22
GOOD PRACTICE GUIDE AN INTRODUCTION TO DIFFERENTIAL-PRESSURE FLOW METERS www.tuvnel.com

An Introduction to Differential-Pressure Flow Meters

Embed Size (px)

Citation preview

Page 1: An Introduction to Differential-Pressure Flow Meters

GOOD PRACTICE GUIDE

AN INTRODUCTION TO DIFFERENTIAL-PRESSURE FLOW METERS

www.tuvnel.com

Page 2: An Introduction to Differential-Pressure Flow Meters

Introductory guide to differential-pressure flow meters

Contents

Foreword 2

1 What is a differential-pressure (Δp) meter? 3

2 What are the different types of Δp meter? 3

3 Advantages and disadvantages of Δp meters 3

4 How a Δp flow meters works 4

5 Common terminology 5

5.1 Beta (β) 5

5.2 Discharge coefficient (C) 5

5.3 Turndown of a Δp meter 6

6 Calculating the mass flowrate using Δp devices 6

7 Different types of commonly used Δp meters 7

7.1 Orifice plates 7

7.1.1 Effect of using different values of beta 9

7.2 Venturi tubes 9

7.3 Cone meters 11

7.4 Flow nozzles 12

7.5 Variable area meters 13

7.6 Averaging pitots 14

8 Choosing a Δp meter 15

8.1 Accuracy 15

9 Common issues 15

9.1 Static hole error 15

9.2 Calibration issues 16

9.3 Wear and tear 16

9.4 Incorrect installation 16

9.5 Maintenance 16

10 Standards 17

11 Summary 17

12 Learning more…… 19

13 References 19

14 Glossary 19

This introductory guide to differential-pressure (Δp) flow meters provides information on the common types of meters available and their main advantages and disadvantages. Fundamental background information to understand how the meters work and some common issues encountered when using Δp meters has been included.

1 2

Page 3: An Introduction to Differential-Pressure Flow Meters

Foreword

This introductory guide to differential-pressure (Δp) flow meters has been produced for people who are relatively

inexperienced in using this type of meter or who would just like to learn more about the subject. The guide provides useful

information on the maintenance of existing meters and for purchasing a new metering system.

The introduction covers the basic theory of how differential-pressure flow meters work and fundamental background

information. Some of the most common types of meters are described together with their advantages and disadvantages.

This includes orifice plates, Venturi tubes, cone meters, nozzles, variable area meters and averaging pitots. Practical

information and guidance are provided on the use of differential-pressure meters including important considerations for

the selection and ongoing maintenance of a suitable meter. A useful table on the advantages and disadvantages of a more

extensive range of differential-pressure meters is provided in the summary.

By reading this guide you will not become an expert in differential-pressure meters, but it will give you some useful and

practical information on their use. This should enable readers to save valuable time and money, as the measurement of

flow can be a costly business if mistakes are made from lack of experience.

2

Page 4: An Introduction to Differential-Pressure Flow Meters

1 What is a differential-pressure (Δp) meter?

Differential-pressure meters work on the principle of partially obstructing the flow in a pipe. This creates a difference in the

static pressure between the upstream and downstream side of the device. This difference in the static pressure (referred to

as the differential pressure) is measured and used to determine the flowrate.

Differential-pressure meters are hugely popular and it is estimated that at least 40% of industrial flow meters in use at

present are differential-pressure devices, with the orifice plate being the most popular. Differential-pressure devices have

been used to meter a wide variety of different fluids from gases to highly viscous liquids.

The popularity of differential-pressure flow meters is in part due to their simple design and low cost. By reading this guide

you will have a much clearer idea of the benefits, viable metering options and applications for using differential-pressure

meters.

2 What are the different types of Δp meter?

The most common types of differential pressure meter are:

• Orificeplates

• Venturitubes

• Conemeters(e.g.V-cones)

• Nozzles

• Lowlossmeters(e.g.Dalltubes)

• Variableareameters

• Inletflowmeters

• Venturicones

• Venturinozzles

• Dragplates

Several of these meters are discussed in more depth in later sections and there is a summary table at the end of the guide.

3 Advantages and disadvantages of Δp meters

There are a number of general advantages common to most Δp meters. These include:

• Theyaresimpletomake,containingnomovingparts

• Theirperformanceiswellunderstood

• Theyarecheap–especiallyinlargerpipeswhencomparedwithothermeters

• Theycanbeusedinanyorientation

• Theycanbeusedformostgasesandliquids

• Sometypesdonotrequirecalibrationforcertainapplications

Good Practice Guide

3

Page 5: An Introduction to Differential-Pressure Flow Meters

3 Advantages and disadvantages of Δp meters (cont.)

The main disadvantages to Δp meters are:

• Rangeability(turndown1) is less than for most other types of flowmeter

• Significantpressurelossesmayoccur

• Theoutputsignalisnon-linearwithflow

• Thedischargecoefficientandaccuracymaybeaffectedbypipelayoutornatureofflow

• Theymaysufferfromageingeffects,e.g.thebuild-upofdepositsorerosionofsharpedges

4 How a Δp flow meters works

The concept of using the pressure drop caused by a fluid flowing through a restriction in a pipe as a measurement of

flowrate dates back to the 18th Century, when it was discovered by Bernoulli.

The basic principle of how a Δp flowmeter operates is described in the figure below.

The differential pressure principle. Manometer tubes measure the difference in static pressure upstream and

downstream of the restriction

When a fluid flows through a restriction, it accelerates to a higher velocity (i.e. V2 > V1) to conserve the mass flow and, as a

consequence of this, its static pressure drops. This differential pressure (Δp) is then a measure of the flowrate through the

device.

• Insimpletermsforagivensizeofrestriction,thehighertheΔp, the higher the flowrate.

The relationship between the differential pressure and flowrate is derived from Bernoulli’s equation. Using Bernoulli’s

equation, and conservation of mass2 , it can be shown that the differential pressure generated is proportional to the square

of the mass flowrate, Qm (kg/s).

(1)

Many of the Δp meters available work on this principle of measuring the difference in pressure between upstream and

downstream but there are some meters which use the differential pressure in other ways, for example, variable area meters.

This type of meter is explained in Section 7.5.

Introductory guide to differential-pressure flow meters

1 The turndown of a meter is the ratio of the maximum-to-minimum flowrate that can be accurately measured. Ideally a large turndown ratio is desirable to measure a wider range of flowrates.2 Conservation of mass means that the mass of fluid flowing through the pipe must be the same mass of fluid flowing through the restriction.

A1 V1 V2

Manometer tubes

Qm=pA1V1

4

Page 6: An Introduction to Differential-Pressure Flow Meters

5 Common terminology

5.1 Beta (β)

The diameter ratio or beta (sometimes referred to as the beta ratio) is the ratio between the diameter of the orifice or

throat of device to that of the pipe.

(2)

Often Δp meters are described in terms of their beta value and diameter to fit a certain pipeline size, for example, a 4-inch

β= 0.6 Venturi tube.

To state that a Δp meter has a low beta ratio, for example β = 0.2, means the plate has a small hole or restriction size. This

causes the pressure loss across the Δp meter to be higher, which may mean that a pump with a higher discharge pressure

(hence more expensive) or compressor will be needed to overcome the increased pressure loss and maintain a flowrate

achievable with a larger beta Δp meter. On the other hand a higher differential pressure can generally be measured more

accurately than a lower one.

5.2 Discharge coefficient (C)

The discharge coefficient, C, is a parameter that takes account of non-ideal effects, for example energy losses due to

friction, when using Δp meters. The discharge coefficient is basically the ratio of the actual to the measured mass flowrate.

The discharge coefficient can either be:

• determinedfromastandard

provides good flow measurement at a reasonable price

is especially suitable where repeatability is more important than accuracy

or

• determinedbycalibration providesloweruncertaintiesontheflowmeasurement.

In nozzles and Venturi tubes the flow follows the boundary of the tube closely and the value of C is usually close to one.

However, for orifice plates C has a value of approximately 0.6. Values of C can be obtained from the standard (ISO 5167)

for nozzles, Venturi tubes and orifice plates that are manufactured to the specified tolerances of the standard.

Good Practice Guide

5

Page 7: An Introduction to Differential-Pressure Flow Meters

5.3 Turndown of a Δp meter

The turndown of a meter is the ratio of the maximum to the minimum flowrate that can be accurately measured. Ideally a

large a turndown ratio is desirable to measure a wide range of flowrates.

Square relationship between flowrate and Δp:• Iftheflowrateis50%ofthefullscale,thentheΔp is at 25% of the full Δp scale

• Iftheflowrateis25%ofthefullscale,thentheΔp is at 6.25% of the full Δp scale

The following graph illustrates the square relationship between the differential pressure and the flowrate.

Turndown of a differential pressure meter

This shows that a turndown of 10:1 on flowrate will require a 100:1 turndown on pressure measurement (this is provided

the density is approximately constant e.g. for a liquid). As it is very difficult to obtain accurate measurements over such a

large range of Δp values from a simple transmitter, this means that the typical turndown of a Δp meter is actually limited to

approximately 4:1 to obtain an accurate measurement of flowrate.

At low values of the flowrate the Δp transmitter uncertainty increases significantly. For example, a Δp cell can typically

offer an uncertainty of 0.2% of Full Scale (FS). This means that, at 1% FS, the uncertainty of the differential pressure would

be 20%. Therefore at low flowrates it becomes much more difficult to measure the Δp.

The turndown of the meter can be extended to around 10:1 if multiple range transducers are used. For example, one Δp transducercouldberangedfor1–10%FSandanotherovertherangeof10–100%FS.

6 Calculating the mass flowrate using Δp devices

Once a value for the Δp has been obtained, the mass flow can be calculated using the following formula.

For liquids the mass flow is given by:

(3)

Introductory guide to differential-pressure flow meters

Δp

(mba

r)

Flowrate (kg/s)

6

Page 8: An Introduction to Differential-Pressure Flow Meters

6 Calculating the mass flowrate using Δp devices (cont.)

Where:

• Cisthedischargecoefficient

• At is the throat area (restriction)

• Δp is the differential pressure

• is the density of the fluid

(4) and (2)

• disthediameterofthethroat

• Disthepipediameter

Owing to the compressibility of gases an additional parameter called the expansibility factor, ε, is used within the mass flow

equation to account for the gas density changing as the pressure drops at the restriction.

For gases the mass flow is given by:

(5)

7 Different types of commonly used Δp meters

7.1 Orifice plates

Orifice plates are the most common type of Δp meter and are basically a machined metal plate with a hole, as shown

below. The plate has a sharp upstream edge and usually a bevelled edge downstream of the flow.

Upstream face Downstream face

Good Practice Guide

7

Page 9: An Introduction to Differential-Pressure Flow Meters

7.1 Orifice plates (cont.)

The diagram below illustrates an orifice plate installed in a pipe. To allow the differential pressure to be measured, a set of

pressure tappings are located upstream and downstream of the plate.

When fluid passes through the hole of an orifice plate the pressure drops suddenly. The flow continues to contract and

converges downstream of the plate with the point of maximum convergence (or minimum area) called the vena contracta

(see diagram below). The fluid then expands and re-attaches to the pipe wall, and the velocity profile approaches that

before the constriction. There is a relatively large net pressure loss across the orifice plate which is not recovered; this

should be taken into account in choosing a meter as orifice plates are not suitable for applications where a large pressure

drop is undesirable.

Care should be taken in the installation of orifice plates as there have been cases where they have been installed the wrong

way round. This can cause significant measurement errors and has led to expensive recalculations of the actual flowrate.

The face with the sharp edge must face the upstream flow as the sharp edge is needed to force the flow to detach from

the plate and allow the flow to contract downstream of the plate and form the vena contracta.

Introductory guide to differential-pressure flow meters

8

Page 10: An Introduction to Differential-Pressure Flow Meters

7.1 Orifice plates (cont.)

Orifice plates should be checked regularly to ensure that the edge is sharp and that no contamination is deposited on the

front of the plate as a round edge or contamination will affect the flow measurement.

Orificeplatesareverysensitivetothevelocityprofileoftheflow–ifthevelocityprofileisasymmetricalorskewedthis

affects the flow measurement. There are specified requirements for using orifice plates which are detailed in the standard

(ISO 5167-2) for their use in dry gas and liquids.

7.1.1 Effect of using different values of beta

The effect of using larger values of beta include:

• anincreaseinthedischargecoefficientuncertainty

• alowerdifferentialpressurebeingmeasuredacrosstheorificeplate(andthiscanbemoredifficulttomeasure)

• longerlengthsofupstreamstraightpipebeingrequiredtoensurethevelocityprofileoftheflowthroughthe

orifice plate is stable and symmetrical

• theflowprofileoftheflowthroughtheorificebeingmoreaffectedbytheroughnessofthepipewalls

There are a number of sizing packages for orifice plates available, which will calculate the dimensions of the plate required.

The software uses empirical formulae based on actual testing. Most of the results are available for beta values of 0.3 to 0.7.

Advantages:

• Lowcost

• Easeofinstallation

• Availabilityofcomprehensivestandard(ISO5167-2)

• Norequirementforcalibration-valueofCfromthestandard

• Availabilityofdifferentdesigns,e.g.forviscousfluids,bi-directionalflows,suspendedsolids

Disadvantages:

• Lowturndown(canbeimprovedwithdualrangeΔp cells)

• Highpressureloss(35toalmost100%ofmeasuredΔp depending on beta)

• Errorsduetoerosion/damagetoupstreamedges

• Errorsduetohighsensitivitytoupstreaminstallation(especiallylargebetadevices)

7.2 Venturi tubes

Venturi tubes are used extensively in industry: the design of a classical Venturi tube is shown below. This type of meter has

a gradual reduction in the pipe area, a parallel throat section and then a gradual expansion back to the full pipe diameter.

The long expansion section (diffuser) enables an enhanced pressure recovery compared with that of an orifice plate, which

is useful in some metering applications.

Good Practice Guide

9

Page 11: An Introduction to Differential-Pressure Flow Meters

7.2 Venturi tubes (cont.)

The differential pressure is measured from the upstream tapping to the throat section, shown by the high-pressure and low-

pressure connections.

Venturi meters are must less susceptible to damage than orifice plates owing to their robust and solid design. It should be

noted that the edges of the pressure tappings need to be sharp and there should be no rough edges, particularly on the

throat tappings, as this can affect the flow measurement.

Venturi meters have a much lower pressure loss across the meter (approximately 10% of Δp); this means less energy is lost

compared with that across an orifice plate with identical beta and pipe diameter. They are also less sensitive to installation

effects than orifice plates and more suitable for gas flows with entrained liquid, as owing to their design they do not

dam the flow. Venturi tubes are more expensive than orifice plates owing to the increased machining necessary for their

manufacture.

Venturi tubes are covered by a comprehensive standard (ISO 5167) and a value for C from the standard can be used. In

reality many Venturi tubes are calibrated to determine a value for C for applications where higher measurement accuracy is

required especially in gas when the uncertainty of an uncalibrated Venturi tube is high.

Advantages:

• Lowpressuredrop(around10%ofΔp)• Lowersensitivitytoinstallationeffectsthanorificeplates

• Lesssusceptibilitytodamage

• Moresuitableforgasflowswithentrainedliquid

• Comprehensivestandards(ISO5167)

Disadvantages:

• Lowturndown(canbeimprovedwithdualrangeΔp cells)

• Greatercosttomanufacture

• Greatersusceptibilityto“tappingerrors”inhighReynoldsnumbergasflowsowingtothehighvelocityfluid

passing the pressure tapping at the throat.

• Lessexperimentaldatathanorificeplates

Introductory guide to differential-pressure flow meters

10

Page 12: An Introduction to Differential-Pressure Flow Meters

Dow nst ream Tap

Dow nst ream Tap

7.3 Cone meters

Cone meters (e.g. V-cones) are proprietary meters and are essentially an inverted Venturi tube. Instead of a contraction in

the pipe, the fluid flows around a central cone as shown in the following diagram.

V-cone meter

Various designs are available and the downstream tapping can either be located in the base of the cone or machined

through the wall of the meter body at the widest part of the cone, as shown below. The upstream pressure tapping is

located before the cone.

V-Cone Δp meter (a) flanged design (b) wafer design (www.mccrometer.com).

Cone meters have proved popular as it is claimed they require very little upstream straight pipework before the meter

to provide accurate measurements. This benefit is due to the fluid flowing around the cone which is described as

“conditioning”theflow.Oneofthedownsidesofconemetersisthelackofstandardsgoverningthistypeofmeter,as

they have been a proprietary device, and there has been a lack of independent data available to provide confidence in

claimed performance. Unlike Venturi tubes, orifice plates and nozzles, which are manufactured to tolerances specified in

ISO 5167, cone meters are not manufactured to a specified tolerance and must be individually calibrated before use.

Good Practice Guide

10 11

Page 13: An Introduction to Differential-Pressure Flow Meters

7.3 Cone meters (cont.)

Advantages:

• LowersensitivitythanorificeplatesandVenturitubestoinstallationeffects

• Shorterinstallationlengths

• Lesspressurelossthanorificeplates

• Wafer(betweenflange)versions

• Effectiveforwetgasflowmeasurementapplications

Disadvantages:

• Lackofstandards

• Notasmuchdataavailableas“ISO5167”meters

• PressurelosshigherthanVenturitubes

7.4 Flow nozzles

Flow nozzles are mainly used in the electrical power generation industry. They have a curved entry and a cylindrical throat,

but no divergent outlet section. Therefore, the discharge coefficient is similar to that of a Venturi tube, but the overall

pressure loss is similar to that of an orifice plate of comparable size used at an equivalent flowrate and pressure difference.

The diagram below shows some examples of nozzles (a Low and high β long-radius nozzles (b) ISA 1932 nozzle).

In order to reduce the pressure loss caused by a nozzle, it can be fitted with a divergent section similar to that used for a

Venturi, hence becoming a Venturi nozzle, see (c) below.

One advantage of a nozzle over an orifice plate is that there is no sharp edge to erode, but they are more expensive to

manufacture and are generally more difficult to install or remove from the pipe for maintenance purposes.

Introductory guide to differential-pressure flow meters

(a) Low -

(a) High -

(b)

(c)

12

Page 14: An Introduction to Differential-Pressure Flow Meters

7.5 Variable area meters

The meters discussed so far have all had flow going through a fixed area; hence the differential pressure varies with the

flowrate and Δp is measured to determine the flowrate.

Variable area meters operate at a constant Δp and the area changes with the flowrate. The area will increase as the

flowrate through the meter increases to preserve a constant Δp.

The most common design of variable area meter is the cone-and-float type, which is also known as a rotameter.

The basic design of a variable area meter is a tapered tube (usually glass) containing a self-centring float that is pushed

up by the flow and pulled down by gravity. At higher flowrates the float rises to increase the area between the tube and

the float and maintain a constant Δp. The flowrate is determined from how far the float has risen up the tube: there are

graduations on the side of the tube.

Variable area meters are widely used for metering gas but different types are available for a variety of different fluids. A

buoyancy correction term is required for liquids and dense fluids.

Advantages:

• Lowcost

• Versionsavailableformostfluids

• Instantaneousvisualindicationofflowchanges

• Lowsensitivitytoinstallationeffects.

Disadvantages:

• Lowaccuracy–uncertaintyonvolumetricflowrateis~2%ofreading

• Generallysmallturndown

• Tendencyoffloatto‘stick’atlowflows

• Requirementforbuoyancycorrectioninliquids

(www.alicatscientific.com) (www.coleparmer.com)

Good Practice Guide

12 13

Page 15: An Introduction to Differential-Pressure Flow Meters

7.6 Averaging pitots

AveragingpitotsaresometimesreferredtoasAnnubarsandcontainmultiplepressuretappingsto‘average’theflow;thisis

to try to compensate for a non-ideal flow profile.

The averaging pitot tube is inserted across the pipe as show below. One side of the bar has pressure taps facing the

flowingfluidthatarecoupledintoan“averaging”chamberthatmeasuresthetotal(i.e.static+dynamic)pressureofthe

fluid.

There may be a single port or multiple tapping ports on the opposite side of the bar to measure the low static pressure in

the downstream region. The difference between the total and static pressures is effectively a measure of the fluid velocity

head, which together with the pipe area enables the volumetric flowrate to be determined.

Advantages:

• Lowpressuredrop

• Lowcost

• Easeofinstallationinexistinglocations

• Possibilityofinstallationinlivelines(“hottapping”)

Disadvantages:

• Loweraccuracy

• Unsuitablefordirtyflow–asportscangetblocked

Introductory guide to differential-pressure flow meters

14

Page 16: An Introduction to Differential-Pressure Flow Meters

8 Choosing a Δp meter

Selecting the correct meter for a particular application and ensuring the metering system, including the pipework,

is correctly designed is vital for the success of any metering system. In principle this sounds easy but is not always

straightforward.

8.1 Accuracy

In choosing a flow meter, there are many factors to consider, and among them the question of accuracy is often very

important. While it is pointless to pay for a more accurate meter than is necessary, a cheap meter that has a lower accuracy

may in the long term become an expensive option.

For example, if you are actually selling more product than you get paid for or in shared pipeline allocation systems for

oil and gas supplies where one operator may be getting financially undercut in receiving less revenue than was actually

produced. Retrofitting or replacing a meter into existing pipework can be expensive especially if a process or system must

be shutdown to install the meter.

Once you have chosen a suitable meter it may need to be calibrated to reduce the measurement uncertainty and it will

need to be installed correctly to achieve its potential accuracy.

Example: If a typical orifice plate is designed and manufactured according to a recognised standard, it is reasonable

to expect that the flowrate uncertainty will be approximately 1% (or less) when the system is flowing at the maximum

flowrate under ideal conditions.

• However,theflowmeasurementuncertaintycouldincreasebyasmuchas4%becauseofinstallationeffects,

for example, inadequate upstream and downstream straight pipe lengths.

By calibrating a differential-pressure meter an uncertainty of less than 0.5% should be obtainable.

9 Common issues

There a number of issues that can occur with the use of certain types of Δp meters. Some of these are discussed below.

9.1 Static hole error

Venturi tubes when used for metering gases at high Reynolds numbers can have a discharge coefficient greater than one,

which is surprising as it would be natural to expect the value would be less than one [2]. This is due to static hole error

caused by high velocity fluid passing the throat tapping, which causes the measured pressure to be higher than the true

pressure. Static hole error can also occur in cone meters when the downstream (or low pressure tapping) is located on the

meter wall at the widest part of the cone, owing to the increased velocity of the fluid at this position. The problem does

not occur in cone meters where the tapping is located on base of the cone.

Good Practice Guide

14 15

Page 17: An Introduction to Differential-Pressure Flow Meters

9.2 Calibration issues

When Venturi tubes are calibrated in gases humps in the calibration curve may be observed. This is the result of resonance

effects in the tappings and is dependent on the velocity of the fluid. Calibration data giving the discharge coefficient are

usually plotted as a function of the Reynolds number. However, if the gas density (or gas pressure) changes but the velocity

of the gas remains constant, then the humps in the calibration graph shift as the Reynolds number has changed (Reynolds

number is proportional to the gas density). This should be taken into account if a function is fitted to a calibration graph.

It is recommended to perform a calibration of Venturi tubes in gas at two different pressures to account for this effect.

9.3 Wear and tear

Having obtained a suitable meter and calibrated it, one might suppose that to be the end of the matter. This is not so.

As time goes by gradual changes may occur which eventually cause significant errors to be introduced unless remedial

measures are taken. The sharp edge of an orifice plate may be eroded away causing the discharge coefficient to rise.

Orifice plates should ideally be checked regularly. Film growth may occur on the throat of a nozzle or a Venturi tube

resulting in a lower discharge coefficient.

Unfortunately, no general rules for the rate of deterioration are available; each installation must be assessed according to

its own set of circumstances, and renewals or recalibrations made as appropriate. The vital thing is to be aware of the

dangers.

9.4 Incorrect installation

There have been many reports of orifice plate meters not functioning properly. When the meter was checked it was found

the orifice plate had gone! This could have been due to corrosion, erosion, or possibly even failure to reinstall it after

maintenance. It may sound obvious but check the meter is where it is supposed to be! Orifice plates have been known to

be installed the wrong way round.

Always check with the meter manufacturer to obtain the correct installation requirements for the meter and operating

instructions.

9.5 Maintenance

If the flow conditions change then this may be outside the specified operating envelope in which the meter is designed to

work and to provide accurate measurements. If the flowrate changes the Δp transducer can become saturated or not be

sensitive enough to measure the Δp.

It has been known for Δp transducers to stop working or become saturated and for this output value to be used to

calculate the flow measurement for a long time before the problem was identified.

As with any piece of plant machinery or instrumentation, it is important that the meter is properly looked after. It may

require routine maintenance or recalibration to ensure accurate and/or consistent flow measurement.

Introductory guide to differential-pressure flow meters

16

Page 18: An Introduction to Differential-Pressure Flow Meters

9.5 Maintenance (cont.)

In some applications the accuracy of the meter may not be important but the repeatability of the meter could be vital to

detect and monitor changes in the flowrate, for example, to ensure consistency in products.

10 Standards

The most significant standard for differential-pressure meters, ISO 5167, has recently been revised in four parts [1].

• Part1coversgeneralprinciplesandrequirements

• Parts2to4coverorificeplates,nozzlesandVenturitubes,respectively

These documents provide information on how the flow in full pipes should be measured to ensure measurement

consistency across industries. It includes information required to meter-single phase flows of liquid and gas, such as

equations to calculate the flow, tolerances for the manufacture of the meter, uncertainty of the flow measurement and

installation requirements etc.

Additional standards are available which cover metering applications outside the scope of ISO 5167 and guidance for the

use of ISO 5167. These include:

• Measurementoffluidflowbymeansofpressure-differentialdevices-Guidelinesforspecificationofnozzlesand

orifice plates beyond the scope of ISO 5167. ISO/TR 15377.

• Measurementoffluidflowbymeansofpressuredifferentialdevices-Guidelinesontheeffectofdeparturefrom

the specifications and operating conditions given in ISO 5167. ISO/TR 12767.

• GuidelinesfortheuseofISO5167.ISO/TR9464.

11 Summary

Differential-pressure meters remain the most common meters in use worldwide. They are simple to make, normally

without moving parts, and well understood. They have generally accepted standards based on years of research, and the

standards have been revised in the light of the latest research. They have the advantage that most differential-pressure

meters, especially the orifice plate, the most common meter, can generally be used without flow calibration.

However, most differential-pressure meters have an output which is non-linear with flow; they are affected by upstream

installations and ageing, and they may cause significant irrecoverable pressure loss.

As with any meter care should be given to the installation and maintenance of the meter to ensure accurate measurements.

The advantages and disadvantages of the common types of differential pressure meters are summarised in the table

overleaf.

Good Practice Guide

16 17

Page 19: An Introduction to Differential-Pressure Flow Meters

11 Summary (cont.)

Relative advantages and disadvantages of Δp meters [3]

Meter Type Relative Advantages Relative Disadvantages

Orifice Plate Cheap.

Easy to install.

Nomovingparts.

Comprehensive guidance (ISO 5167).

Can be installed uncalibrated.

Fiscal uncertainty.

Low turndown (typically <4:1).

High pressure loss

Susceptible to erosion and/or damage from

constituents of the fluid.

Very sensitive to upstream installation.

Venturi Tube Moderately low pressure drop (about 10% of Δp for long Venturis).

Nomovingparts.

Upstream straight length requirement much

shorter than for orifice plates.

Less susceptible to wear.

Suitable for wet gas flow.

Fiscal uncertainty.

Low turndown (typically < 4:1).

High initial material cost.

Susceptibleto“tapping”errorsinhigh-Regas

flows.

Dall Tube Lowest pressure drop of traditional Δp devices.

3 to 4 times shorter than Venturi tubes.

Notwidelyused.

Suitable for clean fluids only.

Nozzle Ease of installation.

Covered by ISO 5167.

High pressure loss - similar to orifice plates.

Uncommon in oil / gas applications.

V-Cone Short installation lengths.

Wafer designs easy to install.

Suitable for wet gas flow.

Alternativetappositionscanreduce“tapping”

errors in high-Re gas flows

Notincludedin

ISO 5167.

Averaging Pitot Easy to install.

Self averaging; compensates for non-ideal

velocity profiles.

Almost no pressure loss in moderate line sizes.

Requires purge system in dirty gas.

Subject to vibration problems.

Inlet Meter Utilises existing inlet sections.

Noupstreamlengthrequired.

Some designs standardised.

Low pressure drop.

Application limited to fan inlets or flows from

liquid reservoirs.

Variable Area Cheap.

10:1 turndown.

Linear output.

Noupstreamlengthrequirement.

Line pressure limited.

Moderately high uncertainty.

Target Meter Relatively cheap.

Electronic, linear output.

Medium turndown of 15:1.

Limited installed base.

Plate subject to damage.

High uncertainty (point measurement).

Introductory guide to differential-pressure flow meters

18

Page 20: An Introduction to Differential-Pressure Flow Meters

12 Learning more……

MoreinformationcanbeobtainedfromtheTUVNELwebsite(www.tuvnel.com),whichcontainsmanydocumentsonΔp flow metering.

13 References

[1] INTERNATIONALORGANIZATIONFORSTANDARDIZATION.Measurementoffluidflowbymeansofpressure

differential devices inserted in circular-cross section conduits running full. ISO 5167-4:2003. Geneva: International

Organization for Standardization, 2003

[2] Reader-Harris,M.J.,Brunton,W.C.,Gibson,J.J.,Hodges,D.,Nicholson,I.G.DischargecoefficientsofVenturi

tubeswithstandardandnon-standardconvergentangles.FlowMeasurementandInstrumentation,Vol.12,No2,pp135-

145, April 2001.

[3] Principlesandpracticeofflowmeasurement.Trainingcoursenotes.TUVNEL,EastKilbride,UK.

14 Glossary

This glossary contains some of the commonly used words which are explained in a bit more detail. The descriptions here are

not meant as thorough explanations, but as a handy tool to make reading this guide easier.

Accuracy - Qualitative term used to compare an indicated value with the true value

Uncertainty - Parameter associated with the result of a measurement that characterises the dispersion of the values that

could reasonably be attributed to the measurement

Error - The difference between the indicated value and the true value

Beta - The ratio of the diameter of the restriction (referred to as the orifice or throat) to the pipe diameter

Discharge coefficient - The ratio of the actual to the measured mass flowrate.

Reynolds number - Ratio of inertial forces to viscous forces.

Vena contracta - The point downstream of a constriction in flow where the area of the fluid stream is smallest

Pressure tapping - Small perpendicular holes in the wall of a pressurized, fluid-containing pipe or vessel used for

connection of pressure-sensitive elements for the measurement of static pressures

Orifice plate - Plate with a small machined hole, which can be used to measure flow by measuring a difference in pressure

Venturi tube - Short pipe with a constricted inner surface, used to measure flowrate

Good Practice Guide

18 19

Page 21: An Introduction to Differential-Pressure Flow Meters

14 Glossary cont.

Cone meter - A cone around which fluid flows, which is used to measure flowrate. In essence an inverted Venturi tube

Static hole error - The difference between the static pressure measured using a pressure tapping and the static pressure in

the absence of a pressure tapping and the reason why the discharge coefficient can sometimes be found to be greater than 1.

Turndown ratio - The range a flow meter is able to measure with acceptable accuracy.

Introductory guide to differential-pressure flow meters

For further information, contact:

TUVNEL,EastKilbride,GLASGOW,G750QF,UK

Tel:+44(0)1355220222Email:[email protected]

Page 22: An Introduction to Differential-Pressure Flow Meters