Measurement of fluid flow in closed conduits — Guidelines on the effects of flow pulsations on flow-measurement instruments

Mesure de débit des fluides dans les conduites fermées — Lignes directrices relatives aux effets des pulsations d'écoulement sur les instruments de mesure de débit

Measurement of fluid flow in closed conduits - Guidelines on the effects of flow pulsations on flow-measurement instruments

General Information

Status
Withdrawn
Publication Date
22-Jul-1998
Withdrawal Date
22-Jul-1998
Current Stage
9599 - Withdrawal of International Standard
Start Date
10-Dec-2013
Completion Date
11-Dec-2013

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TECHNICAL ISO/TR
REPORT 3313
Third edition
1998-08-01
Measurement of fluid flow in closed
conduits — Guidelines on the effects of
flow pulsations on flow-measurement
instruments
Mesure de débit des fluides dans les conduites fermées — Lignes
directrices relatives aux effets de puisations d’écoulement sur
les instruments de mesure de débit
Reference number
ISO/TR 3313:1998(E)
---------------------- Page: 1 ----------------------
ISO/TR 3313:1998(E)
Contents Page

1 Scope ........................................................................................................................................................................1

2 Normative reference ................................................................................................................................................1

3 Definitions ................................................................................................................................................................1

4 Symbols and subscripts .........................................................................................................................................2

4.1 Symbols.................................................................................................................................................................2

4.2 Subscripts .............................................................................................................................................................4

5 Description and detection of pulsating flow.........................................................................................................4

5.1 Nature of pipe flows .............................................................................................................................................4

5.2 Threshold between steady and pulsating flow..................................................................................................4

5.3 Causes of pulsation..............................................................................................................................................6

5.4 Occurrence of pulsating flow conditions in industrial and laboratory flowmeter installations ...................6

5.5 Detection of pulsation and determination of frequency, amplitude and waveform.......................................6

6 Measurement of the mean flowrate of a pulsating flow.......................................................................................9

6.1 Orifice plate, nozzle, and Venturi tube................................................................................................................9

6.2 Turbine flowmeters.............................................................................................................................................19

6.3 Vortex flowmeters...............................................................................................................................................23

Annex A (informative) Orifice plates, nozzles and Venturis — Theoretical considerations..............................25

Annex B (informative) Orifice plates, nozzles and Venturis — Pulsation damping criteria...............................31

Annex C (informative) Turbine flowmeters — Theoretical background and experimental data .......................36

Annex D (informative) Bibliography ........................................................................................................................39

© ISO 1998

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic

or mechanical, including photocopying and microfilm, without permission in writing from the publisher.

International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
---------------------- Page: 2 ----------------------
ISO ISO/TR 3313:1998(E)
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 main task of technical committees is to prepare International Standards, but in exceptional circumstances a

technical committee may propose the publication of a Technical Report of one of the following types:

 type 1, when the required support cannot be obtained for the publication of an International Standard, despite

repeated efforts;

 type 2, when the subject is still under technical development or where for any other reason there is the future

but not immediate possibility of an agreement on an International Standard;

 type 3, when a technical committee has collected data of a different kind from that which is normally published

as an International Standard (“state of the art”, for example).

Technical Reports of types 1 and 2 are subject to review within three years of publication to decide whether they

can be transformed into International Standards. Technical Reports of type 3 do not necessarily have to be

reviewed until the data they provide are considered to be no longer valid or useful.

ISO/TR 3313, which is a Technical Report of type 2, was prepared by Technical Committee ISO/TC 30,

Measurement of fluid flow in closed conduits, Subcommittee SC 2, Pressure differential devices.

This document is being issued in the Technical Report (type 2) series of publications (according to subclause

G.3.2.2 of part 1 of ISO/IEC Directives, 1995) as a “ prospective standard for provisional application ” in the field of

fluid flow in closed conduits because there is an urgent need for guidance on how standards in this field should be

used to meet an identified need.

This document is not regarded as an “ International Standard ”. It is proposed for provisional application so that

information and experience of its use in practice may be gathered. Comments on the content of this document

should be sent to the ISO Central Secretariat.

A review of this Technical Report (type 2) will be carried out not later than three years after its publication with the

options of: extension for another three years; conversion into an International Standard; or withdrawal.

This third edition cancels and replaces the second edition (ISO/TR 3313:1992), of which it constitutes a technical

revision.
Annexes A to D of this Technical Report are for information only.
iii
---------------------- Page: 3 ----------------------
TECHNICAL REPORT ISO ISO/TR 3313:1998(E)
Measurement of fluid flow in closed conduits — Guidelines on
the effects of flow pulsations on flow-measurement instruments
1 Scope

This Technical Report defines pulsating flow, compares it with steady flow, indicates how it can be detected, and

describes the effects it has on orifice plates, nozzles or Venturi tubes, turbine and vortex flowmeters when these

devices are being used to measure fluid flow in a pipe. These particular flowmeter types feature in this Technical

Report because they are amongst those types most susceptible to pulsation effects. Methods for correcting the

flowmeter output signal for errors produced by these effects are described for those flowmeter types for which this is

possible. When correction is not possible, measures to avoid or reduce the problem are indicated. Such measures

include the installation of pulsation damping devices and/or choice of a flowmeter type which is less susceptible to

pulsation effects.

This Technical Report applies to flow in which the pulsations are generated at a single source which is situated

either upstream or downstream of the primary element of the flowmeter. Its applicability is restricted to conditions

where the flow direction does not reverse in the measuring section but there is no restriction on the waveform of the

flow pulsation. The recommendations within this Technical Report apply to both liquid and gas flows although with

the latter the validity may be restricted to gas flows in which the density changes in the measuring section are small

as indicated for the particular type of flowmeter under discussion.
2 Normative reference

The following standard contains provisions which, through reference in this text, constitute provisions of this

Technical Report. At the time of publication the edition indicated was valid. All standards are subject to revision, and

parties to agreements based on this Technical Report are encouraged to investigate the possibility of applying the

most recent edition of the standard indicated below. Members of IEC and ISO maintain registers of currently valid

International Standards.

ISO 5167-1:1991, Measurement of fluid flow by means of pressure differential devices — Part 1: Orifice plates,

nozzles and Venturi tubes inserted in circular cross-section conduits running full.

3 Definitions
For the purposes of this Technical Report, the following definitions apply.
3.1
steady flow

flow in which parameters such as velocity, pressure, density and temperature do not vary significantly enough with

time to prevent measurement to within the required uncertainty of measurement
3.2
pulsating flow

flow in which the flowrate in a measuring section is a function of time but has a constant mean value when averaged

over a sufficiently long period of time, which will depend on the regularity of the pulsation

---------------------- Page: 4 ----------------------
ISO
ISO/TR 3313:1998(E)
NOTE 1 Pulsating flow can be divided into two categories:
 periodic pulsating flow;
 randomly fluctuating flow.

NOTE 2 For further amplification of what constitutes steady or pulsating flow see 5.1 and 5.2.

NOTE 3 Unless otherwise stated in this Technical Report the term “pulsating flow” is always used to describe periodic

pulsating flow.
4 Symbols and subscripts
4.1 Symbols
A area
A area of the throat of a Venturi nozzle
A turbine blade aspect ratio

a , b , c amplitude of the rth harmonic component in the undamped or damped pulsation

r r r
Bbf/q , dimensionless dynamic response parameter
b turbine flowmeter dynamic response parameter
C turbine blade chord length
C contraction coefficient
C discharge coefficient
velocity coefficient
c speed of sound
D internal diameter of the tube
d throat bore of orifice, nozzle or Venturi
E residual error in time-mean flowrate when calculated using the quantity D p
E total error in the time-mean flowrate
f turbine flowmeter output signal, proportional to volumetric flowrate
f pulsation frequency
f resonant frequency
f vortex-shedding frequency
H harmonic distortion factor
Ho Hodgson number
I moment of inertia

I , I moments of inertia of turbine rotor and fluid contained in rotor envelope respectively

R F
kD/ relative roughness of pipe wall
L turbine blade length
L effective axial length
---------------------- Page: 5 ----------------------
ISO
ISO/TR 3313:1998(E)
l impulse line length for differential pressure (DP) measurement device
m = b orifice or nozzle throat to pipe area ratio
N number of blades on turbine rotor
p pressure (absolute)
q mass flowrate
volume flowrate
R turbine blade mean radius
Re Reynolds number
r , r turbine blade hub and tip radii respectively
h t
Sr Strouhal number
Sr Strouhal number based on orifice diameter
t time
t turbine blade thickness
U axial bulk-mean velocity
U bulk-mean velocity based on orifice diameter
V volume
X temporal inertia term for short pulsation wavelengths
a U′ /U
rms
b orifice or nozzle throat to pipe diameter ratio
Dp differential pressure
Dv pressure loss
e expansibility factor for steady flow conditions
g ratio of specific thermal capacities (c /c )
p V
h blade ‘airfoil efficiency’
k isentropic index (= g for a perfect gas)
q phase angle
r fluid density
r turbine blade material density
t =pp/ pressure ratio
w = 2pf angular pulsation frequency
y maximum allowable relative error
---------------------- Page: 6 ----------------------
ISO
ISO/TR 3313:1998(E)
4.2 Subscripts
o pulsation source
p measured under pulsating flow conditions, possibly damped
po measured under pulsating flow conditions before damping
rms root mean square
ss measured under steady flow conditions
U an over-bar indicates the time-mean value
1,2 measuring sections
¢ indicates fluctuating component about mean value, e.g. U¢
5 Description and detection of pulsating flow
5.1 Nature of pipe flows

Truly steady pipe flow is only found in laminar flow conditions which can normally only exist when the pipe Reynolds

number, Re, is below about 2 000. Most industrial pipe flows have higher Reynolds numbers and are turbulent which

means that they are only statistically steady. Such flows contain continual irregular and random fluctuations in

quantities such as velocity, pressure and temperature. Nevertheless, if the conditions are similar to those which are

typical of fully developed turbulent pipe flow and there is no periodic pulsation, the provisions of such standards as

ISO 5167-1 apply.

The magnitude of the turbulent fluctuations increases with pipe roughness and for this reason ISO 5167-1 stipulates

a maximum allowable relative roughness, / , of the upstream pipe for each type of primary device covered by

k D
ISO 5167-1.

ISO 5167-1, however, cannot be applied to flows which contain any periodic flow variation or pulsation.

5.2 Threshold between steady and pulsating flow

If the amplitude of the periodic flowrate variations is sufficiently small there should not be any error in the indicated

flowrate greater than the normal measurement uncertainty. It is possible to define amplitude thresholds for both

differential pressure (DP) type flowmeters and turbine flowmeters without reference to pulsation frequency. It is also

possible to do this for vortex flowmeters but extreme caution is necessary if even the smallest amplitude is known to

be present in the flow.

For DP-type flowmeters, the threshold is relevant when slow-response DP cells are being used. In the case of

turbine flowmeters, the threshold value is relevant when there is any doubt about the ability of the rotor to respond

to the periodic velocity fluctuations. In the case of a vortex flowmeter the pulsation frequency relative to the vortex-

shedding frequency is a much more important parameter than the velocity pulsation amplitude.

5.2.1 Differential pressure (DP) type flowmeters

The threshold can be defined in terms of the velocity pulsation amplitude such that the flow can be treated as

steady if
rms
¶ 0,05 . . . (1)
where U is the instantaneous bulk-mean axial velocity such that
UU= ′+U . . . (2)
---------------------- Page: 7 ----------------------
ISO
ISO/TR 3313:1998(E)
where
U′ is the periodic velocity fluctuation;
U is the time-mean value.
The threshold in terms of the equivalent DP pulsation amplitude is:
Dp′
p,rms
¶ 0,10 . . . (3)

where Dp is the instantaneous differential pressure across the tappings of the primary device such that

DDpp=+Dp′
pp p . . . (4)
where
Dp is the time-mean value;
Dp¢ is the periodic differential pressure fluctuation.

To determine the velocity pulsation amplitude it will be necessary to use one of the techniques described in 5.5 such

as laser Doppler or thermal anemometry. To determine the DP pulsation amplitude it will be necessary to use a fast-

response DP sensor and to observe the rules governing the design of the complete secondary instrumentation

system as described in 6.1.3.
5.2.2 Turbine flowmeters

At a given velocity pulsation amplitude a turbine flowmeter will tend to read high as the frequency of pulsation

increases and exceeds the frequency at which the turbine rotor can respond faithfully to the velocity fluctuations.

The positive systematic error will reach a plateau value depending on the amplitude and thus the threshold

amplitude can be defined such that the resulting maximum systematic error is still within the general measurement

uncertainty. For example, if the overall measurement uncertainty is greater than or equal to 0,1 % then we can

assume that a systematic error due to pulsation of 0,1 % or less will not significantly increase the overall

measurement uncertainty.

The velocity amplitude of sinusoidal pulsation, /U , that will produce a systematic error of 0,1 % in a turbine

rms
flowmeter is 3,5 %. Thus the threshold for sinusoidal pulsation is given by:
rms
¶ 0,035 . . . (5)

Techniques such as laser Doppler and thermal anemometry can be used to determine the velocity pulsation

amplitude. If the flowmeter output is a pulse train at the blade passing frequency and if the rotor inertia is known,

then signal analysis can be used to determine the flow pulsation amplitude as described in 6.2.

5.2.3 Vortex flowmeters

A vortex flowmeter is subject to very large pulsation errors when the vortex-shedding process locks in to the flow

pulsation. There is a danger of this happening when the pulsation frequency is near the vortex-shedding frequency.

At a sufficiently low amplitude, locking-in does not occur and flow-metering errors due to pulsation will be negligible.

This threshold amplitude, however, is only about 3 % of the mean velocity and is comparable to the velocity

turbulence amplitude. The consequences of not detecting the pulsation or erroneously assuming the amplitude is

below the threshold can be very serious. This issue is discussed further in 6.3.
---------------------- Page: 8 ----------------------
ISO
ISO/TR 3313:1998(E)
5.3 Causes of pulsation

Pulsation occurs commonly in industrial pipe flows. They may be generated by rotary or reciprocating positive

displacement engines, compressors, blowers and pumps. Rotodynamic machines may also induce small pulsation

at blade passing frequencies. Pulsation can also be produced by positive-displacement flowmeters. Vibration,

particularly at resonance, of pipe runs and flow control equipment is also a potential source of flow pulsation, as are

periodic actions of flow controllers, e.g. valve “hunting” and governor oscillations. Pulsation may also be generated

by flow separation within pipe fittings, valves, or rotary machines (e.g. compressor surge).

Flow pulsation can also be due to hydrodynamic oscillations generated by geometrical features of the flow system

and multiphase flows (e.g. slugging). Vortex shedding from bluff bodies such as thermometer wells, or trash grids,

or vortex-shedding flowmeters fall into this category. Self-excited flow oscillations at tee-branch connections are

another example.

5.4 Occurrence of pulsating flow conditions in industrial and laboratory flowmeter installations

In industrial flows, there is often no obvious indication of the presence of pulsation, and the associated errors,

because of the slow-response times and heavy damping of the pressure and flow instrumentation commonly used.

Whenever factors such as those indicated in 5.3 are present, there is the possibility of flow pulsation occurring. It

should also be appreciated that pulsation can travel upstream as well as downstream and thus possible pulsation

sources could be on either side of the flowmeter installation. However, amplitudes may be small and, depending on

the distance from pulsation source to flowmeter, may be attenuated by compressibility effects (in both liquids and

gases) to undetectable levels at the flowmeter location. Pulsation frequencies range from fractions of a hertz to a

few hundred hertz; pulsation amplitudes relative to mean flow vary from a few percent to 100 % or larger. At low

percentage amplitudes the question arises of discrimination between pulsation and turbulence.

Flow pulsation can be expected to occur in various situations in petrochemical and process industries, natural gas

distribution flows at end-user locations and internal combustion engine flow systems. Flow-metering calibration

systems may also experience pulsation arising from, for example, rotodynamic pump blade passing effects and the

effects of rotary positive-displacement flowmeters.

5.5 Detection of pulsation and determination of frequency, amplitude and waveform

If the presence of pulsation is suspected then there are various techniques available to determine the flow pulsation

characteristics.
5.5.1 Characteristics of the ideal pulsation sensor

The ideal sensor would be non-intrusive, would measure mass flowrate, or bulk flow velocity, and would have a

bandwidth from decihertz to several kilohertz. The sensor would respond to both liquids and gases and not require

any supplementary flow seeding. The technique would not require optical transparency or constant fluid

temperature. The sensor would be uninfluenced by pipe wall material, transparency or thickness. The device would

have no moving parts, its response would be linear, its calibration reliable and unaffected by changes in ambient

temperature.
5.5.2 Non-intrusive techniques
5.5.2.1 Optical: laser Doppler anemometry (LDA)

This technology is readily available, but expensive. Measurement of point velocity on the tube axis will allow an

estimate only of bulk flow pulsation amplitude and waveform but, for constant frequency pulsation, accurate

frequency measurements can be made. Optical access to an optically transparent fluid is either by provision of a

transparent tube section, or insertion of a probe with fibre-optic coupling. With the exception of detecting low

frequency pulsation, supplementary seeding of the flow would probably be required to produce an adequate

bandwidth. LDA characteristics are comprehensively described in reference [1] (see annex D).

---------------------- Page: 9 ----------------------
ISO
ISO/TR 3313:1998(E)
5.5.2.2 Acoustic: Doppler shift; transit time

Non-intrusive acoustic techniques are suitable for liquid flows only, because for gas flows there is poor

acoustic-impedance match between the pipe wall and flowing gases. For the externally mounted transmitter and

receiver, usually close-coupled to the tube wall, an acoustically transparent signal path is essential. The Doppler

shift technique may require flow seeding to provide adequate scattering. Instruments for point velocity

measurements are available which, as for the LDA, allow only an estimate of bulk flow pulsation amplitude and

waveform. More recently, Doppler derived “instantaneous” full-velocity profile instruments [2] allow much closer

estimates of bulk flow pulsation characteristics. Transit time instruments measure an average velocity, most

commonly along a diagonal path across the flow. All acoustic techniques are limited in bandwidth by the

requirement that reflections from one pulse of ultrasound must decay before transmission of the next pulse. It

should be noted that many commercial instruments do not provide the signal processing required to resolve

unsteady flow components. A recent investigation by Hakansson [3] on a transit time, intrusive-type ultrasonic

flowmeter for gases subjected to pulsating flows showed that only small shifts in the calibration took place and that

these were attributable to the changing velocity profile.
5.5.2.3 Electromagnetic flowmeters

When the existing flowmeter installation is an electromagnetic device, then, if it is of the pulsed d.c. field type (likely

maximum d.c. pulse frequency a few hundred hertz), there is the capability to resolve flow pulsation up to

frequencies approximately five times below the excitation frequency. This technique is only suitable for liquids with

an adequate electrical conductivity. It provides a measure of bulk flow pulsation, although there is some

dependence upon velocity profile shape [4].
5.5.3 Insertion devices
5.5.3.1 Thermal anemometry

The probes used measure point velocity and relatively rugged (e.g. fibre-film) sensors are available for industrial

flows. These probes generally have an adequate bandwidth, but the amplitude response is inherently non-linear. As

with other point velocity techniques, pulsation amplitude and waveform can only be estimated. Estimates of

pulsation velocity amplitude relative to mean velocity may be made without calibration. The r.m.s. value of the

fluctuating velocity component can be determined by using a true r.m.s. flowmeter to measure the fluctuating

component of the linearized anemometer output voltage. Mean-sensing r.m.s. flowmeters should not be used as

these will only read correctly for sinusoidal waveforms. Accurate frequency measurements from spectral analysis

can be made for constant frequency pulsation.

Applications are limited to clean, relatively cool, non-flammable and non-hostile fluids. Cleanness of flow is very

important; even nominally clean flows can result in rapid fouling of probes with a consequent dramatic loss of

response. A constant temperature flow is desirable although a slowly varying fluid temperature can be

accommodated.
5.5.3.2 Other techniques

Insertion versions of both acoustic and electromagnetic flowmeters are available. Transit-time acoustic

measurements can be made in gas flows when the transmitter and receiver are directly coupled to the flow [5],

although this may require a permanent insertion. Again there is the limitation of a lack of commercially available

instrumentation with the necessary signal processing to resolve time-varying velocity components.

Insertion electromagnetic flowmeters are not widely available and are subject to the same bandwidth limitations as

the tube version, due to the maximum sampling frequency of the signal.
5.5.4 Signal analysis on existing flowmeter outputs: software tools
5.5.4.1 Orifice plate with fast-response DP sensor

A fast-response secondary measurement system is capable of correctly following the time-varying pressure

difference produced by the primary instrument provided the rules given in 6.1.3.2 can be followed. In principle, a

---------------------- Page: 10 ----------------------
ISO
ISO/TR 3313:1998(E)

numerical solution of the pressure difference/flow relationship derived from the quasi-steady temporal inertia model,

equation (A.9) would then provide an approximation to the instantaneous flow. The square-root error would not be

present, although other measurement uncertainties (e.g. C variations, compressibility effects) produced by the

pulsation would be. Successive numerical solutions would then provide an approximation to the flow as a function of

time and, hence, amplitude and waveform information. Frequency information can be determined directly from the

measured pressure difference. At present, there is no software tool described for this implementation.

However, the maximum probable value of q¢ can be approximately inferred from a measurement of Dp¢

Vo,rms po,rms
using one of the following two inequalities:
Dp′
Vo,rms po,rms
< . . . (6)
2 Dp
1/2
 
Vo,rms
< −1 . . . (7)
 
1/2
 
11+−DDpp′ /
po,rms po
 
where

Dp¢ is the r.m.s. value of the fluctuating component of the differential pressure across the primary element

po,rms
measured using a fast-response secondary measurement system;

Dp is the differential pressure that would be measured across the primary element under steady flow

conditions with the same time-mean flowrate;

Dp is the time-mean differential pressure that would be measured across the primary element under

undamped pulsating flow conditions;

Dp is the instantaneous differential pressure across the primary element under undamped pulsating flow

conditions where
DDpp=+Dp′
po po po

NOTE 1 Reliable measurements of Dp and Dp¢ can only be obtained if the recommendations given in 6.1.2 and 6.1.3

po,rms
are strictly adhered to.

NOTE 2 If it is possible to determine Dp , equation (6) is to be preferred. Equation (7) will only give reliable results if

0,5.
DDpp′ ,
( )
po,rms po
5.5.4.2 Turbine flowmeter

The raw signal from a turbine flowmeter is in the form of an approximately sinusoidal voltage with a level which

varies with the flow but is usually in the range 10 mV to 1 V peak to peak. In most installations this signal is

amplified and converted to a stream of pulses. The extraction of information about the amplitude and waveform of

any flow pulsation from the variations in the frequency of this pulse train depends on the value of the dynamic

response parameter of the flowmeter. Flowmeter manufacturers do not normally specify the response parameter for

their flowmeters and the measurements which would be necessary to determine it are unlikely to be possible on an

existing flowmeter installation. However, the dependency of the parameter on the geometry of the turbine rotor and

on the fluid density is discussed in 6.2.1.3, and the range of values which have been found for typical flowmeters is

presented in 6.2.1.5, table 1.

The response of a turbine flowmeter to flow pulsation is discussed in 6.2.1. It can range from the ability to follow the

pulsation almost perfectly (medium to large flowmeters in liquid flows) to an almost total inability to follow the

pulsation (small to medium flowmeters in gas flows with moderate to high frequencies of pulsation). This latter

---------------------- Page: 11 ----------------------
ISO
ISO/TR 3313:1998(E)

condition is a worst case for a turbine flowmeter installation because not only does the flowmeter output not show

significant pulsation but if the flow pulsation is of significant magnitude, the apparently steady flowmeter output will

not be a correct represen
...

SLOVENSKI STANDARD
SIST ISO/TR 3313:2001
01-november-2001
Measurement of fluid flow in closed conduits - Guidelines on the effects of flow
pulsations on flow-measurement instruments

Measurement of fluid flow in closed conduits -- Guidelines on the effects of flow

pulsations on flow-measurement instruments

Mesure de débit des fluides dans les conduites fermées -- Lignes directrices relatives

aux effets des pulsations d'écoulement sur les instruments de mesure de débit
Ta slovenski standard je istoveten z: ISO/TR 3313:1998
ICS:
17.120.10 Pretok v zaprtih vodih Flow in closed conduits
SIST ISO/TR 3313:2001 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST ISO/TR 3313:2001
---------------------- Page: 2 ----------------------
SIST ISO/TR 3313:2001
TECHNICAL ISO/TR
REPORT 3313
Third edition
1998-08-01
Measurement of fluid flow in closed
conduits — Guidelines on the effects of
flow pulsations on flow-measurement
instruments
Mesure de débit des fluides dans les conduites fermées — Lignes
directrices relatives aux effets de puisations d’écoulement sur
les instruments de mesure de débit
Reference number
ISO/TR 3313:1998(E)
---------------------- Page: 3 ----------------------
SIST ISO/TR 3313:2001
ISO/TR 3313:1998(E)
Contents Page

1 Scope ........................................................................................................................................................................1

2 Normative reference ................................................................................................................................................1

3 Definitions ................................................................................................................................................................1

4 Symbols and subscripts .........................................................................................................................................2

4.1 Symbols.................................................................................................................................................................2

4.2 Subscripts .............................................................................................................................................................4

5 Description and detection of pulsating flow.........................................................................................................4

5.1 Nature of pipe flows .............................................................................................................................................4

5.2 Threshold between steady and pulsating flow..................................................................................................4

5.3 Causes of pulsation..............................................................................................................................................6

5.4 Occurrence of pulsating flow conditions in industrial and laboratory flowmeter installations ...................6

5.5 Detection of pulsation and determination of frequency, amplitude and waveform.......................................6

6 Measurement of the mean flowrate of a pulsating flow.......................................................................................9

6.1 Orifice plate, nozzle, and Venturi tube................................................................................................................9

6.2 Turbine flowmeters.............................................................................................................................................19

6.3 Vortex flowmeters...............................................................................................................................................23

Annex A (informative) Orifice plates, nozzles and Venturis — Theoretical considerations..............................25

Annex B (informative) Orifice plates, nozzles and Venturis — Pulsation damping criteria...............................31

Annex C (informative) Turbine flowmeters — Theoretical background and experimental data .......................36

Annex D (informative) Bibliography ........................................................................................................................39

© ISO 1998

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic

or mechanical, including photocopying and microfilm, without permission in writing from the publisher.

International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
---------------------- Page: 4 ----------------------
SIST ISO/TR 3313:2001
ISO ISO/TR 3313:1998(E)
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 main task of technical committees is to prepare International Standards, but in exceptional circumstances a

technical committee may propose the publication of a Technical Report of one of the following types:

 type 1, when the required support cannot be obtained for the publication of an International Standard, despite

repeated efforts;

 type 2, when the subject is still under technical development or where for any other reason there is the future

but not immediate possibility of an agreement on an International Standard;

 type 3, when a technical committee has collected data of a different kind from that which is normally published

as an International Standard (“state of the art”, for example).

Technical Reports of types 1 and 2 are subject to review within three years of publication to decide whether they

can be transformed into International Standards. Technical Reports of type 3 do not necessarily have to be

reviewed until the data they provide are considered to be no longer valid or useful.

ISO/TR 3313, which is a Technical Report of type 2, was prepared by Technical Committee ISO/TC 30,

Measurement of fluid flow in closed conduits, Subcommittee SC 2, Pressure differential devices.

This document is being issued in the Technical Report (type 2) series of publications (according to subclause

G.3.2.2 of part 1 of ISO/IEC Directives, 1995) as a “ prospective standard for provisional application ” in the field of

fluid flow in closed conduits because there is an urgent need for guidance on how standards in this field should be

used to meet an identified need.

This document is not regarded as an “ International Standard ”. It is proposed for provisional application so that

information and experience of its use in practice may be gathered. Comments on the content of this document

should be sent to the ISO Central Secretariat.

A review of this Technical Report (type 2) will be carried out not later than three years after its publication with the

options of: extension for another three years; conversion into an International Standard; or withdrawal.

This third edition cancels and replaces the second edition (ISO/TR 3313:1992), of which it constitutes a technical

revision.
Annexes A to D of this Technical Report are for information only.
iii
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TECHNICAL REPORT ISO ISO/TR 3313:1998(E)
Measurement of fluid flow in closed conduits — Guidelines on
the effects of flow pulsations on flow-measurement instruments
1 Scope

This Technical Report defines pulsating flow, compares it with steady flow, indicates how it can be detected, and

describes the effects it has on orifice plates, nozzles or Venturi tubes, turbine and vortex flowmeters when these

devices are being used to measure fluid flow in a pipe. These particular flowmeter types feature in this Technical

Report because they are amongst those types most susceptible to pulsation effects. Methods for correcting the

flowmeter output signal for errors produced by these effects are described for those flowmeter types for which this is

possible. When correction is not possible, measures to avoid or reduce the problem are indicated. Such measures

include the installation of pulsation damping devices and/or choice of a flowmeter type which is less susceptible to

pulsation effects.

This Technical Report applies to flow in which the pulsations are generated at a single source which is situated

either upstream or downstream of the primary element of the flowmeter. Its applicability is restricted to conditions

where the flow direction does not reverse in the measuring section but there is no restriction on the waveform of the

flow pulsation. The recommendations within this Technical Report apply to both liquid and gas flows although with

the latter the validity may be restricted to gas flows in which the density changes in the measuring section are small

as indicated for the particular type of flowmeter under discussion.
2 Normative reference

The following standard contains provisions which, through reference in this text, constitute provisions of this

Technical Report. At the time of publication the edition indicated was valid. All standards are subject to revision, and

parties to agreements based on this Technical Report are encouraged to investigate the possibility of applying the

most recent edition of the standard indicated below. Members of IEC and ISO maintain registers of currently valid

International Standards.

ISO 5167-1:1991, Measurement of fluid flow by means of pressure differential devices — Part 1: Orifice plates,

nozzles and Venturi tubes inserted in circular cross-section conduits running full.

3 Definitions
For the purposes of this Technical Report, the following definitions apply.
3.1
steady flow

flow in which parameters such as velocity, pressure, density and temperature do not vary significantly enough with

time to prevent measurement to within the required uncertainty of measurement
3.2
pulsating flow

flow in which the flowrate in a measuring section is a function of time but has a constant mean value when averaged

over a sufficiently long period of time, which will depend on the regularity of the pulsation

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NOTE 1 Pulsating flow can be divided into two categories:
 periodic pulsating flow;
 randomly fluctuating flow.

NOTE 2 For further amplification of what constitutes steady or pulsating flow see 5.1 and 5.2.

NOTE 3 Unless otherwise stated in this Technical Report the term “pulsating flow” is always used to describe periodic

pulsating flow.
4 Symbols and subscripts
4.1 Symbols
A area
A area of the throat of a Venturi nozzle
A turbine blade aspect ratio

a , b , c amplitude of the rth harmonic component in the undamped or damped pulsation

r r r
Bbf/q , dimensionless dynamic response parameter
b turbine flowmeter dynamic response parameter
C turbine blade chord length
C contraction coefficient
C discharge coefficient
velocity coefficient
c speed of sound
D internal diameter of the tube
d throat bore of orifice, nozzle or Venturi
E residual error in time-mean flowrate when calculated using the quantity D p
E total error in the time-mean flowrate
f turbine flowmeter output signal, proportional to volumetric flowrate
f pulsation frequency
f resonant frequency
f vortex-shedding frequency
H harmonic distortion factor
Ho Hodgson number
I moment of inertia

I , I moments of inertia of turbine rotor and fluid contained in rotor envelope respectively

R F
kD/ relative roughness of pipe wall
L turbine blade length
L effective axial length
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l impulse line length for differential pressure (DP) measurement device
m = b orifice or nozzle throat to pipe area ratio
N number of blades on turbine rotor
p pressure (absolute)
q mass flowrate
volume flowrate
R turbine blade mean radius
Re Reynolds number
r , r turbine blade hub and tip radii respectively
h t
Sr Strouhal number
Sr Strouhal number based on orifice diameter
t time
t turbine blade thickness
U axial bulk-mean velocity
U bulk-mean velocity based on orifice diameter
V volume
X temporal inertia term for short pulsation wavelengths
a U′ /U
rms
b orifice or nozzle throat to pipe diameter ratio
Dp differential pressure
Dv pressure loss
e expansibility factor for steady flow conditions
g ratio of specific thermal capacities (c /c )
p V
h blade ‘airfoil efficiency’
k isentropic index (= g for a perfect gas)
q phase angle
r fluid density
r turbine blade material density
t =pp/ pressure ratio
w = 2pf angular pulsation frequency
y maximum allowable relative error
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4.2 Subscripts
o pulsation source
p measured under pulsating flow conditions, possibly damped
po measured under pulsating flow conditions before damping
rms root mean square
ss measured under steady flow conditions
U an over-bar indicates the time-mean value
1,2 measuring sections
¢ indicates fluctuating component about mean value, e.g. U¢
5 Description and detection of pulsating flow
5.1 Nature of pipe flows

Truly steady pipe flow is only found in laminar flow conditions which can normally only exist when the pipe Reynolds

number, Re, is below about 2 000. Most industrial pipe flows have higher Reynolds numbers and are turbulent which

means that they are only statistically steady. Such flows contain continual irregular and random fluctuations in

quantities such as velocity, pressure and temperature. Nevertheless, if the conditions are similar to those which are

typical of fully developed turbulent pipe flow and there is no periodic pulsation, the provisions of such standards as

ISO 5167-1 apply.

The magnitude of the turbulent fluctuations increases with pipe roughness and for this reason ISO 5167-1 stipulates

a maximum allowable relative roughness, / , of the upstream pipe for each type of primary device covered by

k D
ISO 5167-1.

ISO 5167-1, however, cannot be applied to flows which contain any periodic flow variation or pulsation.

5.2 Threshold between steady and pulsating flow

If the amplitude of the periodic flowrate variations is sufficiently small there should not be any error in the indicated

flowrate greater than the normal measurement uncertainty. It is possible to define amplitude thresholds for both

differential pressure (DP) type flowmeters and turbine flowmeters without reference to pulsation frequency. It is also

possible to do this for vortex flowmeters but extreme caution is necessary if even the smallest amplitude is known to

be present in the flow.

For DP-type flowmeters, the threshold is relevant when slow-response DP cells are being used. In the case of

turbine flowmeters, the threshold value is relevant when there is any doubt about the ability of the rotor to respond

to the periodic velocity fluctuations. In the case of a vortex flowmeter the pulsation frequency relative to the vortex-

shedding frequency is a much more important parameter than the velocity pulsation amplitude.

5.2.1 Differential pressure (DP) type flowmeters

The threshold can be defined in terms of the velocity pulsation amplitude such that the flow can be treated as

steady if
rms
¶ 0,05 . . . (1)
where U is the instantaneous bulk-mean axial velocity such that
UU= ′+U . . . (2)
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where
U′ is the periodic velocity fluctuation;
U is the time-mean value.
The threshold in terms of the equivalent DP pulsation amplitude is:
Dp′
p,rms
¶ 0,10 . . . (3)

where Dp is the instantaneous differential pressure across the tappings of the primary device such that

DDpp=+Dp′
pp p . . . (4)
where
Dp is the time-mean value;
Dp¢ is the periodic differential pressure fluctuation.

To determine the velocity pulsation amplitude it will be necessary to use one of the techniques described in 5.5 such

as laser Doppler or thermal anemometry. To determine the DP pulsation amplitude it will be necessary to use a fast-

response DP sensor and to observe the rules governing the design of the complete secondary instrumentation

system as described in 6.1.3.
5.2.2 Turbine flowmeters

At a given velocity pulsation amplitude a turbine flowmeter will tend to read high as the frequency of pulsation

increases and exceeds the frequency at which the turbine rotor can respond faithfully to the velocity fluctuations.

The positive systematic error will reach a plateau value depending on the amplitude and thus the threshold

amplitude can be defined such that the resulting maximum systematic error is still within the general measurement

uncertainty. For example, if the overall measurement uncertainty is greater than or equal to 0,1 % then we can

assume that a systematic error due to pulsation of 0,1 % or less will not significantly increase the overall

measurement uncertainty.

The velocity amplitude of sinusoidal pulsation, /U , that will produce a systematic error of 0,1 % in a turbine

rms
flowmeter is 3,5 %. Thus the threshold for sinusoidal pulsation is given by:
rms
¶ 0,035 . . . (5)

Techniques such as laser Doppler and thermal anemometry can be used to determine the velocity pulsation

amplitude. If the flowmeter output is a pulse train at the blade passing frequency and if the rotor inertia is known,

then signal analysis can be used to determine the flow pulsation amplitude as described in 6.2.

5.2.3 Vortex flowmeters

A vortex flowmeter is subject to very large pulsation errors when the vortex-shedding process locks in to the flow

pulsation. There is a danger of this happening when the pulsation frequency is near the vortex-shedding frequency.

At a sufficiently low amplitude, locking-in does not occur and flow-metering errors due to pulsation will be negligible.

This threshold amplitude, however, is only about 3 % of the mean velocity and is comparable to the velocity

turbulence amplitude. The consequences of not detecting the pulsation or erroneously assuming the amplitude is

below the threshold can be very serious. This issue is discussed further in 6.3.
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5.3 Causes of pulsation

Pulsation occurs commonly in industrial pipe flows. They may be generated by rotary or reciprocating positive

displacement engines, compressors, blowers and pumps. Rotodynamic machines may also induce small pulsation

at blade passing frequencies. Pulsation can also be produced by positive-displacement flowmeters. Vibration,

particularly at resonance, of pipe runs and flow control equipment is also a potential source of flow pulsation, as are

periodic actions of flow controllers, e.g. valve “hunting” and governor oscillations. Pulsation may also be generated

by flow separation within pipe fittings, valves, or rotary machines (e.g. compressor surge).

Flow pulsation can also be due to hydrodynamic oscillations generated by geometrical features of the flow system

and multiphase flows (e.g. slugging). Vortex shedding from bluff bodies such as thermometer wells, or trash grids,

or vortex-shedding flowmeters fall into this category. Self-excited flow oscillations at tee-branch connections are

another example.

5.4 Occurrence of pulsating flow conditions in industrial and laboratory flowmeter installations

In industrial flows, there is often no obvious indication of the presence of pulsation, and the associated errors,

because of the slow-response times and heavy damping of the pressure and flow instrumentation commonly used.

Whenever factors such as those indicated in 5.3 are present, there is the possibility of flow pulsation occurring. It

should also be appreciated that pulsation can travel upstream as well as downstream and thus possible pulsation

sources could be on either side of the flowmeter installation. However, amplitudes may be small and, depending on

the distance from pulsation source to flowmeter, may be attenuated by compressibility effects (in both liquids and

gases) to undetectable levels at the flowmeter location. Pulsation frequencies range from fractions of a hertz to a

few hundred hertz; pulsation amplitudes relative to mean flow vary from a few percent to 100 % or larger. At low

percentage amplitudes the question arises of discrimination between pulsation and turbulence.

Flow pulsation can be expected to occur in various situations in petrochemical and process industries, natural gas

distribution flows at end-user locations and internal combustion engine flow systems. Flow-metering calibration

systems may also experience pulsation arising from, for example, rotodynamic pump blade passing effects and the

effects of rotary positive-displacement flowmeters.

5.5 Detection of pulsation and determination of frequency, amplitude and waveform

If the presence of pulsation is suspected then there are various techniques available to determine the flow pulsation

characteristics.
5.5.1 Characteristics of the ideal pulsation sensor

The ideal sensor would be non-intrusive, would measure mass flowrate, or bulk flow velocity, and would have a

bandwidth from decihertz to several kilohertz. The sensor would respond to both liquids and gases and not require

any supplementary flow seeding. The technique would not require optical transparency or constant fluid

temperature. The sensor would be uninfluenced by pipe wall material, transparency or thickness. The device would

have no moving parts, its response would be linear, its calibration reliable and unaffected by changes in ambient

temperature.
5.5.2 Non-intrusive techniques
5.5.2.1 Optical: laser Doppler anemometry (LDA)

This technology is readily available, but expensive. Measurement of point velocity on the tube axis will allow an

estimate only of bulk flow pulsation amplitude and waveform but, for constant frequency pulsation, accurate

frequency measurements can be made. Optical access to an optically transparent fluid is either by provision of a

transparent tube section, or insertion of a probe with fibre-optic coupling. With the exception of detecting low

frequency pulsation, supplementary seeding of the flow would probably be required to produce an adequate

bandwidth. LDA characteristics are comprehensively described in reference [1] (see annex D).

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5.5.2.2 Acoustic: Doppler shift; transit time

Non-intrusive acoustic techniques are suitable for liquid flows only, because for gas flows there is poor

acoustic-impedance match between the pipe wall and flowing gases. For the externally mounted transmitter and

receiver, usually close-coupled to the tube wall, an acoustically transparent signal path is essential. The Doppler

shift technique may require flow seeding to provide adequate scattering. Instruments for point velocity

measurements are available which, as for the LDA, allow only an estimate of bulk flow pulsation amplitude and

waveform. More recently, Doppler derived “instantaneous” full-velocity profile instruments [2] allow much closer

estimates of bulk flow pulsation characteristics. Transit time instruments measure an average velocity, most

commonly along a diagonal path across the flow. All acoustic techniques are limited in bandwidth by the

requirement that reflections from one pulse of ultrasound must decay before transmission of the next pulse. It

should be noted that many commercial instruments do not provide the signal processing required to resolve

unsteady flow components. A recent investigation by Hakansson [3] on a transit time, intrusive-type ultrasonic

flowmeter for gases subjected to pulsating flows showed that only small shifts in the calibration took place and that

these were attributable to the changing velocity profile.
5.5.2.3 Electromagnetic flowmeters

When the existing flowmeter installation is an electromagnetic device, then, if it is of the pulsed d.c. field type (likely

maximum d.c. pulse frequency a few hundred hertz), there is the capability to resolve flow pulsation up to

frequencies approximately five times below the excitation frequency. This technique is only suitable for liquids with

an adequate electrical conductivity. It provides a measure of bulk flow pulsation, although there is some

dependence upon velocity profile shape [4].
5.5.3 Insertion devices
5.5.3.1 Thermal anemometry

The probes used measure point velocity and relatively rugged (e.g. fibre-film) sensors are available for industrial

flows. These probes generally have an adequate bandwidth, but the amplitude response is inherently non-linear. As

with other point velocity techniques, pulsation amplitude and waveform can only be estimated. Estimates of

pulsation velocity amplitude relative to mean velocity may be made without calibration. The r.m.s. value of the

fluctuating velocity component can be determined by using a true r.m.s. flowmeter to measure the fluctuating

component of the linearized anemometer output voltage. Mean-sensing r.m.s. flowmeters should not be used as

these will only read correctly for sinusoidal waveforms. Accurate frequency measurements from spectral analysis

can be made for constant frequency pulsation.

Applications are limited to clean, relatively cool, non-flammable and non-hostile fluids. Cleanness of flow is very

important; even nominally clean flows can result in rapid fouling of probes with a consequent dramatic loss of

response. A constant temperature flow is desirable although a slowly varying fluid temperature can be

accommodated.
5.5.3.2 Other techniques

Insertion versions of both acoustic and electromagnetic flowmeters are available. Transit-time acoustic

measurements can be made in gas flows when the transmitter and receiver are directly coupled to the flow [5],

although this may require a permanent insertion. Again there is the limitation of a lack of commercially available

instrumentation with the necessary signal processing to resolve time-varying velocity components.

Insertion electromagnetic flowmeters are not widely available and are subject to the same bandwidth limitations as

the tube version, due to the maximum sampling frequency of the signal.
5.5.4 Signal analysis on existing flowmeter outputs: software tools
5.5.4.1 Orifice plate with fast-response DP sensor

A fast-response secondary measurement system is capable of correctly following the time-varying pressure

difference produced by the primary instrument provided the rules given in 6.1.3.2 can be followed. In principle, a

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numerical solution of the pressure difference/flow relationship derived from the quasi-steady temporal inertia model,

equation (A.9) would then provide an approximation to the instantaneous flow. The square-root error would not be

present, although other measurement uncertainties (e.g. C variations, compressibility effects) produced by the

pulsation would be. Successive numerical solutions would then provide an approximation to the flow as a function of

time and, hence, amplitude and waveform information. Frequency information can be determined directly from the

measured pressure difference. At present, there is no software tool described for this implementation.

However, the maximum probable value of q¢ can be approximately inferred from a measurement of Dp¢

Vo,rms po,rms
using one of the following two inequalities:
Dp′
Vo,rms po,rms
< . . . (6)
2 Dp
1/2
 
Vo,rms
< −1 . . . (7)
 
1/2
 
11+−DDpp′ /
po,rms po
 
where

Dp¢ is the r.m.s. value of the fluctuating component of the differential pressure across the primary element

po,rms
measured using a fast-response secondary measurement system;

Dp is the differential pressure that would be measured across the primary element under steady flow

conditions with the same time-mean flowrate;

Dp is the time-mean differential pressure that would be measured across the primary element under

undamped pulsating flow conditions;

Dp is the instantaneous differential pressure across the primary element under undamped pulsating flow

conditions where
DDpp=+Dp′
po po po

NOTE 1 Reliable measurements of Dp and Dp¢ can only be obtained if the recommendations given in 6.1.2 and 6.1.3

po,rms
are strictly adhered to.

NOTE 2 If it is possible to determine Dp , equation (6) is to be preferred. Equation (7) will only give reliable results if

0,5.
DDpp′ ,
( )
po,rms po
5.5.4.2 Turbine flowmeter

The raw signal from a turbine flowmeter is in the form of an approximately sinusoidal voltage with a level which

varies with the flow but is usually in the range 10 mV to 1 V peak to peak. In most installations this signal is

amplified and converted to a stream of pulses. The extraction of information about the amplitude and waveform of

any flow pulsation from the variations in the frequency of this pulse train depends

...

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