Vortex and Fluidic Flowmeters

A. Vortex B. Fluidic-shedding Coanda effect C. Vortex precession (Swirlmeter™) Services A. Gas, steam, reasonably clean liquids B. Gas, reasonably clean liquids C. Gas, steam, reasonably clean liquids Size Ranges Available A. 0.5 to 12 in. (13 to 300 mm), also probes B. 0.5 to 4 in. (13 to 100 mm); up to 12 in. (300 mm) in bypass versions C. 0.5 to 12 in. (13 to 300 mm) Detectable Flows A. Water, 2 to 10,000 GPM (8 l/min to 40 m3/hr); air, 3 to 12,000 SCFM (0.3 to 1100 SCMM); steam (D&S at 150 PSIG [10.4 bars]), 25 to 250,000 lbm/hr (11 to 113,600 kg/hr) B. Water, 0.033 to 1000 GPM (0.125 to 4000 l/min); fluids, to 80 cSt C. Water, 2 to 10,000 GPM (8 l/min to 40 m3/hr); air, 3 to 12,000 SCFM (0.3 to 1100 SCMM); steam (D&

2.30 Vortex and Fluidic Flowmeters

J G KOPP (1969) D J LOMAS (1982) B G LIPTÁK (1995)

W H BOYES (2003)

B Fluidic-shedding Coanda effect

C Vortex precession (Swirlmeter™)

B Gas, reasonably clean liquids

C Gas, steam, reasonably clean liquids

B 0.5 to 4 in (13 to 100 mm); up to 12 in (300 mm) in bypass versions

C 0.5 to 12 in (13 to 300 mm)

1100 SCMM); steam (D&S at 150 PSIG [10.4 bars]), 25 to 250,000 lbm/hr (11

to 113,600 kg/hr)

B Water, 0.033 to 1000 GPM (0.125 to 4000 l/min); fluids, to 80 cSt

C Water, 2 to 10,000 GPM (8 l/min to 40 m3/hr); air, 3 to 12,000 SCFM (0.3 to 1100 SCMM); steam (D&S at 150 PSIG [10.4 bars]), 25 to 250,000 l/hr (11 to 113,600 kg/hr)

Gas and steam, 20 to 262 ft/s (6 to 80 m/s)

sizes above 4 in.

B Re = 3000; some models claim Re = 400 at specified inaccuracy, with reading down to Re = 75.

C Same as A.

B 600 PSIG (41 bars) below 2 in (50 mm); 150 PSIG (10.3 bars) above 2 in.

C 2000 PSIG (138 bars)

B 0 to 250 ° F ( − 18 to 120 ° C)

C − 330 to 750 ° F ( − 201 to 400 ° C)

Materials of Construction A Mostly stainless steel, some in plastic

B Cast bronze, plastic, stainless, and some specialty metals

C Mostly stainless steel, specialty alloys available

B Reynolds number at maximum flow divided by minimum Re of 3000 (400 for some models)

C Reynolds number at maximum flow divided by minimum Re of 20,000 or more

FE

Flow Sheet Symbol

FI

2.30 Vortex and Fluidic Flowmeters 385

outputs; for analog outputs, add 0.1% of full scale

B 1 to 2% of actual flow for liquids, 1% of rate for gases claimed

C 0.5 to 1% of rate for liquids, 1 to 1.5% of rate for gases and steam with pulse outputs; for analog outputs, add 0.1% of full scale

Cost A Plastic and probe units cost between $250 and $1500; stainless steel units in small

sizes cost about $2500; insertion types cost about $3000

B Small versions for domestic water or heat metering cost between $50 and $125; larger versions including bypass meters cost between $300 and $1500

C Stainless-steel units in small sizes cost about $2500, specialty materials are extra

Partial List of Suppliers A Aaliant Div of Venture Measurement ( www.venturemeas.com )

ABB Instrumentation ( www.abb.com ) Asahi America ( www.asahi-america.com ) Bopp & Reuther (Heinrichs)

Daitron (Saginomiya) Delta Controls ( www.deltacontrols.com ) Eastech Badger ( www.eastechbadger.com ) EMCO ( www.emcoflow.com )

Endress+Hauser Inc ( www.endress.com ) The Foxboro Co ( www.foxboro.com )

GF Signet ( www.gfsignet.com ) Hangzhou Zhenhua Meter Factory Honeywell ( www.honeywell.com ) J-Tec Associates ( www.j-tecassociates.com ) Krohne America ( www.krohne.com ) Metron Technology ( www.metrontechnology.com ) Nano-Master ( www.nanomaster.com )

Rosemount (now Emerson Process Measurement) ( www.rosemount.com ) Sparling ( www.sparlinginstruments.com )

Spirax Sarco Inc ( www.spiraxsarco.com ) Tokyo Keiso ( www.tokyokeiso.co.jp/english/index-e.htm ) Vortek

Yamatake ( www.yamatake.co.jp ) Yokogawa ( www.yca.com ) Yuyao Yinhuan Flowmeter Instrument Co.

Zheijiang Tancy Instrument Co.

B Actaris Metering Systems (formerly Schlumberger) ( www.actaris.com ) Fluid Inventor AB ( www.fluidinventor.se )

Severn Trent Services (formerly Fusion Meter) ( www.severntrentservices.com ) Sontex BV ( www.sontex.com )

C ABB Instrumentation ( www.abb.com ) This section is devoted mainly to the vortex-shedding

flow-meter and its variations, including the earlier designs of

vortex-precession (swirl) meters and the recent combination designs

of vortex bypass elements around orifices Included in this

category of devices are oscillating fluidic flowmeters using

the Coanda effect

THE VORTEX SHEDDING PHENOMENON

It was Tódor von Kármán who discovered that, when an

obstruction (a nonstreamlined object) is placed in the path of

a flowing stream, the fluid is unable to remain attached to the

object on its downstream sides and will alternately separate

(shed) from one side and then the other The slow-moving

fluid in the boundary layer on the bluff body becomes detached on the downstream side and rolls into eddies and

flow velocity Stated in terms of a flag fluttering in the wind, what von Kármán discovered is that the intervals between

is only a function of the diameter of the flag pole (d) There-fore, the faster the wind, the faster the vortices are formed,

changing its wavelength.

Later, Strouhal determined that, as long as the Reynolds number of the flowing stream is between 20,000 and 7,000,000, the ratio between the shedder width (d) and the vortex interval

386 Flow Measurement

a detector that is sensitive enough to count the vortices and

flowing velocity of any substances as

2.30(1)

In building a flowmeter based on Kármán’s principle, the

one-quarter of the pipe diameter (ID) As long as the

obstruc-tion is not eroded or coated, as long as the pipe Reynolds

number is high enough to produce vortices, and as long as

the detector is sensitive enough to detect these vortices (for

gases such as hydrogen, the forces produced by the vortices

are very small), the result is a flowmeter that is sensitive to

flow velocity and insensitive to the nature of the flowing

media (liquid, gas, steam), the density, the viscosity, the

temperature, the pressure, and any other properties

THE DETECTOR

As a vortex is shed from one side of the bluff body, the fluid

velocity on that side increases, and the pressure decreases

On the opposite side, the velocity decreases, and the pressure

increases, thus causing a net pressure change across the bluff

body The entire effect is then reversed as the next vortex is

shed from the opposite side Consequently, the velocity and

pressure distribution adjacent to the bluff body change at the

same frequency as the vortex shedding frequency changes

Various detectors can be used to measure one of the

following:

1 The oscillating flow across the face of the bluff body

2 The oscillating pressure difference across the sides of

the bluff body

3 A flow through a passage drilled through the bluff

body

4 The oscillating flow or pressure at the rear of the bluff

body

5 The presence of free vortices in the downstream to the

bluff body

A flow-sensitive detector can be either a heated ther-mistor element or a spherical magnetic shuttle (with the movement of the shuttle measured inductively) Detectors that are sensitive to pressure use metal diaphragms or vanes Pressure exerted on diaphragms can be converted into a vari-able capacitance or a varivari-able strain on a piezoresistive, piezoelectric, or inductive sensor Pressure exerted on vanes can similarly be converted into an electrical signal through any of the aforementioned sensors Alternatively, the velocity components in the free vortices downstream of the bluff body can be used to modulate an ultrasonic beam diametrically traversing the meter housing Depending on the characteris-tics of the sensing system, the flowmeter will be suitable for liquid, gas, or both

The earliest detector designs were highly sensitive to plugging and required frequent maintenance (Figure 2.30b) These devices were later replaced by units that could not plug

of these designs are still marketed and are well received by users who are not concerned about quick and convenient access to, and replacement of, the detector or about the reli-ability and sensitivity of heat transfer or ultrasonic detectors Still, the trend seems to be toward detectors that are modular, inexpensive, and interchangeable so they can be quickly replaced when necessary Several vortex flowmeter detectors

this design, the detector is a liquid-filled, double-faced dia-phragm capsule with a piezoelectric crystal in the center that detects the vortex-produced pressure changes as they are transmitted through the filling liquid

Other design modifications aim at compensating for back-ground noise by using two detectors, one of which is exposed

to vortex forces and the other is not, and using their difference

FIG 2.30a

The distance between the Kármán vortices (l) is only a function of

the width of the obstruction (d), and therefore the number of vortices

per unit of time gives flow velocity (V).

I

V

d

1D

flow velocity = (f ×d)/( 0 17) = kfd FIG 2.30b

Shuttle-ball and shuttle-flow-type early vortex flowmeter detectors.

Nickel Shuttle Ball

Thermistor Sensor

Magnetic Pickup

Flow

2.30 Vortex and Fluidic Flowmeters 387

modifications aim at amplifying the signal generated by low-energy vortices, such as by low-density gases One approach

is to use two detector elements (capacitance or piezoelectric) and measure the difference between their signals This tends

to amplify the detector output because, as the vortices emerge

on alternate sides of the flow element, the two detectors sense the forces acting on the two different sides of the element Still another method of amplifying the vortex forces is by physically separating the vortex shedding element and the vortex force detector (Figure 2.30f) If the vortex forces are

FIG 2.30c

Solid-state vortex flowmeter designs with limited accessibility to their sensors.

Thermistor Sensors

Strain Gauge Cantilevered Strut

Fixed Vortex Generating Strut

Vortex Generating Strut

Receiver

Ultrasonic Transmitter

Free Vortices

Oscillator Preamplifier

Pressure at Rear of Bluff Body

Flow Velocity

Across Front

Face

Velocity

Change

Flow

FIG 2.30d

Piezoelectric capsule detector element is removable from flow

ele-ment (Courtesy of The Foxboro Co.)

FIG 2.30e

Dual detector serves noise compensation (Courtesy of Johnson

Yokogawa Corp of America.)

Detector

Flange Nuts (4)

Flow Dam Body Washer

Sensor Assembly

Flow +Noise

Noise

Piezo

Elements

Amplifier

Output

Bluff Body

Lift Force

FIG 2.30f

Separating the rugged obstruction and the detector allows the detec-tor to be much more sensitive to the pressure waves The increases

in the forces detected allows for the use of more rugged (less sensitive and therefore less fragile) sensors (Courtesy of EMC Co.)

388 Flow Measurement

amplified, the force detectors can be made less sensitive and

therefore more rugged and reliable

The types of detectors in use as of this writing are listed

below:

It would seem that the piezoelectric designs (particularly their

dual or differential versions) dominate the market, but other

designs claim superior performance under certain operating

conditions The manufacturers of the capacitance design, for

example, claim superior immunity to pipe vibration effects

The fundamental meter output is a frequency signal in

all cases, which can be fed directly into digital electronic

units for totalization and/or preset batching, into computers,

or into data loggers The frequency signal also can be

con-verted into a conventional 4- to 20-mA DC analog signal for

flow rate indication, recording, and control purposes Most

meters are available in either a standard form or in a design

to satisfy Division 1 explosion-proof area requirements

Features

The vortex-shedding meter provides a linear digital (or

converters, simplifying equipment installation Meter

accu-racy is good over a potentially wide flow range, although this

range depends on operating conditions The shedding

fre-quency is a function of the dimensions of the bluff body and,

being a natural phenomenon, ensures good long-term

rate There is no drift, because this is a frequency system

The meter does not have any moving or wearing

compo-nents, which provides improved reliability and reduced

main-tenance Maintenance is further reduced by the fact that there

are no valves or manifolds to cause leakage problems The

absence of manifolds and valves results in a particularly safe

installation, an important consideration when the process

fluid is hazardous or toxic

If the sensor utilized is sufficiently sensitive, the same vortex-shedding meter can be used on both gas and liquid

In addition, the calibration of the meter is virtually indepen-dent of the operating conditions (viscosity, density, pressure, temperature, and so on) whether the meter is being used on gas or liquid (see Figure 2.30g)

The vortex-shedding meter also offers a low installed cost, particularly in pipe sizes below 6 in (152 mm) diameter, which compares competitively with the installed cost of an orifice plate and differential pressure transmitter

The limitations include meter size range Meters below 0.5 in (12 mm) diameter are not practical, and meters above

12 in (30.0 mm) have limited application as a result of their high cost (compared to an orifice system) and their limited output pulse resolution The number of pulses generated per unit volume decreases on a cube law with increasing pipe diameter Consequently, a 24-in (610-mm) diameter vortex-shedding meter with a typical blockage ratio of 0.3 would have a full-scale frequency output of only approximately 5

Hz at 10 ft/s (3 m/s) fluid velocity

Selection and Sizing

As the first step in the selection process, the operating con-ditions (process fluid temperature, ambient temperature, line pressure, and so on) should be compared with the meter specification The meter wetted materials (including bonding agents) and sensors should then be checked for compatibility with the process fluid with regard to both chemical attack and safety With oxygen, for example, nonferrous materials should be used because of the reactive nature of oxygen Applications in which there are large concentrations of solids, two-phase flow, or pulsating flow should be avoided or approached with extreme caution The meter minimum and maximum flow rates for the given application should then be

A typical performance curve for a vortex-shedding flow-meter is shown in Figure 2.30g The flow-meter minimum flow rate is established by a Reynolds number of 10,000 to 10,500, the fluid density, and a minimum acceptable shedding fre-quency for the electronics The maximum flow rate is gov-erned by the meter pressure loss (typically, two velocity heads), the onset of cavitation with liquids, and sonic velocity flow (choking) with gases Consequently, the flow range for

FIG 2.30g

Typical calibration curves for a 3 in (76 mm) vortex meter showing the close correlation between water and atmospheric air calibrations.

2430 2420 2410 2400

2

Pipe Reynolds Number

Water

Audible Cavitation Air at 14.7 lb/in2

± 0.5%

2.30 Vortex and Fluidic Flowmeters 389

any application depends totally on the operating fluid

viscos-ity, densviscos-ity, and vapor pressure, and the application’s

maxi-mum flow rate and line pressure On low-viscosity products

such as water, gasoline, and liquid ammonia, and with an

application maximum velocity of 15 ft/s (4.6 m/s),

vortex-shedding meters can have a rangeability of about 20:1 with

a pressure loss of approximately 4 PSIG (27.4 kPa)

output signal make its application over wide flow ranges a

practical proposition The rangeability declines

proportion-ally with increases in viscosity, decreases in density, and

reductions in the maximum flow velocity of the process

high-viscosity liquids

For liquid applications, it is necessary to verify that

suf-ficient line pressure exists to prevent cavitation in the vortex

meter The maximum pressure drop in a vortex-shedding

meter is in the region of the bluff body, and there is a consid-erable pressure recovery by the meter outlet Upstream line pressure requirements vary from one meter design to another, but a typical minimum acceptable upstream pressure require-ment (to protect against cavitation) is given by the expression,

loss across the meter) Cavitation conditions must be avoided at all costs, as no material can stand up to the damage caused by cavitation One might approximate the minimum upstream pressure

2.30(2)

FIG 2.30h

Sizing chart for liquid flow measurement Note that minimum flows are limited by both specific gravity (water SG = 1) and viscosity limitations (To convert to metric units use: 1 in = 25.4 mm, 1 GPM = 3.78 lpm) (Courtesy of Endress+Hauser Inc.)

Flow Rate (GPM) 100

Maximum Flow Rates

10,300 GPM

7170 GPM

Minimum Flow

Rate Based on

Specific Gravity

(Accuracy is 0.75% FS)

Flow Rate at Which Accuracy Improves

to 0.75% of Rate Based

on Kinematic Viscosity

2660 GPM

680 GPM

309 GPM

187 GPM 79.3 GPM

22.0 GPM 0.5"

1"

1.5"

2"

3"

8"

6"

4"

1.2 to 6 S.G.

S.G.

1170 GPM

1

1.2

1.21

.6

S.G.

.6

1.21 .6 S.G.

1.2 6 S.G.

1

S.G.

.6

1.21 6 S.G.

8

4

0.9 0.6

S.G.

0.6

S.G.

0.4

32

1

10"

12"

P min= ( )1 3P v+( )2 5V max 2g

390 Flow Measurement

where

liquid head

operating temperature in feet of liquid head

second

the units square feet per second

Vortex-shedding flowmeters cannot survive cavitation,

the incoming liquid stream is permanently vaporized in the

will not be mechanically damaged (although the meter output

will be seriously in error)

Installation Requirements

Vortex-shedding meters require a fully developed flow profile

The length of upstream pipework necessary to ensure

satis-factory approach conditions depends on the specific design

of meter, the type of upstream disturbance present, and the

level of accuracy required Typical upstream and downstream pipework requirements for a variety of disturbances are given

in Figure 2.30k Where there is a severe upstream disturbance, the resulting long, straight lengths of pipe can be reduced by fitting a radial vane or bundle-of-tubes flow-straightening element in the upstream pipework Wherever possible, how-ever, the meter should be installed upstream of any severe source of disturbance such as regulating control valves The downstream straight pipe requirement is five times nominal meter diameter The meter can be installed in any attitude (horizontal or vertical), but it is not suitable for reverse flowmetering

Other instrument connections (pressure, temperature) all should be located downstream of the flowmeter and more than five diameters away from it The flowmeter should be the same size as (or smaller than) the pipeline, but never larger The unit can be insulated for cryogenic or high-temperature services and can be provided with extension bonnets It should be installed in self-draining low points in the piping or in vertical upward flows to keep the meter flooded and to avoid air bubbles and standing liquid pools Block and bypass valves should be provided if the meter is

FIG 2.30i

Sizing chart for gas and vapor flow detection: For extremely dense gases, the maximum flow may be less than shown Gases with extremely low densities (e.g., hydrogen, helium) may not be measurable Note that minimum flows are a function of flowing density To convert to metric units use: 1 in = 25.4 mm, 1 ACFM = 0.02832 ACMM, and 1 lb/ft 3= 16 kg/m 3 (Courtesy of Endress+Hauser Inc.)

.05 lb/ft3

.05 05

0.1

0.1 0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.1

0.5

0.5

0.5 0.5

0.5

0.5

0.5 0.5

0.5

0.5

.05

.05

.05

.05 05

.05

.05

.05

2

2

2 2

2

2

2 2

2 2

0.5"

1"

2"

4"

6"

8"

10"

12"

3"

15.0 ACFM

1.5"

lb/ft3 lb/ft3 lb/ft 3

lb/ft3

lb/ft3

lb/ft 3

lb/ft3 lb/ft 3

lb/ft 3

88.3 ACFM

208 ACFM

344 ACFM

757 ACFM MaximumFlow Rates

1300 ACFM

2960 ACFM

5140 ACFM

7980 ACFM

11,500 ACFM

Flow Rate (ACFM)

Minimum Flow Rate Based on Density (lb/ft 3 )

TABLE 2.30j

Sizing for Steam Flow in Lb/m/Hr Units*†

Steam Pressure (PSIG) Meter

max

10 55 12 75 13 95 15 115

16 134

17 154

19 193

21 231

25 326

28 421

31 516

34 610

36 707

39 803

40 997

46 1197

51 1401

57 1611

63 1826

max

30 322 36 442 40 560 44 677

48 792

51 907

57 1140

63 1360

75 1920

85 2490

94 3040

102 3600

110 4170

117 4740

130 5880

143 6440

154 6970

166 7470

176 7950

max

72 761 84 1040

95 1320 104 1600

113 1870

121 2150

135 2690

148 3220

176 4550

200 5880

221 7190

241 8510

259 9850

276 11,200

308 13,900

337 15,200

365 16,500

391 17,700

417 18,800

max

119 1250 139 1720 156 2180 172 2640

186 3090

199 3530

223 4420

244 5310

290 7490

330 9680

365 11,900

397 14,000

427 16,200

455 18,500

507 22,900

556 25,100

601 27,100

645 29,100

686 31,000

max

261 2760 306 3790 344 4800 379 5800

410 6800

439 7780

491 9740

537 11,700

639 16,500

726 21,300

803 26,100

873 30,900

940 35,800

1000 40,600

1120 50,400

1220 55,200

1320 59,800

1420 64,100

1510 68,200

max

450 4760 528 6530 594 8280 653 10,000

707 11,700

756 13,400

846 16,800

927 20,200

1100 28,500

1250 38,800

1390 45,000

1510 53,200

1620 61,700

1730 70,100

1930 86,900

2110 95,200

2280 103,000

2450 110,000

2610 118,000

max

1020 10,800

1200 14,800

1350 18,800

1480 22,700

1600 26,600

1720 30,500

1920 38,100

2100 45,700

2500 64,600

2840 83,400

3140 102,000

3420 121,000

3680 140,000

3920 159,000

4370 197,000

4790 216,000

5180 234,000

5550 251,000

5910 267,000

max

1780 18,800

2080 25,700

2340 32,600

2570 39,400

2790 46,200

2980 52,900

3340 66,200

3650 79,400

4340 112,000

4930 145,000

5460 177,000

5940 210,000

6470 243,000

7120 276,000

8370 343,000

9600 375,000

10,800 406,000

12,000 435,000

13,200 464,000

max

2750 29,100

3230 39,900

3630 50,600

3990 61,200

4320 71,700

4630 82,100

5180 103,000

5670 123,000

6740 174,000

7660 225,000

8470 275,000

9210 326,000

10,000 377,000

11,000 429,000

13,000 532,000

14,900 582,000

16,800 630,000

18,600 676,000

20,500 720,000

max

3970 42,000

4660 57,600

5240 73,000

5760 88,300

6240 103,000

6670 118,000

7470 148,000

8180 178,000

9720 251,000

11,000 324,000

12,200 397,000

13,300 470,000

14,500 544,000

15,900 618,000

18,700 767,000

21,500 840,000

24,200 909,000

26,900 975,000

29,500 1,040,000

Densitysat. lb/ft 3

392 Flow Measurement

to be serviced while the process is in operation There should

be no excessive pipe vibration in the area where the meter is

installed, and gaskets should not protrude into the pipeline

VORTEX-PRECESSION (SWIRL) METERS

A predecessor of the shedding meter, the

vortex-precession meter or Swirlmeter™, is currently manufactured

by a single vendor and sold in combination with that vendor’s

vortex-shedding product line, sharing common sensors,

elec-tronics, and programming features

Construction of a typical vortex-precession (swirl) meter

and the operating principles are illustrated in Figure 2.30l

The fixed, swirl-inducing helical vanes at the entrance to the

meter introduce a spinning or swirling motion to the fluid

After the exit of the swirl vanes, the bore of the meter

con-tracts progressively, causing the fluid to accelerate, but with

the axis of rotation still on the centerline of the meter The

swirling fluid then enters an enlarged section in the meter

housing, which causes the axis of fluid rotation to change

from a straight to a helical path The resulting spiraling vortex

is proportional to velocity and, hence, volumetric flow rate

above a given Reynolds number

The velocity of fluid in the vortex is higher than that

of the surrounding fluid Consequently, as each vortex

passes the sensor, there is a change in the local fluid

veloc-ity The frequency at which the velocity changes occur is

proportional to volumetric flow rate and can be detected

by piezoelectric or thermistor sensors Currently, the only

vortex-precession meter in manufacture uses piezoelectric

sensors

A flow straightener is fitted at the meter outlet to isolate the meter from downstream piping effects that might other-wise impair the development of the precessing vortex The internal components of the swirl meter required a significant amount of complex machining; thus, it is more expensive than some other meter types

The swirl meter operates in most of the same applications

as the vortex-shedding flowmeter but has the advantage that, since flow conditioning is done at the inlet and outlet of the meter body, virtually no upstream or downstream straight run

is required for optimal installation The sole supplier cur-rently furnishes the swirl meter and the vortex-shedding meter in interchangeable “kits.”

FIG 2.30k

Straight pipe-run requirements as a function of upstream disturbance (Courtesy of Endress+Hauser Inc.)

50 × D 5 × D

2 × D 2 × D

8 × D

12 × D

5 × D

20 × D 5 × D

25 × D 5 × D

Flow Straightener

Control Valve

90 ° Elbow

or T-Fitting

Reducer

Flow

2 - 90 ° Elbows

in Two Planes

2 - 90 ° Elbows

in a Single Plane

FIG 2.30l

Construction of a typical vortex-precession (swirl) meter.

Swirl Guide Vanes

Swirl Pressure

Tap

Precessing Swirl

Flow

Sensor Probe Detector Amplifier

2.30 Vortex and Fluidic Flowmeters 393

FLUIDIC (COANDA EFFECT) METERS

In fluidic meters, fluid entering the meter is entrained into a

turbulent jet from its surroundings, causing a reduction in

pressure The internal geometry of the meter body causes the

jet to be deflected from its central position and initially attach

itself to one of the side walls The jet curvature is sustained

by the pressure differential across the jet If a sufficient

vol-ume of fluid is then introduced into the control port on that

side, it will cause the jet to switch to the opposite side wall

oscillate by one of two methods The simplest method is a

relaxation oscillator In this system, the two ports are

con-nected Fluid is sucked from the high-pressure side to the

low-pressure side causing the jet to switch to the other wall

The jet thus continues to oscillate as the fluid is sucked

alternately from one side to the other

The more commonly used system is the feedback

oscil-lator (see Figure 2.30m) The deflected jet causes a

low-pressure area at the control port At the upstream feedback

passage, the pressure is higher due to a combination of the

jet expansion and the stagnation pressure Thus, a small

portion of the main stream of fluid is diverted through the

feedback passage to the control port The feedback flow

intersects the main flow and diverts it to the opposite side

wall The whole feedback operation is then repeated,

result-ing in a continuous, self-induced oscillation of the flow

between the side walls of the meter body The frequency of

oscillation is linearly related to the volumetric flow rate

above a minimum Reynolds number As the main flow

oscil-lates between the side walls, the flow in the feedback

pas-sages oscillates between zero and a maximum value This

frequency is detected by means of a sensor (either a thermistor

or magnetic inductive pickup), providing a frequency output signal

Characteristics

The principal features include a lack of moving components, fixed calibration based on the geometry of the housing, linear digital or analog output, and good rangeability One advan-tage over vortex meters is that fluidic meters can operate down to a Reynolds number of 3000 The maximum flow range (dependent on size and viscosity) is 30:1 The complex housing shape largely dictates the operating pressure and maximum practical pipe diameter In practice, a 4-in (100-mm) diameter unit is the largest commercially available, and the operating pressure in this diameter is typically limited to 150 PSIG (1.03 MPa) Some vendors provide larger diameters up

to 12 in (300 mm) by using a bypass flow tube design In this design, a flow restriction is placed in the tube, forcing fluid through the fluidic flowmeter mounted on top of the flow tube

Although theoretically suitable for gaseous applications, fluidic meters have been used almost exclusively in liquid applications Recent experimentation by several manufactur-ers has produced fluidic flowmetmanufactur-ers that appear to be able to meet AGA certification requirements for household gas meters, and one manufacturer has placed a fluidic-principle gas meter in distribution for industrial and commercial nat-ural gas metering applications

A special, separate converter is required for the meter, which, in some instances, can incorporate a pneumatic

volume of flow passed remains within 1%, and therefore the measurement error remains well within 2% of actual flow between the Reynolds numbers of 3000 and 100,000

CONCLUSION

The advantages of vortex-shedding flowmeters include their suitability for liquid, gas, and steam service; independence from viscosity, density, pressure, and temperature effects; low installed cost in smaller sizes; good accuracy and lin-earity without requiring calibration; wide rangeability; low maintenance using simple, easily accessible and inter-changeable spare parts; simple installation; and direct pulse output capability

In terms of disadvantages, they are not suitable for ser-vices that are dirty, abrasive, viscous, or mixed-flow (gas with liquid droplets, liquid with vapor bubbles), or that have low Reynolds numbers (below 20,000); the available choices

in materials of construction are limited; the pulse resolution (number of pulses per gallon or liter) drops off in larger sizes; the pressure drop is high (two velocity heads); and substantial straight runs are required both upstream and downstream

FIG 2.30m

Diagram of the mode of operation of a feedback oscillator.

Side

Wall

Sensor

Feedback Passage

Control

Port

(c)