EN ISO 7278-2:1995

SLOVENSKI STANDARD
01-maj-1998
7HNRþLRJOMLNRYRGLNL'LQDPLþQDPHULWHY6LVWHPL]DRYHUMDQMHYROXPHWURYGHO
1DSUDYH]DRYHUMDQMHFHYRYRGRY,62
Liquid hydrocarbons - Dynamic measurement - Proving systems for volumetric meters -
Part 2: Pipe provers (ISO 7278-2:1988)
Flüssige Kohlenwasserstoffe - Dynamische Messung - Prüfsysteme für volumetrische
Meßgeräte - Teil 2: Rohrprüfer (ISO 7278-2:1988)
Hydrocarbures liquides - Mesurage dynamique - Systemes d'étalonnage des compteurs
volumétriques - Partie 2: Tubes étalons (ISO 7278-2:1988)
Ta slovenski standard je istoveten z: EN ISO 7278-2:1995
ICS:
75.180.30 Oprema za merjenje Volumetric equipment and
prostornine in merjenje measurements
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

ISO
INTERNATIONAL STANDARD
7278-2
First edition
1988-12-15
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION
ORGANISATION INTERNATIONALE DE NORMALISATION
MEXJ/YHAPOJJHAFl OPTAHM3A~Mfl I-IO CTAH,4APTM3A~MM
Liquid hydrocarbons - Dynamit measurement -
Proving Systems for volumetric meters -
Part 2:
Pipe provers
S ystkmes d% talonnage des comp teurs
Hydrocarbures liquides - Mesurage dynamique -
volume triques
Partie 2: Tubes t+talons
Reference number
ISO 7278-2 : 1988 (E)
ISO 7278-2 : 1988 (El
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of
national Standards bodies (ISO member bodies). T.he work of preparing International
Standards is normally carried out through ISO technical committees. Esch 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, govern-
mental 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.
Draft International Standards adopted by the technical committees are circulated to
the member bodies for approval before their acceptance as International Standards by
the ISO Council. They are approved in accordance with ISO procedures requiring at
least 75 % approval by the member bodies voting.
International Standard ISO 7278-2 was prepared by Technical Committee ISO/TC 28,
Petroleum produc ts and lubrican ts.
Users should note that all International Standards undergo revision from time to time
and that any reference made herein to any other International Standard implies its
latest edition, unless otherwise stated.
0 International Organkation for Standardkation, 1988
Printed in Switzerland
ii
ISO 7278-2 : 1988 (EI
Page
Contents
.........................................................
0 Introduction
..........................................
1 Scope and field of application
2 References .
3 Definitions. .
................................................
4 Description of Systems
....................................
5 Essential Performance requirements.
6 Equipment .
7 Design of pipe provers .
8 Installation .
9 Calibration .
Annexes
............................. 15
A The use of pipe provers with four detectors
........ 18
Example of the calculation of the design Parameters of a pipe prover
B
Figures
..........................
1 Typical unidirectional return-type prover System
....................
2 Typical bidirectional straight-type Piston prover System
.........................
3 Typical bidirectional U-type sphere prover System
..............
4 Simultaneous use of two counters with a four-detector prover
...................
5 Temnorarv connection of counters to measure nl and n7
. . .
Ill
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INTERNATIONAL STANDARD ISO 7278-2 : 1988 (E)
Dynamit measurement -
Liquid hydrocarbons -
Proving Systems for volumetric meters -
Part 2:
Pipe provers
1.2 Most of the material in this part of ISO 7278 is general in
0 IntroductioF
that it applies to pipe provers for use with different liquids and
types of meters and for proving them in different Services. This
Pipe provers are used as volume Standards for the calibration of
liquid meters. The purpose of this part of ISO 7278 is to outline part of ISO 7278 does not apply to the newer “small volume” or
“compact” provers.
the essential elements of a pipe prover, to provide speci-
fications for its Performance, and to give guidance on its
design, installation and calibration. Pipe provers discussed in
1.3 The Standard reference conditions for Petroleum
this part of ISO 7278 are of the running-start/running-stop
measurement are a temperature of 15 OC and a pressure of
type, in which flow is uninterrupted during proving, thus
101 325 Pa as specified in ISO 5024.
permitting the meter to be proved under its normal operating
conditions. This type of prover includes a calibrated section of
NOTE - In some countries other reference temperatures are used,
pipe in which a displacer travels, actuating detection devices e.g. 20 OC and 60 OF.
which produce electrical Signals as the displacer Passes each
end of the calibrated Portion. The displacer finally Stops at the
end of the run as it enters a region where the flow bypasses it.
2 References
ISO 2715, Liquid h ydrocarbons - Volumetric measurement b y
Both stationary and mobile provers may be constructed on this
turbine me ter s ys tems.
principle. The calibrated section of the prover may be straight
or folded (U-shaped), and the design may be such that the
ISO 4267-2, Petroleum and liquid Petroleum products -
displacer moves around a closed loop in only one direction
Calcula tion of oil quantities - Part 2: Dynamit
(unidirectional) or, alternatively, in both directions
measuremen t. 1)
(bidirectional) .
ISO 5024, Petroleum liquids and gases - Measuremen t -
ISO 7278 consists of the following Parts, under the general title
Standard reference conditions.
Liquid h ydrocarbons - D ynamic measuremen t - Pro ving
s ys tems for volume tric me ters :
ISO 7278-3, Liquid hydrocarbons - Dynamit measurement -
Proving s ystems for volumetric meters - Part 3: Pulse
- Part 7: General principles
in terpola tion techniques.
- Part 2: Pipe provers
ISO 8222, Petroleum measurement Systems - Calibration -
- Part 3: Pulse in terpolation techniques
Tempera ture corrections for use with volumetric reference
measuring s ystems.
Annex A forms an integral part of this part of ISO 7278.
Annex B is for information only.
3 Definitions
1 Scope and field of application For the purposes of this part of ISO 7278, the following
definitions apply:
1.1 This part of ISO 7278 provides guidance for the design,
Calculation 3.1 base volume: The volume of a prover calibrated
installation and calibration of pipe provers.
techniques for use when calibrating and sperating provers are section, i.e. the length between the detectors, at specified
detailed in ISO 4267-2. reference conditions of temperature and pressure.
1) At present at the Stage of draft.
ISO 7278-2 : 1988 (El
a) The manual-return unidirectional prover is an elemen-
3.2 K-factor: The ratio of the number of electrical pulses
tary form of in-line prover which uses a section of Pipeline
emitted by a meter during a proving run to the volume of liquid
as the prover section. The entire metered stream may flow
passed through the meter.
continuously through the prover even when the prover is
not being used for proving. Detectors are placed at selected
of the actual volu me of a liquid
3.3 meter factor: The ratio
Points which define the calibrated volume of the prover sec-
passed throu gh a meter to the volume indicated the meter.
bY
tion. A displacer launching device is upstream of the prover
section, and receiving facilities are installed at some Point
calibration : The procedure for determining the downstream of the prover section. Usually, conventional
34 prover
base volume of a prover. launching and receiving scraper traps are used for this pur-
pose. To make a proving run, a displacer (a sphere or
specially designed Piston) is launched, allowed to traverse
proof: The determination of the meter factor
3.5 proving;
the calibrated section, received downstream and then
or K-factor.
manually transported back to the launching site.
3.6 range: The differente between the highest and the
b) The automatic-return unidirectional (endless loop)
lowest values within a batch of results.
prover has evolved from the prover described in 4.2.1 a) and
is shown in figure 1. In this endless loop, the piping is
arranged so that the downstream end of the looped section
crosses over and above the upstream end of the loop. The
4 Description of Systems
interchange is the means whereby the displacer is transfer-
red from the downstream end to the upstream end of the
4.1 General
loop without removing it from the prover. The displacer
detectors are located at a suitable distance from the inter-
Change inside the looped Portion. Such endless prover
4.1.1 There are several types of pipe prover, all of which are
loops may be manually operated or they may be automated
relatively simple and commercially available. All types operate
so that the entire sequence for proving a meter tan be ac-
on a common principle, namely the precisely measured
tuated by a Single action. The metered stream may be per-
displacement of a volume of liquid in a calibrated section of
mitted to run through the prover when the prover is not
pipe between two signalling detectors, by means of a displacer
being used for proving, and the prover need not be isolated
(a slightly oversized sphere or Piston) being driven along the
from the carrier line unless desired. This permits the move-
pipe by the liquid stream being metered. While the displacer is
ment of several different types of liquid in succession
travelling between the two detectors, the output of the meter is
through the prover, and affords a self-flushing action which
recorded automatically. Pipe provers may be operated auto-
minimizes intermixing between them, as well as providing
matically or manually.
temperature stabilization.
4.1.2 A meter being proved on a continuous-flow basis shall,
at the time of proof, be connected to a counter which tan be
4.2.2 A meter proof run in a unidirectional prover consists of a
started or stopped instantly by the signalling detectors. The
Single one-way run, therefore the base volume of a unidirec-
counter is usually of the electronie-pulse-counting type. The
tional prover is the volume of liquid, corrected to Standard
counter is started and stopped by the displacing device ac-
temperature and pressure conditions, displaced between the
tuating the detector at each extremity of the calibrated section.
detectors during a Single trip sf the displacer.
4.1.3 There are two main types of pipe prover: unidirectional
and bidirectional. The unidirectional prover allows the displacer
to travel in only one direction through the proving section, and
4.3 Bidirectional provers
has a transfer arrangement for returning the displacer to its
starting Position. The bidirectional type allows the displacer to
The bidirectional prover has a Sength of pipe in which the
move first in one direction, then in the other. lt therefore incor-
displacer travels back and forth, actuating a detector at each
porates a means of reversing the flow through the pipe prover.
end of the calibrated section and stopping at the end of each
(See figures 1, 2 and 3.)
run when it emers a region where the flow tan bypass it or
when the action of a valve diverts the flow. Suitable sup-
plementary pipework and a reversing valve, or valve assembly,
4.1.4 Both unidirectional and bidirectional provers shall be
constructed so that the full flow through the meter being either manually or automatically operated, make possible the
reversal of the flow through the prover. The main body of the
proved Passes through the prover.
prover is often a straight piece of pipe (see figure 21, but it may
be contoured or folded (sec figure 3) so as to fit in a limited
4.2 Unidirectional provers
space or to make it more readily mobile. Normally, a sphere is
used as the displacer in the folded or contoured type and a
Piston is used in the straight-pipe type. A meter proof run
4.2.1 Unidirectional provers may be subdivided into two
usually consists of a “round trip” of the displacer, and the
categories depending on the manner in which the displacer is
displaced volume in this type of prover is expressed as the sum
handled, namely the manual-return in-line type sometimes
of the displaced volumes in two consecutive one-way trips in
referred to as a “measured distance” type, and the automatic-
opposite directions.
return or circulating type, often called the “endless loop” type.
ISO 7278-2 : 1988 (EI
and the design shall provide
5 Essential Performance requirements ab, for this requiremen t. All
weld ing shall be in accordance with applicable Codes.
shall ensure that the followi ng per-
The design of a pipe prover
formante requirements are met.
6.1.3 Internal coating of the prover section with a material
which will provide a hard, smooth, long-lasting finish will
reduce corrosion and wear and will prolong the life of the
5.1 Short-term repeatability
displacer and prover. Experience has shown that internal
coatings are particularly useful when the prover is used with
When a unidirectional prover is calibrated using the master
liquids having poor lubricating properties, such as gasoline or
meter method, the results of five successive calibration runs
LPG.
shall lie within a range of 0,02 %. When a bidirectional prover is
calibrated with a master meter, the results of five successive
runs each comprising a round trip of the displacer, shall be 6.2 Temperature stabilization
within a range of 0,02 %.
Temperature stabilization of the proving System is normally ac-
complished by the continuous circulation of liquid through the
The short-term calibration repeatability when using the
prover section, with or without insulation. When large portions
volumetric or gravimetric water draw methods shall be such
of the prover are buried and the liquids are at or near ground
that the results of three successive calibration runs are within a
temperature, additional insulation is usually not required. When
range of 0,02 %.
provers are installed above ground, the application of thermal
insulation will contribute to better temperature stabilization.
When a prover is used to prove a high-performance flow meter
Where a high temperature gradient tan appear along the prover
such as one suitable for custody transfer or fiscal measure-
Pipe, as with heated products, thermal insulation is recom-
ment, the results of five successive provings shall lie within a
mended.
range of 0,05 %.
63 . Temperature measurements
5.2 Valve seating
Temperatures shall be measured with an Overall uncertainty not
exceeding + 0,5 OC. This requires temperature Sensors with a
The sphere interchange in a unidirectional prover or the flow
certified accuracy of + 0,2 OC or better. The temperature
reversing valve or valves in a bidirectional prover shall be fully
Sensors shall be installed in thermowells near the inlet and
seated and sealed (so that the displacer is travelling at full
outlet of the prover and in positions which receive active fluid
velocity) before the displacer meets the first detector. These
flow during both normal and calibration operations. The
and any other valves whose leakage tan affect the accuracy of
thermowells shall be inserted to a minimum of 100 mm in large
proving shall be provided with some means of demonstrating
pipes and as closely as possible to one-half the diameter in
that they are sealed during the proving run.
small pipes. Thermowells shall be filled with a suitable heat
transfer medium. If mercury-in-glass thermometers are used,
5.3 Freedom from shock
they shall be of such a design that they tan be read while re-
maining immersed in the heat transfer medium to the recom-
When the prover is operating at its maximum design flow rate,
mended depth for the thermometer in use. lt is important to
the displacer shall come to rest safely and without shock at the
match the thermowell with a temperature Sensor of suitable im-
end of its travel. mersion requirements.
54 P Freedom from cavitation
64 . Pressure measurement
Pressure measurement devices shall be capable of measuring
When the prover is operating at its maximum design flow rate
pressure with an uncertainty of less than k 50 kPa ( + 0,5 bar)
and with the liquids for which it was designed, there shall be no
at pressures of up to 2 500 kPa (25 bar) and + 2 % of operating
risk of cavitation in the prover, valves or elsewhere, over the
pressure at higher pressures.
specified pressure and temperature range.
6.5 Displacing devices
6 Equipment
6.5.1 One type of displacing device commonly used in pipe
provers is the elastomer sphere filled with a liquid under
6.1 Materials and fabrication
pressure, and expanded so that its minimum diameter is slightly
larger than the inside diameter of the prover Pipe. The diameter
6.1.1 The materials selected for a prover shall conform with
shall be such that a seal is provided without excessive friction;
the applicable Codes specifying the pressure rating and the area
this tan usually be achieved by inflating the sphere to a
where the prover is to be used. Pipes, pipe fittings and bends
diameter which is at least 2 % greater than the inside diameter
shall be selected for internal roundness and smoothness.
of the prover Pipe. In general, the larger the sphere, the greater
this percentage should be. Too little expansion of the sphere
6.1.2 In the fabrication of provers, care shall be exercised to tan lead to leakage past the sphere and consequent measure-
ensure proper alignment and concentricity of pipe joints. All ment error. Too great an expansion of the sphere may not im-
welds within the path of the displacer shall be ground intern- prove sealing ability and will generally Cause the sphere to wear
ISO 7278-2 : 1988 (El
precision with which the detector in a prover tan detect the posi-
more rapidly and to move erratically. Care shall be exercised to
tion of the displacer (which is one of the governing factors in
ensure that no gas remains inside the sphere. The elastomer
determining the length of the prover section) shall be ascertained
shall be as impervious as possible to the operating liquids and
as accurately as possible (see annex AL The diameter of any
retain its mechanical properties (especially its elasticity) under
opening in the wall of the calibrated section of the Pipe, including
operating conditions. The liquid employed to fill the sphere
the holes accommodating the detectors, shall be appreciably less
shall have a freezing Point below any anticipated temperatures.
than the width of the sealing zone of the displacer.
Water or water-glycol mixtures are commonly employed.
6.9 Meter pulse generator
6.5.2 A second type of displacing device is the cylindrical
Piston with suitable Seals. This is often used with straight pipe
An externally fitted pulse generator shall generate electrical
provers that have been internally honed to ensure adequate
pulses of satisfactory characteristics for the type of proving
sealing.
counter employed. The device shall generate a sufficient
number of pulses per unit volume to provide the required
resolution. The pulse emitter shall be designed to eliminate the
6.5.3 Other displacers are acceptable if they give a perform-
ante equal to the two types mentioned in 6.5.1 and 6.5.2. generation of spurious pulses due to mechanical vibrations or
other influences.
6.6 Valves
6.10 Electronie pulse counter
An electronie pulse counter is usually used in meter proving
6.6.1 All valves used in pipe prover Systems which tan con-
because of the ease and accuracy with which it tan count high-
tribute to a bypass of liquid around the prover, the displacer or
frequency pulses and because of its ability to transmit its count
the meter, or which tan Cause leakage between prover and
to remote locations. The pulse-counting devices are equipped
meter, shall be bubble-tight on low differential pressure tests. A
with a Start-stop electronie switching circuit actuated by the
means of checking valve seal leakage during the proving run
prover’s detectors. Proving Systems tan also be equipped with
shall be provided for such valves. If a sphere or spheres are
a pulse interpolation System as defined in ISO 7278-3.
used to provide this sealing mechanism in lieu of a valve, they
shall be provided with some means of testing for leakage.
6.11 Equipment for automatic- retu
unidi rectional provers
6.6.2 The entire Operation of the flow reversing valve or
valves in a bidirectional prover, or of the interchange valve in a
6.11.1 Equipment necessary for the proper Operation of the
return type unidirectional prover, shall be completed before the
automatic-return or endless-loop unidirectional prover is cen-
displacer actuates the first detector. This is to ensure that dur-
tred around the sphere interchange unit. lt is within this unit
ing the trip of the displacer through the calibrated section no li-
that the sphere is diverted from the flowing stream at the
quid is allowed to bypass the prover. The necessary distance
downstream end of the prover, Passes through the interchange
between the initial Position of the displacer and the first detec-
and is then reinserted at the upstream end of the prover, all
tor, commonly called the pre-run, is dependent on valve opera-
automatically.
tion time and the velocity of the displacer. Any method tan be
used to shorten this pre-run, whether by faster Operation of the
valve or by delaying the launching of the displacer. However,
6.11.2 Sphere interchange may be accomplished with several
caution shall be exercised in the design so that hydraulic shock
different combinations of valves or other devices. Esch com-
or additional undesired pressure drop is not introduced. If more
bination comprises a System of devices designed to arrest the
than one flow directing valve is used, all valves shall be linked
sphere and pass it through the interchange, yet prevent any
by some means to ensure that shock cannot be caused by in-
flow of liquid through the interchange which would bypass the
correct sequence of Operation.
prover section during the proving period. Typical combinations
of devices are
6.7 Calibration connections
a) a Single special ball valve modified for sphere handling;
a dual power-operated check valve assembly;
b)
Connections shall be provided on the prover to allow for water
draw or master meter calibration at a later date (see figures 1,2
c) a combination of a ball or gate valve with a power-
and 3).
lrated check valve;
OPe
d) a dual through-conduit gate or ball valve;
6.8 Detectors
e) a valveless two- or three-sphere assembly;
f) an interchange using a plunger-type valve to block the
Detection devices and switches shall indicate the Position of
flow.
the displacer accurately, and in a bidirectional prover they shall
operate equally well in both directions. Various types of detec-
6.11.3 The controls and actuators used in connection with
tor are in use, the most common of which is the mechanically
actuated electrical switch. Other types, including the electronie unidirectional provers will depend primarily on the degree of
automation with which it is desired to operate the proving
proximity, the induction pickup or the ultrasonic type, may be
used, provided the required repeatability criteria are met. The System.
ISO 7278-2 : 1988 (EI
the degree of automation that will be incorporated
hl in
6.11.4 Separator tees, as shown in figure 1, are sized at least
the proving Operation;
one pipe size larger than the nominal size of the sphere or loop.
The design of the separator tee shall ensure dependable separa-
the size and type of meter that will be proved;
i)
tion of the sphere from the stream for all flow rates within the
j) the facilities that will be required for safely installing and
flow range of the prover.
removing the displacer;
facilities that
k) the will be required for safely venting and
6.11.5 Launching tees are generally one pipe size larger than
draining the prover.
the displacer sphere and shall have smooth transition fittings
leading into the prover. The launching tee shall have a slight in-
clination downwards toward the prover section, or some other
7.2 Diameter
means of ensuring movement of the sphere into the prover dur-
ing periods of low flow, such as might occur during calibration
In determining the diameters of the pipes to be used in the con-
by the water draw method.
necting lines, or manifolding, and the prover, the head loss
through the pipe prover System shall be compatible with the
head loss considered tolerable in the metering installation.
6.12 Equipment for bidirectional provers
Generally, the diameter of the pipe prover and manifolds shail
not be iess than the outlet diameter of any Single meter to be
6.12.1 In Piston-type bidirectional provers of the design
proved.
shown in figure 2, the outlets and inlets on the prover ends
shall be provided with holes or Slots. These shall be deburred
7.3 Volume
and shall have a total area greater than 1,5 times the cross-
sectional area of the pipe beyond the outlet. In sphere-type
bidirectional provers with oversized end chambers (see In determining the volume of a prover between detectors, the
figure 3), the chambers shail be designed so that the displacer following facto Irs shail be considered by the designer:
cannot obstruct the inlet or outiet openings and thus prevent
liquid from flowing. The receiving chambers shali be sized to
the Overall repeatability required of the proving System;
a)
ensure that the dispiacer is arrested without shock under
b) the repeatability of the detectors (see annex A,
maximum flow conditions.
clause A. 5);
the ability of the eiectronic counter to indicate oniy to
c)
6.12.2 A Single multiport valve is commonly used for revers-
the nearest pu lse, unless pulse interpolation is emp loyed;
ing the direction of liquid flow, and hence that of the displacer.
d) the discrimination of the meter Signal generator, that is,
Other means of fiow reversal may also be used. All valves shall
the volu me passing th rough the meter per pulse registered;
allow continuous flow through the meter during proving. The
valve size and actuator shail be selected to minimize pressure
the maximum permissibie fiow rate of the System.
drop and hydraulic shock.
7.4 Displacer velocity
7 Design of pipe provers
7.4.1 lt is not the intention of this part of ISO 7278 to limit the
velocity of displacers and, provided acceptable Performance is
7.1 Initial considerations
guaranteed, there shall be no arbitrary limit imposed upon
velocity.
Before considering the design of a pipe prover, it is necessary
to estabiish the type of prover required for the installation and
the manner in which it will be connected with the meter piping. 7.4.2 The maximum and minimum velocities of the displacer
From a study of the application, intended usage and space tan be determined from the diameter of the prover pipe and the
limitations, establish the following : maximum and minimum flow rates of the meters to be proved.
Clearly, some practical limit to maximum velocity of a displacer
must exist, partiy to avoid mechanical darnage to the prover,
a) whether the prover will be stationary or mobile;
partiy to limit surges and partly to prevent darnage to the
be dedicated (on-line) or
b) if stationary, whether it will
displacer and the detectors. Nevertheless, the developing state
used as part of a centrai System;
of the art is such that it is inadvisable to set a firm limit on
displacer velocity as a criterion of design. The minimum veioc-
c) if a stationary, dedicated prover, whether it will be kept
ity shaii be set at a ievel that ensures smooth travel of the
in Service continuously or will be isolated from the metered
displacer and that prevents intermittent travel of the displacer
stream when not being used to prove a meter;
in fluids with poor iubricating properties.
d) if stationary, what portions, if any, will be beiow
grou nd level;
7.4.3 A velocity of 3 m/s is a typicai design specification for
e) the permissible range of temperature and pressure;
unidirectional provers, whereas the displacer velocity in bidirec-
tional provers is usually Iower. However, the use of special
f) the permissible maximum and minimum flow rates;
iaunching techniques allows bidirectional provers to be used at
higher displacer velocities.
g) the physical properties of the fluids that will be handled;

ISO 7278-2 : 1988 (El
The degree of uncertainty is represented mathematicaiiy as
7.5 Repeatability and accuracy
foilows :
7.5.1 The ultimate requirement for a prover is that it shall
prove meters accurately. However, this accuracy cannot be Uz -
. . .
(1)
n
established directly as this is dependent on both the repeat-
ability of the meter and the systematic uncertainty in the deter-
where
mination of the base voiume of the prover. The repeatabiiity of
any prover/meter combination, however, tan always be deter-
U is the degree of uncertainty of the recorded pulse count
mined experimentaliy by carrying out a series of repeated
arising from this Source aione, commoniy calied the resolu-
measurements under carefuiiy controlied conditions and
tion;
anaiysing the resuits statistically. lt is therefore usuai to adopt
repeatability as the oniy avaiiabie criterion of a prover’s accep-
n is the number of Pulses coiiected during a proving run.
tabiiity. But it should always be remembered that, whereas
poor repeatabiiity is an immediate indication that a prover is not
Having estabiished the degree of uncertainty, the minimum
repeatabiiity does not
performing satisfactoriiy, good
voiume between the prover detectors is determined as foiiows:
necessarily indicate good accuracy since there is aiways the
possibiiity of unknown systematic errors having occurred, and
v= -
. . .
(2)
Operators must always be on their guard against such errors.
UK
7.5.2 The repeatability of a proving System will depend upon
its components and, in particular, upon the repeatability of the
V is the minimum voiume between the detectors;
detector’s abiiity to iocate the Position of the displacer.
K is the mini mum K-factor (number of
Pulses per unit
7.5.3 The selection of displacer detectors will have a direct
volume) of meter to be proved by the prover.
anY
bearing on the uitimate iength of the proving section. A more
precise detector will aliow a shorter iength. The required
7.5.6 lt foiiows that prover voiumes tan be reduced by in-
repeatability of the detector’s abiiity to iocate the dispiacer may
creasing the pulse generation rate of the meters to be proved.
be measured experimentaliy by the method described in
Caution shaii be exercised, however, in the use of gear-driven
annex A, clause A.5.
pulse generators on dispiacement meters to obtain very high
pulse generation rates, because with these devices mechanicai
7.5.4 When replacing worn or damaged Parts in a detector,
Problems, inciuding backlash, drive-shaft torsion and cyciic
great care shall be taken to make sure that neither the
variations, tan Cause irreguiar pulse generation. Eiectronic
detector’s actuating depth nor any of its electrical switch com-
means of pulse interpoiation tan also be used to reduce the
ponents is altered to the extent of changing the caiibrated
resolution; such techniques shaii be used with discretion,
voiume of the prover by more than the iimit allowed (0,02 %).
however, and provisions governing their use are contained in
This is especially important in unidirectional provers because,
ISO 7278-3. Pulse interpoiation is most effective for meters
uniike bidirectional provers, errors due to changes in detector
emitting Pulses at regular intervais.
actuation depth in unidirectionai provers are not reduced by the
compensating effect of round-trip sphere travel. To avoid such
7.5.7 Optimum dimensions of provers
errors, one or more of the following shail be done whenever
this type of maintenance is carried out:
When seiecting the minimum acceptabie dimensions of a
prover for a particuiar duty, the foiiowing considerations shaii
with a
a) the detector assembly shall be replaced
be taken into account:
precaiibrated duplicate unit;
b) if the prover is fitted with twin detectors at each end,
a) Decreasing the diameter of the prover pipe necessitates
the repaired detector shali be reset by the procedure
an increase in the length between detectors for a given
described in clause A.4 of annex A;
volume, and thus reduces the effect of errors due to detec-
tor resolution. However, it also increases dispiacer velocity
c) if neither of the above is done, the prover shaii be
which may become a limiting factor.
recalibrated.
b) Increasing the diameter of the prover pipe has the op-
NOTE - Provided that either a) or b) above has been foilowed, and
posite effect, i.e. the veiocity of the dispiacer is reduced.
subject to the agreement of the Parties involved, recalibration of the
However, the resulting decrease in length increases the ef-
prover is not required.
fett of errors due to detector resolution, and this may
become a limiting factor.
7.5.5 An important Source of random error when a meter is
proved by a meter prover is counter resolution. A digital
counter has a resolution of unity, and hence the pulse count
7.6 Examp Ie of the calculati on of the design
has a random uncertainty of + 1. For example, if it were
para meters pipe prover
of a
desired to limit the uncertainty from this Source aione to
+ 0,Ol %, without the method of pulse interpolation de- An example of a calculation of the design and sizing of a pipe
scribed in ISO 7278-3, it would be necessary to collect at least prover is provided for reference in annex B. The caiculation is
10 000 pulses during a proving run or during a Single one-way illustrative only and is not an integral part of this part of
travel of the displacer in a bidirectional prover. ISO 7278.
ISO 7278-2 : 1988 (El
given to the inclusion of suitable valving to isolate the prover
8 Installation
unit from line pressure when not operating, thus facilitating
maintenance and displacer removal. The System shall be pro-
8.1 In relation to their method of installation, pipe provers
vided with an adequate number of vent and drain connections.
may be classified as either mobile (portable) provers, dedicated
Vent valves shall be installed on the topmost portions of the
provers or central provers.
pipe to ensure that all air tan be vented from dead spaces not
swept by the displacer. Provision shall be made for the safe
8.1 .l Mobile prover
disposal of liquids or vapours drained or vented from the prover
section. This may be accomplished by pumping liquids or
A mobile prover is normally mounted on a road vehicle or trailer
vapours back into the System or by diverting them to some coi-
so that it tan be taken to various sites for on-site proving of
lecting Point. Thermometers and pressure gauges shall be in-
meters in their installed positions while they are in normal
stalied in suitable iocations near both the meter and the prover
Operation. Occasionally, mobile provers are mounted in con-
in Order to determine the temperature and pressure of each.
tainers or on seif-contained skids so that they may be
transported by road, rail or sea. Mlobile provers are always pro-
8.2.2 Provision shaii be meide for recalibrating the prover.
vided with some means of connecting them conveniently to the
Exampies of suitable connections are shown in figures 1, 2
metering System where they are to be used; this usually takes
and 3.
the form of flexible hoses, but any other System compiying with
the applicable safety Standards may be used.
8.2.3 All wiring and controls shali conform to applicable
Codes. Components shall conform to the class and group ap-
8.1.2 Dedicated prover
propriate to the iocation and Operation. All electrical controls
and components shaii be placed in a convenient location for
A dedicated prover is connected through a System of pipework
Operation and maintenance. Manufacturers’ instructions shaii
and valves to a battery of meters in parallel. Its sole function is
be strictiy foilowed for instailation and grounding of all eiec-
to prove those meters one at a time, at intervais as required.
tricai components, such as eiectronic counters, controis,
Aithough dedicated provers normaiiy serve severai meters, the
power units and Signal cabies. 1)
term may also be used where a prover is permanentiy con-
nected to one meter.
Automatic or manuai pressure reiief vaives, compiete
8.2.4
8.1.3 Central prover with discharge piping and ieakage detection faciiities, are
usualiy instaiied to ailow for the thermal expansion of the liquid
in the prover whiie it is isoiated from the main System. Where
A central prover is permanently instaiied at a iocation where a
supply of liquid and pumping faciiities are availabie, but is not there are both iocai and remote controis, either iockout
switches or iockout circuits, or both, shail be provided between
permanently connected to a battery sf meters. lnstead, it is used
for the proving of meters that are periodicaliy brought to the the two sets sf controis to prevent accidentai remote Operation
of a unit whiie it is being controiied iocaliy. Suitabie safety
prover, usualiy compiete with their upstream and downstream
pipework and flow-conditioning System, and temporariiy con- devices and locks shaii also be instaiied to prevent inadvertent
Operation of, or unauthorized tampering with, equipment.
nected to the prover for this purpose. The centrai proving
System shail be capabie of proving a meter under all conditions Automated or power-actuated provers or proving Systems shaii
that will be encountered during the Operation of the meter, and have emergency manuai actuators for use in the event of a
especially over its full working range sf fiow rate and viscosity. power faiiure.
After centrally proving a meter, care shali be taken to ensure
that the meter is not mishandled in a way that couid Cause its
8.2.5 All types of pipe prover shaib be instailed downstream
calibration to have changed by the time it is reinstalied at the
from straining or fiitering equipment.
operating site.
8.2 General installation guidelines
9 Calibration
All components of the prover instaliatio including connecting
n,
ing, valves and nifolds, shali be in accordance with ap- 94 . Principles
ma
PiP
piicabie piping and safety Codes. Once the prover is onstream,
it becomes a part of the System under pressure.
9.1.1 Before being placed in Service, a pipe prover shali be
caiibrated to determine its base voiume, that is the voiume of
8.2.1. The prover and its accessories shaii have suitabie the prover under Standard reference conditions (see ISO 5024).
hangers and supports in accordance with appiicabie codes and Periodic recaiibration of a prover is also required; the intervai
Sound engineering principies. In the design and construction of between caiibrations will depend on the frequency of use of the
prover and the nature of the liquid or iiquids being metered.
proving Systems, Provision shali be made for expansion, con-
traction, Vibration, pressure surges and any other adverse con- The prover shaii be thoroughiy fiushed and cieaned before
ditions that may affect the installation. Consideration shaii be caiibration or recalibration.
Dnstrumentation and anciliary equipment for liquid-hydrocarbon metering Systems will be covered in a future International Standard.
1)
ISO 7278-2 : 1988 (El
from the prover into the volumetric referenceis). At least one
9.1.2 The base volume shall be documented on a calibration
trial calibration run shall be made to determine the approximate
certificate. The accuracy of the base volume, as determined,
volume of the prover between its detectors, so that the approx-
cannot be better than the accuracy of the certified volumetric
imate number of fillings of the appropriate certified volumetric
referencels) used in determining it. Base volumes are usually
referencels) tan be determined. The temperature and pressure
quoted to five significant digits, with intermediate calculations
at the discharge of the prover shall then be recorded as the
having one more significant digit than the final value shown on
temperature and pressure in the prover at the Start of caiibra-
the certificate (sec ISO 4267-2 for further details on calcula-
tions). The calibration certificate or report shall be dated and tion.
signed by a person authorized to do so.
9.2.4 Uninsulated provers which are calibrated outdoors
under extreme weather conditions shall be temporarily in-
9.1.3 The base volume of a unidirectional prover is the
volume, corrected to Standard temperature and pressure con- sulated and/or sheltered to reduce variations in temperature.
ditions, of liquid displaced between the detectors during a
Single trip of the displacer. The base volume of a bidirectional
9.2.5 Certified volumetric references for the calibration of a
prover is the sum of the volumes displaced between the detec-
prover shall have been caiibrated by reference measures
tors during the two trips in opposite directions which comprise
traceable to national primary Standards of volume.
one round-trip of the displacer, corrected to Standard
temperature and pressure conditions. There are two methods
9.2.6 The prover may be calibrated using small-diameter
for calibrating a pipe prover, the water draw method (sec 9.2)
water lines and temporary valves, or by using the vaives and
and the master meter method (see 9.3).
piping which are part of the installation. Solenoid valves ac-
tuated by the detector switches are normally used to Start and
9.1.4 If a prover is to be operated exclusively at some fairly
stop the calibration run.
high pressure, e.g. above 2 000 kPa (20 bar), and will thus not
be subjected to widely varying operating pressures, an alter-
9.2.7 The caiculations required to derive the prover base
native to the procedure outlined here is to calibrate such a
volume from the calibration data are described in detail in
prover at its working pressure and then to refer its b
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