Quantifying and optimizing flow meter accuracy in real world.
Real-World Error Sources
In flow measurement, accuracy means a combination of bias--consistent error--and repeatability--random error. (1) Standards, including those provided by bodies such as the American Gas Association (AGA), International Standards Organization (ISO) and American Society of Mechanical Engineers (ASME), or guidelines from agencies such as Measurement Canada, describe how to correctly install and maintain a flowmeter to minimize bias error. (2) This is--of course--critical in custody applications because even a very small bias error can, over time, significantly favor either the buyer or seller. Repeatability also is important, allowing users to:
* Minimize disagreements between billing and check meters.
* Improve the ability to identify operational problems (for example, leak detection, pipeline capacity).
* Improve process control. By definition, a correctly installed and maintained flowmeter should suffer from zero bias error, so accuracy is repeatability. To quantify, suppliers will typically submit their devices to a third-party laboratory for testing. Unfortunately, this third-party laboratory test data is not useful in comparing flowmeters or predicting real-world performance. Installed or "real world" accuracy and repeatability are much worse for every flow technology, since few flowmeters are installed in laboratory conditions:
* Rigidly controlled ambient temperature: Many flowmeters are installed outdoors. In some climates, ambient temperatures can vary [+ or -] 25 degrees C or more from calibration temperature. While standards bodies require users to compensate for fluid temperature variation, ambient temperature variations cause transmitter output shifts, including the transmitter used to compensate for density changes in linear meters such as ultrasonic or turbine meters.
* Constant static line pressure: With orifice meters, a changing line pressure can significantly affect the differential pressure transmitter used to infer flow.
* Calibration before every test (no drift): The output of any analog component varies over time and with changing environmental conditions.
How can the user quantify the impact of "real-world" sources of error? Suppliers of equipment intended for industrial applications publish specifications which allow users to calculate and predict the impact of "real-world" effects on installed flow accuracy and repeatability. The spreadsheet shown in the sidebar article uses published specifications to calculate flow error caused by the DP transmitter in an orifice meter installation. The results at 100% flow, under "typical" installed conditions, are shown in Figure 1.
[FIGURE 1 OMITTED]
Note that reference (laboratory) accuracy is a trivial component of total transmitter installed error:
* Two DP transmitters with identical 0.075% "reference accuracy" can provide dramatically different installed accuracy and repeatability.
* Although the example of an orifice meter is used below for illustration, installed accuracy is worse than laboratory accuracy for all flow technologies.
Since DP varies with the square of flow, DP declines twice as fast as flow, and small errors at 100% flow are magnified at lower flowrates.
Low Flow Characterization
Suppliers minimize real-world effects by characterizing--sometimes called "footprinting"--their transmitters over a broad operating range. Completely different from calibration, characterizing a transmitter involves exposing it to a range of conditions--in particular, ambient temperature--and observing the impact on the measurement. Better smart transmitters include a built-in temperature sensor to measure ambient temperature--during operation in the field, observed ambient temperature variations can be automatically compensated in the microprocessor.
The integrated temperature sensor is also commonly used to help detect the conditions under which hydrate formation is likely to occur so necessary precautions can take place.
Figure 3 shows a typical DP transmitter characterization in which the impact of varying ambient temperature on the transmitter's output is measured.
As shown in Figure 3, suppliers normally characterize transmitters using reference inputs evenly distributed over the transmitter DP range. This approach provides transmitters which can be used in any "differential pressure" application--including flow, but also level, pressure drop across a filter, etc. Unfortunately, while it maximizes flexibility and minimizes spares, evenly distributed characterization is not optimized for flow applications, because:
[FIGURE 3 OMITTED]
* Half the characterization points are negative, hence not useful in the vast majority of (unidirectional) flow applications.
* The positive points, while evenly distributed by DP, are clustered toward the high flows due to the square root relationship between DP and flow.
The result of this clustering is that no characterization is performed between 0% and 45% flow. This is, as noted previously, the flow range where DP transmitter accuracy is most important, since uncertainty contributed by the transmitter is usually dominant only at low flows. To avoid this problem, users should instead ask their supplier to provide DP flow transmitters "characterized for flow." As shown in Figure 4, for these transmitters the supplier will apply the majority of the characterization effort to low-flow conditions, where the DP transmitter accuracy has the greatest impact on flowmeter performance.
[FIGURE 4 OMITTED]
This will provide significantly better real-world performance at low flows, resulting in dramatically better flowmeter turndown. For example, a DP transmitter which is characterized for flow can more than double flow turndown in a given installation, for a given installed accuracy and repeatability. Increased turndown allows a wider range of measurement without reducing the accuracy. This allows for better operational efficiency by eliminating drive time to the remote location whether it's a compressor station or field measurement.
While ambient temperature variations usually dominate in outdoor applications, line pressure variations can also be important. Newer smart transmitter designs offer built-in compensation for line pressure variations, for similar improvements.
What Are The "Best Practices?"
The first step in designing any flow measurement system is to determine mutual performance expectations--accuracy and repeatability--over the range of flow (the "turndown"). In custody transfer applications, the key stakeholders are of course the buyer and seller. In non-custody applications, the key stakeholders are the users of the data--the individual(s) that would object if the flowmeter were to disappear entirely. This determination of mutual performance expectations should be made by individuals who have a clear understanding of all of the costs of measurement disputes caused by poor accuracy and repeatability.
The second step is to quantify the operating conditions which are not controllable. For a flow measurement, these can include:
* Expected ambient temperature variation.
* Maximum static line pressure.
* Line pressure and temperature variation.
* Maximum allowable pressure loss.
* Minimum flow that needs to be measured (turndown).
The third step is to select hardware and installation and maintenance procedures which will ensure that the measurement provides the required installed performance under the expected (uncontrollable) operating conditions.
When using DP-flowmeters, specify DP transmitters "characterized for flow" to dramatically improve flow turndown.
This article is an update of one published in Pipeline & Gas Journal in July 2001 and after being updated, was presented at the Canadian School of Hydrocarbon Measurement, March 2005.
(1.) Menezes, "Calculating and Optimizing Repeatability of Natural Gas Flow Measurements", Pipeline & Gas Journal, July 2001.
(2.) Miller, R.W., Flow Measurement Engineering Handbook, McGraw-Hill, Toronto, 1996.
By Mark Menezes and Phil Schwarz, Emerson Process Management (Rosemount)
Mark Menezes, P.Eng., is Measurement Business Manager (Canada), for Emerson Process Management (Rosemount).
Phil Schwarz, MBA and MSE is Global Oil & Gas Marketing Manager, for Emerson Process Management (Rosemount).
Figure 2 shows the impact of this error at lower flowrates. Flowrate "Better" "Worse" (scfm) DP .075% .075% Analog 1000 100 0.09% 0.21% 0.65% 750 50 0.16% 0.38% 1.16% 500 25 0.37% 0.85% 2.60% 250 6.25 1.48% 3.40% 10.40% 125 1.56 5.9% 13.6% 41.6%
|Printer friendly Cite/link Email Feedback|
|Author:||Menezes, Mark; Schwarz, Phil|
|Publication:||Pipeline & Gas Journal|
|Date:||Nov 1, 2007|
|Previous Article:||Cyber security threats to pipelines and refineries.|
|Next Article:||Vortex pilot gas heater design needs no external energy source.|