Gas Flow Measurement
Transcript of 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.
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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.