquid
metering. These include the properties and nature of the liquid to be metered, the correct installation
and operation of the meter, environmental effects, and the wide choice of secondary and ancillary
equipment. Aspects of meter proving and maintenance are also discussed.
This document is applicable to the metering of any appropriate liquid. Guidance is given on the use of
positive displacement meters in the metering of two-component mixtures of the same phase such as
water and oil.
It is not applicable to two-phase flow when gases or solids are present under metering conditions (i.e.
two-phase flow). It can be applied to the many and varied liquids encountered in industry for liquid
metering only. It is not restricted to hydrocarbons.
Guidance on the performance expected for fiscal/custody transfer applications for hydrocarbons is
outlined.
This document is not applicable to cryogenic liquids such as liquefied natural gas (LNG) and refrigerated
petroleum gas. It does not cover potable water and fuel dispenser applications.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO/IEC Guide 99, International vocabulary of basic and general terms in metrology (VIM)
ISO 4006, Measurement of fluid flow in closed conduits — Vocabulary and symbols
3 Terms, definitions, symbols and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological 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/
3.1.1
accuracy
closeness of the agreement between the measured quantity value and a true quantity value of a
measurand
Note 1 to entry: The concept “measurement accuracy” is not a quantity, and should not be given a numerical value.
The quantitative expression of accuracy should be in terms of uncertainty. “Good accuracy” or “more accurate”
implies small measurement error. Any given numerical value should be taken as indicative of this.
[SOURCE: ISO/IEC Guide 99:2007, 2.13, modified]
3.1.2
adjustment
set of operations carried out on a meter or measuring system so that it provides prescribed indications
corresponding to given values of the quantity measured
EXAMPLE This entails bringing a measuring instrument (meter) into a satisfactory performance and
accuracy.
Note 1 to entry: Adjustment can be of zero point, span, linearity or other factors affecting the performance of
the meter.
Note 2 to entry: Adjustment should not be confused with calibration, which is a prerequisite for adjustment.
Note 3 to entry: After adjustment, a recalibration is usually required.
[SOURCE: ISO/IEC Guide 99:2007, 3.11]
3.1.3
calibration
set of operations that establish, under specified conditions, the relationship between quantities
indicated by an instrument and the corresponding values realized by standards
Note 1 to entry: Calibration should not be confused with adjustment of a measuring system.
[SOURCE: ISO/IEC Guide 99:2007, 2.39, modified]
3.1.4
cavitation
phenomenon related to, and following, flashing (3.1.6), where vapour bubbles or voids form and
subsequently collapse or implode
Note 1 to entry: Cavitation causes significant measurement error and also potentially cause damage to the pipe
and meter through erosion.
3.1.5
error
measured value minus a reference value
Note 1 to entry: Relative error is error divided by a reference value. This can be expressed as a percentage.
[SOURCE: ISO/IEC Guide 99:2007, 2.16, modified]
3.1.6
flashing
phenomenon which occurs when the line pressure drops to, or below, the vapour pressure of the liquid,
allowing gas to appear from solution or through a component phase change
Note 1 to entry: Vapour pressure of the fluid can reduce with increasing temperature.
Note 2 to entry: Flashing is often due to a local pressure drop caused by an increase in liquid velocity, and
generally causes significant measurement error.
2 © ISO 2017 – All rights reserved

Note 3 to entry: The free gas produced by flashing will remain for a considerable distance downstream of the
meter even if pressure recovers.
3.1.7
K-factor
ratio of the number of pulses obtained from a meter and the quantity passed through the meter
3.1.8
linearity
total range of deviation of the accuracy curve from a constant value across a specified measurement range
Note 1 to entry: The maximum deviation is based on the mean of derived values at any one flow point.
Note 2 to entry: The deviation is the largest minus the smallest value of mean values at each flowrate.
Note 3 to entry: Relative linearity is the range of values divided by a specified value, e.g. the independent linearity
as defined in ISO 11631.
3.1.9
lubricity
liquid property which affects friction between moving surfaces
Note 1 to entry: Good lubricity allows the formation of a liquid film between surfaces, and thereby reduces
friction. Poor lubricity, where little or no film is formed, can result in accelerated component wear.
3.1.10
meter factor
ratio of the quantity indicated by the reference standard and the quantity indicated by the meter
3.1.11
performance indicator
derived value which may be used to indicate the performance of the meter
EXAMPLE Error, K-factor, or meter factor.
3.1.12
proving
calibration (3.1.3) with comparison to defined acceptance criteria
Note 1 to entry: Proving is a term used in the oil industry and is similar to “verification”.
Note 2 to entry: Proving is a calibration, sometimes of limited measurement range, according to methods defined
by standards, regulation or procedures providing a determination of the errors of a meter and showing (proving)
it performs to defined acceptance criteria.
3.1.13
pulse interpolation
means of increasing the effective resolution of the pulses output from a meter by multiplying the pulse
frequency or measuring the fraction of a pulse associated with the total collected across a time period
Note 1 to entry: The latter is the most common method through a double timing technique.
3.1.14
range
measuring range
set of values of flowrate for which the error (3.1.5) of a measuring instrument (flowmeter) is intended
to lie within specified limits
[SOURCE: ISO Guide 99:1993]
3.1.15
range
range of values
difference between the maximum and minimum values of a set of values
Note 1 to entry: This can be expressed as a half range (±) number. Relative range is normally expressed as a
percentage of a specified value, e.g. mean, minimum or other calculated value.
3.1.16
repeatability
measurement precision
closeness of agreement between indications or measured quantity values obtained by replicate
measurements under specified conditions
Note 1 to entry: Specified conditions normally imply the same reference, same conditions, same operators and
procedures and that the data are obtained sequentially over a short period of time.
Note 2 to entry: Repeatability can be expressed as the range (difference between the maximum and minimum)
values of error or K-factor. Alternatively, repeatability can be expressed as a function of the standard deviation
of the values.
Note 3 to entry: Dividing repeatability by the mean gives the relative value which can be expressed as a
percentage. Some standards suggest dividing by the minimum value.
[SOURCE: ISO/IEC Guide 99:2007, 2.21, modified]
3.1.17
slip
measure of the fluid which passes through the meter without being directly measured
3.1.17.1
dynamic slip
slip measured when the meter is rotating
3.1.17.2
static slip
slip measured when the meter is not rotating
3.1.18
standard conditions
conditions of temperature and pressure to which measurements of volume or density are referred to
standardize the quantity
Note 1 to entry: These are the specified values of the conditions to which the measured quantity is converted.
Note 2 to entry: For the petroleum industry, these are usually 15 °C, 20 °C or 60 °F and 101,325 kPa.
Note 3 to entry: Quantities expressed at standard conditions are shown by prefixing the volume unit by “S”, e.g.
3 3
4 Sm or 700 kg/Sm .
Note 4 to entry: Definition has been adapted from Energy Institute HM 0 and OIML R 117. Some other petroleum
standards employ the term “base” conditions.
Note 5 to entry: In some other documents, “standard” conditions are described as “base” conditions and,
incorrectly, as “reference” conditions. Reference conditions are conditions of use (influence quantities) prescribed
for testing the performance of a measuring instrument.
[SOURCE: ISO Guide 99:1993]
3.1.19
swirl
condition where the liquid flowing through a pipeline rotates with an associated high tangential
component of velocity relative to the axial component
4 © ISO 2017 – All rights reserved

3.1.20
uncertainty
non-negative parameter characterizing the dispersion of the quantity values attributed to a measurand
based on the information used
[SOURCE: ISO/IEC Guide 99:2007, 2.26, modified]
Note 1 to entry: The uncertainty is normally expressed as a half width range along with the probability
distribution with that range. It can be expressed as a value or as a percentage of the perceived true value.
3.2 Symbols and abbreviated terms
For the purposes of this document, the symbols given in ISO 4006 and ISO/IEC Guide 99 apply.
NOTE The preferred unit for kinematic viscosity is metre squared per second (m /s) or millimetres squared
per second (mm /s). The practical unit used in this document is the industry recognized unit centistoke (cSt);
1 cSt = 1 mm /s.
4 Design and operation of positive displacement meters
4.1 Basic characteristics and mode of operation
Positive displacement (PD) flowmeters, as the name implies, are devices which continuously divide the
flowing stream into volumetric segments, and momentarily isolate these segments for measurement
purposes. The total of the volumes contained within the segments as the meter rotates over a period of
time is the total volume passed. The frequency at which the segments pass is a measure of the volume
flowrate. PD meters are driven by the flow, and it is the pressure drop across the meter internals that
creates a hydraulic imbalance which causes rotation.
All PD meters can be considered as possessing three basic elements: the external housing, the metering
element and the output shaft. The housing can be of single-case or double-case construction. The
external housing contains the fluid, and is designed to suit the operating conditions of temperature
and pressure. A double-case design minimizes the effect of pressure expansion on the outer external
housing by having a secondary internal housing around the metering element. The measuring unit is a
precise component (or series of components), which performs the liquid segmentation and comprises
a chamber and displacement mechanism. The cyclic volume displaced is a function of the number of
chambers or the precise design of the volume being swept by rotation or reciprocation.
The metering chamber (and the associated readout registers) is often sealed to prevent tampering and
fraud. Widespread type approval for trade use by relevant authorities is more common with PD meters
than most other types of flowmeter.
The output shaft is used to drive mechanical or electrical (pulsed) outputs. This could be a direct drive
to a pulse generator or through a gear box to a mechanical readout. Various calibration devices and
drives to compensators and printers can be attached. Some designs might have electronic pickups
fitted to detect rotation through the meter casing, thus, avoiding shaft seals.
PD meters can be subdivided into five classes, based on the type of motion:
a) reciprocating motion (single and multiple pistons);
b) rotating motion (vanes and gear types);
c) oscillating motion (semi-rotary types);
d) nutating motion (disc types);
e) intermeshing screw type.
Eight of the more common types are shown in Figure 1.
a) Reciprocating piston meter b) Sliding vane meter c) Oval gear meter
d) Bi-rotor meter e) Tri-rotor meter f) Nutating disk meter
g) Oscillating piston meter h) Screw (spindle) meter
Key
1 pistons slide
2 outlet
3 flow
4 inlet
Figure 1 — Eight common types of displacement flowmeter
More details of the basic components of one of these, the sliding vane type, are shown in Figure 2.
Although the means for separating and counting the liquid pockets are many and varied, the whole
group possesses similar basic characteristics. The performance indicator is usually given in terms of
meter error, meter factor or meter K-factor as a function of volumetric flowrate or Reynolds number.
A general performance curve is shown in Figure 3 for a small rotary device to illustrate the basic
performance.
At low flowrates, the metering mechanism has to overcome frictional resistance before motion
commences and, as a result, liquid slip may be significant. As flowrate increases, the percentage of slip
diminishes and metering error reduces. Certain designs, when operated within controlled conditions,
have measurement uncertainties, which are comparable with the method of proving in the laboratory
or in the field, and a full assessment of actual potential performance cannot be realized.
6 © ISO 2017 – All rights reserved

The pressure drop follows a classical relationship, increasing with the square of the flowrate when the
flow regime is turbulent, and linearly proportional when the flow regime is laminar.
In certain applications, component wear (and hence slip) can be accelerated by excessive pressure drop.
This might increase linearity at low flowrates, particularly if gear trains and other drag producing
components are fitted and are in poor condition. Most modern designs use shaft encoders or pulse
generators to enable the meter to resolve smaller volumes with reduced frictional loading. Such devices
have shown that some designs are capable of very low repeatability specifications even in the non-
linear region at the lower end of the measurement range. This allows these meters to be used as master
meters, transfer standards or custody transfer devices for the bulk shipment of high value liquids such
as refined hydrocarbons.
4.2 Reciprocating displacement types
In this class of PD meter, the measuring element is a piston (or series of pistons arranged in a line
or radially) that drives a common crank connected to the output shaft. The crank synchronizes the
movement of the pistons. Slide or rotary valves allow liquid to alternately fill and exhaust from the
measuring cylinders. In other designs, ports in the cylinder walls are used instead of valves. These are
covered and uncovered in sequence by the reciprocating motion of the pistons.
The volume measured in one cycle is the product of piston stroke, cylinder area, and the number of
pistons. This volume may be adjusted by altering the stroke length, but more usually, by adjusting
the readout mechanism driven from the crank. Good sealing is essential through fine tolerances or
appropriate ring seals. Frictional effects can increase if sealing is too tight. Where high viscosity liquids
are metered, drag on the metering elements might result in increased pressure drop.
4.3 Rotating displacement types
In this group are found the majority of PD meter types used for metering hydrocarbons. The two basic
designs are vane and gear types.
The major elements of a vane type are shown in Figure 2. There is a cylindrical rotor mounted within
a profiled body. The rotor assembly carries vanes (usually four or five) so that they slide freely within
slots machined within the assembly. The proximity of the rotor assembly and the vanes to the outer
casing causes a good seal during motion through the measuring crescent. The radius of this section is
constant, so the liquid trapped between the inlet and outlet porting is maintained at constant volume.
An output shaft connected to the rotor assembly drives the volume registration equipment. The
performance is usually de-rated (in flowrate terms) with dry (non-lubricating) or with abrasive liquids
to avoid component wear.
Key
1 outer casing 6 outer covers
2 rotor 7 register or indicator
3 vanes 8 counter
4 coupling rod 9 gear box, calibrator and pulse generator
5 inlet manifold 10 measuring crescent
Figure 2 — Details of a typical vane type PD meter
The other concept is the gear meter, of which there are many designs. One common basic design is the
oval gear meter. This design consists of two oval shaped rotors, which are forced to contra-rotate. This
is driven by the flow created by the pressure on the inlet. The close fit between the inter-meshing rotors
and the measuring chamber walls traps discrete liquid volumes, which are then continually discharged
through the outlet port.
Helical gear meters are design variants where the two rotors mesh in the longitudinal plane. They are
more commonly used on more viscous products. Because the sealing surfaces are relatively long, they
are more susceptible to over-speed and bearing damage.
Also in this group are the bi-rotor and tri-rotor meters, which are a combination of the vane and
gear types. Two or three synchronized rotors revolve within the chamber, again, trapping liquid in
discrete pockets. Each rotor shaft carries a timing gear, and each of the bladed displacement rotors
moves alternately through the measuring chamber half cylinder bores. In the tri-rotor design, the
single blocking rotor produces a continuous capillary seal between inlet and outlet, forcing the liquid
to flow through the measuring section. The blocking rotor is geared to revolve at half the speed of the
displacement elements.
4.4 Intermeshing screw spindle type
Screw-type meters consist of an axial mounted screw element with a second intermeshing screw
creating the sealed measured volume(s) and seal. Liquid trapped between the screw elements is
transferred from the inlet to outlet as the screws rotate.
Traditionally, this type of meter was restricted to high viscosity, low accuracy applications. More
specialized designs are now available with excellent performance, even in low viscosity liquids, and
perform well in custody transfer applications. These precision-made (spindle type) devices are used
as master meters due to the excellent linear performance comparable with that of the other common
types of PD meters.
8 © ISO 2017 – All rights reserved

4.5 Oscillating displacement types
The semi-rotary or oscillating piston meter, Figure 1 g), is one of the most widely used PD meters in
low accuracy process applications. Typical applications include any clean non-abrasive liquids where
flow measurement range is not important and where low flowrates are not involved. These meters tend
to exhibit increased slip as both viscosity and flowrate decrease. The metering piston is constrained
by the chamber wall, the barrier plate and the central boss, so that it moves in an oscillatory manner,
sweeping liquid from inlet to outlet during the cycle. They are better suited to viscosity applications
in excess of 100 mm /s, and also to cryogenic applications, where they tend to be used in customized
measurement systems.
4.6 Disc type meters
This type of meter is generally applicable to clean, non-abrasive liquids. The operating principle is
shown in Figure 1 f). The metering disc is mounted within the casing, but is constrained from pure
rotation by a radical partition attached to the measuring chamber wall. It does, however, swivel about
its vertical axis, with the incoming liquid alternately filling the spaces above and below the disc. The
pressure differential causes the metering element to nutate (wobble), and a central drive pin mounted
on the upper surface of the disc transfers this movement into rotary motion to drive the meter register.
5 Performance aspects
5.1 General
This clause discusses the general performance of displacement meters and the various factors which
can affect the characteristic curve. Performance is normally stated in terms of variation in performance
indicator as function of volumetric flowrate through the meter. The performance indicator is usually
meter factor. However, error and K-factor are also used.
Meters can have a single value determined and applied across the flowrange, or a number of values
determined across the range and by appropriate interpolation applied to the operating flowrate.
5.2 Factors affecting meter performance
The performance of displacement meters is affected by a number of variables, depending on the
metering element design and the geometry of body and measuring chambers. The most important are:
a) liquid flowrate;
b) liquid viscosity;
c) liquid temperature;
d) liquid pressure and pressure drop through the meter;
e) meter construction and metering element design;
f) lubricating properties of the liquid;
g) debris and deposits;
h) wear characteristics affecting clearances between key components;
i) secondary components, e.g. solids or gases.
5.3 General performance characteristics
While displacement meters are supplied with a nominal meter factor and performance characteristic,
to achieve accurate measurement, all displacement meters would need calibration. This establishes the
meter factor and general performance characteristics such as the sensor output signal and pressure
drop. Typical characteristics of the variation in error as a function of flowrate through the meter for low
viscosity hydrocarbon usage are shown in Figure 3. The error curve can be divided into distinct parts.
The length of the linear portion of this curve depends primarily on meter size, metering element design
and liquid viscosity. If the meter is sized and operated within this portion of its measurement range, the
use of an arithmetic or weighted mean value of the meter factor (or K-factor) would cause only a small
additional measurement error. This practice can be acceptable in situations where the flowrate is fairly
constant for extended periods, such as in pipeline operations.
As flowrate decreases, the value of the meter factor (error or K-factor) changes sharply. At very low
flowrates, the retarding torque overcomes the driving torque and the rotor stops, even though a very small
flow is still slipping through the meter clearances. This is evident for low viscosity fluids, e.g. gasoline,
where the low liquid viscosity reduces the measurement range, as well as affecting the meter factor.
y
,
,
,
,
,
,
,
,
,
Key
A low flow; high slip; negative error x flowrate (% of full measurement range)
B almost no slip y error (%)
C linear operating range
D increased pressure loss and friction: higher slip
NOTE This is a typical performance curve, with representative error given as percentage error. It might not
represent any particular meter or type. The meter characteristics show a general bias of +0,05 % error.
Figure 3 — General performance curve for PD meter
Meter performance can be significantly affected by liquid viscosity. Increasing the viscosity reduces
slippage and so improve low flow performance, but at high flowrates, higher viscosities cause
an increase in pressure drop, hence internal forces on the meter, which might cause damage to the
internal mechanism. Increasing the clearances within the meter could allow better performance for
high viscosity fluids. However, it would create unacceptable leakage if the meter is then used for low
viscosity liquids.
Generally, a measurement range of 10:1 can be achieved within a linearity band of 0,2 % for fluids with
viscosity less than 20 cSt. However, this might reduce to 2:1 for higher viscosity liquids. Using a meter
in the non-linear portion of the measurement range might be invalid, since the non-repeatability, which
is present in this portion of the characteristic, causes unacceptable errors.
An assessment of meter performance can best be made through calibration at different viscosities and
subsequent use of a meter control chart (see ISO 4124) to record performance with time and fluid. The
control chart plots successive meter factors (or meter K-factors) obtained on a given liquid over a period
of time and calibrations. It enables change in meter factor (or meter K-factor) to be identified over short
or long periods, and gives confidence in the reproducibility of the meter. Further guidance is given in 11.4.
10 © ISO 2017 – All rights reserved

5.4 Pressure drop and back pressure considerations
Pressure drop and outlet pressure are both important to meter linearity. The pressure drop through
the meter is proportional to the square of the flowrate when the meter is operated in the turbulent
flow regime. The pressure drop is, however, proportional to flowrate when the meter operates in
the laminar flow regime (at low velocity and/or in high viscosity fluid). For a constant inlet pressure,
there is a maximum flowrate at which a PD meter can be operated within acceptable error limits
before performance deteriorates. Pressure drop below that of the vapour pressure of the liquid could
allow cavitation or flashing of the fluid to occur locally within the meter or at the outlet. Adequate
performance might require a higher operating pressure than that dictated by overall pressure drop
considerations to ensure this does not occur.
It is difficult to provide firm guidelines on back pressure requirements, as they are dependent on
the fluid, the meter design, and the speed of flow and rotor. Low pressure in the metering chambers,
cavitation, or flashing causes significant measurement error and might damage the meter. Their
occurrence can be indicated by cavitation noise and a sharp change in the value of the meter factor.
5.5 Flow profile
In general, due to the nature of a positive displacement meter physically interrupting the flow, these
meters are mostly unaffected by the flow profile entering the meter. It is prudent, however, to avoid
extremes of fl
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