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tabl f CsKp mi pblms fm bbbli p 4Entrained gas can aect Coriolis meters but you can take steps to optimize perormance.
usa h Aaci f Mams 9Magnetic owmeters provide accuracy and can be used in a variety o applications and environments
tak a diff Lk a Cifal Pmps 13An unconventional assessment can provide insights or eective control.
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Kp mi pblms fm bbbli pEntrained gas can aect Coriolis meters but you can take steps to optimize perormance.
and environments
By Tim Patten, Micro Motion, Inc.
CorIoLIS MeterS have long been used verysuccessully on single-phase luids. However,
liquids that contain bubbles (air or gas) cause
dynamic changes to a Coriolis meter that are not
present in a single-phase luid and that lead to
measurement errors.
A Coriolis meter operates by driv ing one or
two tubes at a resonant, or natural, requency. In
the meter, the electronics (or transmitter) send a
drive signal to the sensor that tracks the requen-
cy o the tube and maintains the proper vibration
amplitude. Driving on the resonant requency is
important because it enables luid density mea-
surement and minimizes power requirements.All modern Coriol is meters are intrinsical ly
sae (IS), which limits the amount o power that
is allowed to drive the sensor. Bubbles moving
around in the l iquid tremendously increase uid
damping, which results in power requirements
that ar exceed what IS restrict ions permit. So, the
tube amplitude signicantly decreases. Tis condi-
tion is sometimes called stall, although the tubes
usually do continue to vibrate to some extent.
When the tube amplitude decreases, the
signal-to-noise ratio also alls, making it a chal-
lenge to extract the mass ow signal rom the
Figure 1. When uid velocity cant overcome buoyancy, bubbles get caught in
inlet leg.
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relatively high level o noise. Older analog signal-
processing techniques are highly sensitive to
entrained air because signa l amplitude is low and
noise is high; no algorithm is available to enhance
the measurement signal, thereby improving the
signal-to-noise ratio. In contrast, digital signal
processing (DSP) can eectively lter the noise
to yield a good stable measurement so long as the
tube is vibrating, even at reduced amplitudes.Note: Even with DSP, when gas is present in a liq-
uid stream the meter can only provide total-product
density (including the gas), not liquid-only density.
tHe IMPACt oF FLuId dynAMICS
Coriolis meters are not sensitive to ow prole
and other disturbances that aect other metering
technologies. For instance, since the undamental
measurement o delta comes rom the relative
values o each o two tubes in bent-tube designs,
swirl upstream o the meter doesnt impact the mea-
surement because it doesnt matter how much ow
goes through one tube or the other. Accuracy is not
degraded even when one tube is completely plugged.
However, when gas is present in a liquid, the
ow prole can become a concern. Although the
undamental measurement is unaected (that is,
the relative delta ), the tubes can become imbal-
anced due to the large density dierence between
them (air in one, liquid in the other, or instance).
An imba lance can cause meter zero errors; there-ore, measuring low ow rates can be problematic.
An equally signicant problem occurs at rates
too low to sweep bubbles out o the tubes. I the
uid velocity is less than approximately 0.6 m/s,
air will hang up in tube regions where the ow
is against gravity (Figure 1). Bubbles get caught in
the inlet tube leg because uid velocity is not great
enough to push the bubbles down and out against
gravity orcing the bubbles up. Tis issue is present
in any bent-tube meter design because at some loca-
tion in the tube the uid velocity is ghting gravity.
Te solution is to keep ow rate high enough
Figure 2. Even at low ow rate, measurements or 10,000-cP toothpaste with 2-5% void raction are within specifcation.
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such that uid velocity can purge the sensor o
air. A rate o 20% o meter nominal ow (1 m/s in
the ow tube) or higher is adequate to completely
purge the meter o bubbles and give good peror-
mance. In a U-shaped meter, mounting the sensor
in a vertica l pipe run with ow going up helps to
keep the bubbles moving through the meter.
tHe roLe oF FLuId ProPertIeS
Pressure, luid temperature and viscosity all
impact how a Coriolis meter deals with varying
levels o entrained air.
As pressure increases or decrea ses , the appar-ent void raction changes, o course. his means,
or instance, i two meters are piped in series, the
downstream meter is at a distinct disadvantage
because the pressure is lower and thereore the
void raction is higher.
emperature plays a minor role, in that it a-
ects viscosity and surace tension. It also impacts
void raction to a small degree (higher tempera-
ture results in higher void raction).
Viscosity is a very important uid parameter
because it directly inuences the propensity o
the uid to hold up air (or gas). In a low-viscosity
liquid such as water, air bubbles coalesce rom nely
distributed small bubbles into large ones that collect
at high points in the line. In contrast, i the bubbles
stay nely distributed, as happens in high-viscosity
liquids, they will be purged rom the meter easily
and not collect and metering will be accurate.
Figure 2 shows results or toothpaste with a viscosity
o 10,000 cP and entrained air level between 2 and
5%. Rates are quite low or a 2-in. meter (
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be measured because o the previously described
separation issues. However, noise rejection im-
provement with new DSP techniques will allow
the minimum rate to be pushed lower.
A signicant problem with any two-phase
ow (water/air, dog ood with solids suspended in
water, oil/gas, etc.) is at zero ow. When the ow
is stopped, the multiple phases separate by gravity,
prompting an imbalance in the tube. Tis imbal-ance causes an apparent meter zero change. Work
on signal processing improvements to address this
problem is currently a signicant area o research.
A SPeCIAL CASe
Empty-ull-empty batching can pose a related measure-
ment issue. Such batching is most common to avoid
cross-contamination o products when lling large
tanks such as rail cars or trucks. Tereore, the loading
line is purged with air or other inert gas between loads,
leaving the meter empty beore and ater the batch.
Generally, this application is not too difcult
because the batches tend to be long (greater than
one minute). Any transient meter behavior at the
beginning and end o the batch is sma ll compared
to the whole batch, so errors a re washed out.
However, when batches are short (less than one
minute), the transient errors can account or a signi-
cant raction o the total error. Air may be entrained
or a brie period, but the main issue is the time ittakes to ll the meter with uid. For instance, an ap-
plication running at 3 m/s will take about 0.1 s to ll
i the tube length is 0.3 m; an application at 0.3 m/s
will take a ull second simply to ll the meter. Experi-
ence has shown that i the meter ll-time is less than
0.1 sec., good batching perormance can be achieved,
regardless o the meters tube geometry.
tIM PAtten is director o measurement technology or
Micro Motion, Inc., Boulder, Colo. E-mail him at Tim.Pat-
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usa h Aaci f MamsMagnetic owmeters provide accuracy and can be used in a variety o applications
and environments
By David W. Spitzer
MAgnetIC FLowMeterS are among themost versatile o lowmeter technologies. hese
meters measure liquid velocity, rom which the
volumetric low rate is inerred. he measure-
ment is linear with liquid velocity and exhibits a
relatively large turndown. In addition, the range
o accurate low measurement is relatively large
and easy to change a ter installation.
Straight-run requirements are relatively short,
so magnetic lowmeter technology can be applied
where l imited straight run is available. In addi-
tion, the technology has no Reynolds number
constraints, so it can be used or liquids with high
or varying viscosity. However, liquid electricalconductivity constraints must be satisied or
these lowmeters to unction.
he only wetted parts o the lowmeters are
the liner and electrodes, both o which can be
made rom materials that can withstand cor-
rosion. his makes the lowmeters suitable or
use in chemical plants where corrosion may be
a concern. wo-wire magnetic lowmeters are
available that do not require power wiring. hese
can replace an existing lowmeter using the exist-
ing conduit or wiring with little or no electrical
rework.
PrInCIPLe oF oPerAtIon
Magnetic owmeters use Faradays Law o electro-
magnetic induction to determine the velocity o a
liquid owing through a pipe. Following Faradays
Law, ow o a conductive liquid through a magnetic
eld will generate a voltage signal. Tis signal is sensed
by electrodes located on the ow tube walls. When
the coils are located externally, a non-conductive liner
is installed inside the pipe to electrically isolate the
electrodes and prevent the signal rom being shorted.
For similar reasons, non-conductive materials are used
to isolate the electrodes or internal coil designs.
Te uid itsel is the conductor that will move
through the magnetic eld and generate a voltagesignal at the electrodes. When the uid moves aster,
it generates more voltage. Faradays Law states that the
voltage generated is proportional to the movement o
the owing liquid. Te transmitter processes the volt-
age signal to determine liquid ow.
SeLeCtIon FACtorS
Many actors must be considered when selecting
a owmeter, including the ambient conditions to
which the owmeter primary and transmitter will be
exposed. For the most part, the ambient temperature
rating o the owmeter primary is higher than that
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o the transmitter and does not limit applicability.
Many primary and transmitter enclosures that are
rated or NEMA 4X or IP67 provide adequate pro-
tection against ambient humidity and precipitation
encountered in outdoor installations.
Operating conditions inside the pipe include
pressure, temperature and liquid conductivity. In
addition, the liquid can be corrosive or abrasive. Tese
conditions are typically addressed using appropriatemechanical design and material selection. Pressure
requirements are addressed by appropriate design o
the ow tube or the application. One supplier makes
a specially designed magnetic owmeter that can
withstand 1,500 to 2,000 bar (more than 20,000 psi).
Many primaries are available with polytetrauoro-
ethylene (PFE) or peruoroalkoxy (PFA) liners that
are rated to about 266 Degrees F and 356 Degrees F
(130 Degrees C and 180 Degrees C), respectively.
Less expensive liners rated to lower temperatures are
oten available to handle less demanding applications.
Appropriate electrode and liner material selection can
reduce the eects o corrosion and abrasion. ake care
when using ceramic liners because they can shatter
when temperature gradient constraints are exceeded.
Whereas the conductivity o the liquid in a typical
magnetic owmeter must be maintained above about 5
mixro-Siemens/cm (micro-S/cm), special low-conduc-
tivity designs are available that operate as low as about
0.01 micro-S/cm. Some owmeters require more than
50 micro-S/cm, however, they are low-cost units that
are oten applied to water or wastewater service where
this conductivity is usually not a constraint.Te amount o straight-run pipe required to
achieve the stated accuracy o the owmeter is a
reection o the quality o the design and the tight-
ness o the accuracy specication. In many applica-
tions, these owmeters will unction accurately
with about three nominal pipe diameters upstream
and two nominal pipe diameters downstream o
the electrode.
Magnetic owmeter operation requires good
electrical connections between the electrodes and
the liquid. Te quality o this connection can
degrade i an electrode becomes coated or corroded;
this can compromise AC owmeter accuracy by
shiting the zero, and may cause the owmeter to
ail to operate. Te advent o DC-pulse excitation
transmitters reduced much o the need to address
this issue. In addition, some manuacturers have
designed their transmitters to exhibit a relatively
high input impedance to help decrease the eects o
connection quality.
Magnetic owmeter coils can use and store sig-nicant amounts o energy relative to the amount o
energy needed to cause ignition. Most magnetic ow-
meter transmitters are designed to be non-incendive, so
normal transmitter operation will not cause ignition.
However, when installed in some hazardous locations,
ormal approval is required, and the transmitter must
be designed and installed to address the hazard.
A hazard may be present not only in the general
location o the primary and transmitter, but also inside
the pipe where the electrodes can provide a source o
ignition. o mitigate this hazard, the circuits o some
designs limit the energy available at the electrodes to an
amount less than that required or ignition.
Maintaining equipment is simplied when sel-
diagnostics are available to help the user. Te extent
and quality o the diagnostics and their ease o use
varies by manuacturer. Changing ranges is easier
and more accurately perormed in a digital manner.
Potentiometer adjustments and step switches are more
prone to problems.
otHer ConSIderAtIonS
Te market or magnetic owmeters is competitive,so prices are relatively low. Magnetic owmeters or
water and wastewater service can be economical due
to the economies o scale and the relatively low cost o
liners and electrodes or this service. However, applying
magnetic owmeters to corrosive or abrasive services
can signicantly increase the cost o the meters.
Magnetic owmeters or use in the chemical in-
dustry are typically more expensive than vortex shed-
ders. In some applications, the cost can rival that o
turbine owmeter or orice-plate owmeter systems.
Magnetic owmeters are typically more economical
than Coriolis mass owmeters.
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FLowMeter PerForMAnCe
Te purpose o installing a owmeter system is to
accurately measure ow in a reliable manner. Issues
related to physical properties, process parameters,
electronic eatures and interconnections are oten
given much consideration. Relatively little empha-
sis, however, is given as to how well the owmeter
will perorm its intended purpose. Adding to the
conusion are the dierences in how perormance isexpressed and the incomplete nature o the avail-
able inormation. Nevertheless, the quality o ow
measurement should be a concern.
Te perormance o a owmeter is quantied by
its accuracy statements. Te reader must under-
stand not only which parameter is being described,
but also the manner in which the statement is
expressed. In ow measurement, parameters are
commonly described in terms o a percentage o
the actual ow rate, a percentage o the ull scale
o rate, or a percentage o the meter capacity. Tese
terms are mathematically related, so it is possible to
convert one to another (able 1). Note that when
compared on a common basis, such as percent o
rate, these statements describe signicantly dier-
ent perormance.
Other terminology may be used to express these
concepts. When this occurs, conrm exactly what
the other terms mean so they can be understood.
Perormance statements apply to a range o
ow or, stated dierently, between a minimum and
maximum ow velocity. It is important to identiy
the range in which the statement applies becauseperormance can be signicantly degraded or
undened when the owmeter operates outside o
this range.
Complicating the issue are some owmeters that
have dierent perormance statements or dier-
ent measurement ranges. For example, a owmeter
may have a reerence accuracy o 0.25% o rate
rom velocities o 1 to 10 m/s, and an absolute er-
ror o 0.0025 m/s rom 0.1 to 1 m/s. Perormance
is undened below 0.1 m/s. able 2 describes this
perormance using the above inormation. Note how
perormance degrades at low ows.
PerForMAnCe CLAIMS
For the most part, the claims made by suppliers
regarding magnetic owmeters are true statements,
even though they may seem extraordinary. Te
problem is that the statement may be incomplete,
and may not include certain acts and inormation
that clariy the statement. Sometimes claims are
simplied or convenience and easier understand-
ing. However, in many cases, urther investigationmay reveal other motives or doing so.
For example, consider a magnetic lowmeter
that has a reerence accuracy o 0.25% o rate and
a turndown o 1,000:1. he implication is that
the lowmeter can measure within 0.25% o rate
over a 1,000:1 range o low. aken individua lly,
both parts o the claim are likely true statements.
Yet when combined, t hey can be misle ading by
omission. Further investigation will show that
the reerence accuracy o 0.25% o rate applies
only within a range o low rates. Below the
minimum low rate o the range, the reerence
accuracy becomes a ixed absolute error. So as the
low rate decreases, the accuracy expressed as a
percentage o rate will increase.
Assuming that the reerence accuracy o 0.25%
o rate applies between 5% and 100% o meter
capacity, and that between 0.1% and 5% o meter
capacity, the reerence accuracy is xed at the abso-
lute error at 5% o meter capacity. able 3 calculates
reerence accuracy throughout the range o ows.
Tis illustrates that above 0.5 m/s, the reerence
accuracy is 0.25% o rate and that the turndown is10/0.01 or 1,000:1, both a s claimed. What is not
stated in the claim is that the reerence accuracy
degrades below 0.5 m/s and can approach 12.5%
o rate. Also not stated is that in actual installa-
tions, ows near meter capacity would rarely be
encountered, so the 1,000:1 turndown would rarely
be achieved. Assuming a more reasonable ull-scale
calibration range o 0 to 2 m/s, this owmeter
would achieve a 0.25% o rate reerence accuracy
rom 0.5 to 2 m/s, or a 4:1 turndown, and only a
200:1, or 2/0.01, turndown when the stated peror-
mance at low ow rates is included.
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In addition to high turndown, some suppliers
claim that their lowmeter operates at extremely
low low rates. Consider a claim to measure
velocity as low as 0.01 m/s. For a meter with a
capacity o 10 m/s, this corresponds to a 1,000:1
turndown. Although the lowmeter may operate
at this low rate, able 3 shows that it does so
with a reerence accurac y o 12.5% o rate.
Statements about magnetic owmeters otenclaim high reerence accuracy. What oten is not
stated is that it may apply over a range o higher
ows, and much o this range may not be encoun-
tered in actual operation. Furthermore, the reer-
ence accuracy a s a percentage o rate generally
degrades or is undened at lower ow rates (see
tables). When the ca librated ull-scale is low, and
the high reerence accuracy statements are limited
to a small range o high ow rates, the stated
reerence accuracy may not be achieved.
In general, reerence accuracy should be clear-
ly and completely stated or all ow rates prior to
perorming any analysis. Te range o applicabil-
ity o the high accuracy statement and the actual
operating ow range should be compared.
Magnetic owmeters are among the most
versatile o owmeter technologies. However, the
user should be aware o the manner in which their
application and operation are described in orderto ensure that the proper magnetic owmeter is
selected and installed.
dAvId w. SPItzer has more than 25 years o experience in
speciying, building, installing, commissioning and trouble-
shooting process-control instrumentation. Spitzer is a principal
in Spitzer and Boyes LLC, which oers consulting services or
the process industries in addition to product development,
marketing and distribution consulting or manuacturing and
automation companies.
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tak a diff Lk a Cifal PmpsAn unconventional assessment can provide insights or eective control.
By Andrew Sloley, Contributing Editor
oFten, CrItICAL understanding o a systemcomes rom turning the common analysis on its
head. With centriugal pumps, this means thinking
that ow results rom back-pressure on the pump
discharge, not that pump discharge pressure varies
with ow rate.
Tis unconventional approach was crucial in
addressing a troublesome control system or the
overhead o a natural gas liquids plants debutanizer,
which goes to an accumulator downstream o a reux
pump (Figure 1). Varying the rate o liquid product
controls tower pressure. A dual-range controller
handles the reux drum pressure -- one valve lets in
uel gas to pressurize the system when pressure drops,
another vents the drum when pressure rises to too
high a level.
Tis rather odd system did not work. Te reux
pump cavitated all the time and pressure control was
erratic.
Te owner, which acquired the unit during a
company buy-out, lacked tower drawings, exchanger
inormation, pump curves, control valve inorma-
tion and historical operating data. Current operating
personnel had no experience with the unit and neverhad seen it work stably.
Lack o inormation doesnt justiy ignoring the
problem. So, lets examine this systems undamentals
and explore the most serious shortcomings.
Centriugal analysis starts by looking at two
things: the system curve and the pump curve. Te
system curve is the head loss required versus ow rate
through the system. Te pump curve is the dynamic
head generated by the centriugal pump. Te intersec-
tion o the system and pump curves denes the ow
rate the system will get.
We most commonly attain the required ow rate by
adjusting the system curve by adding an extra pressuredrop via a control valve. Alternatively, we can change
the pump curve using an adjustable speed drive.
In Figure 1, the reux control valve is a hand-op-
erated control valve (HCV). Te reux system doesnt
include an automated pressure drop. It essentially has
a xed system curve. Tis brings us to thinking about
the pump operation: pump ow stems rom back-
pressure on the pump.
Now, lets consider the tower pressure-control
system. PC3 adjusts the product ow out o the
system with the intent o changing the liquid level
in the condensers. Varying wetted condenser area
on the process side allows or pressure control. Tis
is a simple, ast-acting and eective system or total
condensation services.
Meanwhile, the product drum pressure-control
system maintains a constant destination pressure or
the net product rom the reux pump.
Te static head to the top o the tower ar exceeds
the pressure change between the tower and product
drum.
Te problem comes rom how the systems interact.
Te pressure control system requires level to existsomewhere up in the heat exchangers. Tink o the
exchangers and piping as a tall narrow vessel -- i more
liquid exits a vessel than goes in, the level drops, and
vice versa. Te pressure control system should perorm
similarly to a tight level control system.
Te system curve or the reux stream includes
two components: static head and system pressure
drop. Static head doesnt change with ow rate, but
system pressure drop does. I static head makes up
most o the system curve, the curve is relatively at.
Flat system curves create large ow rate changes rom
small pressure drop changes.
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Te ow rate out o the vessel (heat exchangers
and piping) is very sensitive to changes in the HCV
position. Te HCVs purpose is to generate enough
pressure drop in the reux line so the unit can operate
in a sweet spot where the pressure control system will
work. In this case, we suspect the sweet spot is toosmall. Te back-pressure on the pump imposed by
the HCV usually is too low. At low back-pressure the
pump capacity exceeds the liquid rate. More liquid
is leaving the vessel (heat exchangers and piping)
than going in. Liquid level drops quickly. Finally, the
pump cavitates.
How can we address this problem?
One way is to try to put as much dynamic pres-
sure drop on the HCV as tolerable. Tis makes the
reux system curve steeper, which gives more stable
control. Tis is cheap and quick.
A second, and better, approach automates the
HCV. Control systems should transer a disturbance
rom where its important to where its not. Whats
important here is the ow rate out o the vessel
-- so we can maintain tight level control. We must
move the disturbance to something unimportant.
Many dierent congurations are possible. Tecloud in Figure 1 shows one o the simplest and
easiest options. A strap-on ultrasonic ow meter
along with a bolt-on actuator on the HCV enables
ully automated control o the overhead system. Te
disturbance now is in the valve pressure drop -- an
unimportant spot.
Tere are other ways to approach this problem.
But a undamental understanding o the system
comes rom looking at the pump backwards.
Andrew SLoLeyis a Chemical Processing Contributing
Editor. You can e-mail him at [email protected]
Debutanizer OverheaD
Figure 1. Lack o an automated pressure drop in the reux system made control difcult.
mailto:[email protected]:[email protected]