gas flow measurement

8/13/2019 Gas Flow Measurement

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UNIT-5

GASFLOWMEASUREMENT


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It is required to enable the determination of the amount ofgas being produced or sold, and also as a basicparameter for almost all of the design procedures.

The produced gas stream is in a continuous state of flowfrom the instant it leaves the reservoir until it is consumedat the delivery end.

Gas measurements must be done mostly on a flowingstream of gas.

Gas is most commonly measured in terms of volumebecause of the simplicity of procedure.


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MEASUREMENTFUNDAMENTALS

Flow is one of the most difficult variables to

measure because it cannot be directly measured

like temperature.

It must be inferred by indirect means such as the

pressure differential over a specified distance,

speed of rotation of a rotating element etc.

Many flow measurement techniques and devices

have been developed for a wide range applications.


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PARAMETERSTOCHARACTERIZETHEFLOW

METER

Accuracy

Rangeability

Repeatability

Linearity


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ACCURACY

This is a measure of a flow meters ability to indicatethe actual flow rate within a specified flow raterange.

It is defined as the ratio of the difference betweenthe actual and measured rates to the actual rate.

Accuracy = Actual rate-Measured rate

----------------------------- X 100%Actual rate

Accuracy is represented in either two ways :percent of full scale, or percent of reading.


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RANGEABILTY

A flow meters rangeability is the ratio of themaximum flow rate to the minimum flow rate at thespecified accuracy.

Rangeability= Maximum rate that can bemeasured

---------------------------------------------------

Minimum rate that can be measured

Rangeability is usually measured as a ratio x : 1


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REPEATABILITY

It is also known as reproducibility or precision

,repeatability is the ability of a meter to reproduce

the same measured readings for identical flow

conditions over a period of time.

It is computed as the maximum difference between

measured readings.


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LINEARITY

This is a measure of the deviation of the calibration

curve of a meter from straight line.

It can be specified over a given flow-rate range, or

at a given flow rate.

A linear calibration curve is desirable because it

leads to a constant metering accuracy.


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SELECTIONOFMEASUREMENT

Accuracy desired

Expected useful life of the measuring device

Range of flow ,temperature

Maintenance requirements

Power availability Liquid or gas

Cost of operation

Initial cost

Availability of partsAcceptability by others involved

Purpose for which measurements are to be used

Susceptibility to theft


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DIFFERENTIALPRESSUREMETHOD

There are basically two types of differential

pressure devices

The pressure difference is measured across a flowrestriction.

Eg: Orifice, venturi

The difference in pressure measured upon impactEg: Pitot tube


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ORIFICEMETER


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ORIFICEMETER

This is the most commonly used device formetering natural gas.

It consists of a metal plate with a circular hole

,centered in a pair of flanges in a straight pipesection.

The pressure differential is measured across thisplate to yield the flow rate .

This is a rugged, accurate, simple, and economicaldevice and can handle a wide range of flow rates.


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Orifice flow meters are used to determine a liquid or gasflow rate by measuring the differential pressure (P1 - P2)across the orifice plate.

They are generally less expensive to install andmanufacture than the other commonly used differentialpressure flow meters;


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VENTURIMETER


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VENTURIMETER

A venturi is a point in a pipe that has been narrowed sothat the flow is restricted slightly.

The venturi is widely used because it has no movingparts and the small amount of restriction it produces toinduce a pressure drop does not disturb the fluid flowtoo much.

The change in cross-sectional area in the venturi tubecauses a pressure change between the convergentsection and the throat, and the flow rate can bedetermined from this pressure drop.

Although more expensive that an orifice plate; theventuri tube introduces substantially lower non-recoverable pressure drops.


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ADVANTAGES

The pressure recovery is much better for the venturi

meter than for the orifice plate.

The venturi tube is suitable for clean, dirty and viscousliquid and some slurry services.

The rangeability is 3.5:1.

Pressure loss is low.

Typical accuracy is 1% of full range.


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PITOT TUBE


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The pitot tube measures the difference between the

static pressure at the wall of the flow conduit and the

flowing pressure at its impact tip where the kinetic

energy of the flowing stream is converted intopressure.

It gives the flow velocity only at a point.

The tip can be easily clogged by liquids or solids.

Because of the relatively poor accuracy of this deviceit is not used very often.


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Turbine flow meter


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These meters are sometimes classified as positive

displacement meters.

They consist of a turbine or propeller that turns at aspeed proportional to the velocity of the gas ,

converting linear velocity to rotational speed.

Turbine meters have been used for measuring liquid

flow rates rather than gas flow rates.

Fluctuations in velocity, caused by pressure

fluctuations, turbulence or unsteady state flow

conditions, will cause the turbine meter to give a

higher than actual value.


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ROTAMETER

A Rotameter is a device that measures the flow rate ofliquid or gas in a closed tube.

It belongs to a class of meters called variable area meters.

A rotameter consists of a tapered tube, typically made ofglass, with a float inside that is pushed up by flow andpulled down by gravity.

The fluid entering at the base of the tube causes the floatto rise until the annular area between the float and the tubewall is such that the pressure drop across this constrictionis just sufficient to support the float.


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ROTAMETER


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ADVANTAGES

A rotameter requires no external power or fuel, ituses only the inherent properties of the fluid, alongwith gravity, to measure flow rate.

A rotameter is also a relatively simple device that

can be mass manufactured out of cheap materials,allowing for widespread use in places such as thirdworld countries


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DISADVANTAGES Due to its use of gravity, a rotameter must always be vertically

oriented and right way up, with the fluid flowing upwards.

Due to its reliance on the ability of the fluid or gas to displace thefloat, the graduations on a given rotameter will only be accurate for agiven substance. The main property of importance is the density ofthe fluid. Either separate rotameters for different substances must beused, or the read out adjusted.

Rotameters normally require the use of glass (or other transparentmaterial), otherwise the user cannot see the float. This limits theiruse in many industries to benign fluids, such as water.

Rotameters are not easily adapted for reading by machine: althoughmagnetic floats that drive a follower outside the tube are available.
http://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Water


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ULTRASONICMETERS

A typical transit-time flow measurement system utilizes two

ultrasonic transducers that function as both ultrasonic transmitter

and receiver. The flow meter operates by alternately transmitting and

receiving a burst of sound energy between the two transducers andmeasuring the transit time that it takes for sound to travel between

the two transducers. The difference in the transit time measured is

directly and exactly related to the velocity of the liquid in the pipe.


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To be more precise, let's assume that Tdown is the transit-

time (or time-of-flight) of a sound pulse traveling from the

upstream transducer A to the downstream transducer B, and

Tup is the transit-time from the opposite direction, B to A. The

following equations hold:

Tdown = ( D / sin) / ( c + V*cos ), (1)

Tup = ( D / sin) / ( c - V*cos), (2)

where c is the sound speed in the liquid, D is the pipe

diameter and V is the flow velocity averaged over the soundpath. Solving the above equations leads to

V = ( D / sin2 ) * T / (Tup * Tdown), (3)

where T = Tup - Tdown. Therefore, by accurately measuringthe upstream and downstream transit-time Tup amd Tdown,

we are able to obtain the flow velocity V. Subsequently, theflow rate is calculated as following,

Q = K *A* V, (4)

where A is the inner cross-section area of the pipe and K is

the instrument coefficient.


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ORIFICEMETERS

Because orifice meters are simple, accurate, relativelyinexpensive they are most widely used of the flow metersfor gases.

An orifice meter consists of a thin plate ,0.115-0.398 in.thick depending upon the pipe size and pressure, held

perpendicular to the direction of flow by a pair of flanges,with a circular sharp square edged orifice accuratelymachined to the required size in the centre of the plate.

Pressure taps are provided on the upstream and downstream end in the fitting that holds the orifice plate.

A pressure measuring and recording device is connectedto the pressure taps.


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ORIFICETYPESDifferent kinds of orifice plates include concentric, eccentric,

and segmental, each of which has different shapes andplacements for measuring different processes.


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The concentric type is the most common ,because of itslow cost, ease of fabrication and ease of calibration.

The eccentric and segmental types are very useful fortwophase flow streams and for flow streams withsuspended solids such as dirty gases or slurries.

Rangeability -3:1

Accuracy; +/- 1.5-2%


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LOCATIONOFPRESSURETAPS

The magnitude of the measured pressure differential is

obviously affected by the location of the points across theorifice between which it is measured.

Pressure taps are designated as P1 and P2. "D" is thediameter of the pipe and "d" is the diameter of the orifice.

The four types of pressure tap locations that are used:

Flange type

Pipe tap

Corner type


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LOCATIONOFTAPS


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Flange type: In this type the pressure is measured 1 in.

from the upstream face of the plate and 1 in from the

down stream face of the orifice plate. This is the mostcommon type of pressure tap.

Pipe taps: 2.5 IDs from the upstream, and 8 Pipe IDs

from the down stream.

Corner types: In this type the pressure taps are located

immediately adjacent to the upstream and down stream

faces of the orifice plate.


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STRAIGHTENINGVANES

Straightening vanes are used to minimize the flowdisturbances in meters.

Flow eddies, rotation swirls and other undesirable

flow patterns are minimized as the flow passesthrough the relatively small tubes.

Straightening vanes are available as pin type or

flange type in carbon steel or stainless steel.


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MEASUREMENTCALCULATIONS

The relationship for orifice meters can be derived fromthe general energy equation written between two points

in the flowing stream point1 being some pointupstream of the orifice plate & point-2 the orifice throat.

1

2

Vdp+1/gc 12

v dv + g/gc12

dz = wslw---------(1)For most meters ,change in elevation between points 1and 2 , dz is zero, and no work is done by the flowing

fluid stream. Therefore equation 1 is written as

12

Vdp+1/gc 12

v dv + lw=0 -----------(2)The lost work term expresses the frictional losses due to

viscosity and turbulence of the fluid.


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These losses can be handled in a manner convenient

for meter calculations without reference to friction

factor.

The basic orifice equation can be written in the form:C212Vdp+1/gc 12v dv=0 -----------(3)

C= Empirical constant that takes care of friction and

other irreversibilities.

Multiplying with

C212 dp+1/gc 12 v dv =0 -----------(4)

Assuming a constant , average density avfor simplicityand integrating the equation (4) we get:

C2(p2- p1) =( av/ 2gc)(v22-v12)=0 -------- (5)

Converting to commonly used pressure units of psia

(lbf/in2) and rearranging we get:


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(v22-v1

2) = 2(144) C2(p1- p2) / av-------------(6)

The mass flow rate ,m (lbm/sec),is given by

m = vA

where A= cross sectional area of flow ,ft2

This analysis assumes steady-state flow conditions , for

which the mass flow rate is constant. Equation (6)

can now be written as:

(m2/ av2 )[1/A22-1/A12]=2(144)gcC2(p1-p2)/ av -------(7)

Taking A22as common

(m2/ A22 )[1-(A2/A1)

2]=2(144) gcC2 (p1-p2) av -------- (8)

Let d1 and d2 be the diameters of the pipe and theorifice , respectively, in inches. Defining =d2/d1andsolving equation (8) for m:


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m= C A2[(2(144)gc(p1-p2) av) /(1- 4) ]0.5

Or

m= C d22 [gc(p1-p2) av) / 1152(1- 4) ]0.5 -------(9)Using the gas law, the gas density can be expressed as

:

av=28.97 g p av / Zav R Tav ------- (10)The pressure differential (p1-p2) is generally expressed

in terms of inches of water. This conversion can be

achieved ,using the relation

p= g h/gc and is written as

(p1-p2) = {62.43 h/(144)(12)} -------(11)

h=pressure differential in inches of water.


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Using equations (10) and (11) ,(9) becomes :

m = C d2

2 [ (28.97) (62.43)gc

gh p

av) / (1,152)(144)(12) (1- 4) Z

avR Tav]0.5 --------------------(12)

Gas flow is generally reported in terms of the flow rate qsc

in scf /hr at standard conditions , which is related to the

mass flow rate m in lbm/sec as follows (m=q)

m=(qsc /3600) {(28.97) gpsc/ ZscRT sc}

Using standard conditions of psc=14.73psia,Tsc=520oR

and Zsc=1 we obtain:

m= {(28.97)(14.73)/(3600)(520)R} g

qsc

-----------(13)

Using equation (13) in (12) substituting R=10.732 psia-

ft3/lb mole-oR and solving for qscwe obtain:

qsc= {7,717.96 Cd22 }/ {(1- 4) gZ avTav }0.5 {hpav}0.5


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In equation 14

qscis in scf/hr

d2is in inches

h is in inches of water

Pav is in psia

Tav is inoR

C, , g,Z av are dimensionlessThe equation 14 is commonly expressed as:

qsc = Ko {h pav}0.5

The constant Ko is given by

Ko= 7717.96 Cd22 / [ (1- 4) g Zav Tav]0.5 ------------(15)


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In metering practice ,the average pressure pavis replaced

by a measurable gauge pressure pf.

Factors are provided to account for this pf being

,measured at the upstream or down stream, or being

measured as the mean of upstream and downstream

static pressures and for the type of pipe tap.

Equation is then written in the following form:

qsc=K [hwpf]0.5-----------(16)

hw=differential pressure at 60oF , inches of water

pf=absolute static pressure of the flowing fluid, psia

And the constant is expressed as a product of severaldifferent as follows.

K F F Y F F F F F F F F (17)


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K= FbFrY Fpb FtbFtf FgFpvFmFlFa ------------(17)

Fb= basic orifce factor, scf/hr

Fr = Reynolds number factor

Y = expansion factor

Fpb = pressure-base factor

Ftb =Temperature base factor

Ftf=flowing temperature factor

Fg= specific gravity factor

Fpv =supercompressibility factor

Fm = manometer factor

Fl=gauge location factorFa = orifice thermal expansion factor


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1) Basic Orifice Factor , Fb :

The factor is simply the constant in equation (16) Its value depends upon the type of pressure taps and

the pipe and orifice diameters.

Fbcan be obtained from tables 10-2 and 10-7 for flange

taps and pipe taps respectively.


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2) Reynolds Number Factor, Fr:

This factor accounts for the variation of the orifice

discharge coefficient with Reynolds number.

Tables 10-3and 10-8 show the value of Fr for flange

taps and pipe taps, respectively.

If Fris small it can be neglected.


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3) Expansion Factor , Y:

This factor accounts for the change in gas density with

the pressure changes across the orifice. The expansion factor can be obtained from Tables 10-

4,10-5,and 10-6 for flange taps, and Tables10-9and 10-

10 for pipe taps.

These tables indicate the pressure tap from which theabsolute static pressure pf is measured Y1 for

upstream,Y2 for down stream and Ym for static pressure

recorded as the mean of the upstream and down stream

static pressures.


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4) PressureBase Factor ,Fpb:

This factor corrects for cases where the base (standard)

pressure , pb

in psia , at which flow is to be measured is

other than 14.73 psia:

Fpb=14.73/pb

5) Temperature- Base Factor, Ftb:

This factor corrects for cases where the base

(standard) temperature ,TbinoR at which flow is to be

measured is other than 520oR:

Ftb=Tb/520


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6) Flowing Temperature Factor, Ftf:

The flowing temperature factor corrects for cases

where the flowing temperature Tf

(oR) , is not 520oR, using

the fact that the gas flow rate varies inversely as the

square root of the absolute flow temperature:

Ftf=[520/Tf]0.5

7) Specific Gravity factor, Fg:

The basic orifice factor ,Fb, is determined assuming a

gas gravity of 1.0 .So, a correction for gas gravity is

required, as follows:

Fg=1/g0.5

8) Supercompressibility Factor F :


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8) Supercompressibility Factor , Fpv:

This factor corrects for the deviation of an actual gas

from ideal-gas behavior . It is calculated as follows:

Fpv=Zb/Z0.5

Zb assumed to be equal to 1.0

Z at operating conditions

Due to variations in gas compressibility factors with

gas composition, pressure and temperature, Fpv isdetermined experimentally or through empirical

techniques.

Specific gravity method

Heating value method

1) S ifi it th d


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1) Specific gravity method:

This uses the specific gravity and the carbon

dioxide(CO2)and nitrogen (N2) contents of the gas to

calculate the pressure and temperature adjustmentindices, fpgand ftgrespectively, as follows:

fpg = g- 13.8yC+5.420yNftg= g- 0.472yC0.793yN

g=specific gravity of the gas (air=1)yC=mole fraction of the CO2in the gas

yN=mole fraction of N2in the gas


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These values are used to determine the pressure and

temperature correction factors from Tables 10-11a and

10-11b , that are added to the actual flowing pressure

and temperature of the gas, respectively. these correctedpressure and temperature values are used in Table 10-11

e to estimate the supercompressibilty factor, Fpv.

2) H ti V l M th d


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2) Heating Value Method:

The heating value method uses the specific gravity , total

heating value (Ht in Btu/scf), and the CO2content of the

gas to calculate the pressure and temperatureadjustment indices, fph and fthas follows:

fph=g-0.0005688Ht+3.690yC

fth=g-0.001814Ht+2.641yC


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These values are used to determine the pressure and

temperature correction factors for Tables 10-11c and 10-

11d, that are added to the actual flowing pressure andtemperature of the gas respectively. These corrected

pressure and temperature values are used in Table 10-

11e to estimate the super compressibility factor ,Fpv.


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9) Manometer Factor, Fm:

This factor is required only where mercury manometer is

used for measuring the differential pressure. It

compensates for the different heads of gas above the two

mercury columns of the manometer.

It is generally negligible and is totally ignored for

pressures below 500 psia. Table 10-12 gives this

correction factor as a function of gas gravity, flowingpressure, and ambient temperature.


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10) Gauge Location Factor, Fl:

The gauge location factor fl given in Table 10-13 is used

where orifice meters are installed at locations other than

sea level elevation and 45o latitude. This is also a verysmall correction.


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11) Orifice Thermal Expansion factor, Fa:

This factor accounts for the expansion or contraction of

the orifice hole with flowing temperature, calculated as

follows:

Fa=1+[0.0000185(Tf-528)] for stainless steel

Fa=1+[0.0000159(T

f-528)] for monel

Tf= gas flowing temperature at the orifice,oR


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ORIFICEMETERSELECTION

Several factors need to be considered in choosing anorifice metering system:

Flow rate : flow rate uniformity, maximum and minimumflow rates expected.

Pressure: expected static and differential pressures and

their range ; permissible pressure variations. the size of the orifice affects the range of flow rates that

can be measured and the pressure differential that will beobtained.

A well designed metering system can only be achieved ifall these factors are carefully considered in choosing thesize and type of orifice and the pressure measuringdevices.


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Question: An orifice meter with a 2-inch orifice

equipped with pipe taps using upstream static

connections in a 6-in nominal(6.065-in. internal

diameter) pipe line shows an average differential

head=60in.water and an average upstream static

pressure=90psia. The flowing temperature is50oF and the gas gravity is 0.65. Using a base

pressure of 14.9 psia and base temperature of

50oF, calculate the gas flow rate indicated by the

meter.


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FACTORSAFFECTINGORIFICEMETER

ACCURACY

Following are the sources of constant errors:

1. Incorrect estimate of orifice size

2. Convex or concave contouring of the orifice plate

3. Thick or dull orifice edge

4. Eccentricity of orifice with respect to the pipe

5. Incorrect estimate of pipe diameter

6. Excessive recess between the end of pipe and

the face of the orifice plate7. Excessive pipe roughness.

1 Flow disturbances


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1. Flow disturbances

2. Imprecise location of the pressure taps

3. Pulsating flow

4. Buildup of solids

5. Liquid accumulation

6. Differences or changes in operating conditions

7. Incorrect zero adjustment

8. Non-uniform calibration

9. Corrosion or deposits in the metal internals

10. Emulsification of liquids with mercury

11. Leakage around the orifice plate12. Formation of hydrates in meter piping

13. Over dampening of the meter response


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COMMONMEASUREMENTPROBLEMS

Some of the common measurement problemsencountered in gas metering are:

Hydrate formation

Pulsating flow

Slugging

Sour gas

H


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HYDRATEFORMATION

Hydrates may be formed at the orifice ,or in the meter-

piping or internals, whenever the gas temperature fallsbelow the hydrate-forming temperature for the gas.

Hydrate formation can be prevented using any of the

following :

Gas dehydration Use of hydrate inhibitors

Installation of heaters along the line or near the meter

Other methods


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PULSATINGFLOW

Pulsating flow is flow comprising suddenchanges in pressure and flow rate of the flowing

fluid.

Common sources of such flow in gas

measurement are:Reciprocating systems-compressors ,or engines

Improperly sized, loose, or worn valves and

regulators Two-phase flow conditions

Intermitters on wells and automatic drips.

Prevention:


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Prevention:

1. Locate the meter along the flow line in a position where

pulsations are minimized.

2. Reduce the amplitude of the pulsations by placing avolume capacity ,flow restriction, or specially designed

filter between the pulsation source and the meter.

3. Operate at pressure differentials as high as possible by

using a smaller diameter orifice.


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SLUGGING

1. Slugging refers to the accumulation of liquids inthe gas flow line.

2. Liquid accumulation

3. Liquid is swept through to the orifice and beyond

4. Prevention is the installation of liquid

accumulators in the flow line.


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SOURGAS

Sour gas is detrimental for two reasons:

1. Corrosion

2. Accelerated hydrate formation

Preventive measures are:

1. To ensure proper gas metering include using H2S

resistant components in the meters.

2. Sealing the meters against the atmosphere.

gas flow measurement

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