Calibration of Ultrasonic Flowmeters G M - 4055

William Johansen and Joel Clancy Colorado Engineering Experiment Station, Inc.

Introduction

Ultra-sonic flowmeters are currently being put into service in large numbers. When used in custody transfer applications, ultrasonic flowmeter calibration is required. There is currently no standard covering the calibration of ultra-sonic flowmeters, although AGA Report No. 9 is used as a guide. The process of calibrating an ultra-sonic meter will be discussed as well as the calculation of a flow weighted mean error (FWME) and a calibration factor in accordance with AGA Report No. 9.

General Description of Calibration

Calibrating an ultrasonic flowmeter is performed by placing a flow standard in line with the ultra-sonic flowmeter being calibrated. The flow standard is used to accurately measure flow and has been calibrated using standards that are traceable to NIST. As long as there are no leaks in the system between the flow standard and the meter being calibrated it can be assumed that the two meters are passing the same amount of flow. There may only be one standard or there may be many that can be placed in parallel in the flowstream to produce a wide flowrate range.

There are two basic types of calibration systems. Figure 1 shows a calibration system on an existing natural gas pipeline. When the large valve on the pipeline is closed slightly a differential pressure across the valve is produced. The differential pressure across the pipeline valve provides a motive force to push flow through the calibration system. As the main pipeline valve is closed further, more flow is pushed through the calibration system. In this manner a wide flowrate range can be passed through the calibration system allowing the calibration of a wide range of meter sizes. At very low flowrates, fine flow control can be accomplished by throttling with a smaller valve inline with the meter being calibrated.

There are advantages to this type of system. Because the pipeline is passing flow constantly, very

long data points or many data points at a single flowrate can be taken. This type of system can hit very high flowrates allowing calibration of the largest ultra-sonic meter sizes. The high velocities allow observation of the meter and meter components under actual pipeline conditions. Some meter components such as temperature wells and flow conditioners fail or generate significant amounts of noise at high velocity. The effect of noise can be immediately determined at a calibration facility with the ability to flow at high meter velocities.

The disadvantages of this type of system vary. The pressure drop across the calibration facility will be determined by the pipeline operator. This should not affect ultra-sonic calibrations but can be a consideration when calibrating meters like orifice meters that need a pressure drop to measure flow. The calibration system has to operate at the pressure in the pipeline. This means that a meter to be used in a low-pressure system may have to have a flange with a higher pressure rating installed temporarily for the calibration.

The second type of natural gas calibration system used is shown in Figure 2. This type of system is referred to as a pressurized loop. Prior to flowing, the loop is pressurized with gas to the desired flowing pressure. Flow through the system is created by a compressor that must run continuously while calibrating. Flow control valves can be placed in the system for flow control. Heat exchangers in the system allow some temperature control.

A pressurized loop system also has certain advantages. The flow in pressurized loop systems can be controlled very precisely; this allows the system to produce high quality data. The temperature in a pressurized loop can be varied over a limited temperature range allowing the effects of different flowing temperatures to be investigated. The composition of the flowing gas can be varied by injecting different components into the loop.

The disadvantages of a pressurized loop calibration system include high operating expenses. This is because the compressor must be operating

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continuously. The suction pressure on the compressor must be maintained above some minimum value, which places a limitation on the differential pressure across the loop. This limitation on differential pressure places a flowrate limit on pressurized loop systems that is typically much lower than that of pipeline based systems.

Pre-Calibration Inspection and Meter Installation

Upon receiving the ultra-sonic flowmeter at the calibration facility, a thorough inspection is started. Ultra-sonic meters are often very large with attached electronic instruments so the inspection of the ultra-sonic flowmeter begins before it comes out of the box. A damaged shipping container indicates that the meter may have visible damage or damage to electronic components that will be harder to find. Open the crate and inspect the electronics. Ensure that all electronic boards are securely fastened to the junction box that houses the electronics. Look for any signs of damage or a n y loose parts or fittings. Inspect the meter body. Ensure that the transducers are not damaged. Ensure all cables are securely fastened.

When installing the meter in the piping system check the holes where the pressure taps penetrate the meter body on the inside surface. Any burrs or protrusions on the pressure taps can create pressure-reading errors and must be removed prior to calibration. Ultra-sonic flowmeters are often sold with upstream and downstream spool pieces. There may be identification stamps on the meter and accompanying spool pieces, make sure the identification numbers match. The meter and spool pieces may have alignment pins. Check the alignment, it is not unusual to find the pins do not provide good alignment. Spool pieces may come separately from a different supplier. In this case make sure that the internal diameters match well. Drawings may accompany the meter and spool pieces. Assemble the meter parts as shown in the drawings. Check the drawings for alignment pins that may not have been installed. Flow conditioners sometimes fail when first used. Inspect the flow conditioner to ensure the manufacturing and assembly is complete. Some flow conditioners need to be pinned as they can move around inside the pipe when installed. It is important that all upstream components that can affect the flow conditions at the meter remain exactly the same in use as they were during the calibration.

Meters that have been in use in the field are often recalibrated. These meters do not have original shipping containers and are often partially

disassembled for shipping. Inspect the cables carefully ensuring all cables are with the meter and that no damage has occurred. Inspect the inside surfaces of the meters. There is often a build-up of contaminants. Ensure the pressure taps are clear. The customer may want the meter calibrated in the condition it arrives in, referred to as an "As Found" calibration, then cleaned and recalibrated clean, referred to as an "As Left" calibration. There is not much data available yet about how well ultra-sonic flowmeters perform with thick layers of contaminants on the transducers and pipe walls. Any difference in performance of the meter between the "As Found" and "As Left" calibration may be very useful to the customer.

Once the meter and spool pieces have been installed in the test section the instrumentation can be installed and the meter can be powered up. Install pressure and temperature transmitters. Dual instrumentation is preferred. When dual instrumentation is used any differences in readings can be identified quickly allowing the calibration to proceed smoothly. Connect the power supply to the ultra-sonic meter. They typically require a 24-volt DC power source, but sometimes require 110 VAC. Connect the RS-485 communication lines to the meter. One RS-485 output is used to communicate flow and meter status information to a computer running software provided by the manufacturer. Another output is also connected to the meter. This output is a second flow signal from the meter. The second output may be an RS-485 output or the meter may produce a frequency output, which is proportional to flow passing through the meter. If an older meter is received from the field for recalibration it may require some communication switch changes to allow communication with the calibration facility. These changes must be accomplished with the support of the manufacturer.

The location of the thermal wells should be noted. AGA Report No. 9 discusses the appropriate placement of a thermal well stating 2 to 5 pipe diameters downstream of the ultra-sonic meter. In the case of a bi-directional meter, AGA 9 calls for a thermal well placement of 3 diameters from either ultra-sonic meter flange face. Although thermal well placement is defined in AGA 9, some users elect to choose a different location for their thermal well(s). Caution should be taken here. Often thermal wells are placed upstream of a flow conditioner. The pressure drop created by the flow conditioner also creates a corresponding temperature drop know as the JT effect. You have, therefore, a different temperature at the meter than is being recorded by the temperature transmitter.

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Meter Calibration

Test section pressurization, leak check, and pre-flow are now performed. As the meter body pressurizes, dual pressure instrumentation is checked for good agreement. Pre-flow is generally conducted at 60 to 80% of the meter capacity. Pre- flow will last for 30 to 60 minutes. The pre-flow allows the meter and test section piping to warm up. Dual temperature instrumentation is checked for good agreement. During pre-flow, several piping and instrumentation conditions are checked. Flow conditioners are often a source of flow noise. The amount of noise being generated by the flow conditioner is monitored during pre-flow. Any unusual mechanical noises may be an indication that the flow conditioner is coming apart or vibrating violently. Installing a thermal well too close to a flow conditioner can cause thermal well vibration. This installation can produce several problems. This vibration can cause problems for the ultra-sonic meter. The introduction of noise inhibits the meter's ability to function properly. The thermal well vibration also creates a heating effect that will produce a temperature measurement error at the ultra-sonic. Pre-flow is very important for multi-path ultra-sonic flowmeters. Multi-path ultra-sonic flowmeters have an internal memory they draw upon when they experience chord failure. When performing pre-flow, the performance of all the flow transducers is monitored to ensure there are no chord failures. Unusual signals can be produced from a variety of problems to include a bad set of transducers, improper characterization of the transducers, or improper internal wiring to name a few. If all the chords are performing well, the flow memory stored in the meter is that of good solid meter performance.

Once pre-flow is finished, the calibration begins. The flow is taken up to the highest flow rate requested by the customer. If no flow rates have been specified by the customer, the flow rate is taken to the maximum flowrate suggested by the manufacturer. At the high flow rate there may be enough flow noise to cause chord failure. That is, the flow noise is of a sufficient level to weaken the signal received by the meter. The flow noise is often produced by system components like pipe reducers and flow conditioners. When chord failure occurs it is very important that the memory of flow stored in the meter is good. This flowrate may not be a flowrate the meter will normally operate at but some indication of the performance of the meter at the high flowrate may be desired. It is important to

monitor the system carefully when increasing flow to the highest flowrate. If any components like flow conditioners are going to fail then this is the time when failure is most likely to happen. Any unusual noises or large changes in noise may indicate that a system component is experiencing failure.

When flow at the highest flowrate has been established, the calibration system is allowed to stabilize. Ultra-sonic meter calibration systems may be composed of large piping systems with a considerable amount of volume between the standards used to accurately measure flow during the calibration and the ultra-sonic meter being calibrated. It is important that any pressure fluctuations that may be present in the system due to changes in flowrate are allowed to dissipate. When stable flow conditions have been observed for an adequate length of time, calibration data can be taken from the ultra-sonic meter being calibrated and the calibration system. Several data points may be taken at a single flowrate. The number of data points may be specified by the customer, or it may be left to the judgement of the calibration system operator. In general, fewer data points are taken at higher flowrates than at the lower flowrates.

Data is typically taken at a minimum of six flowrates. AGA Report No. 9 specifies flowrates of qmin, 0.10qmax, 0.25qmex, 0.40qmax, 0.70qmax, and qmax. Additional data points may be requested at specific flowrates by the customer if the meter is to be used in a specific flowrate range.

Data may be acquired using two separate computer systems. One ~system will be running software supplied by the manufacturer that will interrogate the meter while a data point is taken and another system will acquire data from the calibration system. It is important that the two systems acquire data during the same time period.

Obtaining the calibration .log or .asc from the meter's software data logs can prove to be an important tool once the meter is put into service. Logs collected at the time of the calibration can provide information such as speed of sound, or gain level to limit ratios on a chord by chord basis to name a few. This information can be used as a baseline. When collecting logs throughout the life of the meter, the baseline logs can be used as a reference. Any deviations from the ratios observed at the calibration can be used as a way to troubleshoot potential problems with meter performance.

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Calibration Report

The results of a calibration will include the individual data points taken at each flowrate, the average values from the individual data points, and the calibration factor. These values may not all appear on the calibration report but they should be available to the customer. Meter and electronics identification numbers should also be recorded and maintained in records. There are also software settings, which need to be recorded. These software settings include the meter factor and other settings, which directly affect the performance of the meter.

The meter factor or calibration factor is the final product of the calibration. This factor is input into the software of the ultra-sonic flowmeter to introduce a constant offset in meter performance. The calculation of the calibration factor is shown in an example in Appendix A of AGA Report No. 9. This example is covered in detail in the following paragraphs.

AGA Report No. 9 Calibration Factor Calculation Example

An 8-inch ultra-sonic flowmeter is to be calibrated. The maximum flowrate of this meter is 87,500 ACFH. The customer has requested that calibration data be taken in compliance with AGA Report No. 9 with a minimum flowrate at a meter velocity of about 3 ft/sec. The calibration facility operator sets up the calibration plan shown in Table 1.

Table 1. Calibration Test Plan

Maximum Meter Flowrate of 87500 ACFH

3. ft/sec

AGA Report No. 9 Flowrates (ACFH)

qmax 0.75qma~ 0.40qm~ 0.25qr~ 0.10qn~

qmi,

Calibration Flowrates (ACFH) 87,500 61,250 35,000 21,875 8,750 3,750

Table 2. Calibration Results

Desired Flowrate (ACFH)

87,500

Actual Flowrate (ACFH)

86,500

Flowrate Indicated by Meter being Calibrated (ACFH) 86,183.4

Error (%)

-0.366 61,250 60 ,415 60,190.3 -0.372

2.0 ...............................................................................................................................................................

1.5

0.5 uJ "~ 0.0

no -0.5

-1.0

-1.5

-2.0

D

0 20000 40000 60000 80000 100000

Flowrate (ACFH)

35,000 37 ,801 37,681.9 -0.315 21,875 21 ,980 21,910.1 -0.318 8,750 6,890 6,915.9 0.376 3,750 3,475 3,508.1 0.953

The calibration now proceeds with pre-flow and recording data at the 6 flowrates listed above. After all the individual data points have been averaged the data would appear as shown in Table 2.

The plot shown in Figure 3 below shows the results of the calibration. It appears that the meter consistently under-measured flow through most of the range and then appeared to begin over- measuring flow at the low flowrates.

A flow weighted mean error (FWME)is now calculated. The FWME is a single value that describes the performance of the meter just calibrated. The FWME is found by first expressing each flowrate as a percent of full-scale flow. The percent full-scale calculation for the high flowrate values is shown below.

Percent Full Scale = Indicated Flowrate

Maximum Flowrate xlO0.

The percent full-scale values are then multiplied by the percent error values. The values in the percent full-scale column and the percent full-scale times

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percent error column are summed. This is shown in Table 3 below.

Table 3. Summed Values Actual

Flowrate (ACFH) 86,500

Error (%) Percent

Full Scale Percent F.S.x.

Error

-0.366 0.9886 -0.3618 60,415 -0.372 0.6905 -0.2568 37,801 -0.315 0.4320 -0.1361 21,980 -0.318 0.2512 -0.0799 6,890 0.376 0.0787 0.0296 3,475 0.953 0.0397 0.0378

Sum = 2.4807 Sum =-0.7672

The FWME is then found by dividing the summed percent full scale times percent error values by the summed percent error values as shown in the following equation.

- 0 .7672 F W M E - = - 0 . 3 0 9 3

2 . 4 8 0 7

The FWME value is not the value entered into the ultra-sonic flowmeter software to correct metering performance. The calibration factor is the value used to correct meter performance and is calculated in the following manner.

C a l i b r a t i o n F a c t o r = 100

(100 + F W M E ) = 1.0031

Each of the indicated flowrate values can now be multiplied by the calibration factor. The adjusted flowrates are shown in the Figure 4.

If the meter just calibrated is to be operated primarily in the 20,000 to 30,000 ACFH range should the low flowrate range data be used in the calculation of the FWME and calibration factor? If the 2 lowest flowrates were deleted from the calculation of the FWME the new calibration factor would be 1.0035, a difference of less than 0.05%. The small change is due to the flow weighting.

0.5 w

0.0 o

#. -0.s

-1.0

-1.5

-2.0

0 20000 40000 60000 80000 100000

Flowrate (ACFH)

Figure 4. Adjusted Calibration Results

Summary Ultra-sonic flowmeters are being installed in natural gas pipelines in large numbers. The calibration of ultra-sonic flowmeters is performed in accordance with AGA Report No. 9. The inspection, installation, and calibration of ultra-sonic flowmeters were discussed. The FWME calculation example in AGA Report No. 9 was covered in detail.

Figure 3. Calibration Results

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Direction of Flow through Pipeh'ne Natural Gas t~peh~e

Pipeh'ne disserential Pressure Control Valve

Flow Control Valve Ultra-sonic Meter Flow Standard

Compressor Heat Exchanger ( ,)

Flow Control Valve U/tra-sonic Meter Esl

Flow Standard

J Direction of Flow Around Loop

Figure 1. Pipeline Based Calibration System

Figure 2. Pressurized Loop Calibration System

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