ultrasonic gasmeters handbook

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ULTRASONIC GASMETERS

HANDBOOK



INTRODUCTION

1. OPERATION

1.1 Operating principle1.2 Pulse generation and detection

1.2.1 Transducer technology1.2.2 Pulse detection

2. PERFORMANCE

2.1 The accuracy of the travel time of a single path meter2.2 The uncertainty due to the flow pattern

2.2.1 Reynolds’ number2.2.2 Perturbed flow2.2.3 The effect of turbulence

2.3 Possible path configurations2.3.1 Single path meters

2.3.1.1 CheckSonic spool piece meter2.3.1.2 Insertion meter

2.3.2 Multipath meters2.3.2.1 Q.Sonic spool piece meter

3. INSTALLATION

3.1 Installation of single path meter3.2 Installation of Q.Sonic multipath meter3.3 Ultrasonic noise caused by regulators and control valves3.4 Pressure drop3.5 Wet gas3.6 Pressurising and depressurising3.7 Physical size3.8 Transducer installation

4. DIAGNOSTICS

4.1 AGC4.2 Number of pulses accepted4.3 Velocity of sound4.4 Noise and signal analysis

5. OUTPUTS



ULTRASONIC GASMETERS

INTRODUCTION

This handbook is one of a series that Instromet has prepared for thegas industry. It describes the operating principle, the performance, the

installation and the output facilities of the different types ofInstromet’s ultrasonic gasmeters. Comparisons are made with othertypes of meters such as orifice plates, turbine meters and rotary piston

meters.

Other handbooks in this series deal with turbine meters, rotarydisplacement meters, gas chromatographs and with complete GasMetering Systems.

1. OPERATION

1.1 Operating principle

Sound waves travel with a specific velocity through a medium.The velocity of sound in gas is determined by its composition and also

by its pressure and temperature.

Some indicative figures for different gases are given in Table 1.

Table 1. Velocities of sound for different gases under differentconditions

Substance p [bar] T [K] C [m/s]

Methane 1 275 432

Methane 1 320 463

Methane 60 275 414Methane 60 320 456

Air 1.0133 275 333

Hydrogen 1.0133 273.15 1022

Ethylene 1 273.15 318

Natural gas (Groningen) 1.0133 273.15 400



For gases that behave as ideal gases the velocity of sound (C) is equalto:

C =√ (k•p)

For natural gas [1] the velocity of sound is approximately equal to:

C =√ (k•p)

with k Poisson’s constantP pressureZ compressibilityρ density

If a sound wave is created in a flowing medium its speed ofpropagation will be equal to the vector sum of the velocity of theoriginal wave and the velocity of the medium. This effect is used tomeasure gas velocity.

In figure 1 the basic system set-up is shown. On both sides of the pipe,at positions A and B, transducers are mounted, capable of transmitting

and receiving ultrasonic waves. The acoustic waves are generated as abeam perpendicular to the surface of the transducer.

Figure 1. Basic system set-up

The Instromet ultrasonic gasmeters use a short pulse signal. The form

of this signal, which is really a short burst of a very high frequency(figure 2), is recognised at the receiving end and the time elapsed since

emission is measured digitally.

φ

vm

A

B

ρ

z•ρ



Figure 2. Typical form of high frequency pulse

With zero flow the travel time from A to B (tAB

) is equal to the travel time

from B to A (tBA

). This is equal to the average travel time for the acoustic

pulses t0:

tAB

= tBA

= t0

= L

where L is the length of the acoustic path and C is the velocity of sound in

the gas.

If there is a flow of gas with velocity vm in the direction indicated infigure 1, the travel time of the acoustic pulse from A to B will decrease andfrom B to A will increase according to:

tAB = LC + v

mcos (ϕ)

LtBA =C - vm cos (ϕ)

where ϕ is the angle between the path A - B and the pipe axis.

When the two acoustic pulses are transmitted at the same time, the

velocity of sound is identical for both measurements and can therefore be

eliminated, resulting in:

C



Lv

m= • ( 1 1 ) (1)

2 cos (ϕ)

where vm

is the averaged flow velocity along the ultrasonic path. It isclear from (1) that the flow meter is truly bidirectional.

Alternatively when the gas velocity vm

is eliminated, the velocity ofsound can be calculated:

C = L• ( 1 + 1 ) (2)

Because the velocity of sound is related to the density of the medium,it can also be used in some applications to calculate an approximatevalue for the mass flow in the system. This technique has been applied,for example, in flare gas measurement and in vapour recovery.

1.2 Pulse generation and detection

One of the reasons why ultrasonic gasmeters developed much laterthan their equivalents for liquids, is that it is much more difficult totransmit a sound wave into a gas than into a liquid. High efficiencytransducers are needed that emit a well defined pulse in a directed

beam into the gas.

It is also desirable to be able to change transducers without the needfor recalibration. These requirements demand a special design and

extreme quality control.

1.2.1 Transducer technology

Specially designed transducers are used in the Instromet meters. Thesetransducers are capable of both transmitting and receiving ultrasonicpulses.The main component within a transducer performing thesefunctions is a piezoceramic element. In the transmitting mode thesepiezoceramic elements are excited with a characteristic electrical pulsewhich results in the emission of a well determined acoustic pulse. Whenused as a receiver, the incoming pulse generates a small signal which,after amplification, can be processed. The shape of the pulse

generated and the directional pattern of a transducer depend, to a

large extent, on the dimensions and material characteristics of thepiezoceramic element.

tAB tBA

2 tAB tBA



1.2.2 Pulse detection

Before pulse detection and recognition take place, the received signalis pre-processed using Automatic Gain Control (AGC) and a filtersection. The AGC section is used to cope with a wide spectrum of gasdensities, pressures and composition. After the pre-processing stage

the pulse is presented to the detection circuitry. In the detection circuitthe signal is digitized and compared with a ‘fingerprint’ of theexpected pulse signal, making it highly immune to other acousticsignals that might otherwise influence the measurement. Themeasurement result, based on the two transmitted pulses, is either:

* accepted, if the signal transmission is completely in agreementwith the preset quality standards, or

* rejected, if a deviation from these quality standards isdetected.

Only after the received pulse is accepted will the travel time bedetermined and used in the calculation of the gas velocity and thespeed of sound. Matching the received signal with its fingerprint notonly eliminates spurious signals, it also makes it possible to determinethe time of arrival more accurately. This method results in the highest

measurement quality that can presently be achieved.

Depending on the pipe diameter some 20 to 60 pulses are emitted peracoustic path every second. The average travel time of accepted pulsesis used every second for further processing.

2. PERFORMANCE

Instromet’s ultrasonic gasmeters are manufactured in three types, eachdirected to a particular market need. Two are intended to be used forcontrol purposes. One of these is designed to be mounted on existingpipelines by hot tapping, the second is designed as a spool piece. Boththese types are influenced by the velocity profile. The third type isdesigned specifically for custody transfer. The recommendations of theOIML [2] for turbine meters and the international standard ISO 9951 [3]were used as a guideline in awaiting specific standards or

recommendations.



As a result Instromet's Q.Sonic ultrasonic gasmeter was the world’s firstto be officially approved. It has now been approved for use in fiscalmeasurement by the Dutch Official Authority NMi, by the GermanOfficial Authority PTB and by Industry Canada, the Canadian Weightsand Measures Authority. Other approvals are pending.

The difference in design and in path configuration of the three types ofmeters result in a different performance.

A single path ultrasonic flow meter measures the average velocityalong the path in accordance with equation (1). If the velocity wasuniform over the cross-section, it would suffice to multiply this averagevelocity with the cross-sectional area. In practice, the velocity reducesfrom the centre towards the wall and the actual shape of this velocityprofile is a function of the Reynolds’ number.

For the flow rate we find therefore :

LQ = • k • A • ( 1 1 ) (3)

2•cos ϕ

where A denotes the cross-sectional area of the pipe and k the velocityprofile correction factor.

The uncertainty in the values of tAB

and tBA

is determined by theelectronics. The path length L, angle ϕ and the surface area A aredetermined by the geometry and any uncertainty in these parameterswill result in an uncertainty in the flow rate.

2.1 The accuracy of the travel time of a single path meter

To illustrate the required accuracy in the travel time measurement,practical values will be substituted in the preceding equation, using avalue of 60° for the ultrasonic path angle ϕ. The equations then

convert to:L t

0tAB

= = = ≈ t0

- t0. (4)

C + V.cos ϕ1+ . 1 +

L_C

M_2

M_2V_

C

1_2

tAB

tBA



L t0t

BA= = = ≈ t

0+ t

0. (5)

C - V.cos ϕ1 - . 1 -

where M, the Mach number of the flow (v/C), must be much smaller

than unity. These relations show that the measured travel times (bothup and downstream) are equal to the mean travel time t0, with a small

correction A • M depending on the average gas velocity.

A typical design velocity for an ultrasonic meter is 30 m/s. For a 1:75range the minimum velocity is then 0.4 m/s. As an example, when thisvelocity has to be measured with an accuracy of 0.5%, the meter musthave a resolution of 2mm/s. Combining this with a typical soundvelocity of 400 m/s and an ultrasonic path length of 0.4 m gives the

following results:

t0

= 1 ms ; ∆t0

= M • t0

= 2.5 ns

The small value of the mean travel time indicates that an ultrasonicflow meter is capable of measuring with high repetition rates. In surgecontrol and other applications, where the flow drops from its set pointto its minimum in less than 0.5 s, this high repetition rate is of primeimportance. Typical repetition rates are 10 to 30 Hz but can be set to ahigher value if necessary. The advanced signal processing used inInstromet’s ultrasonic meters make it even possible to measure volumein pulsating flow with little additional error [4].

The achievement of a high resolution in travel time measurementrequires the use of high speed, high accuracy and well designedelectronics.

The uncertainty in the time resolution in all Instromet’s ultrasonicgasmeters is 10 ns or better.

The velocity error ∆v is proportional to the time error ∆t and given by:

C2 tan(ϕ)∆v = ___________ ∆t

4D

L_C

M_2

M_2V_

C

1_2

t0 •M2

t0 •M2



As a function of the pipe diameter this results in:

Table 2. Velocity error as a function of the pipe diameter for a10 ns uncertainty in time measurement

Table 3. Envelope of meter errors

2.2 The uncertainty due to the flow pattern

2.2.1 Reynolds’ number

After entering the pipe the velocity profile will graduallyaccommodate itself until it is axisymmetric and fully developed. Thiswould normally take some 80 D. Most theoretical and experimental

work in flow in pipes is related to fully developed flow.

For straight circular ducts the flow profile is determined by theReynolds’ number (Re) of the flow and the relative roughness of thepipe wall. The dimensionless Reynolds’ number is calculated using the

D[mm] D [inch] V [mm/s] V[inch/s]

100 4 7.0 0.3125 5 5.0 0.2

250 10 3.0 0.1500 20 1.5 0.061000 40 0.7 0.031600 64 0.4 0.016



velocity, the duct diameter, the density and the dynamic viscosity of theflowing medium. For low Reynolds’ numbers the flow is laminar andhas a parabolic velocity profile (Hagen Poiseuille); for high Reynolds’numbers the flow is turbulent and the velocity profile assumes theform of a plug.

The transition from laminar to turbulent in a straight pipe normallytakes place at a Reynolds’ number of about 2300.

Several relations have been put forward to describe the velocity profilein a round pipe. From these relations one can calculate a theoreticalvalue for k. Instromet's experience has been that the one given byRothfus and Monrad [5] in a study for Shell results in an uncertainty of

approximately 1% for a single path meter operated at Re ≥ 105.

2.2.2 Perturbed flow

In a practical installation a fully developed flow will always beperturbed to a certain extent. There may be bends, headers, risers,valves or even filters generating all sorts of perturbations. Even in avery long pipe the roughness of the surface, welds and the matchingbetween two subsequent pipe sections may influence the profile. Thesensitivity of the flow profile to minor deviations in the geometry isexemplified by the tough requirements on the quality of the upstreampiping in the ISO 5167 [6] standard for orifice plate metering.

As a result the flow pattern will normally not be axisymmetric and maycontain a swirl component. Practical installations may also showpulsating flow.

The difference between swirl and turbulence is defined by the size of

the vortices. In case of turbulence, the size of the vortices is much smaller than the pipe diameter. Since a single (small) vortex automatically generates other vortices in different directions, this small vortex breaks up into smaller and smaller vortices until it dissipates intoheat. In gas pipelines this process will last between 0.1 and 10 seconds,depending on the pipe diameter. In the case of swirl, however, its size isof the same magnitude as the inner pipe diameter. Therefore there isno space to generate vortices in other directions. Consequently, thisvortex does not break up into smaller ones but remains stable over

pipe lengths of hundreds of pipe diameters long. In the extreme case,one single vortex will occupy the full pipe diameter and encounter very little damping other than created by the wall roughness.



Swirl can only be eliminated within a short distance by blocking it bymeans of a flow straightener, making the installation unsuitable for‘pigging’. This also generates a pressure drop.

In all cases except the case of one single vortex, the presence of swirlwill also mean that there are radial velocity components present,

so-called cross flows (figure 3).

Figure 3. Single and double vortices and possible acoustic paths

As can be seen in figure 3 cross flow influences the average velocityover a flow path and therefore generates an error in the flow rate.A single vortex also produces an error in any path not going throughthe centre. By choosing a different path configuration, the effect ofswirl can be completely eliminated. One method, shown in figure 4a,uses another pair of transducers in the same plane parallel to the pipeaxis . By taking the average of the velocities measured by both pairs oftransducers, the cross flow is eliminated. The same effect can beobtained by using the reflection of a centric ultrasonic beam as infigure 4b. For this solution highly efficient transducers are needed.

The efficiency of the sophisticated Instromet transducers is such thateven multiple reflections can be used. All Instromet’s ultrasonicgasmeters use the reflecting beam principle.



a. Two paths in the same plane

b. One single reflected path

Figure 4. Two ways to eliminate the effect of swirl

2.2.3 The effect of turbulence

Turbulence consists of small vortices in random directions rapidlybreaking down. The effect of turbulence can therefore be eliminated

by a sufficiently long averaging process. It shows in a random, smallapparent variation in the flow rate.

2.3 Possible path configurations

In figure 5 some possible path configurations are shown.



Figure 5. Path configurations

In arrangement A swirl as in figure 3 will give an error. It is also aconstruction that is less suitable for buried pipe. Configuration B isinsensitive to these flow patterns and, because access from only oneside is needed, lends itself for application to buried pipe.Configurations C and D are less influenced by asymmetric flow profilesbut are quite sensitive to vortices as in figure 4a. By adding a similarsecond path rotating in the opposite direction, this effect can be

eliminated. These latter two configurations have also a longer pathlength which increases accuracy. Configuration D is difficult to installand C is therefore the better choice.

Single reflection

Double reflection

Fourfold reflection

No reflection

A

B

C

D



2.3.1 Single path meter

For a fully developed flow the uncertainty in the flow rate is mainlydetermined by the geometry of the meter and, for low flow rates, theturbulent fluctuations and the offset error. The flow rate is calculatedfrom the average velocity over the path assuming a fully developed

velocity profile. If this is not the case the uncertainty in the flow ratewill increase.

Orifice plate flow meters are similarly affected by the velocity profile. It isreasonable to assume that, if the same installation conditions areobserved as for orifice plates, similar uncertainties can be expected. For aninstallation that satisfies ISO 5167 [6] this would give a basic uncertaintyof 1% for a ß of 1. As the installation requirements according to thestandard increase with ß, and are only listed up to a value of 0.8, theuncertainty may still be somewhat higher in practice, even though theinstallation conditions of ISO 5167 for ß = 0.8 are fully met.

For installations that do not satisfy these installation conditions theuncertainty introduced by inadequate knowledge of the flow profileincreases and can go up to 2%.

2.3.1.1 CheckSonic spool piece meter

The CheckSonic is a single reflection, single path meter with a pathconfiguration as in figure 4b, mounted in a machined spool piece. Inthese meters the distance between the transducers can be accuratelycontrolled and the cross-sectional area of the pipe is known with greatprecision. As a result the dimensions contribute very little to the uncertainty.

When averaging over 1 second, the uncertainty due to turbulenceamounts to approximately 2% for low velocities, a few meters persecond, decreasing for higher velocities.

For long averaging times or conversion of flow rate to a quantity, theeffect of turbulence goes to zero. Then, the uncertainty in the velocityprofile becomes the determining factor. For very low velocities theabsolute error in the traveltime becomes a significant addition.

For fully developed flow the uncertainty would be approximately 1.5%

at velocities down to 1 m/s if the installation conditions of ISO 5167 areobserved. For long averaging times in a good installation, theuncertainty approaches 1%.



2.3.1.2 Insertion meter

The transducers are inserted through a full bore 2” valve perpendicularto the pipe and in a plane through the pipe axis (figure 6). Thetransducers are constructed to transmit a sound wave whose anglewith the tube is 45° or more. The meter is normally located on a trans-

mission line and if needed the pipe can be tapped under pressure usinghot tapping techniques.

Figure 6. CheckSonic insertion meter

Compared to a spool piece meter additional uncertainties in thefollowing parameters have to be considered:

* the diameter of the pipe,

* the position and orientation of the transducers, affecting thelength of the ultrasonic path,

* the roundness of the pipe affecting the surface area.



These factors are very much determined by local practical conditions. Ingeneral larger diameters give smaller errors except for roundness,when large diameter pipes are becoming thin shelled structures.

2.3.2 Multipath meter

In a practical installation the velocity profile will mostly differ from theundisturbed velocity profile due to the actual piping configuration.The piping may result in:

* asymmetric velocity profiles,

* swirl, generating tangential and / or radial velocity components,

* pulsations.

2.3.2.1 Q.Sonic spool piece meter

Ultrasonic meters utilizing a multiple path configuration, f.i. parallelpaths or crossed paths, could eliminate their effects to a certain extent.

The multipath Q.Sonic has been designed to fully eliminate the effect of

the distortions of the velocity profile. The path configuration has beenchosen in such a way that it is possible to detect the type of distortionand to measure its strength (figure 7). Two double reflection triangularcorkscrew paths, one rotating clockwise and the other counter clockwise,are used to measure the swirl strength. Three single reflection paths areused to measure the asymmetry of the flow pattern.

Figure 7. Q.Sonic path configuration

Swirl paths Diagonal paths Cross-sectional view



For the determination of the flow rate, the mean velocity over thecross-section of the pipe is needed. To calculate the value, severalthousands of flow profiles were measured and analysed. On the basisof this analysis Instromet has developed a proprietary procedure thattakes into account:

* the Reynolds’ number

* the measured velocities along the individual paths

* the measured swirl strength

* the asymmetry of the flow

The velocities are measured almost instantaneously. The pulses aregenerated in a semi-random way and averaging over a sufficiently longperiod eliminates any bias due to pulsations.

As a result, the uncertainty in measurement with a Q.Sonic can bepredicted from the dimensions to within 0.5%. Individual (wet)calibration in a test installation can reduce this even further.

For the smaller diameters a 3 paths configuration is used and the

uncertainty with dry calibration only increases to 0.7%.

3. INSTALLATION

3.1 Installation of single path meter

As discussed earlier the flow profile has a similar effect as for orificeplates. Present indications are that observing the installationconditions as stipulated in ISO 5167 will result in a basic uncertainty ofabout 1%. Similarly, if the smaller, bracketed values listed in thisstandard are used for the straight lengths preceding the meter, one canexpect to have to add an additional 0.5% arithmetically.

3.2 Installation of Q.Sonic multipath meter

In absence of any standards for ultrasonic meters or their installation, theQ.Sonic was tested to the standard for turbine meters ISO 9951.



The standard defines a set of flow perturbators that have to be installedupstream of the meter for a performance test. The flow perturbators giveeither a low level or a high level of perturbations.

The high level perturbation as defined in ISO 9951 consists of twobends in perpendicular planes, with a segmental orifice located

between the two bends blocking half the pipe area. The objective is tomodel the flow pattern generated by a regulator. The low levelperturbation is similar to the high level perturbation except for theomission of the segmental orifice.

Independent tests carried out by users [7] have confirmed that theQ.Sonic only needs 10 nominal diameters of straight upstream pipe tosatisfy the requirements of ISO 9951.

3.3 Ultrasonic noise caused by regulators and control valves

Some types of regulators and control valves generate very high levelsof ultrasonic noise. If the acoustic energy that is generated is in thesame frequency band as where the meter operates there is a likelihoodof interference.

Instromet has developed a mathematical model to predict the effect ofthe piping configuration on the behaviour of the ultrasonic gasmeter.Regulators and control valves produce noise that interferes with themeasuring signals of an ultrasonic gasmeter. If the noise level is toohigh corrective measures have to be taken to reduce the noise. Bends,tees, filters, turbine meters or heat exchangers all have a reducingeffect on the noise that reaches the ultrasonic gasmeter. In general, asingle bend accounts for an attenuation of 5 dB, a tee 10 dB, a turbinemeter 15dB and a heat exchanger 30 dB. As a first order estimation thegraph can be used for designing a measurement station withultrasonic gasmeters. If the pressure drop over the regulator or controlvalve is known, the required noise attenuation is given for a particularmeter size. This attenuation can be obtained by adding noise reducingelements.

The graph gives an example of a 6" and 12" ultrasonic gasmeter bothoperating in the safe area (with no noise interference) at a pressure

drop of 20 bar over the regulator or control valve; at that point theyboth operate below the safe line. The configuration with the 24" flowmeter requires additional attenuation by at least one extra bendbetween the regulator or control valve and the ultrasonic gasmeter.



Remark: Instromet is not liable for any consequences by using thisgraph for first order estimation. Please ask Instromet for detailed

calculation to help with your design.

If an ultrasonic meter has to be installed in the same installation as aregulator or control valve that operates above the critical pressureratio, the following measures should be considered:

1. Install the regulator or control valve and the meter as far apartas possible.

2. Avoid a design where regulator or control valve and meter arein line, some bends or tees in between help attenuate the noise.

3. If possible, install the meter upstream of the regulator or

control valve.

3.4 Pressure drop

Ultrasonic meters produce no pressure drop.



3.5 Wet gas

Under certain conditions small amounts of liquid can deposit fromnatural gas on the pipe wall. This liquid film is propelled through thepipe by the gas velocity, at low velocities mostly along the bottom, andat higher velocities also along the rest of the surface. At the same time

droplets are being carried in the gas stream.

Instromet’s ultrasonic gasmeters are not sensitive to the presence ofliquid. The meter will continue operating even if totally flooded.However, in these circumstances the uncertainty of the measurementresult will increase with the amount of liquid present. This is for a largepart due to the decrease in free area available for gas flow.

A temporary flooding of the meter with water or condensate will notnormally affect its performance afterwards. However, the meter shouldbe installed so that liquids do not collect in the meter.

The performance of the Q.Sonic for wet gas has been demonstrated intrials carried out by ARCO British Limited on the Thames platform inthe North Sea [8].

3.6 Pressurising and depressurising

Pressurising and depressurising the meter can be done at any rate. Themeter does not suffer any damage from high gas velocities.

3.7 Physical size

Sizes and ranges are given in table 3 for the CheckSonic and in table 4for the Q.Sonic.



Table 4. Sizes and ranges for the CheckSonic single path meter

Table 5. Sizes and ranges for the Q.Sonic multipath meter

3.8 Transducer installation

The transducers have proven to be extremely reliable. There ishowever, an option available which permits the removal of thetransducers under pressure. The transducers are in this case insertedthrough full bore ball valves.

The transducers are fully exchangeable without invalidating the official

calibration. There is no need for recalibration after installing adifferent transducer.

Diameter Product Flow range Qmin

Qmax Meter body

[m3 /h] [m

3 /h] length

4” CheckSonic 1:40 20 800 5D

6” CheckSonic 1:60 30 1800 5D

8” CheckSonic 1:60 50 3000 5D10” CheckSonic 1:80 60 5000 3D

12” CheckSonic 1:100 80 8000 3D

16” CheckSonic 1:125 95 12000 3D

20” CheckSonic 1:140 135 19000 3D

24” CheckSonic 1:175 160 28000 3D

30” CheckSonic 1:250 225 45000 3D

Diameter Product Flow range Qmin

Qmax Meter body

[m3 /h] [m

3 /h] length

6” Q.Sonic 1:40 45 1800 5D

8” Q.Sonic 1:50 60 3000 5D

10” Q.Sonic 1:65 75 5000 5D

12” Q.Sonic 1:90 90 8000 3D16” Q.Sonic 1:120 100 12000 3D

20” Q.Sonic 1:150 130 19000 3D

24” Q.Sonic 1:140 200 28000 3D

30” Q.Sonic 1:225 225 45000 3D



The transducers of insertion meters can routinely be retracted to beable to give passage to a pig. A hydraulic system to automaticallyretract the transducers at the approach of a pig is available as anoption.

4. DIAGNOSTICS

The fact that all relevant data are available in digital electronic formallows for sophisticated diagnostic techniques. These diagnostic datacan be accessed on line and used to generate control charts. In this wayany degradation in performance can be detected in an early stage andremedied. The following diagnostics are available on Instromet’sultrasonic gasmeters:

4.1 AGC

The received signals are amplified. An Automatic Gain Control adjuststhe amplification to achieve a specific signal level. Decrease of signallevel could for example be caused by thick deposits on the transducers.Monitoring the AGC provides an excellent diagnostic tool.

4.2 Number of pulses accepted

The received signal is compared with a template of the expected signal.If the resemblance is sufficient the signal is accepted. The percentage ofrejected signals has to remain under a certain level however.

4.3 Velocity of sound

From equation (2), the velocity of sound can be calculated.Depending on the fluid and the pressure and temperature range thatcan be expected a maximum and minimum value for this figure can bedetermined. Values outside of these limits would indicate abnormalconditions or a malfunction somewhere in the system. In multipathmeters the velocity of sound can be determined for each of the pathsindividually.



4.4 Noise and signal analysis

The noise received in absence of signals is analysed typically every10 seconds. Its level determines the value to which the gain can beincreased by the AGC. The AGC level strength and the noise level areboth available from the meter.

5. OUTPUTS

The outputs of ultrasonic flow meters consist of a pulse signal and aserial digital data signal available according to RS 232 or RS 485 standards.The pulse rate is proportional to the flow rate so that each pulse equalsa certain volume.

Additional status signals are available to indicate flow direction and toindicate validity of the signal.

Special software is provided to communicate with the gasmeterthrough a PC. This PC can also serve to adjust parameters as far asallowed for legal metrological purposes.



REFERENCES

[1] R.L. Andsager, R.M. Knapp: Acoustic measurement of distance innatural gas systems, Society of Petroleum Engineers, paper SPE1640, 1966.

[2] Organisation Internationale de Métrologie Légale (OIML),recommendation R32: Rotary piston gas meters and turbinegasmeters.

[3] ISO 9951, Measurement of gas flow in closed conduits - Turbinemeters.

[4] H.J. Dane: Ultrasonic measurement of unsteady gas flow, Paperpresented at the 1995 meeting of the AGA operating section.

[5] R.R. Rothfus, C.C. Monrad:Correlation of turbulent velocities fortubes and parallel plates, Indust. and Engng. Chem. 47 (6) (1955)p.1144.

[6] ISO 5167-1, Measurement of fluid flow by means of pressuredifference devices-Orifice plates, nozzles and Venturi tubes inserted

in circular cross-section conduits running full.

[7] F. Vulovic, B. Harbrink, K. van Bloemendaal: Installation effects on amultipath ultrasonic flow meter designed for profile distortions,North Sea Flow Measurement Workshop, October 1995,Lillehammer.

[8] P. Robbins: Thames Alpha gas metering ultrasonic meter (USM)trial, North Sea Flow Measurement Workshop, October 1996,

Peebles Hydro, Scotland.



Sales Offices:In Argentina:INSTROMET S.A.

In Australia:INSTROMET SYSTEMS AUSTRALIA PTY. LTD.

In Austria:

INSTROMET B.V. GES.M.B.H.In Belgium:INSTROMET INTERNATIONAL N.V.

In Brazil:INSTROMET MEDIÇÃO E CONTROLE LTDA.

In Croatia:INSTROMET CROATIA

In France:INSTROMET S.A.R.L.

In Germany:INSTROMET G.M.B.H.

In Hungary:INSTROMET HUNGARY SERVEX HUNGARY KFT.

In India:SIDDHA GAS INSTROMET INDIA PVT. LTD.

In Italy:INSTROMET ITALIA S.R.L.

In Korea:INSTROMET KOREA LTD.

In Malaysia:INSTROMET INTERNATIONALREGIONAL OFFICE, SOUTH EAST ASIA

In the Netherlands:

INSTROMET B.V.

In Nigeria:INSTROMET WEST AFRICA B.V.

In Poland:INSTROMET POLSKA

In Portugal:INSTROMET PORTUGAL LDA.

In Spain:INSTROMET ACUSTER S.L.

In Switzerland:INSTROMET AG.

In the UK:INSTROMET UK

In the Ukraine:INSTROMET UKRAINE LTD.

In the USA:INSTROMET INC.

Products & Services:

q Ultrasonic gas meters

q Turbine gas meters

q Rotary gas meters

q Insertion gas meters

q Electronic volume correctors

q Flow computer systems

q Calorimeters

q Gas chromatographs

q Supervisory Systems

q Gas filters

q Gas pressure regulators

q Safety shut-off valves

q Telemetering systems

q Electronic metering and control systems

q Calibration and test installations

q Complete gas measurement and control stations

q Commissioning, servicing, training and consulting

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INSTROMET has agents and representatives worldwide.

YOUR SALES OFFICE OR REPRESENTATIVE:

For your nearest sales office orrepresentative please contact:INSTROMET INTERNATIONALRijkmakerlaan 9 - B-2910 ESSEN - BELGIUMTel: +32 3 6700 700 - Fax : +32 3 667 6940

E-mail: [email protected]: http://www.instromet.com

ultrasonic gasmeters handbook

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