2_process variables

8/7/2019 2_PROCESS VARIABLES

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Basic Instrumentation

ByEng. Mohamed Salah-Eldin


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Course Contents

Introduction to Measurement

Process Variables & Primary SensingElements

Process Instruments

Instrument Air Systems


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The main four process variables

PressureTemperature

FlowLevel

All Engineers in the plant must understand thesevariables in order to operate / maintain / Improve the

plant equipments



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P ressure Measurement



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P ressure is:

A measure of force applied onto a unit area

PRESSURE = FORCE D IVIDED BY AREA

Ex ample:

Then, P = 5 pounds per square inch (psi)

inches square 10pounds 50

!P



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P ressure Measuring Units

S.I. UNITSIM P ER IAL UN ITS

mm Wcbar lnch WcLbf/in2 or (PS I)UNIT

703 .10.0689527.731Lbf/in2 or (PS I)

25.42.4 87x 10-310.036 13lnch Wc

10.22 x1031402.114. 50 4bar

197 .98x 10-60.039371.422 x10-3mm Wc



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P ressure S ca les

G auge Pressure Scale:The gauge pressure scale uses atmosphericpressure as the starting point of allmeasurements . (14 . 7 psi at sea level) .

Absolute Pressure Scale:Absolute pressure = gauge pressure + atmosphericpressure .Vacuum Scale:

A perfect vacuum is a space that containsa bsolutely no matter The vacuum scale is used to measure pressures invessels which are below atmospheric pressure .



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R e lationship Between Abso lute and Gage P ressure



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Main Types of P ressure Measuring

Sensors:1 . M anometer (Liquid column instruments),

direct reading, for very low pressures .2 . B ourdon Tu be Gauge (C shape, S piral and

heli x), for low and high pressures .3 . D iaphragm Gauge , for low pressures .4 . B ellows Gauge , for low pressure .

5 . Electrical sensors ( i.e . capacitance, S traingauge, resonant wire or piezoelectric) for allpressure ranges .



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U-Tube Manometers

The mercury moves to either side of themanometer, depending on the difference inpressures at the two ends of the instrument



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Bourdon Tube Gauge

B ourdon tu be changes in its shape due topressure changes

C-Shaped Bourd on Tube principle



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Types of bourdon tubes

Spiral Boudon Spring Heli x Bourdon tube



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D iaphragm Gauge

a change in pressure on a diaphragm causesthe indicating needle to change position



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D iaphragm Gauge

Two Inputs forD

ifferential P

ressure

difference in the two pressures determines thepointer position



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Be ll ows Gauge

An increase in pressure causes the bellowsto e xpand . The e xpanding bellows will movethe pointer to indicate a higher pressure .



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P IS T ON TY PE

The pressure actsdirectly on the pistonand compresses the

springThe position of thepiston is directly relatedto the pressure



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S train-Gauge S ensors

W hen metallic conductors or semiconductors aresu bjected to mechanical strain, there is a change inresistance



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P iezoe lectric-type S ensor

A certain class of crystals produce an electrical

signal when they are mechanically deformed . The voltage level of the signal is proportional tothe amount of deformation .



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P ressure Transmitter


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P ressure Transmitters

availa ble in gagepressure, a bsolutepressure anddifferential pressuretypically 0 .07 5% range accuracy50:1 turndown



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E lectric P ressure Switches

An electric pressure switch senses pressure and opensor closes a switch element at a set pressure to signalanother electrical device .

Pressure switches are set to trip at a certain point withrising or falling pressure .

W hen the pressure is returned to within the accepta ble

range, the switch does not reset at e xactly the samepoint that it tripped .



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P ressure Switches



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F low Measurement



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F low Rate

Volumetric flow rate (Q) is defined as thevolume of fluid that passes a given point in apipe per unit of time .

Q = Av

M ass flow rate (W) is the mass or weightflowing per unit time .

W = Q



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F low Measuring Units(examp les for common ly used units)

Volumetric Flow Rate:GP M (Gallons per M inute), Liquid

LP M (Liters Per M inute), LiquidCu M eters /Hr (Cu bic M eters Per Hour),

Liquid or GasCF D (Cu bic Feet Per D ay), Gas

Mass Flow Rate:

T/D

Tons per day

Kg/ Hr (Kilogram Per Hour)

Lb/Hr (Pounds Per Hour)



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Ve locity P rofi les

Re < 2 000

Re > 4 000

2000 < Re < 4 000

Re is the Reynolds num ber



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Ve locity P rofi les

For tur bulent flow, after a sufficient straightpipe run the flow profile becomes fullydevelopedfully developed flow is critical to good flowmeasurement system design .In a fully developed flow, the velocity at the

center of the pipe is only a bout 1 .2 times themean velocity .



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Cl assifying F luid F low Measurement



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D P F lowmeter El ements

creating a differential pressure proportional to thesquare of the flow rate

P F (E



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O rifice Pl ate

The Orifice Plate is the most commondifferential pressure flow primary element .It is based on proven technology, has nomoving parts and is suita ble for hightemperature and pressure applications .O rifice plates are recommended for cleanliquids, gases and low velocity steam flows .They are often chosen for their low initialcost, simplicity of construction and ease of installation .



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C oncentric O rifice



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Integrated Differential P ressure

Orifice Meters


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R otameters (Variab le Area F lowmeter)

The rotameter uses a varying restriction to keep

the difference in pressure constant.The varying restriction is made of a tapered tubeand a float.



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Magnetic F lowmeter



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Vortex S hedding

The frequency of vortices shed from a bluff bodyplaced in the flow stream is proportional to thevelocity of the fluid .

V elocity times area gives the volumetric flowrate .V orte x flowmeters provide good measurementaccuracy with liquids, gases, or steam .

They have no moving parts and are fouling tolerant .

V orte x meters can be sensitive to pipeline noise andrequire flow rates high enough to generate vortices .



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V ortex F lowmeterdetectors respond topressure oscillation with alow voltage output signal

with same frequency asthe oscillation



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Vortex F lowmeter


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Time of F light O peration



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Time of F light O peration

the time of flight of the ultrasonic signal is measuredbetween two transducersone upstream and onedownstream . The difference in elapsed time going with or against theflow determines the fluid velocity . W hen the flow is zero, the time for the signal T1 to get toT2 is the same as that required to get from T2 to T1 .W hen there is flow, the effect is to boost the speed of thesignal in the downstream direction, while decreasing it inthe upstream direction .



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Turbine Meter

Fluid passing through a tur bine flowmeter spins arotor .

The rotational speed of the rotor is related to thevelocity of the fluid .M ultiplying the velocity times the cross-sectionalarea of the tur bine provides the volumetric flow rate



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T urbine Flow M eter

accuracy is 0.25% of rategood for clean liquids, gases5 to 10 pipe diametersupstream/downstream

10:1 turndown3 to 5 psig pressure drop


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Coriolis Mass F lowmeters

artificially introduce aCoriolis accelerationinto the flowing stream

measure mass flow bydetecting the resulting

angular momentum



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C orio lis F low Meter


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Coriolis F T



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P ositive D isp lacement ( P D )

P D flowmeters measure the volumetric flow rate of aliquid or gas by separating the flow stream intoknown volumes and counting them over time .

V anes, gears, pistons, or diaphragms are used toseparate the fluid .

P D flowmeters provide good to e xcellent accuracyand are one of only a few technologies that can be

used to measure viscous liquids .However, they create a nonrecovera ble pressureloss and have moving parts su bject to wear .



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S traight P ipe D iameter R equirements

Flow Installation Rules of Thum bUp-stream Down-stream

M agnetic meter 5 2M ass flow meter 1 1V orte x meter 10 2 5Tur bine meter 1 5 10O rifice plate 10 5(B eta = .5)O rifice plate 1 5 5(B eta = . 7 )



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L evel Measurement



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Leve l MeasurementLeve l Measurement

L evel is another common process variable that is measured in many industries. The method used will vary widely depending on the

nature of the industry, the process, and the application.

Inventory:a constant supply or storage of material

Control:continuous, batch, blending, and mi xing controlsta bilize flow to the ne xt process

Alarming:

hi/lo limits, safety shut downData Logging:

material quantities for inventory and billing purposesand where regulatory requirements are necessary



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W hat is measured?W hat is measured?

The measured medium can be liquid, or solid and stored in vessels (open/closed tanks),silos, and bins

Units of level can be expressed in:feet (meters)gallons (liters)pounds (kilograms)cu bic volume (ft 3, m 3)

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D irect MethodsD irect Methods

D irect methods sense the surface or interface of theliquid and is not affected by changes in material density (Specific Gravity)

Examples:D ip S tickResistance TapesS ight Glass

FloatsUltrasonic



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Indirect Methods (Inferentia l)Indirect Methods (Inferentia l)

I ndirect methods infer liquid level by measuring some other physical parameter such as pressure or weight

Changing materials means a corrective factor must be used or recalibrating the instrument

Examples:Hydrostatic head methodsLoad CellsCapacitance

Conductivity



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S e lection C riteriaS e lection C riteria

When determining the type of level sensor that should be used for agiven application, there are a series of questions that must beanswered:

O pen tank or closed tank?Can the level sensor be inserted into the tank or should it becompletely e xternal? Contact or non-contact ? Continuous measurement or point measurement?D irect or Indirect measurement?W

hat type of material is being measured? Liquid or S

olid? Cleanor S lurry?



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D ip S tick D ip S tick

Simple and cheapCan be used with any wet material and not affected by

density.Can not be used withpressurized tanksVisual indication only (electronic versions areavailable)



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R esistance TapeR esistance Tape

The pressure of the fluid in the tank causes the tape to short-circuit, thus changing the total resistance of the measuring tape.An electronic circuit measures the resistance; it's directly related to the liquid level in the tank.



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S ight G lassS ight G lass

Another simple direct method of measuring liquids.

Can be used inpressurized tanks(as long as the glassor plastic tube canhandle the pressure)



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Magnetic Leve l S ensorsMagnetic Leve l S ensors

Used where the sight glass level gauge can

not be.M agneto-resistive types

can provide anelectrical output.

Liquid/liquid interface (such as water and oil) can be measured by changingthe buoyancy of the magnetic float



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Magnetic L evel Indicator


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F loatsF loatsF loat rides the surface level to provide the measurement.

M any different styles are available.

Liquid density does not affect measurement



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D isp lacerD isp lacerD isplacer Type works on The Principle of Buoyancy The displacer pushes liquid aside. I ts weight is reduced by

the weight of the liquid it displaces.



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H ydrostatic H ead Leve l S ensorsH ydrostatic H ead Leve l S ensors

These methods infer level by measuring thehydrostatic head produced by the liquid column.A pressure sensing element is installed at the

bottom of the tank and pressure is converted tolevel.D ifferent liquid densities or closed tank applicationsmust be accounted for.



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Genera l Theory for H ead MeasurementGenera l Theory for H ead Measurement

The P ressure exerted by theH eight of the liquid is:

P = H x Density*

I f the D ensity of the liquid isknown then

H = Pr essu r e

Density*

H eight ( H )

Pressure PSI

Liquid Density (D)

*Note: For liquids other than water, use the density of water 0 .03 6 1 lb/in3as a reference and multiply by the S G of the liquid .



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D ifferentia l P ressure Leve l Measurement

C ontacting dP Levelsuitable for liquids onlyuses same dP transmitter as in dP flow measurement



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C onductivity P robeW hen the liquid makescontact with any of theelectrodes, an electric

current will flowbetween the electrodeand groundThe current energizesa relay



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O ptions for Leve l Measurement

Non-Contacting Radar Level

suita ble for liquids and solidsfoaming, tur bulence, vesselwalls and internals can affectsignal if not installed correctly



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E lectro-mechanical vibratory switch forlevel detection


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Temperature Measurement

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W hat is Temperature?

Temperature is the degree of hotness or coldness, measured on a definite scale.

Heat is a form of energy and is measuredin calories or BTUs

(British Thermal Units )


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H eat Transfer

1. Convection:By moving molecules in gases or liquidsby natural circulation.

2. Conduction:Conduction is the transfer of heatthrough or across a conductor.

3. Radiation:Transfer of heat by rays the sun heatsthe earths atmosphere.


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H eat Transfer by C onvection


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H eat Transfer by C onduction


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H eat Transfer by R adiation


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Temperature Scales



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Temperature Units Conversion

Deg. C = (Deg. F- 32) x 5/9

Deg. F = (Deg. C x 9/5) + 32

Deg. K =(Deg. C + 2 73 . 1 5)

Deg. R =(Deg. F + 4 59 .7 )


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Temperature Measuring Instruments

1. Thermocouples

2. RTD, Resistance Temperature Detectors

3. Bimetal Thermometers

4. Filled System Thermometers

5. G lass Stem Thermometers



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1

R T D R i


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R T D, R esistance Temperature D etectors

RTD are sensors used to measure

temperature by correlating the resistance of the RTD element with temperature .

Common Resistance Materials for R TDs:Platinum (most popular and accurate)

NickelCopper B alco (rare)Tungsten (rare)


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O ti f L l R t T t


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O ptions for Loca l or R emote TemperatureMeasurement

T hermocouples & R T D

Bi l Th


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Bimeta l Thermometers

two thin strips of metal, with different coefficients of e xpansion, W hen one end is fi xed, the other end will

deflect in nearly direct proportion to the change intemperature .


F ill d S t Th t


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F ill ed S ystem Thermometers

The system is completely filledwith fluid (liquid or gas)W ith an increase in measuredtemperature, the fluid in the bulb

will expand and increase thepressure in the bulb, capillary,and bourdon tu be . The pressure sensing elementresponds to the increase inpressure by moving the pointer up the scale


C2


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G lass S tem ThermometersM ercury is used for hot temperaturesand colored alcohol for coldtemperatures

pro blems with glass thermometersB rittleM ercury solidifies at - 40C

Alcohol boils at around 120 CAccurate ones are e xpensive


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Slide 97

C2 for research pupose, as a demonstration of DMs CommitteeComputer, 3/5/ 2007

2_process variables

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