Pressure Measurement 5 07 09

7/28/2019 Pressure Measurement 5 07 09

1/70

1

PROCESS INSTRUMENTATION . MEASUREMENT

AND CONTROL

Pressure Measurement

Pressure Transducers


2/70

2

Section 2.Pressure Transducers

4.2.1 Introduction

Whilst there are a wide range ofpressure measuring devices on themarket, they may be divided into maingroups:

mechanical and electromechanical


3/70

3

4.2.2 Mechanical Sensors

Mechanical pressure measuring elementsinclude:

Manometer Dead weight tester

Bourdon tubes

Bellow elements

Diaphragm element


4/70

4

4.2.1 Manometers

In any column of liquid (Figure 4.2.1),thehead pressure p is given by:

P ghWhere :

P head in pressure

density

h height


5/70

5

4.2.1 Manometers

In any column of liquid (Figure 4.2.1),the head pressure p isgiven by:

P gh

Where :

P head in pressure

density

h height


6/70

v

In its simplest form the manometer is a U tubeabout half with liquid (commonly water, mercury oralcohol).

With both ends of the tube open, the liquid is atsame height in each leg (Figure 4.2.2(a)).When positive pressure P is applied to one leg, theliquid is forced down in that leg and up in theother(Figure 4.2.2(b)).

The height h, indicates the difference in appliedpressure P and the atmospheric pressure P.


7/70

v

Since the pressure in both tubes must balance:P P + gh

h (P -P)/gFor water and mercury the conversion intoPascals is:

Water: Pa = mmHO x 9,80665

Mercury: Pa= mmHg x133,332


8/70

v

Alternatively, when a vacuum is applied to oneleg, the liquid in that leg and falls in the other

(Figure 4.2.2(c)).This time the difference in height h indicates theamount of vacuum. Because the difference inheight the two columns is always a trueindication of the pressure, regardless of variation

in the internal diameter of the tubing, the U-tubemanometer is a primary standard.


9/70

9

PROCESS INSTRUMENTATION . MEASUREMENT AND CONTROL




10/70

v

A disadvantage of the U-tube manometer is thatreading must be taken at two different places.

This shortcoming is overcome in the well-typemanometer (Figure 4.2.3)where thewell(reservoir) is sufficiently large that thechange of level in the reservoir is negligible.

Alternatively, the change in reservoir liquid levelmay be compensated for on the scale.


11/70

11





12/70

v

To increase readability and sensitivity furtherthe well-type manometer indicating tube can be

inclined.(Figure 4.2.4) to produce a greater linearmovement along the tube for a given pressuredifference. Because the inclined manometer isfrequently used for determining the over- fired

draft in boiler uptakes and it often is called aDraft Gauge.


13/70

13





14/70

v

Suitable for measuring pressure in the rangesfrom about 3 to kPs (gas) up to 1 GPa

(hydraulic), the dead-weight tester is usedessentially as a primary pressure calibrationstandard and provides accuracies of down to0.02% of reading.

4.2.2.2 Dead weight tester


15/70

v

The dead-weight tester is based on

measuring the force acting on knownarea. As illustrated in Figure 4.2.5,hydraulic fluid, contained within apressure cylinder, acts on a position,

having a known cross-section area, whichsupport a known weight.


16/70

16





17/70

v

The operation, the screw press is operated until the pressure within the

system is sufficient to raise the position, with its associated weights, off

its stop. At this point, ignoring frictional losses, the pressure acting on thepiston is given by:

P force /cross-sectional area

P m.g /A

Where:

P pressure

g acceleration due to gravity

A cross sectional area


18/70

v

Two points should be noted. In most system, the apparatus is surrounded

by atmospheric pressure and the calibrated pressure is thus gauge

pressure. Some system are mounted within an evacuated chamber inorder to derive absolute pressure.

A second point is that, as indicated above, performance will be affected

by the acceleration due to gravity. Thus gives rise to a variation of about

to 0.5% around the globe. Consequently, it is important that local value of

the acceleration due to gravity is known and corrected for.


19/70

v

Figure 4.2.3 illustrates the C shaped or C Bourdon tube.The tube is commonly manufactured of phosphor bronze

having a flattened cross-sectional area and is sealed at oneend.

When pressure is applied to the open end of the tube itwill tend to straighten and the relatively small travel of theend of tube is amplified by means of a link to drive apointer through a drive segment and toothed gear.


20/70

v

Although the movement of the tip of the tube is non-linear this can be

compensated for in the link and rear mechanism.

The link also usually incorporates a bi-metallic element for temperature

compensation.(Figure 4.2.7)

Bourdon tubes are available to cover the range from 0-30 k Pa up to 0-50

M Pa

4.2.2.3 Bourdon tube


21/70

21





22/70

22




23/70

23





24/70

24




25/70

25




26/70

26


27/70

v

The Bourdon tube can be fabricated from

a variety of materials including phosphorBronze beryllium copper, 4130 alloysteel, 316 and 403 stainless, Monel, andtitanium

With the choice determining both therange and corrosion resistance.


28/70

v

The C- Bourdon tube usually has a relativelyshort arc of around 250and provides a typical

accuracy of 2%. It is used for local pressureindication connected directly to process vesselsand lines. Inexpensive, C-Bourdon tubeinstruments feature a wide operating range, good

sensitive and fast response.


29/70

v

The main problem with C- Bourdon tube is theirsusceptibility to damage due to shock or

vibration although this can be overcome by fillingthe instrument case with a damping fluid such asglycerin or silicon oil. Apart from reducingresonance-induced fracturing of the measuringelement, the liquid filling prevents aggressive orcorrosive gases from entering the instrument andprevents condensation from forming.


30/70

v

For the lower measurement range use is often made of thespiral Bourdon tube (Figure 4.2.8) in which the tube makes

several turns to increase the effective angular length andthus increase the movement of the free end for a givenpressure input. Because the need for further mechanicalamplification is reduced, the tube end is mechanicallylinked direct to the pointer.

This eliminates the necessity for the toothed quadrant-with the consequent reduction backlash and friction errors.


31/70

31


32/70

v

Whilst the C Bourdon is generally suitable for pressure up toabout 1 Mpa,In any bourdon tube element, the higher the pressure required,the thicker the wall of the tubing. Thus whilst spiral tube low

pressure elements may have only two or three coils, highpressure elements with their thicker wall, may require up to 20coil in order not maintain their sensitivity. Generally the spiraltube is suitable for pressure up to 30 Mpa.A variation on the spiral tube is the helix tube where the tube iswound longitudinally to provide ranges up to 50 Mpa.The mail disadvantage of both spiral and helix elements is thatvery expensive.


33/70

v

The bellow measuring device is made up of aseries of thin-walled cylindrical elements thatfrom the bellows arrangement and is used wherea large degree of travel is required in a restrictedspace. Bellows element are also used for lowerpressure ranges and for ranges that cross fromvacuum into positive gauge pressure.

4.2.2.4 Bellow elements


34/70

34


35/70

v

The bellows measuring device(Figure 4.2.9)ismade of series of thin-walled cylindrical elementthat from the bellow arrangement and used

where degree of travel is required in a restrictedspace.

Bellows elements are also used for lowerpressure ranges and for ranges that cross from

vacuum positive gauge pressure.



36/70

36


37/70

37


38/70

v

The principle of operation is based on the fact when a pressure isapplied to the bellows element, its length changes.

In the arrangement shown in Figure 4.2.9, the spring loaded

bellows elements is enclosed within a pressure container that tothe process pressure source.

When pressure is applied the bellows compress against theopposing pressure source. When pressure is applied the bellowscompress against the opposing force of the spring- with thevertical movement transmitted, through a suitable linkage, to

pointer or actuating device.


39/70

v

Because pressure is exerted over a large area,the bellows element produces a considerable

force per unit change in pressure. As a result,bellows elements are used in the range from 0-500 Pa up to 100 k Pa.

Bellows elements are often used to actuate anon/off switch for in the air conditioning industry.


40/70

v

In the simple diaphragm a flexible disc, whichcan either be flat or have concentric corrugation,

is fabricated from sheet metal to exacting high totolerance dimensions. In the instrument shown inFigure 4.2.10, the diaphragm is usedindependently as a pressure sensor. Appliedpressure deflects the diaphragm which move a

push rod.



41/70

41


42/70

42


43/70

v

Due to their and position ,diaphragmsdisplay high mechanical resistance and

are less shock-sensitive. Compared toBourdon tubes, the travel of a diaphragmis very small and thus both quality andtolerances must meet very exactingstandards.


44/70

v

The diaphragm is also the basic component of acapsular element comprising two diaphragms

joined together by crimping or fusion-welding. InFigure 4.2.11, the orientation of the corrugationof two diaphragms is oppose and again a pushrod is used to actuate a toothed drive segments,gear and pointer.


45/70

45


46/70

v

In modern process control system, pressuremeasurement is normally carried out using a

different pressure transducer. The role of such apressure transducer is to measure the differentialpressure and convert it to an electrical signalthat can be transmitted from the field to thecontrol room or the pressure controlling system.

4.2.3 Electrical displacement sensors


47/70

v

As illustrated in figure 4.2.12, most industrialdifferential cell make use of isolation diaphragms

that isolate the transmitter. Movement of theisolation diaphragms is transmitted via theisolating fluid(e.g. silicon fluid) to the measurediaphragm whose deflection is a measure of thedifferential pressure.


48/70

48


49/70

v

Deflection of the measuring diaphragm isextremely small(of the order of a few

millimeters)and measure is normally carried outby one of five basic methods:1. Inductance2. Strain gauge3. Capacitance

4. Piezoresistive5. Piezoelectric


50/70

v

The inductive based sensor(often referred to asa linear differential transformer)

(Figure 4.2.13) senses the displacement ofmagnetic core mounted on the measuringdiaphragm.

This is shown the displacement of a magneticcore mounted on the measuring diaphragm. This

is show schematically in Figure 4.2.14.

4.2.3.1 Inductance


51/70

v

Displacement of the magnetic core changes thecoupling between the primary and the two

secondarys.With no differential pressure, the measuringdiaphragm is not deflected and the voltageinduced in the measuring coil is equal andopposite with a net output of zero.


52/70

v

When the measuring diaphragm isdefected the output voltagemagnitude and phase of eachsecondary will vary in directproportional to the pressure applied

to the movable element.


53/70

53


54/70

54


55/70

v

In the simplest form, adhesive is used tobond four metal strain gauge elements

directly to the diaphragm(Figure 4.2.15).In most instances, the strain gaugeselements are arranged to form the fourarms of a Wheatstone bridge.

4.2.3.2 Strain gauges


56/70

56


57/70

57


58/70

v

Thus , a change in pressure producesmechanical deformation which result in a

change of electrical resistance that isproportional to the change in pressure.The greater the pressure applied to thediaphragm, the more it will deflect.


59/70

v

The bonded foil strain gauge is the mostreliable and durable of four technologies

and can be used in ultra high pressureapplication(0-600 kPa through to 700MPa). Its durability makes it suitable forapplication that experience pressure

cycling, shock, and vibration.


60/70

v

Another major advantage of thistechnology is that foil strain gauge can be

matched and bonded with extremeaccuracy, these is no need to include anytemperature compensating devices withinthe transmitter.


61/70

v

The mail limitation of this technology is its poorperformance at below 500 k Pa. If the diaphragm

is too thin, the strain gauges begin to interferewith the diaphragms motion. Increasedsensitivity can be obtained using a metallic thinsensor where the strain gauge is vapor depositedor spattered onto the diaphragm sensing

element.


62/70

v

The variable capacitance transmitter(Figure 4.2.16) is themost widely used method of measuring differentialpressure. The upstream and downstream pressure are

applied to isolation diaphragms on the high and lowpressure sides, and are transmitted to the sensingdiaphragm(movable electrode)- usually through a fail oil.Movement of the sensing diaphragm changes its distancefrom the fixed plate electrodes, resulting in a change in

capacitance.

4.2.3 Capacitance


63/70

63


64/70

v

Capacitance based transmitters aresimple, reliable , accurate, small, in size

and weight, and remain stable over a widetemperature range. The main advantageof the capacitive transmitter is that it isextremely sensitive to small changes in

pressure- down to 250 Pa pressure.


65/70

v

Similar in operation to the strain gauge , thepiezoresistive element employs four nearly

identical piezio-resistive diffused the surface of athin circular wafer of N- type silicon. Thediaphragm is formed by chemically etching acircular cavity- with the un etched portionforming a rigid boundary and surface. The

mechanical strength of silicon generally imposean upper limit of around about 3 MPa.

4.2.3.4 Piezoresistive


66/70

v

Through not as rugged as a foil strain gage, piezoresistivegauges are generally more sensitive than metallic thick filmdevice and thus produce a measurable signal at lower

strain. However, silicon piezoresistive sensing technology isnot suited for application that experience extreme pressurecycles, shocks, or vibration, due to the weakness of thesilicon piezoresistors. Further, the upper temperature limitfor diffused silicon strain gauge based transmitters runs

between 125and 200

C.


67/70

v

Ceramic piezoresistive sensing technology uses conductivelink deposition on the reference side of a ceramic sensingdiaphragm. Like silicon piezoresistive, this technology

provides a strong, sensitive output signal in lower pressureranges. Ceramic piezoresistive technology is slightly morerugged than silicon piezoresistive and is used in pressureranges of 0- 100 kPa to 0 -10MPa. Further, the ceramicwetted face may be used in corrosive fluid application


68/70

v

When a mechanical stretching or compressingforce is applied to an asymmetrical crystalline

material, such as barium titanite or quartz(Figure4.2.17), equal and opposite electrical changesappear across it . the magnitude of the chargesappear across it. The magnitude of the chargesdepends on the dimension of the quartz crystal

and the magnitude of the applied force.

4.2.3.5 Piezoelectric


69/70

69


70/70

Pressure transducers based on this phenomenon producean out that is proportional to a change in applied pressureand do not thus respond to static conditions. Available in a

wide range of dynamic pressure from to 200 kPa to 100MPa with an accuracy down to 0.075%, piezoelectricpressure transducer have a very fast response(

Pressure Measurement 5 07 09

Documents

Transcript of Pressure Measurement 5 07 09