SYSTEMSHANDBOOK

CONTENTS

INTRODUCTION 5

PURPOSE 5

DESIRED OUTPUTS 6

Nature 6Values to be determined 7Definition of units 7Additional values 9

INPUT VARIABLES 10

Flow rate 10The “State” of the gas 15Pressure measurement 15Temperature measurement 16Density measurement 17Composition and compressibility 18Composition measurement 21Calorimeters 21

CONVERTING TO BASE CONDITIONS 22

Linearisation of flow meter 25Legal and/or contractual requirements 25

ACCESSING STATION DATA 26

RELIABILITY AND DIAGNOSTICS 27

3

5

GAS METERING SYSTEMS

INTRODUCTION

This manual is one of a series that Instromet has prepared for the Gas indus-try. It describes the systems that can be used to determine a gas quantityor a flow rate. The total gas measurement system is described rather thanthe individual instruments. Individual instruments are dealt with in sepa-rate handbooks or in brochures.

The specifications to which a measuring station is constructed, largely deter-mine the optimal system. The first chapter is therefore devoted to therequired outputs and their accuracy.

The different ways to generate these outputs are the subject of the nexttwo chapters dealing with the primary instruments and the processingof the information.

The last chapters deal with practical conditions and constraints such as howto handle multiple streams, fiscal controls and regulations, other regula-tions and with maintenance, diagnostics and reliability.

This handbook will only consider volumetric meters to determine gas flow.Details on the use of or the calculation procedures for orifice plates are notincluded.

PURPOSE

There are two main purposes for which a system for flow measurementmay be constructed: Process control or Custody transfer.

Examples of process control applications are pipeline balancing and opti-misation. No money is directly involved and accuracy of flow measurement is normally not critical. Instead, speed of response may beof crucial importance.

Custody transfer, on the other hand, will always involve a reciprocal trans-fer of money, whence the importance of accuracy. Custody transfer involvesa quantity determination rather than a flow rate. Speed of response is onlyof secondary importance.

6

This handbook will concentrate on meters for custody transfer but it willoccasionally refer to specific process control applications.

Apart from flow rate or quantity, there may also be quality related valuesor operational values that the contract requires to be within specified brack-ets and need to be recorded. These will only be dealt with if they can bederived from the measurements needed to determine quantity.

DESIRED OUTPUTS

Nature

When measuring natural gas, its energy is generally the ultimate para-meter sought as it quantifies the value of the gas in the market. This ener-gy is determined from the quantity of gas and the gas’ heating value. Inthe case where the heating value of a gas can be assumed to be constant,the energy can be directly calculated by multiplying the quantity beingmeasured by this value. The heating value may either be checked con-tinuously or sampled periodically to verify the validity of this assump-tion. If the heating value is not constant, then this calculation may be donecontinuously on-line.

The quantity of the gas is usually expressed in terms of volume at “base”or “standard” atmospheric conditions. This is the volume that the gas wouldoccupy if the pressure and temperature were adjusted to a standard atmos-pheric temperature and pressure. This standard however, is not everywheresimilarly defined and different units -metric or imperial- may be used.

Though quantity may be expressed in terms of mass instead of volume,it is in most cases still desirable to obtain the standard volume as it is a valu-able parameter in downstream processes.

The actual volume (not reduced to standard conditions) that flows throughan installation is mostly needed for operational and for contractual or legalmetrology purposes.

Apart from the above quantities that determine fiscal values, other infor-mation can be determined as a by-product of the fiscal metering.

7

One is the Wobbe index which is a measure of the quantity of energy deliv-ered at the burner injector and is the common basis for comparing dif-ferent gases. Other data, such as sulphur content or dew point may alsobe required in a few cases but will not be discussed further in this hand-book as they are only used in specific circumstances and have to be measured by special instruments.

Values to be determined

Energy : The total amount of energy that was delivered by the station ina certain specified time interval.

Quantity : The volume that the gas delivered over this time interval wouldhave occupied if it had been delivered under standard (or base) conditions.These conditions are roughly atmospheric pressure and temperature.

Heating value : The amount of heat released at combustion per unit of vol-ume (standard). The heating value may vary with time.

Definition of units

The units in which the measured quantity is specified may be differentfrom case to case. Contracts and/or legal requirements determine what thedesired output units are.

Attention should be given to the exact definitions of the units of the val-ues to be determined. These will normally be given in the contract or bylegal authorities. As even small discrepancies may add up to large sums ofmoney, the definitions should be carefully checked. The following needparticular attention:

Quantity: The base conditions need to be verified. Pressure is mostly takenat 1.013 25 bar which is the average absolute atmospheric pressure atsea level. For the base temperature, however, one finds currently 0 °C, 15 °C and 60 °F.

8

Heating value : Though the international standard ISO* 6976 [1] has nowbeen universally accepted as basis for the calculation of heating value ofa natural gas from its composition, it does still leave a number of options.

The statement that the heating value shall be according to ISO 6976 is notsufficient to define the units of heating value.The heating value is the amount of energy released by the complete com-bustion of a base volume unit of the gas. There are three definitions ofbase temperature. This gives three different base volume units.

The standard furthermore leaves the choice between superior heatingvalue and inferior heating value. In the first case the water vapour formed at combustion is condensed andin the second it remains in the vapour phase. “Superior” also stands for“gross”, “higher”, “total” or “upper” and similarly “inferior” is the sameas “nett” or “lower”. The gas industry predominantly uses superior heat-ing value, whereas the heating and combustion industry traditionally usesthe inferior heating value. Sometimes the term “Calorific value” is usedinstead of “Heating value”.

A further complication is caused by the fact that the reference tempera-ture to which the gas and air, and subsequently the products of combus-tion are brought before and after combustion can either be 0, 15, 20 or25 °C.

As a consequence ISO 6976 gives twelve different definitions of heatingvalue and it should be clearly established which one should be used. Thestandardised notation is as given below :

H [ t , V (p ,t )]

V (p ,t )

t

x 1 2 2

1

2 2

x = i : inferiorx = s : superior

: reference t for combustion

: p and t of base volume

{

* ISO: International Organisation for Standardisation.

9

Table 1 Conversion factors for Energy and Heating value

The standard unit defined by ISO for the quantity of energy is the Joule(J). The gas industry uses the Megajoule (MJ). Occasionally the kcal is stillin use. In some Anglo-Saxon countries the Btu is used. In places where elec-trical energy is dominant one may find the kWh. Heating value in ISO 6976is given in terms of MJ/m3. Table 1 gives conversion factors.

Energy: As the total quantity of energy delivered is equal to the volumeof gas multiplied by the heating value, it will be on the same basis as thelatter. So “Superior” or “Inferior” and for a reference temperature for com-bustion of either 0, 15, 20 or 25 °C.

Additional values

Apart from the values listed above that are generally sufficient to deter-mine the value of the gas supplied, additional values may be requiredfor metrological or contractual purposes.

Line volume: The total volume that was delivered at the pressure and tem-perature prevailing at that time. This figure is only relevant for the quan-tity if pressure and temperature during the time are relatively stable.

Energy units

1 kcal = 4.1868 . 10-3 MJ1 kWh = 3.6 MJ1 Btu = 1.05506 . 10-3 MJ

Heating value

1 kcal/m3 = 4.1868 . 10-3 MJ/m3

1 Btu/scf #1 = 0.03929 MJ/m-3 #2

#1: H[ t1 ,V (14.73 psia, 60 °F)]#2: H[ t1 ,V (1.013 25 bar, 0 °C)]

10

Relative density : This is also known as specific gravity. It is the ratio betweenthe density of the gas and the density of dry air at base pressure and tem-perature.

Wobbe index : This parameter is also known as “Wobbe number” and relat-ed to the suitability of the gas to be used in a class of appliances. It is cal-culated by dividing the heating value by the square root of the relativedensity. The maximum capacity in terms of energy of an appliance witha certain fixed geometry is proportional to the Wobbe index. Increasingthe Wobbe index by 10 % without changing the settings of the appliancewill result in a 10% higher gas energy flow.

INPUT VARIABLES

The input variables can be split in three categories:

Flow rateStateComposition

Flow rate

Volumetric flow meters generate a signal that is proportional to the flowof the gas at the prevailing pressure and temperature.

The types of meters that are in regular use in the gas industry are:Diaphragm meters, Rotary piston meters, Turbine meters and Ultrasonicmeters, in ascending order of capacity.

The international organisation for legal metrology OIML has proposeda standard range of gas meter sizes in terms of maximum flow rate : theso-called G-series. Each size has approximately a 60% higher capacity thanthe size below. The number following the G is meant to be the averagedesign flow in m3/h. This is about 60 % of the maximum flow rate. The G-series is given in Table 2.

The piping in a station is generally designed for a maximum gas velocity.The gas meter is usually chosen to match the diameter thus found.

11

Table 2 G-series gas meter sizing, pipe size required for specificmaximum velocities and meter types available from INSTROMET in these sizes

The reason to limit gas velocity is that the risk caused by vibrations to equip-ment that is attached to the pipe or inserted into the pipe, increases withincreasing velocity. There is also an increased risk of organ pipe type res-onances being set up in attached piping. These phenomena only rarelyoccur for velocities under 10 m/s. For higher velocities increasingly moreattention has to be paid to detailed design to prevent this from happen-ing. Table 2 also lists the diameters for particular maximum design veloc-ities. In this table the INSTROMET meters available in these sizes can alsobe found.

Important parameters for the different meter types are listed in Table 3.

46.510162540651001602504006501000160025004000650010 00016 00025 000

6.510162540651001602504006501000160025004000650010 00016 00025 00040 000

3/43/411.51.523346681012162024304048

1/23/4111.52233466810121620243040

1/21/23/43/411.51.52334668101216202430

G-v

alue

Max

. flo

w r

ate

( m

3 /h)

V <

15

m/s

V <

20

m/s

Pipe size (inches)

V <

10 m

/s

S &

L ty

peD

iaph

.mtr

s.

IRM

Infin

ity

Rot

ary

Pis

ton

met

ers

I-t

ype

Indu

stria

l m

eter

s

SM

-RI a

nd Q

-ser

ies

turb

ine

met

ers

P.S

onic

U.S

. met

er

Q.S

onic

U.S

. met

er

12

Table 3 Typical properties of different types of volumetric meters

Typical accuracyQ.Sonic 0.5 %P.Sonic 1.0 %SM-RI 0.5 % (0,3% with curve linearisation)Q-series 1.0 %I-series 2.0 %IRM 0.3 %

Installation influenceQ.Sonic Satisfies requirements of ISO 9951 (High level perturbations)

with 10D straight length upstream.P.Sonic Needs fully developed flow profile for best accuracy.SM-RI 2D straight length suffice for all ISO 9951 perturbations.Q-series For perturbed flows use external straightener as per ISO 9951,

AGA Report No. 7 or ISO 5167.I-series External straighteners recommended if installation influence

should be eliminated.IRM None at all in normal installations.

RangeQ.Sonic Within 1 % from 1 : 40 for 6” to 1 : 160 for 30”P.Sonic Within 2 % from 1 : 40 for 4” to 1 : 160 for 30”SM-RI Varies from 1 : 20 for low pressure to 1: 160 for 70 barQ-series Varies from 1 : 20 for low pressure to 1: 160 for 70 barI-series 1 : 10IRM From 1:50 for G16 to 1 : 300 for G 250

Pressure drop (mbar)Q.Sonic NoneP.Sonic NoneSM-RI < 0.015 ρQ2/ D4 for D ≤ 6”

< 0,01 ρQ2/ D4 for D > 6”Q-series < 0.015 ρQ2/ D4 for D ≤ 6”

< 0,01 ρQ2/ D4 for D > 6” I-series < 0.02 ρQ2/ D4IRM < 0.01 ρQ2/ D4

MaintenanceQ.Sonic NoneP.Sonic NoneSM-RI Lubricate every two monthsQ-series Lubricate every two monthsI-series NoneIRM Change oil every five years

} ρ in kg/m3, Q in m3/h, D in inches

13

OverloadQ.Sonic Insensitive to overload P.Sonic Insensitive to overloadSM-RIQ-seriesI-seriesIRM

DiagnosticsQ.Sonic Extensive on-line on at least 4 criteriaP.Sonic On-line on three criteriaSM-RI Consistency of HF and LF outputs on-line. In situ visual inspec-

tion of rotor and spin test off-line. Q-series Consistency of HF and LF outputs on-line. In situ visual inspec-

tion of rotor and spin test off-line. I-series NoneIRM Any malfunction results in blockage of the meter

Electrical outputsQ.Sonic RS 485 accessing flow and diagnostics; flow rate as frequencyP.Sonic RS 485 accessing flow and diagnostics; flow rate as frequencySM-RI Flow rate as HF and LF frequency Q-series Flow rate as HF and LF frequencyI-series Flow rate as LF frequencyIRM Flow rate as HF and LF frequency

Electrical requirementsQ.Sonic Ex-proof power supply P.Sonic Ex-proof power supplySM-RI Intrinsic safe power supply for electrical outputsQ-series Intrinsic safe power supply for electrical outputsI-series Suitable supply for voltage-free contactsIRM Intrinsic safe power supply for electrical outputs

Body materialQ.Sonic Any available steel to spec.P.Sonic Any available steel to spec.SM-RI Any available steel to spec. Small sizes also in ductile iron (19 bar)Q-series Any available steel to spec. Small sizes also in ductile iron (19 bar)I-series Aluminium only (10 bar)IRM Aluminium and ductile iron (19 bar) or steel for higher pressures

These are typical properties - see brochures for pertinent data.

}Short periods up to 120% of maximum flow rate. Opening of valves should be done carefully. Accelerationfrom 0 to maximum flow should not take less than 3seconds.

14

For fiscal purposes, the integrated volume is displayed in a register andin some meters this may be the only value that is accessible. This is for exam-ple the case with most diaphragm meters where there is no signal pro-portional to flow rate available.

Most meters have electronic outputs. Ultrasonic meters of course, by nature,do. The flow rate is available on an RS 485 or RS 232 serial interface. Anyavailable data for diagnostic purposes can be obtained from the same bus.A high frequency signal (0 - 10 kHz), proportional to the flow rate is alsoavailable from all INSTROMET Ultrasonic meters.

Mechanical meters can either have low frequency (below 1 Hz) or high fre-quency outputs, proportional to the flow rate. Low frequency outputs aregenerally in the form of voltage-free contacts. High frequency outputs aregenerated electronically and may result from a proximity detector sensingthe passage of the rotor blades of a turbine meter. The frequency at max-imum flow rate ranges from 2500 Hz for small meters to 200 Hz for largeones. Many meters can be provided with a choice of outputs.

To ascertain the accuracy, meters are normally tested or calibrated againsta reference standard before delivery. Calibration of a meter against areference standard can also be used to verify its proper operation afterfield service, for example in case of a dispute. The relevance of the resultsof a calibration for its performance in real life operation is determined bythe following considerations:

• Sensitivity of the meter to differences in calibration and operatingconditions. The lesser the sensitivity of a meter to operating con-ditions (pressure, temperature), the lesser the need for calibratingunder operating conditions.

• The relationship between calibration conditions and practical oper-ating circumstances. The closer the relation, the more accurate prac-tical performance can be predicted.

• The stability of the meter. High accuracy of the calibration is onlyuseful for sufficiently stable meters.

• The stability of the operating conditions. For widely varying oper-ating conditions, the performance of the meter should be eitherindependent of those conditions or its dependence should be known.

15

The calibration of a meter to the highest possible accuracy is of little use ifthe meter is to be used in highly perturbed flows and with heavily pol-luted gas.

For a highly stable meter for which the influence of operating condi-tions (pressure and installation) is small and/or that operates at moder-ate pressures, a single test at atmospheric conditions may be sufficient formost applications. However, calibration under conditions that resemblethe practical application to a greater extent will only increase meteringaccuracy.

The “State” of the gas

To calculate quantity in terms of base or standard volume one needs toknow the quantity of matter, e.g. the number of moles, that occupiesthe actual volume measured under operating conditions.

This is done by using a suitable “Equation of State” for the type of gasmeasured and by using measured pressure and temperature.

Pressure measurement

The pressure used in the Equation of State is always the absolute pressure.Absolute pressure is 0 in vacuum and approximately 100 kPa (1 bar, 14.7psi) in the atmosphere at sea level.

At present absolute pressure sensors are readily available and thus thereis no necessity to use gauge pressure together with the atmospheric pres-sure. Moreover, absolute pressure sensors have proven to be more sta-ble.

If the use of gauge pressure cannot be avoided, the absolute pressure canbe calculated from the gauge pressure by adding the atmospheric pressure :

Pabs

=Pgauge

+ Pbaro

, where Pbaro

stands for the atmospheric pressure. Inascending steps of complication one can determine the atmospheric pres-sure in the following way:

16

a. Use a constant pressure approximately equal to the local aver-age atmospheric pressure to add to P

gauge. Care should be taken

that the altitude of the station is taken into account, as the aver-age atmospheric pressure is normally taken at sea level. Measuredatmospheric pressure decreases when going to higher altitudesby approximately 0.12 mbar per meter.

b. Use actual on site measured atmospheric pressure.

Unless prevented by existing procedures or regulations, an absolute pres-sure sensor should be used in view of these complications.

The temperature dependence of modern pressure sensors at different pres-sures is charted during manufacture and the effect is automatically com-pensated for in operation. These sensors can therefore be highly linearover a wide range of temperatures and pressures, and are often referredto as “Smart” sensors.

If the flow meter has been provided with a pressure reference point (Pr

point), the pressure sensor should be connected to that point. If no pres-sure point has been provided on the meter it should be drilled at rightangles with the pipe wall and the tapping should be without burrs.

Temperature measurement

There is a large number of accurate temperature gauges available. Absolutetemperature (expressed in Kelvin) is used in the Equation of State but dif-fers only from temperature in degrees Celsius by a constant:

Tabs

= TC

+ 273.15

Platinum resistance temperature sensors and semiconductor sensors bothgive very good accuracy. Platinum resistance sensors are less vulnerable tothe effects of electric storms and therefore preferable in open field instal-lations.

Care has to be taken that the temperature of the sensitive part of the sen-sor assumes indeed the temperature of the gas. In laboratories naked sen-sors are used preferably. In practical installations thermowells are used butcare should be taken that their influence is minimised.

17

Thermowells therefore have to protrude to sufficient depth in the conduit.A length of 3/4 of the pipe diameter or 150 mm, whichever is less, is oftenrecommended. The sensitive part of the transmitter should be located asas close as posible to the bottom of the well.The heat leakage to ambientshould be small, and for best accuracy the part of the transmitter protrudingfrom the pipe should be insulated.

If a thermowell has been provided in the meter it should be used. In absenceof a built-in thermowell it should preferably be mounted downstreamof the meter within 4D.

Density measurement

Densitometers of the vibrating cylinder type have been used successfullyin gas metering stations.

It is essential that the density of the gas in the densitometer is equal to thedensity at the appropriate point in the volumetric flow meter. If the flowmeter has been provided with a pressure reference point (Pr point), thepressure in the densitometer should be identical to the pressure at thatpoint. The temperature of the gas in the densitometer should be equal tothe gas temperature in the meter. The best way to achieve this is to insertthe densitometer in a well in the conduit. For best accuracy heat leak-age to ambient should be prevented by suitable insulation.

The instrument is slightly sensitive to the velocity of sound in the gas,the VOS effect. For highest accuracy this has to be corrected for or the densitometers have to be calibrated with a similar gas.

The gas in the density meter should be of the same composition as the gasin the flow meter. As composition tends to vary slowly, it is sufficient tohave a small bleed of gas through the densitometer.

Dirt or condensation is catastrophical for the accuracy and a filter is essen-tial. A schematical drawing of a densitometer installation is given in Figure1.

18

Fig. 1 Connecting a densitometer

Correlations for compressibility have now developed to a point that measurement on the basis of pressure, temperature and compressibility(PTZ) is equally accurate to using a densitometer. As a result many com-panies presently prefer PTZ as this is easier to install and maintain.

Composition and compressibility

The composition of the gas influences the constants in the Equation ofState. This is mostly translated in the “Compressibility factor” or “Z”. Severalmethods exist to calculate Z. In many cases the particular one to be used isdictated by contract.

The most recent and most accurate methods are given by AGA [2] andGERG [3,4]. They describe two methods to calculate Z.

The first is based on a detailed chemical analysis of the gas. It is valid forthe pressure and temperature area given in Figure 2. The uncertainties arealso given in this figure. The calculations are valid for concentrations ofthe individual components listed in Table 4. AGA Report no. 8 also lists a wider concentration range where the equa-tions can be applied with lesser accuracy.

The GERG method that uses a detailed analysis is based on a different modeland the results are therefore not identical to those obtained with the com-parable AGA Detailed Characterization Method. The GERG equation is onlyvalid in the innermost region of Figure 2.

Densitometer

Filter

Pr

19

Fig. 2 Pressure and temperature field for validity of compress-ibility equations

The second method to calculate Z is simpler and only needs three of thefour following quantities:

* the relative density (specific gravity)* the gross heating value* the mole fraction of nitrogen (N

2)

* the mole fraction of carbon dioxide (CO2)

`This method can only be applied in the central area of figure 2.

1%

0.5%

0.3%

0.1%

-130 -60 -8 62 120 200

140

70

17

12

0

Pre

ssur

e M

Pa

Temperature Co

20

Table 4 Gas mixture ranges covered by AGA report number 8.The normal range is covered by both SGERG and AGA 8Gross Characterization Method

It can also handle mixtures of natural gas and manufactured gas. In thatcase the mole percentage of hydrogen (H

2) is also needed. The method

is not applicable to gas-air mixtures.

This simpler method is known as the “Gross Characterization Method”in AGA 8 [2] and it is identical to the one listed in SGERG [4].

Quantity Normal Expanded Units range range

Relative density 0.56 to 0.87 0.07 to 1.52 SuperiorHeating value 477 to 1150 0 to 1800 Btu/scf Superior heating value 18.7 to 45.1 0 to 66 MJ/m

3

Methane 45.0 to 100.0 0 to 100.0 Mole %Ethane 0 to 10.0 0 to 100.0 Mole %Propane 0 to 4.0 0 to 12 Mole %Butanes 0 to 1.0 0 to 6.0 Mole %Pentanes 0 to 0.3 0 to 4.0 Mole %Hexane plus 0 to 0.2 0 to dewpoint Mole %Nitrogen 0 to 50.0 0 to 100.0 Mole %Carbon dioxide 0 to 30.0 0 to 100.0 Mole %Helium 0 to 0.2 0 to 3.0 Mole %Hydrogen none 0 to 100.0 Mole %Carbon monoxide none 0 to 3.0 Mole %Argon none 0 to 1.0 Mole %Oxygen none 0 to 21.0 Mole %Water 0 to 0.05 0 to dew point Mole %Hydrogen sulphide 0 to 0.02 0 to 100.0 Mole %

21

Composition measurement

The chemical composition of natural gas can be measured conveniently bymeans of a gas chromatograph such as the ENCAL 2000. In large stationssuch as in transmission lines or at power stations, the total value of tradeis so high that the on-line application of a gas chromatograph is often war-ranted. Gas chromatographs can also be used to monitor the compositionfor a number of stations, provided these are supplied from one point.

Gas chromatographs need a

* sampling system with sample conditioning set to provide a rep-resentative sample to the chromatograph

* a sample conditioner

* carrier gas (helium)

* calibration gas of a suitable composition for the range of gasesto be measured

A separate Instromet Handbook is available on gas chromatography giving more detailed information.

Gas chromatographs provide primarily the chemical composition. From thecomposition one can determine the heating value of the gas, the rela-tive density and also the Wobbe index. These data are calculated in theENCAL 2000. As the chromatograph has determined the mole percentagesnitrogen and carbon dioxide, it provides all data necessary for calcula-tion of Z either from full composition or by the Gross CharacterizationMethod (SGERG).

Calorimeters

To measure the energy supplied in smaller stations, a more compact andeconomic calorimeter can be used such as an instrument from the Tru-Therm range.

22

For the Tru-Therm one needs :

* sampling system to provide a representative sample to the chro-matograph

* a pressure regulator

* compressed air, instrument-grade

* ultra-high purity methane as calibration gas

The Tru-Therm provides the heating value, the specific gravity and theWobbe index.

For the calculation of the compressibility, the concentration of either carbon dioxide or nitrogen has to be known. In many cases a good esti-mate of one of these components can be made on the basis of known aver-age compositions of the different gases that make up the mixture. At lowerpressures (<10 bar) a slight error in this value usually does not mattertoo much for the end result.

CONVERTING TO BASE CONDITIONS

To convert the volume or flow rate measured at line conditions to baseconditions, two types of instruments exist. These are the “Flow Computer” and the “Electronic Volume Corrector Device, EVCD”. Thesetwo types of devices differ in a number of aspects. These are listed in table 5.

Conversion of the measured line volume to base volume relies on the equa-tion of state for the particular gas.

PV0

= ZRT (1)

In this equation R is the universal gas constant and equals 8.31451 J/mol K.

This equation is valid for one mole of gas and describes the relation betweenthe volume V

0, the (absolute) pressure P and the (absolute) temperature T.

23

The compressibility factor is dependent on the gas composition and onpressure and temperature. For very low pressures the compressibility fac-tor for any gas is equal to 1.

Instrument

Property 900 & 500 series Model 999 700 seriesElectronic correctors Electronic Correctors Flow computers

Installation on/close to meter on/close to meter remote in safe area

Calculation rate at every LF impulse at every LF impule fixed, determined by FC

Power supply battery battery (5 yrs) mains

Z factor:

fixed yes yes yesadjustable on most models SGERG/AGA 8 yeson-line no no yes

Best uncertainty <0.3 % <0,2% only limited by sensors

Multistream no no available

Orifice plates no no available

Ultrasonic meters no no available

Data logging on some models extensive and flexible available

Safety Intrinsic up to 80 bara barriers to hazardous areas

Table 5 Comparison of volume correctors and flow computers

A typical plot of compressibility as a function of pressure and temperatureis given in Figure 3 .

Ideal gases, such as helium, have a very simple equation of state which isalso known as Boyle’s and Gay-Lussac’s law or Boyle’s and Charles’ law. Forthese gases the compressibility factor Z is equal to 1 for a wide range ofpressures and temperatures.

24

Fig. 3 Compressibility as a function of pressure for a typical lowmolecular mass natural gas

We are generally dealing with n moles of gas and can rewrite (1) as

V = nZRT (2)

with V the volume occupied by n moles of gas.

Applying (2) to n moles of gas at the conditions in the meter (subscript m)and the same number of moles at base conditions (subscript b), we find

Vb

= Vm

(Pm

ZbT

b) / (P

bZ

mT

m) (3)

0.4

0.5

0.6

0.7

0.8

0.9

1.0

50 100 150 200 250 300 350

0

25

50

75

OC

OC

OC

OC

0

Pressure (bar)

Com

pres

sibi

lity

Z

25

Equation (1) can also be rewritten in terms of density

P/ρ = ZRT/M

where M is the molar mass (Molecular mass) of the gas.

In a similar way we can then write

Vb

= Vm

ρm

/ ρb

(4)

It is the volume correction device or flow computer that performs calcu-lation (3) or (4).

Linearisation of flow meter

If the error of a flow meter is known, it can be corrected for. Some flowcomputers have the ability to carry out this correction. First the correctiondata resulting from calibration are fed into the instrument. Next, the appro-priate correction factor at the particular flow rate is determined andapplied.The result will be perfectly linear. The 700 series of INSTROMETflow computers provides for two systems of correction: linear interpola-tion between the test points or a polynomial, the so-called Straatsma poly-nomial. Correction can either be on the basis of flow rate or on the basisof Reynolds’ number (see turbine meter Handbook).

Legal and/or contractual requirements

The figure resulting from the calculations will serve for invoicing. The reli-ability of this figure is from utmost importance and it is therefore that spe-cific requirements have to be met for contractual reasons and, in manycases, to conform with legislation. Almost all INSTROMET equipment hasbeen at least approved by Weights and Measures Officials in Europe.

Requirements vary slightly from country to country though the aims arethe same. These aims are:

* Sufficient accuracy over a range of operating temperatures andpressures.

26

* Sufficient accuracy over a range of ambient temperatures.

* Result not influenced by electromagnetic interference.

* Safeguarding from interference.

The last is traditionally implemented by sealing of the instrument or partsof it. There is now a trend for modern instruments to use passwords anda built-in non-erasable audit log stored in memory.

As gas meters are situated in areas that are often zoned as hazardous,all INSTROMET instruments are constructed according to stringent EuropeanCENELEC safety requirements for at least zone 2.

Instruments are either explosion-proof, intrinsically safe (field mountedEVCD’s) or connected to the hazardous areas by safety barriers. The ENCAL2000 is explosion-proof.

Approval consists of a type approval and tests on individual equipment ormanufacturing process to assure conformity with the approved type.

ACCESSING STATION DATA

Traditionally a meter reader visually reads all registers and records someoperational data during his visit. These are checked for consistency andthen used for invoicing. Remote meter reading using telephone lines isnow becoming more popular and equipment such as the Metretek rangeprovides adequate means for this purpose.

For very large stations, the data are of such importance that it is oftenimperative to continuously monitor both the station flow and the oper-ation of the equipment at a control centre. A station may consist of a num-ber of meter runs, each equipped with their own flow computer, back-upor check meters, alarms and diagnostics and a gas chromatograph. As the instruments all operate independently, interrogation can only effi-ciently be carried out through a buffer effectively decoupling the teleme-try from the operation of the instruments. INSTROMET provides a StationController for this purpose.

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Dependent on the design philosophy, all or part of the data relating to theequipment operation are transmitted to the control centre. Data are storedlocally or remotely to serve for diagnostics, trend and fault analysis. A localPC may be provided to facilitate analysis.

RELIABILITY AND DIAGNOSTICS

Present gas metering equipment is very reliable. If used with normal com-mercial, clean gas and with normal maintenance the technical life expectan-cy of a system is in the order of at least 20 years.

Failures can be classified in two types:

* Catastrophic failure

* Degradation of performance.

Catastrophic failure is mostly obvious and alarms are installed to indi-cate gross malfunction.

There are three main methods to check on degradation:

* Calibration

* History

* Diagnostics

Calibration is traditionally carried out on a regular basis, starting at com-missioning. As far as possible, all equipment related to the process is com-pared in the field with a reference. The instruments are then adjusted toread accurately. Logging the adjustments provides a control chart that givesan early warning on degradation. This used to be particularly suitablefor pressure gauges and analog electronic instruments.

In the field, calibration of flow meters is not possible. Calibration in thefield of pressure and temperature transducers has become counter- pro-ductive as the stability and initial factory calibration of modern high qual-ity transducers give superior accuracy than what is achievable with fieldreferences.

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Gas composition and calorimetry are the measurements in a gas stationthat are most sensitive to drift. The ENCAL 2000 and the Tru-Therm haveboth built-in automatic calibration procedures to check their operationwith the aid of a reference gas and automatically compensate for drift.

When one can be sure that a parameter (pressure, temperature) has to bewithin certain limits or when there is a relation between two or more para-meters, we can use diagnostic systems. For example, pressure at no placein the station can be significantly higher than the inlet pressure. If a pres-sure is found well above the inlet pressure one can conclude that thereis a malfunction of some kind. Another example is the correlation betweenthe corrected and uncorrected volumes. Average pressure and tempera-ture are known to be within certain limits. The figure calculated from theuncorrected volume with this average pressure and temperature shouldnot be too different from the corrected volume.

Some instruments have extensive built-in diagnostics. An example is theINSTROMET multipath ultrasonic meter. In this meter the data from all 5paths are continuously compared. Each path yields the velocity of sound,and differences between the paths have to be within certain limits.Furthermore the automatically adjusted gain has to remain below acertain value. The noise received by the transducers is automaticallyanalysed. Also, the shape of all received pulses is checked, and pulses thatdo not fulfill certain criteria are rejected. The percentage of rejected puls-es is an excellent indication for health monitoring.

Diagnosis relies often on a degree of redundancy. This is the case with the5 paths of the INSTROMET multipath meters. For very important stationsadditional redundancy may be built in. Using redundant flow meters it hasbeen proven that deviations of 0.2 % or less of a single meter run canbe detected within one hour in a well designed and maintained station.

Two volumetric meters in series can be very accurately compared. By cor-recting for the differential pressure between the two meters, the line vol-umes are compared, rather than the base volumes, thus minimising theinfluence of additional instrumentation.

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Fig. 4 Direct comparison of line volumes in two meters

In figure 4 the line volumes relate as:

V2= V

1(1+∆P/P)

In this figure an SM-RI-X turbine meter and a Q.Sonic ultrasonic meter aredrawn but the same system can be used for any two volumetric meters ofeither the same or different kind.

Slight errors in either ∆P or P have little effect on the result. If the gas tem-perature is much different from ambient, the gas cannot be assumed tohave the same temperature in both meters. In that case the tempera-ture difference can be also corrected for.

If a station consists of a number of meter runs in parallel, this offers anoth-er opportunity to check meter performance. It has been shown in practicethat the flow distribution over a manifold is very constant. The ratio offlows in the parallel runs can therefore be logged and used as anotherdiagnosis tool. To be able to use this most efficiently, all runs should oper-ate continuously.

In figure 5 a schematic drawing is given of a system that would give thehighest accuracy presently achievable and have powerful and sensitivediagnostics.

The system consists of two parallel runs with each two volumetric metersin series. The meters in series are continuously compared as are the tworuns. Even a small drift in any one meter can be detected in a very shorttime, and by comparison with the data from the other run, it can be

P

∆P+ -

V V1 2

30

P

∆P+ -

P

∆P+ -

793-SC

793-1

793-4

793-1

793-4

Encal 2000Remote

SCADARemote computer

Local computer

T T

TT P

P

Encal2000

Fig. 5 Example of high accuracy system with extensive diagnosticsand a high level of redundancy

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established which one is the cause. Gas composition is measured by anENCAL 2000 gas chromatograph and provides data to continuously updateheating value and compressibility. Temperature and pressure are digitally read using a HART bus, and a 793flow computer calculates Quantity in base volume and Energy withoutadding any additional error. A 793 SC provides the interface betweenthe flow computers and the outside world. It distributes the ENCAL basedcomposition to the FC’s, compares volumetric meters, determines total flowand collects diagnostics of individual instruments. It can communicate datato a local or remote computer or both, tying in with user’s existing instru-mentation systems if necessary.

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REFERENCES

[1] ISO 6976, Natural gas - Calculation of calorific values, density, rela-tive density and Wobbe index from composition

[2]. AGA Report No. 8, Compressibility Factors of Natural Gas andOther Related Hydrocarbon Gases (Second Edition, November1992)

[3] GERG Technical Monograph, High Accuracy Compressibility FactorCalculation for Natural Gases and Similar Mixtures by Use of aTruncated Virial Equation

[4] Standard GERG Virial Equation for Field Use, Simplification of theInput Data Requirements for the GERG Virial Equation - AnAlternative Means of Compressibility Factor Calculation forNatural Gases and Similar Mixtures

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In the USA:INSTROMET ULTRASONIC TECHNOLOGIES INC.BADGER INSTROMETINSTROMET TECHNOLOGY CORP.

Products & Services:● Ultrasonic gas flow meters

● Turbine gas meters

● Rotary gas meters

● Diaphragm gas meters

● Insertion gas meters

● Electronic volume correctors

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● Gas filters

● Gas pressure regulators

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● Calorimeters

● Gas chromatographs

● Pressure transmitters

● Telemetering systems

● Electronic metering and control systems

● Calibration and test installations

● Complete gas measurement and control stations

● Commissioning, servicing, training and consulting

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INSTROMET has agents and representatives worldwide.

INSTROMET has a continuing program of product research and development.Technical specifications and construction may change due to improvements.This publication serves as general information only, and all specifications aresubject to confirmation by INSTROMET.

YOUR SALES OFFICE OR REPRESENTATIVE:

Gas measurement and control equipment

For your nearest sales office orrepresentative please contact:INSTROMET INTERNATIONALRijkmakerlaan 9 - B-2910 ESSEN - BELGIUMTel: +32 3 667 3440 - Fax : +32 3 667 6940E-mail: instromet.intl@tornado.beWeb-page: http://www.zeb.be/instromet/