Pressure Measurement New 29 04 09

7/28/2019 Pressure Measurement New 29 04 09

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PROCESS INSTRUMENTATIONMEASUREMENT AND CONTROL

Pressure Measurement

Pressure Transducers


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SECTION 2.PRESSURE TRANSDUCERS

4.2.1 Introduction

Whilst there are a wide range of pressure measuring

devices on the market, they may be divided into

main groups: mechanical and electromechanical


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4.2.2 MECHANICAL SENSORS

Mechanical pressure measuring elements

include:

Manometer

Dead weight tester Bourdon tubes

Bellow elements

Diaphragm element


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4.2.1 MANOMETERS

In any column of liquid (Figure 4.2.1),the head

pressure p is given by:

P gh

Where :P head in pressure

density

h height


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In its simplest form the manometer is a Utube about

half with liquid (commonly water, mercury or alcohol).

With both ends of the tube open, the liquid is at same

height in each leg (Figure 4.2.2(a)).

When positive pressure P is applied to one leg, the

liquid is forced down in that leg and up in the

other(Figure 4.2.2(b)).

The height h, indicates the difference in applied

pressure P and the atmospheric pressure P.


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Since the pressure in both tubes must balance:

P P + gh

h (P -P)/g

For water and mercury the conversion intoPascals is:

Water: Pa = mmHO x 9,80665

Mercury: Pa= mmHg x133,332


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Alternatively, when a vacuum is applied to one

leg, the liquid in that leg and falls in the other

(Figure 4.2.2(c)).

This time the difference in height h indicates the

amount of vacuum. Because the difference in

height the two columns is always a true

indication of the pressure, regardless of variation

in the internal diameter of the tubing, the U-tube

manometer is a primary standard.


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A disadvantage of the U-tube manometer is that

reading must be taken at two different places.

This shortcoming is overcome in the well-type

manometer (Figure 4.2.3)where the

well(reservoir) is sufficiently large that thechange of level in the reservoir is negligible.

Alternatively, the change in reservoir liquid level

may be compensated for on the scale.


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To increase readability and sensitivity further

the well-type manometer indicating tube can be

inclined.(Figure 4.2.4) to produce a greater linear

movement along the tube for a given pressure

difference. Because the inclined manometer isfrequently used for determining the over- fired

draft in boiler uptakes and it often is called a

Draft Gauge.


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4.2.2.2 DEAD WEIGHT TESTER

Suitable for measuring pressure in the ranges

from about 3 to kPs (gas) up to 1 GPa

(hydraulic), the dead-weight tester is used

essentially as a primary pressure calibration

standard and provides accuracies of down to

0.02% of reading.


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The dead-weight tester is based on measuring the

force acting on known area. As illustrated in

Figure 4.2.5, hydraulic fluid, contained within a

pressure cylinder, acts on a position, having a

known cross-section area, which support a knownweight.


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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 thispoint, ignoring frictional losses, the pressure acting on

the piston is given by:

P force /cross-sectional area

P m.g /A

Where:

P pressure

g acceleration due to gravityA cross sectional area


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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 in order 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 isimportant that local value of the acceleration due to gravity is

known and corrected for.


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4.2.2.3 BOURDON TUBE

Figure 4.2.3 illustrates the Cshaped or CBourdon

tube.

The tube is commonly manufactured of phosphor

bronze having a flattened cross-sectional area and is

sealed at one end.

When pressure is applied to the open end of the tube it

will tend to straighten and the relatively small travel

of the end of tube is amplified by means of a link to

drive a pointer through a drive segment and toothed

gear.


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Although the movement of the tip of the tube is non-

linear this can be compensated for in the link and rearmechanism.

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


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The Bourdon tube can be fabricated from a

variety of materials including phosphor

Bronze beryllium copper, 4130 alloy steel, 316

and 403 stainless, Monel, and titanium

With the choice determining both the range and

corrosion resistance.


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The C- Bourdon tube usually has a relatively

short arc of around 250and provides a typical

accuracy of 2%. It is used for local pressure

indication connected directly to process vessels

and lines. Inexpensive, C-Bourdon tubeinstruments feature a wide operating range, good

sensitive and fast response.


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The main problem with C- Bourdon tube is their

susceptibility to damage due to shock or vibration

although this can be overcome by filling the

instrument case with a damping fluid such as

glycerin or silicon oil. Apart from reducingresonance-induced fracturing of the measuring

element, the liquid filling prevents aggressive or

corrosive gases from entering the instrument and

prevents condensation from forming.


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For the lower measurement range use is often made of the

spiral Bourdon tube (Figure 4.2.8) in which the tube makes

several turns to increase the effective angular length and

thus increase the movement of the free end for a givenpressure input. Because the need for further mechanical

amplification is reduced, the tube end is mechanically linked

direct to the pointer.

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


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Whilst the CBourdon 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, high pressureelements with their thicker wall, may require up to 20 coil in

order not maintain their sensitivity. Generally the spiral tube is

suitable for pressure up to 30 Mpa.

A variation on the spiral tube is the helix tube where the tube is

wound longitudinally to provide ranges up to 50 Mpa. The mail disadvantage of both spiral and helix elements is that

very expensive.


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4.2.2.4 BELLOW ELEMENTS

The bellow measuring device is made up of a

series of thin-walled cylindrical elements that

from the bellows arrangement and is used where

a large degree of travel is required in a restricted

space. Bellows element are also used for lowerpressure ranges and for ranges that cross from

vacuum into positive gauge pressure.


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The bellows measuring device(Figure4.2.9)is made of series of thin-walled

cylindrical element that from the bellow

arrangement and used where degree oftravel is required in a restricted space.

Bellows elements are also used for lower

pressure ranges and for ranges thatcross from vacuum positive gauge

pressure.


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The principle of operation is based on the fact when a

pressure is applied to the bellows element, its length

changes.

In the arrangement shown in Figure 4.2.9, the springloaded bellows elements is enclosed within a pressure

container that to the process pressure source.

When pressure is applied the bellows compress against

the opposing pressure source. When pressure is applied

the bellows compress against the opposing force of the

spring- with the vertical movement transmitted,

through a suitable linkage, to pointer or actuating

device.


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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 an

on/off switch for in the air conditioning industry.


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4.2.5 DIAPHRAGM ELEMENTS

In the simple diaphragm a flexible disc, which

can either be flat or have concentric corrugation,

is fabricated from sheet metal to exacting high to

tolerance dimensions. In the instrument shown

in Figure 4.2.10, the diaphragm is usedindependently as a pressure sensor. Applied

pressure deflects the diaphragm which move a

push rod.


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Due to their and position ,diaphragms display

high mechanical resistance and are less shock-

sensitive. Compared to Bourdon tubes, the travel

of a diaphragm is very small and thus both

quality and tolerances must meet very exactingstandards.


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The diaphragm is also the basic component of a

capsular element comprising two diaphragms joinedtogether by crimping or fusion-welding. In Figure

4.2.11, the orientation of the corrugation of two

diaphragms is oppose and again a push rod is used to

actuate a toothed drive segments, gear and pointer.


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4.2.3 ELECTRICAL DISPLACEMENT SENSORS

In modern process control system, pressure

measurement is normally carried out using a

different pressure transducer. The role of such a

pressure transducer is to measure the differential

pressure and convert it to an electrical signalthat can be transmitted from the field to the

control room or the pressure controlling system.


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As illustrated in figure 4.2.12, most industrial

differential cell make use of isolation diaphragms

that isolate the transmitter. Movement of the

isolation diaphragms is transmitted via the

isolating fluid(e.g. silicon fluid) to the measurediaphragm whose deflection is a measure of the

differential pressure.


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Deflection of the measuring diaphragm is extremelysmall(of the order of a few millimeters)and measure is

normally carried out by one of five basic methods:

1. Inductance

2. Strain gauge3. Capacitance

4. Piezoresistive

5. Piezoelectric


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4.2.3.1 INDUCTANCE

The inductivebased sensor(often referred to as a

linear differential transformer)(Figure 4.2.13) senses the displacement of magnetic core

mounted on the measuring diaphragm.

This is shown the displacement of a magnetic core

mounted on the measuring diaphragm. This is showschematically in Figure 4.2.14.


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Displacement of the magnetic core changes the

coupling between the primary and the two

secondarys.

With no differential pressure, the measuring

diaphragm is not deflected and the voltageinduced in the measuring coil is equal and

opposite with a net output of zero.


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When the measuring diaphragm is defected the

output voltage magnitude and phase of each

secondary will vary in direct proportional to the

pressure applied to the movable element.


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4.2.3.2 STRAIN GAUGES

In the simplest form, adhesive is used to bond

four metal strain gauge elements directly to the

diaphragm(Figure 4.2.15). In most instances, the

strain gauges elements are arranged to form the

four arms of a Wheatstone bridge.


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Thus , a change in pressure produces mechanical

deformation which result in a change of electrical

resistance that is proportional to the change in

pressure. The greater the pressure applied to the

diaphragm, the more it will deflect.


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The bonded foil strain gauge is the most reliable

and durable of four technologies and can be used

in ultra high pressure application(0-600 kPa

through to 700 MPa). Its durability makes it

suitable for application that experience pressurecycling, shock, and vibration.


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Another major advantage of this technology is

that foil strain gauge can be matched and bonded

with extreme accuracy, these is no need to

include any temperature compensating devices

within the transmitter.


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The mail limitation of this technology is its poor

performance at below 500 k Pa. If the diaphragm

is too thin, the strain gauges begin to interfere

with the diaphragms motion. Increased

sensitivity can be obtained using a metallic thinsensor where the strain gauge is vapor deposited

or spattered onto the diaphragm sensing

element.


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4.2.3 CAPACITANCE

The variable capacitance transmitter(Figure

4.2.16) is the most widely used method of

measuring differential pressure. The upstream

and downstream pressure are applied to isolation

diaphragms on the high and low pressure sides,and are transmitted to the sensing

diaphragm(movable electrode)- usually through

a fail oil. Movement of the sensing diaphragm

changes its distance from the fixed plate

electrodes, resulting in a change in

capacitance.


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Capacitance based transmitters are simple,reliable , accurate, small, in size and weight, and

remain stable over a wide temperature range.

The main advantage of the capacitive transmitter

is that it is extremely sensitive to small changesin pressure- down to 250 Pa pressure.


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4.2.3.4 PIEZORESISTIVE

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

identical piezio-resistive diffused the surface of a

thin circular wafer of N- type silicon. The

diaphragm is formed by chemically etching acircular cavity- with the un etched portion

forming a rigid boundary and surface. The

mechanical strength of silicon generally impose

an upper limit of around about 3 MPa.


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Through not as rugged as a foil strain gage,piezoresistive gauges are generally more

sensitive than metallic thick film device and thus

produce a measurable signal at lower strain.

However, silicon piezoresistive sensingtechnology is not suited for application that

experience extreme pressure cycles, shocks, or

vibration, due to the weakness of the silicon

piezoresistors. Further, the upper temperature

limit for diffused silicon strain gauge based

transmitters runs between 125 and 200C.


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Ceramic piezoresistive sensing technology usesconductive link deposition on the reference side of

a ceramic sensing diaphragm. Like silicon

piezoresistive, this technology provides a strong,

sensitive output signal in lower pressure ranges.Ceramic piezoresistive technology is slightly

more rugged than silicon piezoresistive and is

used in pressure ranges of 0- 100 kPa to 0 -

10MPa. Further, the ceramic wetted face may be

used in corrosive fluid application

P


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4.2.3.5 PIEZOELECTRIC

When a mechanical stretching or compressing forceis applied to an asymmetrical crystalline

material, such as barium titanite or

quartz(Figure 4.2.17), equal and opposite

electrical changes appear across it . themagnitude of the charges appear across it. The

magnitude of the charges depends on the

dimension of the quartz crystal and the

magnitude of the applied force.


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Pressure transducers based on this phenomenonproduce an out that is proportional to a change in

applied pressure and do not thus respond to

static conditions. Available in a wide range of

dynamic pressure from to 200 kPa to 100 MPawith an accuracy down to 0.075%, piezoelectric

pressure transducer have a very fast response(

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Pressure Measurement New 29 04 09