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New Developments to Improve Natural Gas Custody Transfer Applications with Coriolis Meters
Including Application of Multi-Point Piecewise Linear Interpolation (PWL)
Marc Buttler, Midstream O&G Marketing Manager
Tonya Wyatt, Process Gas and Chemical Industry Marketing Manager
Karl Stappert, Americas Flow Solutions Advisor
Emerson Process Management – Micro Motion
Ron Gibson, Senior Engineer
Gary McCargar, Senior Engineer
ONEOK Partners
Abstract
This paper provides an update on the growing use of Coriolis flow meters for natural gas wholesale
custody transfer measurement. Practical applications of provisions found within the 2nd
Edition of the
American Gas Association Report No. 11 (AGA 11) will be covered that deal with calibration fluid
flexibility, compensation for the effect of pressure on the meter, automatic multi-point piecewise linear
interpolation, and in-situ secondary verification methods that can be used after meters are installed in
service as an ongoing check of the calibration accuracy. This paper expands upon a paper previously
presented at the 9th International Symposium of Fluid Flow Measurement (ISFFM)
1.
1. Calibration Adjustment Factors Employed in Natural Gas Custody Transfer The 2
nd Edition of the American Gas Association Report No. 11 (AGA 11)
2 refers to four different
methods for applying adjustment factors to minimize any observed bias errors in meter calibration results.
These include:
Flow-Weighted Mean Error (FWME)
Polynomial Algorithm
Multi-point Linear Interpolation
Piecewise Linearization
The purpose of applying adjustment factors is to allow for the meter to be adjusted to as close to zero
error as possible at flow rates that cover the full expected service flow range.
Any type of calibration adjustment factor relies upon taking laboratory measurements on a flow device
and applying a correction factor(s) to the results in order to reduce the residual differences between the
flow device and the reference. The difference between the four adjustment methods listed above are that
they may be a single adjustment factor for the entire flow range (like the FWME) or multiple factors
applied for the range that both adjust any bias compared to the laboratory reference and adjust the
performance to provide a more linear result. When multiple correction factors are applied, they can
implement a best fit algorithm such as a polynomial fit, can implement multiple individual correction
factors for each point (Piecewise Linearization or Step-wise correction), or can implement multiple
correction factors with interpolation between the points.
Historically, the most common method for adjusting the indication of a typical early flow meter design
(e.g., turbine gas flow meter) was the application of a single FWME correction factor (a.k.a.
Flow-Weighted Final Meter Factor). Although this method provided a single flow-weighted best fit
correction suitable for a narrow range of flow rates, when a meter was applied over a wider flow range,
2
the linearity of some meter types could vary by more than the accuracy requirements for the application
as shown in Figure 1.
Fig.1. Example of a Traditional Flowmeter Potential Non-Linearity
With the advent of microprocessor-based flow computers, the piecewise linearization method was
developed. This method allowed for the improvement of a meter’s performance across the whole flow
application range. Piecewise linearization provides the ability to linearize a flow meter indication so that
it closely tracks the measurement of the calibration flow reference over a wider range of flow rates.
The first implementations of multi-point piecewise linearization were in turbine and positive displacement
flow computers in the 1980s. An example of piecewise multi-point linearization adjustment is shown in
Figure 2. This correction is a step-wise correction process. In liquids standards terminology, this type of
corrections is sometimes called Piecewise Linearization (note that for the remainder of this paper,
Piecewise Linearization will not be used to describe this step-wise correction method) or Multi-Point
Calibration.
Fig.2. Example of Piecewise Linearization Adjustment with No Interpolation
-0.50%
-0.30%
-0.10%
0.10%
0.30%
0.50%
0 2 4 6 8 10Erro
r
Flow Rate, lbm per second
As Found Error
Step-wise Correction
Corrected Data
3
This and other linearization methods have since been applied and are widely accepted in the use of
ultrasonic flow meters and other highly repeatable flow meters. Ultrasonic flow meters very commonly
employ a multi-point piecewise linear interpolation adjustment. With this method, the adjustment that is
applied between each neighboring pair of the piecewise points is the linear interpolation between those
points as shown in Figure 3. When used with ultrasonic meters, this type of adjustment is commonly
referred to as just “piecewise linearization” or “PWL” for short. Note that this may be confusing because
turbine and positive displacement meter correction using the step-wise correction can be called the same
thing. The ability to linearize flow meter performance utilizing a piecewise linearization function has
become so widely accepted that this functionality can be commonly found in the designs of most
ultrasonic flow meter transmitters and flow computers.
Fig.3. Example of Multi-Point Piecewise Linear Interpolation (PWL) Adjustment
2. Coriolis Meter Calibration Options from AGA 11
Coriolis meters continue to grow in natural gas custody transfer applications. The wide range of flow
rates, high level of accuracy, ease of use, low maintenance, and long-term stability of Coriolis meters
make them quite useful in applications ranging from smaller industry and city gates to larger transmission
lines. A typical installation of a Coriolis meter in a natural gas custody transfer application is shown in
Figure 4. To support this growth, studies are being conducted to explore and develop new options for
Coriolis meter calibration to achieve further improvements in accuracy and efficiency.
Fig.4. Typical Natural Gas Custody Transfer Station with Coriolis Meter Installed
-0.10%
-0.05%
0.00%
0.05%
0.10%
0 2 4 6 8 10Erro
r
Flow Rate, lbm per second
As Found Error
Multi-Point Piecewise Linear Interpolation Correction
Corrected As Found Data
4
a. Calibration Fluid Flexibility
AGA 11 states in the beginning of Section 7 Gas Flow Calibration Requirements that it can be valid to
use an alternative calibration fluid, such as water, to calibrate Coriolis meters for gas measurement, so
long as transferability of the calibration from the alternative fluid to gas has been demonstrated by the
meter manufacturer through tests conducted by an independent flow calibration laboratory.
Transferability of the calibration from an alternative fluid will include an added uncertainty relative to gas
measurement that must be quantified by the manufacturer and verified by the independent flow
calibration laboratory.
Emerson has verified transferability of water calibration to gas flow measurement for the Micro Motion®
ELITE® CMF series of flow meters through testing at multiple independent flow calibration laboratories.
The maximum difference observed during testing between the original water calibration and the tests on
natural gas and nitrogen was ±0.5%3. No linearization or adjustment was applied after the original
factory calibration on water was performed. Compensation for the known effect of pressure on the meter
was applied. A Coriolis meter installed in an independent gas calibration laboratory for testing is shown
in Figure 5.
Fig.5. Coriolis Meter Installed for Independent Gas Laboratory Testing4
Coriolis meter designs that have not yet demonstrated transferability of calibration fluids are required to
be flow calibrated on gas as prescribed in Section 7.1 of AGA 11.
b. Pressure Effect Compensation
AGA 11 describes the effect that internal line pressure can have on some Coriolis meter designs and sizes.
AGA 11 includes the equation that is used to perform a linear correction which applies the compensation
factor (PEffect - in units of % of flow rate per unit of pressure) that is required to compensate accurately as
the line pressure deviates from the calibration pressure (PCal). It is important to note that not all Coriolis
meters exhibit a measurable effect. As the size of the meter increases, the likelihood also increases that a
pressure effect will occur. Compensation factors are published for all Micro Motion meters that exhibit a
pressure effect and Micro Motion meters have the ability to automatically apply compensation as it is
described in AGA 11.
5
The pressure compensation applied by a Micro Motion meter is optional and may be activated or
deactivated through the device configuration. When activated, pressure compensation can be
implemented in one of two ways. The first method allows the operator to manually enter the service line
pressure as a fixed value. This is a simple method that can be employed when it is known that the line
pressure will be relatively constant. In cases where it is expected that the line pressure will vary
considerably, the second method can be employed in which a pressure reading is read as a live input by
the Coriolis meter and used for dynamic pressure compensation.
An example of independent laboratory as-found test data that is both uncompensated and then
compensated for pressure effect is shown in Figure 6. The meter was initially calibrated at the factory on
water at approximately 20 psig. The application of the correct pressure effect compensation (applying
correction from 20 psig to 740 psia) causes the measurements to meet the manufacturer’s published
specifications.
Fig.6. Large Coriolis Meter Test Data Uncorrected and Corrected for Pressure Effect
For any specific meter design and size, if the Coriolis meter manufacturer has not provided a pressure
correction value or declared that the meter in question has no pressure effect, then it is necessary for an
independent gas test laboratory to ensure that any pressure effect is addressed. This can be done either by
calibrating the meter at the service average line pressure, if known, or by testing to characterize the
pressure effect compensation that will need to be applied to the meter while it is in service.
c. Multi-Point Piecewise Linear Interpolation
Coriolis meters have traditionally used the technique of applying a single factor over the entire flow range
to adjust meters during calibration. This single factor may often come from the original factory
calibration or by using the FWME method. The original factory calibration is typically established at a
single flow rate and subsequently verified across the range of the flow meter. The FWME method applies
a single factor to all flow rate measurements and is usually applied after testing a meter against an
-2.00%
-1.50%
-1.00%
-0.50%
0.00%
0.50%
1.00%
1.50%
2.00%
0% 20% 40% 60% 80% 100%
Erro
r
% of QMax
Natural Gas at 740 psiaTest Results - CMFHC2M - Pigsar
Uncompensated
Pressure Effect Compensated
6
independent reference. A single-factor adjustment method for calibration is usually quite acceptable
because many Coriolis meters are inherently linear within AGA 11 limits over a wide range of flow rates.
Although gas flow measurement with Micro Motion ELITE CMF meters will meet both the AGA 11
maximum allowable error requirements of ±0.70% and the published manufacturer’s specifications of
±0.35% without the benefit of any form of piecewise linearization adjustment, recent work is showing
that the application of the multi-point piecewise linear interpolation method (PWL) has the potential to
achieve even greater accuracy during calibration than is otherwise possible. ONEOK and Emerson
Process Management have worked together on a program of testing with independent labs to determine
whether or not, and by how much, the accuracy of gas meter test results can be improved.
3. Implementation of PWL and Pressure Compensation of Coriolis Meters by Independent Gas
Calibration Laboratories
Emerson has developed an optional software feature in the Micro Motion ELITE CMF meters that
provides qualified independent gas calibration laboratories with the capability to program the meter with
up to ten adjustment points that will thereafter be applied for PWL adjustment of the meter flow
indication. The adjustment applied by this method is identical to that shown earlier in Figure 3. Meters
must be specially ordered with the necessary software option for PWL capability before they are sent to a
qualified laboratory for the calibration to be performed.
All ELITE CMF meters are equipped with a standard feature that may be configured and activated to
perform pressure compensation.
To date, Emerson has qualified three independent gas calibration laboratories in North America to
perform multi-point linear interpolation calibrations of Micro Motion ELITE CMF meters; Colorado
Engineering Experiment Station, Inc. (CEESI), Southwest Research Institute (SwRI), and TransCanada
Calibrations (TCC). These labs have received the tools, procedures, and training that are needed to
determine and program the PWL settings.
The procedure to perform PWL calibrations of Micro Motion ELITE CMF meters follows these basic
steps:
1. The meter is installed in the laboratory to meet or exceed manufacturer and operating company
recommendations.
2. The meter zero setting is verified and adjusted, if needed, according to manufacturer and
operating company recommendations and policies.
3. The meter pressure compensation is activated, if needed, and configured appropriately to apply
the correct compensation during the collection of all as-found and as-left data.
a. (Alternative method with no pressure compensation activated)
An alternative method for calibrating the meter is possible with the pressure
compensation deactivated. When this method is used, the laboratory calibration pressure
(PCal) value should be changed at the end of the procedure from the original factory
calibration pressure to the independent laboratory calibration pressure. For meters that
do not require pressure compensation, it is not necessary to adjust the PCal value.
4. The initial (as-found) data is collected covering the flow range for the application.
7
5. The initial (as-found) data is reviewed and up to ten flow rates are selected where the
linearization adjustment points will be set so as to optimize the overall results. The correction
values to be applied at the selected flow rates are then determined based on the average error of
the as-found data that was recorded at each of these flow rates. Linearization points will not be
selected at flow rates below the Qt flow rate because adjustment in the range of the lowest flow
rates of the meter is better accomplished with the meter zero setting.
6. The established PWL points are programmed into the meter configuration.
7. Verification test data is collected and reviewed to assess the successful implementation of the
PWL adjustment. Often, flow rates that are different than the flow rates selected for linearization
will be chosen for the verification tests in order to thoroughly test the effectiveness of the
adjustment.
Following the installation of the meter in service, overall system metrics will be monitored to ensure that
no bias shift in the performance of the meter has occurred to adversely impact the flow measurement.
Examples of the resulting improvements that are being realized with PWL adjustment can be seen in
Figures 7 through 12. The “as-found” data shown in these figures represents the meter performance in the
gas calibration laboratory as it was received following the initial factory calibration on water. The
“verification” data shown represents the data that was collected later after the PWL adjustments were
completed.
Fig.7. Example of PWL Results with a CMF100 1-inch meter
Complete Data Set
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5% E
rro
r
lbm per second
As Found Data
Verification Data
8
Fig.8. Example of PWL Results with a CMF100 1-inch meter
Averages at Each Flow Rate
Fig.9. Example of PWL Results with a CMF200 2-inch meter
Complete Data Set
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5% E
rro
r
lbm per second
As Found Averages by Flow Rate
Verification Averages by Flow Rate
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0 2 4 6 8 10 12% E
rro
r
lbm per second
As Found Data
Verification Data
9
Fig.10. Example of PWL Results with a CMF200 2-inch meter
Averages at Each Flow Rate
Fig.11. Example of PWL Results with a CMF300 3-inch meter
Complete Data Set
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0 2 4 6 8 10 12% E
rro
r
lbm per second
As Found Averages by Flow Rate
Verification Averages by Flow Rate
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20 25% E
rro
r
lbm per second
As Found Data
Verification Data
10
Fig.12. Example of PWL Results with a CMF300 3-inch meter
Averages at Each Flow Rate
a. Importance of the Correct Implementation of Pressure Compensation and PWL Together
During Coriolis Meter Calibration
The best practice is to apply PWL adjustment independently of pressure correction. This is achieved by
activating pressure compensation during the collection of the initial (as-found) data that is to be used to
establish the set of adjustment point values. With this method, the linearization can be subsequently
applied regardless of the service line pressure, while the pressure correction can be adjusted
independently and applied with flexibility to accommodate line pressure changes and live pressure
compensation.
An alternative method combines pressure compensation together with linearization as part of the PWL
adjustment. This can be done by collecting the initial (as-found) data at the service line pressure with the
meter pressure effect compensation deactivated. In this implementation, the resulting multi-point
adjustment values will include both linearization and pressure compensation for that service pressure.
The value of PCal will henceforth become the pressure that was applied during the collection of the initial
(as-found) data in the gas calibration laboratory instead of the pressure that was applied during the initial
factory calibration on water. As a result, there will be zero pressure compensation when the line pressure
is equal to this new PCal value, and future pressure compensation will be based on the difference between
any new line pressure and this new PCal calibration pressure baseline.
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0 5 10 15 20 25% E
rro
r
lbm per second
As Found Averages by Flow Rate
Verification Averages by Flow Rate
11
4. In-Situ Secondary Verification of Coriolis Meter Calibration
One of the most sought-after benefits of Coriolis meters is that the calibration remains very constant over
time if nothing occurs to damage the meter flow tubes structurally. This is why diagnostic tools that
accurately monitor the structural health of a Coriolis meter are useful as a tool for secondary verification
of the meter calibration that can be relied on, even after lengthy periods of time have passed since the
most recent calibration against a primary or secondary flow reference standard.
Micro Motion ELITE CMF meters can be equipped with an optional diagnostic feature called Smart
Meter Verification (SMV) that uses a sophisticated analysis the Coriolis meter flow tube vibration
response characteristics to assess and trend the structural consistency of the meter flow tubes. If the
structure of the meter is found by the SMV tool to be consistent over time, this result indicates that the
meter’s flow calibration has remained unchanged.
As discussed earlier in this paper, a Coriolis meter may be installed directly into service for natural gas
custody transfer after the factory calibration on water, or it may be sent to an independent gas calibration
laboratory for calibration that may or may not implement PWL adjustments. In either case, the value of
the investment in equipment and calibration can be prolonged and preserved by using an automated
secondary verification diagnostic tool like the Micro Motion Smart Meter Verification feature.
5. Impact of Zero
For a flow metering device, there are three potential types of offset; zero, span, and linearity. When the
zero of the flow metering device is offset, that specific amount of error results in a consistent bias across
the entire flow range (see figures 13 and 14). The flow calibration, on the other hand, impacts the
measurement by the same percentage across the entire flow range (see figures 13 and 15).
Fig.13. Example of linear device with a positive meter zero offset
and flow calibration factor applied for slope adjustment
12
Fig.14. Example of linear device where the slope or flow calibration
factor (from Fig. 13) remains unchanged, but the zero has shifted down
Fig.15. Example of linear device where the meter zero (from Fig. 13)
remains unchanged, but the flow calibration factor has changed
The meter zero typically will have little effect in terms of percentage of rate offset at high flow rates, but
can become very important at low flow rates where the zero offset is a much higher percentage of the
flow. Since gas applications have low density, the mass flow rates through the meter are typically much
smaller than liquid mass flow rates through a similar line size, so achieving a proper meter zero is
especially important for gas.
All Micro Motion Coriolis meters are zeroed during the factory calibration. In most installations, that
initial zero that is captured may be the best zero for the meter and should not be changed. However, some
things can occasionally impact the zero including temperature, mounting conditions, etc. Some
manufacturers recommend always capturing an installed meter zero, while others claim there is never a
need to zero. Micro Motion offers a zero verification tool that looks at eight parameters to determine if
the flow is stopped and the process conditions are stable enough to determine whether or not the currently
captured zero is the best zero. If conditions are found to be stable and the current zero is not the best zero
for the application, the tool will recommend the user on the appropriate actions. This is a simple way
with a touch of a button to insure the meter has the best zero without having to change the zero value.
13
6. Conclusions
Coriolis meters that are properly designed and constructed for natural gas custody transfer service will
meet the basic AGA11 maximum allowable error requirements of ±0.70% above the Qt flow rate without
the need for any form of linearization. Micro Motion ELITE CMF meters are designed to achieve
±0.35% accuracy in mass flow measurement of natural gas based on the initial factory calibration using
water and without any further adjustment or linearization other than the application of published pressure
compensation, when it is needed.
However, it has been found that accuracy improvements beyond the AGA 11 requirements and the
manufacturer’s specifications can be achieved in independent laboratory tests by applying the PWL
adjustment method over the range of flow rates tested. Results of testing to date have shown that PWL
adjustment of Micro Motion ELITE CMF Coriolis meters during calibration by independent gas
calibration laboratories consistently yields results as good as ±0.10% or better in the averages at each
flow rate over the calibrated flow range during the verification tests and Emerson offers an option to
make this correction in the transmitter. It is important to note that the overall combined uncertainty will
always include the uncertainty of the laboratory reference standards.
The next logical step in this work to further confirm the potential value of PWL adjustment of Coriolis
meters will be to conduct round robin testing of meters that have been calibrated with this method
between multiple independent accredited gas calibration laboratories in order to demonstrate that PWL
adjustment results are transferrable from one lab to the next. Parties interested in learning more about this
ongoing work are invited and encouraged to contact the authors.
1 Buttler, M., Gibson, R., McCargar, G., Stappert, K., Wyatt, T., The Practical Application of Multi-
Point Piecewise Linear Interpolation (PWL) and Other Developing Trends with Coriolis Meters for
Natural Gas Custody Transfer Applications, April 2015 2 AGA Report No. 11, API MPMS Chapter 14.9, Measurement of Natural Gas by Coriolis Meter,
American Gas Association, 400 N. Capitol Street, N.W., 4th Floor, Washington, DC 20001 3 Test Report Number NMi-12200340-02, Project Number 12200340, NMi Certin B.V., Hugo de
Grootplein 1, 3314 EG Dordrecht, The Netherlands 4 Wyatt, T., Stappert, K., Large Coriolis Meters and the Applicability of Water Calibrations for Gas
Service