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6 CHAPTERPROVING SY STEMS
objects traveling in the flow stream. They should beinspected
and cleaned periodically.Turbine meters are especially susceptible
to the effectsfrom deposits because they re velocity devices.
Layering orcoating of the meters internais will change the velocity
ofthe liquid flowing through the meter and cause the meter
toregister incorrectly.Temperature changes can affect the
mechanical clearancesof displacement meters, as well as the
viscosity of the uidbeing metered. This may result in changes n
slippage.4.8.2.8.5 Electronic Equipment andInstrumentation
All electrical and electronic equipment, such as
counters,switches, interconnecting cables, and grounding cables,
shallbe periodically inspected for condition and for proper
instal-lation and operation. Operating procedures may
requirespecial permission or permitsbefore equipment is connected.A
counter may miss some of the puises generated by themeter, in which
case the counter will read low. Counting toofew puises is usually
caused by setting the sensitivity controldeveloped. By adjusting
the sensitivity control or by elimi-nating the electrical fault,
the trouble can usually becorrected.A counter may include signals
from outside sources aspulses. These signals, not generated by the
meter, will causethe counter to read high. Signals not generated by
the metercan originate from electrical power supplying the
counter,electrical welding equipment, radio transmitters, and
soforth. These puises may be intermittent and difficult todetect.
See API MPMS, Chapter 5 Section5.Signal transmission cables should
be kept as far awayfrom power cables s possible and should cross
power cablesat right angles. Shielded signal transmission cable
isnormally grounded only at the instrument-receiving end toprevent
a ground loop current that travels along the shieldand adversely
affects the signal transmission).
on-the counter too low, or by an electrical fault which has
4.8.2.8.6 Flow Rate VariationsMeter performance is dependent
upon flow rate; thus,flow rate during proving shall be maintained
at or near thenormal operating flow rate.4.8.2.9 METER REGISTRATION
HEAD) CHECK
Compare the meter register indicated volume) to
theproving-counter registration. This can be done by manuallygating
starting and stopping) a prover counter connected tothe
transmitter, based on a significant volume registered bythe meter
counter or register. The pulses displayed on theprover counter are
then compared to the volume displayedon the mechanical register. If
the meter generates 8400
puises per barrel, the prover counter should show approxi-mately
84 000pulses for each 10barreis on the register.4.8.2.10 FREQUENCY
OF METER PROVING
The frequency required for proving varies from severaltimes a
day to twice a year or even longer depending uponthe value of the
liquid, costhenefit to prove, meter provinghistory, meter system
stability, and variations of operatingsystems.For large volumes or
different liquids, a permanentlyinstalled prover is normally used.
The meters should beproved whenever the flow rate, temperature,
pressure, APIGravity relative density), or viscosity changes
significantly.Normally, time or volume is used to determine when
themeter should be proved.When metering a single or similar liquid,
the meter factoris normally applied forward to the meters indicated
volumeuntil the meter is reproved. Normally, there is a
prescribeddeviation limit between consecutive meter factors on
thesame or similar liquid. When this deviation limit isexceeded,
the previous and the new meter factors arenormally averaged and
applied to the indicated volumeduring this period. If the deviation
limit is consistentlyexceeded, it may be appropriate to reduce the
intervalbetween meter proving. It may also be appropriate to
inspectand repair the meter and the proving system.When batching
operations permit, a new meter factorshould be determined for each
batch. This applies tobatching operations involving different
liquids or lengthydown time. When a meter is proved during a batch,
the meterfactor should be applied forward until the meter is
reprovedduring the batch. If the meter is reproved during the
batch, adeviation limit may be installed between consecutive
meterfactors, or the meter factors may be averaged. When
thisdeviation limit is exceeded, the previous and new meterfactors
are normally averaged and applied to the metersindicated volume
between these provings. If i t is impracticalto prove each batch,
meter factors are normally appliedforward until the next proving,
as is the case withnonbatching operations.
The proving frequency for new systems should start atshort
intervals and be extended to longer intervals as confi-dence
increases in the system. See API MPMS Chapter 13.2for statistical
evaluation of meter proving data.4.8.2.11 PROVER
RECALIBRATIONFREQUENCY
Typically a provers base volume is originally certified atthe
manufacturer site by the water draw method in the pres-ence of the
purchaser and other interested parties. Provervolumes may change as
the result of worn or faulty detectorswitches; the reduction of
internal coating thickness; or lossof internal material due to
oxidization, abrasion, or the accu-mulation of foreign material
such as wax) buildup. Subse-
yright American Petroleum Instituteoduced by IHS under license
with API Licensee=Perez Companc/5955403001
Not for Resale, 05/09/2005 07:08:49 MDTeproduction or networking
permitted without license from IHS
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Sect ion Operat ion o Proving Systems
quent calibration is required whenever a change in basevolume
could have occurred.Six considerations determine how frequently a
provershould be recalibrated. They are usage, time, calibration
history, calibration costhenefit, contractual requirements,and
value of the metered liquids. Usage causes wear, andtime
contributes to deteriorationof the prover.For the recommended
procedure for calibrating a prover,refer to API MPMS Chapters 4 and
12.2. Recalibration ofprovers should occur when any one of the
following condi-tions exist:a. Alterations or repairs which affect
the certified volume aremade to the prover.b. A meter control chart
indicates a change in provervolume.c. The maximum interval
indicated below has elapsed.1. Three years for small volume provers
and mobileprovers.
2. Five years for permanently installed pipe provers.3. Five
years for permanentIy installed tank provers.4.Three months for
master meter provers.The prover displacer and the inside surface of
the prover
should be inspected periodically. The surface of a sphere orthe
contact edge of a piston cup or seal may indicate theinternal
condition of the prover. If these surfaces or edgesare scored or
worn, this may indicate that the proverrequires further inspection
or repair and may require recal-ibration.4.8.2.12 FILLING AND
PRESSURING THEPROVER
This section refers to conventional pipe provers, smallvolume
provers, and master meter provers.This section doesnot pertain to
tank provers, which are covered in 4.8.5.4.After checking that end
closures and any openablefittings are properly fastened and that
all vent and drainvalves are closed, proceed with filling the
prover in thefollowing sequence:a. Partially open the prover
isolation valve to fill the proverb. Observe the system for leaks.
Wait until the system iscompletely filled and the connections have
been shown to beleak-tight. Verify the seal integrity of all vents,
drains,reliefs, and all double block-and-bleed valves.c. Open the
vents to allow discharge of air/gas when the fluidis admitted into
the prover.d. Fully open the prover inlet and outlet valves.e.
Close the valve to divert all flow through the prover.f. Operate
the prover and continue to vent the high pointsuntil no air is
observed.g. Close the vents when air or vapor is no longer
observed.
slowly.
4.8.2.13 CERTIFICATIONVerify that the prover has a valid
calibration certificateand that the certificate is for the prover
being used, by veri-fying the prover serial number with the serial
number on the
certificate. If a conventional pipe prover is being used,
checkto ensure that the prover volume between detectors is
sufficient to accumulate a minimum of 10,000 pulses. If not,pulse
interpolation techniques are required. Since someprovers have more
than one calibrated volume, verify thatthe proper calibration
certificate is being used.If a tank prover is used, verify that the
prover volume isequal to a minimum of one minute of the maximum
oper-ating flow rate. See API MPMS, Chapter 4.4.If a master meter
is used, all data that is used to developthe master meter factor
s), including the prover calibrationreport, certificate, and master
meter factor s) reports shouldbe available.If a small volume prover
is used, verify that the interpola-tion system has a valid and
current calibration certification.Refer to API MPMS, Chapter4
Sections3 and 6.
4 8 3 Conventional Pipe Provers4.8.3.1 PRINCIPLEOF OPERATION
The basic principle on which the pipe prover operates isshown in
Figure 3. A sphere or piston known as a displ ceris installed
inside a specially prepared length of pipe. Whenthe prover is
connected in series with a meter, the displacermoves through the
pipe and forms a sliding seal against theinner wall of the pipe so
that it always travels at the samespeed as the liquid flowing
through the pipe.
In some conventional provers, the displacer is a piston
withelastomer or plastic seals. However, in most
conventionalprovers, the displacer is an elastomer sphere. To
provide goodsealing, the pipe bore must be smooth.At two or more
points in Figure 3 there are devices knownas detectors fixed to the
pipe wall. These detectors emit anelectric signal when the
displacer reaches them. The signalfrom the first detector switch is
used to start the electroniccounter, which accumulates pulses from
the meter. When thedisplacer reaches the second detector, its
signal stops theproving counter. The number of pulses shown on the
provingcounter is the total pulses generated by the meter while
thedisplacer was travelling between the two detectors.
Conven-tional pipe provers both bidirectional and
unidirectional)arethose that have a volume between detectors that
permits aminimum acculuation of 10 OOo direct unaltered)pulses
fromthe meter. Thus a unidirectional prover typically accumulatesa
minimum of 10,ooO pulses per proving run, and a bidirec-tional,
prover typically accumulates a minimum of 20,000pulses per proving
run. Direct unaltered) puises include thosethat are the output of
high frequency pulse generators, consid-ered to be a part of the
meter. It should also be noted that
yright American Petroleum Instituteoduced by IHS under license
with API Licensee=Perez Companc/5955403001
Not for Resale, 05/09/2005 07:08:49 MDTeproduction or networking
permitted without license from IHS
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