IBP61504
AGA11 and Coriolis Meters for Natural Gas Measurement
Karl B Stappert1
This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2004, held between 4 and 7 October 2004, in Rio de
Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the
abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to
translate or correct the submitted papers. The material as it is presented, does not necessarily represent Instituto Brasileiro de Petróleo e Gás’ opinion,
nor that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference
2004 Annals.
Abstract
Coriolis meters have gained worldwide acceptance in liquid applications since the early 1980’s with an
installed base or more than 600,000 units. Newer designs have shown greatly improved low-flow sensitivity, lower
pressure drop, and immunity to noise; factors which now enable their successful use in gas-phase fluid applications.
With more than 24,000 units on gas around the world, measurement organizations around the world are involved in
writing standards for this “emerging” gas flow technology.
In December of 2003 the American Gas Association and the American Petroleum Institute co-published AGA
Report No. 11 and API MPMS Chapter 14.9.
An overview of theory, selection, installation & maintenance, and benefits of Coriolis meters will be
presented. Application details will be presented to illustrate both the range of natural gas applications, including
production, fuel flow control to gas power turbines, master metering, city/industrial gate custody transfer, and thirdparty test data. Laboratories include the Colorado Engineering Experiment Station Inc. (CEESI), Southwest Research
Institute (SwRI), and Pigsar (Germany).
1. Introduction
Coriolis is one of the fastest growing technologies, and its growth in gas phase applications is approximately
four times faster than that of liquid applications. Older designs were known to have some fairly well justified
limitations for use on gas. In general a relatively high pressure drop (around 1000” H2O) was required to obtain a high
accuracy flow reading, and large meters (3”-4” meter) did not work well due to low flow sensitivity to noise and effects
of process pressure.
Newer designs and technology developments since the early 1990’s have changed this, allowing accurate gas
flow measurement for even low-pressure gases (50-100 psi). Low flow sensitivity has been dramatically improved, and
pressure drop lowered (a typical 500 psi distribution application can now be sized as low as 90” wc pressure drop). All
in all, it can be argued that Coriolis technology solves more problems and offers even more value for gas than liquid
measurement. This is because gases are compressible, and with traditional technologies (orifice, turbine, rotary,
diaphragm), process pressure, temperature, and gas composition must be accurately measured or controlled, the devices
regularly maintained (orifice plates checked, turbine bearings rebuilt), and adequate flow conditioning provided for
profile-sensitive technologies. Since Coriolis measures the flowing mass of the gas, and accuracy is independent of
composition and flow profile/swirl, the meter is more accurate under a wider range of operating conditions, and is often
lower cost to install and maintain.
Coriolis is a smaller line-size technology: the largest offering from any vendor for gas applications is a 6” meter. The
pressure drop and flow range of a Coriolis meter draws a direct relationship to the actual flow area through the meter
when comparing it to other metering technologies. Because of this relationship a Coriolis meter will typically be one
pipe size smaller than a turbine meter and several sizes smaller than an orifice while having approximately the same or
pressure drop and flow range. This is especially true at gas static pressures of approximately 300 psig and higher.
The typical installation, shown in Figure 2, of a Coriolis meter is very cost competitive with other metering
technologies on an installed cost basis, where installed cost includes:
______________________________
1
A.A.S Electronics Technology – Micro Motion, Inc
Rio Oil & Gas Expo and Conference 2004
-
Instrument purchase price
Temperature and pressure compensation
Flow conditioning and meter tube requirements
Engineering and Procurement of these instruments
Labor to install metering equipment (mounting, wiring, and plumbing)
2. Application “Sweet Spots”
The ideal applications where Coriolis meters make sense from a performance, capital cost, and operating cost
standpoint are as follows.
-
Gas delivery locations/Pressure cut locations
Bi-directional metering locations
Measurement locations where high regulator noise is a concern
Line sizes 10” and smaller in ANSI classes of 300 and above
High turndown requirements (20:1 up to 50:1 is common), eliminating parallel metering runs of other technologies
Dirty or wet gas where maintenance can be an issue
No room for adequate straight-runs (i.e. Turbine, Orifice, and Ultrasonic)
Measurement were fluid pulsations are present
Changing gas composition and density
Critical phase fluids such as Ethylene (C2H4) or Carbon Dioxide (CO2), where the requirements to achieve high
accuracy volumetric metering is very expensive
Custody transfer, process control, or system balances where is mass based measurement provides a higher degree
of accuracy
Applications where periodic peak flows will cause over-range damage or loss of measurement with other
technologies (i.e. Turbine, Rotary, Ultrasonic, Orifice)
Currently, as measured by flow meter units sold, around 10% of the worldwide market for Coriolis meters is
for gas phase applications. This is in flow market that is approximately one-fourth (26%) gas, not including steam
(Process gas is thought to be approximately 16% with Natural gas being 10%, and steam being 10%). Coriolis is
primarily a single-phase flow meter, although early testing on gas with liquids up to 5% by weight has shown an
accuracy of approximately 1%.
3. Coriolis Base Volume Output
Coriolis technology measures the mass of fluid (gas or liquid) flowing through the primary element. The
Coriolis meter also has the ability to measure fluid densities comparable to the accuracy of a liquid densitometer. For
liquid applications, the on-line density from the Coriolis meter is used to output actual volume. This is useful for
fiscal transfers of liquid petroleum, and is often corrected to base conditions, such as barrels of oil at 60 deg F using
API volume correction methods.
For gas applications, the meter output can be configured for base (standard or normal) volumetric flow units,
such as MMSCFD or NM3/hr calculated using base density as shown by Equation 1 or relative density as shown by
Equation 2. The on-line gas density from the Coriolis meter is not used; rather the base density as shown by Equation
3 is calculated from gas compositional information determined from system averages, historic gas samples, or on-line
analysis, using a gas chromatograph (GC). Methods and gas physical property information requirements are identical to
those currently utilized in the use of other gas metering technologies. Coriolis technology uses the following
calculations to output a highly accurate base volume output.
Vb ( gas ) =
w( Gas )
ρ b ( gas )
(1)
2
Rio Oil & Gas Expo and Conference 2004
Vb ( gas ) =
w( Gas )
(2)
Gr ( gas ) ρ b ( Air )
ρ b ( gas ) =
Pb M r
Z b RTb
Where:
w(Gas ) =
Weight or mass of gas
Vb (Gas ) =
Volume of gas at base conditions
ρ b (Gas ) =
Density of gas at base conditions
Gr (Gas ) =
Relative Density or Specific Gravity the gas at base conditions
ρ b ( Air ) =
Density of air at base conditions
Pb
Mr
Zb
R
Tb
(3)
=
Pressure of the gas at base conditions
=
Molar mass of the gas
=
Compressibility of the gas at base conditions
=
=
Universal Gas Constant
Temperature at base conditions
Coriolis has been used since the late 1970’s for liquid process applications, and has now been used since 1992
for process gas with more than 10,000 installed units. Another 10,000 have been used for Compressed Natural Gas
(CNG), natural gas at 3000+ psi for vehicle fueling. This paper will discuss why the technology is now a bona fide
option for natural gas applications. Status of major worldwide standards will be presented, with an emphasis on the
Americas and Europe, plus a sampling of applications from “wellhead to burner tip”.
4. Theory of Operation
A Coriolis meter is comprised of two main components, a sensor (primary element) shown in Figure 3. and a
transmitter (secondary). Coriolis meters infer the gas mass flow rate by sensing the Coriolis force on a vibrating tube or
tubes. The conduit consists of one or more tubes and is forced to vibrate at a resonant frequency. Sensing pick-off
coils located on the inlet and outlet sections of the tube(s) oscillate in proportion to the sinusoidal vibration. During
flow, the vibrating tube(s) and gas mass flow couple together due to the Coriolis force causing a phase shift between
the vibrating pick-off coils as shown in Figure 4. The phase shift, which is measured by the Coriolis meter transmitter,
is directly proportional to the mass flow rate.
Drive Coil
and Magnet
Leftt Pickoff Coil
and Magnet
Flow Tubes
Process Connection
Flanges
Case
Right Pickoff Coil
and Magnet
RTD
Process Connection
Flanges
Figure 3. Coriolis Sensor Design
3
Rio Oil & Gas Expo and Conference 2004
Left
∆t
Right
Figure 4. Pickoff Coil Signal Phase Shift
Note that the vibration frequency is proportional to the flowing density of the fluid. For gas applications, the
flowing or “live” density is not used for gas measurement, but can be used as an indicator to change in a Coriolis
meter’s flow factor.
For a more complete discussion of the Coriolis theory of operation, please contact the author.
5. Standards Work, Approvals, and Research
Coriolis meters have long been used for process control, and a number of worldwide approvals or documents
exist for fiscal (custody) transfer of liquids. These include:
USA – NIST and API
German - PTB
Dutch - NMi
Numerous other countries, including Switzerland, Belgium, Austria, and Russia
Beginning in the mid-1990’s, some of these groups and industry also began studying the technology for
gaseous applications. The German weights and measures group (PTB) extended custody transfer approval to include
both gas and liquid phase fluids in 1999. As well, Dutch weights and measures (NMi) has performed testing and
published a statement that the flow calibration factor established on water transfers without field calibration to gas
phase applications, within a tolerance determined in their testing relative to the transferability of a water calibration to a
gas calibration.
6. Sizing and Selection
Selection of a Coriolis meter for gas application is quite straight forward, but different than traditional technologies
used on natural gas such as orifice and turbine meters. There are two reasons for this; one being that Coriolis is
available in discrete sizes (like turbine or rotary); the second being that a Coriolis meter can be sized for a much higherpressure drop than is the industry norm. This can be useful as it increases useable turndown and/or presents no overranging concern when applying the technology.
6.1. Pressure Drop
The sensor geometry, gas density and velocity determine the permanent pressure loss through the meter. This
relationship is expressed by the pressure drop equation shown by Equation 4.
∆P =
Kρ f v 2
2gc
(4)
Any pipe fittings required for meter installation also determines pressure loss. Pipe reducers, valves, and
additional straight pipe requirements should be considered when calculating the loss in pressure for the selected meter.
4
Rio Oil & Gas Expo and Conference 2004
6.2. Velocity in the Coriolis Meter
Some Coriolis meters have performance limitations at higher gas velocities due to noise imposed on the meter
signal. Such signal noise can affect meter accuracy and repeatability. The gas velocity at which signal noise becomes a
problem is design (vendor) specific. Seldom is signal noise a concern when the gas velocity in the meter is below about
200 ft/s. To define the maximum recommended velocity a Mach number limit is usually provided by the meter
manufacturer.
If abrasive contaminants are present in the gas flow stream, erosion of the wetted meter components may be a
concern when the meter is exposed to high gas velocities. This concern is application specific.
Since Coriolis meters are made of stainless steel and nickel alloys like Hastelloy, pipe erosion from high gas
flow rates often experienced with carbon steel pipe is not of concern. The erosion of carbon steel pipe is initiated by
moisture in the gas causing oxidation of the carbon steel and erosion of the oxide layer by high gas flows. Internal
velocities through a Coriolis meter are similar to orifice and turbine meters. Like Coriolis meters, turbine and orifice
meters are made of materials which are relatively immune to oxidation and thus the erosion from high gas velocities.
6.3. Meter Error vs. Flow Rate
Meter error versus flow rate is determined from a performance curve, the error versus flow rate curve is based
on the results of laboratory calibrations. Most manufacturers state the probable meter error as a percentage of rate, plus
the zero stability value.
The error is determined by following expression: % Error = ± 0.50%; When the flowrate is less than (zero
stability/.0050) accuracy equals ± ((zero stability/flow rate) x 100). The base error value (0.50%) in the above equation
was chosen for illustrative purposes. The actual meter error can be established from laboratory calibration. This
should include the effects of laboratory uncertainty, linearity, hysteresis, and repeatability.
6.4. Zero Stability
The zero stability value defines the limits within which the meter zero may drift during operation, and is
constant over the operating range. It may be given as a value in flow rate units, or a percentage of a stated nominal
mass flow rate. The zero stability value is the limiting factor when establishing meter turndown ratio. The stated zero
stability value is achievable when the Coriolis flow meter is installed, and re-zeroed at operating conditions.
Because process temperature will affect the meter zero stability, the estimated value of the zero stability is
usually limited to meters at thermal equilibrium. The affect of changes in this value is typically given. In most gas
applications changes in process temperature are negligible, but to minimize the effect it is recommended that a Coriolis
meter be zeroed at process temperature conditions.
6.5. Temperature and Pressure Compensation
Both pressure and temperature affect the meter vibration characteristics, hence the magnitude of the sensed
Coriolis force. In comparison to zero stability, these effects are small, but should be compensated for to achieve
optimum meter performance. Most meter designs compensate for temperature effect automatically by monitoring the
temperature of the flow tube(s). The pressure effect can be continuously monitored and corrected for using an external
pressure transmitter, or by entering a fixed adjustment for the known average pressure. Other meter designs
periodically check meter sensitivity by applying a waveform reference force to the tube(s), during field operation, and
compare the system response to that achieved under reference flowing conditions. This system will compensate for
both pressure and temperature effects. Errors and compensation methods for pressure and temperature effects should be
stated in the meter performance specifications, and included, if necessary, when establishing meter performance for
sizing considerations.
6.6. Turndown Ratio
5
Rio Oil & Gas Expo and Conference 2004
Flow meter turndown ratio is the ratio of the acceptable maximum mass flow rate to the acceptable minimum
mass flow rate. The turndown ratio is application specific and dependent on gas conditions, allowable pressure loss
across the meter, and allowable meter error.
The maximum pressure loss (at maximum flow rate) across the meter can be determined once the meter
diameter, piping installation configuration, and maximum allowable gas velocity are specified. Typically, the meter
selected is one line diameter smaller than the size of the pipe in which the meter is installed. The comparison to other
flow technologies (Orifice and Turbine) is relative to flow area through the meter. This usually provides comparable
pressure drops to the other flow technologies and more accurate measurement at lower flow rates. However, the
resulting permanent pressure loss for a given flow rate is higher than if the meter diameter is the same as the pipe
diameter. Because higher-pressure gas has a higher mass flow rate for the same velocity, higher pressures will allow
for higher gas velocities through the Coriolis meter lessening pressure drop and producing higher flow turndowns for
the same meter arrangement.
7. Installation and Application Concerns
7.1. Mounting
Proper mounting of the sensor is required. Consideration should be given to the support of the sensor, the
alignment of the inlet and outlet flanges with the sensor. A spool piece should be used in place of the meter to align
pipe-work during the construction phase.
Piping should follow typical industry piping codes. Meter performance, specifically zero stability, can be
affected by axial, bending, and torsion stresses from pressure, weight and thermal effects. Although Coriolis meters are
designed to be relatively immune to these affects, utilizing properly aligned pipe-work and properly designed piping
supports insures these affects remain within acceptable limits.
The Coriolis transmitter should be mounted where it is easily accessed to attach communications equipment, to
view displays, and to use keypads. Coriolis meters are configured in two basic ways – the transmitter mounted to the
sensor or the transmitter mounted remotely.
7.1. Orientation
As a general rule, orient the sensor tubes in such a way as to minimize the possibility of “settling” heavier
components, such as condensate, in the vibrating portion of the sensor. Solids, sediment, plugging, coatings or trapped
liquids can affect the meter performance, especially when present during zeroing. Allowable sensor orientations will
depend on the application and the geometry of the vibrating tube(s). In gas service the ideal orientation of the sensor is
with the flow tubes in the upright position.
7.2. Fluid Swirl and Flow Profile Effects
The effect of fluid swirl and non-uniform velocity profiles caused by upstream and downstream piping
configuration on meter performance differs from one meter design to another. Flow conditioning, straight upstream,
and downstream piping lengths may or may not be required. It is recommended that installation effects data be
requested from the manufacturer to guide the designer in these requirements.
7.3. Effects of contaminants, i.e. compressor oil, liquids and free mists
Testing has shown that liquid carried in a gas stream may not have the same adverse affect on performance as
gas carried in a liquid stream. However, the meter will measure the mass flow rate of the total flow stream, including
the liquid i.e., condensate, glycol, and compressor oil. The allowable liquid fraction will depend on the application and
sensor geometry. Care should be taken to remove liquids before measuring the gas flow.
6
Rio Oil & Gas Expo and Conference 2004
7.4. Vibration and Fluid Pulsation
During product development, extensive analysis and testing have resulted in meter designs that are inherently
stable under a wide range of mechanical vibration and fluid pulsation conditions. Although Coriolis meters are for the
most part immune to mechanical vibration and fluid pulsations, they are very sensitive to vibrations or pulsations at the
resonant frequency of the flow tubes. The resonant frequency of the flow tubes is meter design and fluid density
dependent. In applications where mechanical vibration or fluid pulsations are present it is recommended that the
manufacturer be consulted to determine the resonant frequency of the flow tubes at operating conditions.
8. Operation and Maintenance Considerations
Other than the vibrating sensor element(s), Coriolis meters have no moving parts, requiring minimal
maintenance.
There are three common types of field checks, which include meter zero, sensor checks, and transmitter
checks. Verification of these parameters to be in tolerance is a direct indication of the meters operation within specified
accuracies. Out of tolerance conditions are indicators of adverse external influences on the meter,
malfunctioning/damaged meter components, or the need for meter cleaning in dirty service.
8.1. Meter zero stability
Meter zero and the stability of the meter zero should be checked periodically. Variation of meter zero outside
of specified tolerances with flowing temperature are an indicator of varying piping stresses an improperly aligned
piping or piping support. If the meter zero is out of tolerance and does not meet the manufacturer’s specifications its
value along with flow conditions should be logged for diagnostic purposes and the zero reset.
8.2. Zero Drift
Product buildup, erosion or corrosion will affect the meter performance. Product buildup (coating) may bias
the meter zero. If the buildup is causing a zero drift, cleaning and re-zeroing the meter should bring performance within
specification. If coating of the sensor continues, the zero will continue to drift. Although rare, erosion or corrosion will
permanently affect meter calibration and will compromise sensor integrity. When used within the specified fluid and
ambient condition limits, fatigue of the sensing tubes of a Coriolis meter due to vibration during the stated meter
lifetime is rare, and does not need to be considered when inspecting a meter. However, operating the meter in more
extreme corrosive or erosive applications will shorten the expected lifetime.
8.3. Checking and Adjusting Meter Zero
Improper zeroing will result in measurement error. In order to adjust the zero of the meter there must be no
flow through the flow sensor, and the sensor must be filled with gas at process conditions. The meter zero must be
established at process conditions of temperature, pressure and density. Even though the stream is not flowing, the flow
meter may indicate a small amount of flow, either positive or negative. Causes for the zero error are usually related to
the differences between the calibration conditions and the actual installation, which include but are not limited to the
following:
−
−
−
Differences between the calibration media density and the gas density
Differences in temperature
Differing mounting conditions
The meter should read a mass flow rate that is less than the manufacturer’s zero stability specification under the noflow condition.
The zeroing of the meter must be performed at nominal operating condition with no flow through the meter. Once it
has been confirmed that there is no flow through the meter, the zeroing procedure specified by the meter manufacturer
should be followed.
7
Rio Oil & Gas Expo and Conference 2004
9. Summary
Although a relatively new technology for natural gas applications outside of compressed natural gas (CNG),
Coriolis meters have gained worldwide acceptance for other fluids and in other industries. With a worldwide installed
base of around 600,000 units, Coriolis technology is seeing expanded use for both liquid petroleum and natural gas. A
number of countries and groups have published standards or are in the process of studying the technology. Most
notably is AGA and API who have jointly published AGA Report No. 11 / API MPMS Chapter 14.9, Measurement of
Natural Gas by Coriolis Meter in December of 2003.
Technology limitations of earlier designs have been largely overcome, with high accuracy measurement now
possible at low-pressure drop, typically 150” wc. Coriolis “sweet spots” are mainly in lines of 8” and smaller, where
high turndown is needed, flow conditioning with other technologies to meet new AGA requirements is costly, and/or
the gas is of dirty, sour, or of changing composition. Also, good potential exists for “simple” energy metering, using
the Coriolis meter output directly, scaled for energy units.
Coriolis technology merits serious consideration as a bona fide contender to complement Ultrasonic in low
cost of ownership metering for natural gas applications. These two technologies overlap 4” to 8” line size range.
Third-party data from CEESI, Pigsar, SwRI, and others show little if any effect of flow profile, and little if any
shift in meter factor from factory calibration to natural gas application. AGA testing in the future is planned to quantify
the effects on accuracy for wet gas.
Common Coriolis gas applications range from wellhead separator, medium to high pressure distribution
metering, and fuel gas to power turbines, reciprocating engines, and boilers for combustion control. As users of gas
meters investigate Coriolis they are finding it to be a fiscally responsible choice for gas measurement in today’s
competitive business environment.
10. References
AGA Report Number 11, Measurement of Natural Gas by Coriolis Meter, American Gas Association, 400 N. Capitol
Street, N.W., 4th Floor, Washington, DC 20001.
8