Training - Pressure1

7/27/2019 Training - Pressure1

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Level 1 - Fundamental Training Pressure 1

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Level 1 - Pressure 1

Fundamental TrainingFundamental TrainingLevel 1

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Topics: Slide No:• Why measure pressure? 3• What is pressure? 4 - 5• Pressure terminology 6 - 11• Inferring non-pressure variables 12 - 29• Pressure measurement technology 30 - 44• Pressure calibrators 45• Exercises 46 - 48

ContentsContents





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Why measure pressure?Why measure pressure?4 Common Reasons4 Common Reasons

Safety• prevent pressurized pipes & vessels from burstingProcess Effic iency• variation of pressure below or above a set-point wi ll result in

scrap rather than useable product in some manufacturingprocess

Cost Saving• preventing unn ecessary expense of creating more pressure or

vacuum than is required saves money

Inferred Measurement of Other Variables• rate of flow through a pipe• level of fluid in a tank• density of fluid• how two or mor e liquids in a tank interface

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What is pressure?What is pressure?The Same Weight , Diff erent PressureThe Same Weight , Different Pressure

1 sq ins 100 sq in s

1 sq ins100 sq in s

Weight = 100lb

Pressure = Pressure =1lb/in² 100 lb/in²





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What is pressure?What is pressure?Liquid & Gas PressuresLiquid & Gas Pressures

LIQUIDSThe pressure exerted by a liquid is influenced by 3 main factors.

1. The height of the liquid.2. The density of the liquid.3. The pressure on the surface of the liquid.

GASESThe pressure exerted by a gas is influenced by 2 main factors.

1. Volume of the gas con tainer.2. Temperature of the gas

Note. Gases are compr essible whereas liqu ids are not

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I/P

PT

PIC • Pressure Loop Issues: – May be a Fast Process

» Liquid» Small Volume

– May Require Fast Equipm ent

Pressure terminologyPressure terminology

Pressure Control LoopPressure Control Loop





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Pressure terminologyPressure terminologyEngineering UnitsEngineering Units

Pressure is defined as FORCE applied over a unit AREA.

P = F/AExamples of pressure units:Units of force per un it areaPascals Pa N / m 2 (Newtons / square metre)psi lbs/in 2 (Pounds / square inch)Bar Bar = 100,000 Pa

Units referenced to columns of liquidsins. water gauge in H 2Omm water gauge mm H 2O

ins. mercury in Hgmm mercury mm Hg

Atm osphere atm

Pressure applied by a 1 inch column of mercur y witha density of 13.5951 g/cm³.

Pressure exerted by the earth’s atmosphere at sea level(approximately 14.6959psi)

Pressure applied by a 1 inch column of water at 20°C.

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Gage(psig) - Level of pressure relative to atmospheric– Positive or negative in magnitude

Atm osp her ic Pr essu re App rox. 14.7 ps ia

Abso lu te

Gage CompoundRange

BarometricRange

PressureTotal Vacuum(Zero Abso lute)

Abso lu te(psia) - based from zero absolute pressure - no massTypical atm reference: 14.73 psia

Compound Range (psig) - Gage reading vacuum as negative value

Differential(psid) - difference in pressure between two points

Pressure t erminologyPressure t erminology

Reference PressureReference Pressure





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AbsoluteZero

Total Vacuum

Atm. Pressure 14.7 psi a

5 psig

?

Psia19.7

5 psivacuum

?Psia

?Psig-5 9.7

A s su m e : P a t m = 1 4 . 7 p s ia ; 2 8 i n c h e s H 2 O p e r p s i

1000 in H 2 O = ___________ psi35.71

Pressure terminologyPressure terminologyQuizQuiz

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Pressure terminologyPressure terminology

Measurable PressuresMeasurable PressuresThe four most common types of measurable pressures

used in the process control i ndustr ies are:

1 . Head Pressure or Hydrostatic Pressure.Head Pressure or Hydrostatic Pressure.Pressure exerted by a column of liqui d in a tank open toatmosphere, HEAD PRESSURE = HEIGHT x DENSITY

2. Static Pressure, Line Pressure, or Working pressureStatic Pressure, Line Pressure, or Working pressurePressure exerted in a closed system

3. Vapor PressureVapor Pressure

The temperature at which a liquid b oils, or turn s into a vapor varies depending on the pressure. The higher the pressure, thehigher the boiling point.

4. VacuumVacuum Abso lu te pressure below atmospher ic pressure ( a compo undrange gage transmitter will read a negative pressure)





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Pressure terminologyPressure terminologyMeasurable PressureMeasurable Pressure

Typical Vapor Pressure Curve

P r e s s u r e

( l o g

)

Temperature

liquid

gas Higher Altitute

Lower Altitute(Sea Level)

T1 T2

Vapor pressure increases with temperature.

• Liquid boils when its vapor pressure equalsatmospheric pressure.

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Flow Restriction in Line cause a differential Pressure

Line Pressure

Q V= K DP

Orifice Plate

Inferring nonInferring non --pressure variablespressure variables

FlowFlow





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Theoritical equations come from 3 sources:

Continuity Equation• Flow into pipe equals flow out of pipe and is the same at all pipe

cross sections (Conservation of Mass)

Bernoulli’s Equation• (Conservation of Energy for fluid in a pipe)

Experimentally Determined Correction Factors• Discharge Coefficient• Gas Expansion Factor

Q m = K DP

Inferring nonInferring non --pressure variablespressure variablesFlowFlow

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The volume flowing into a pipe equals the volumeflowing out of pipe, assuming constant density

A1V1 A2V2Flow Flow

v 1 = A2/A1 x v 2v 1 = d 2/D2 x v 2

⌫ π d 2 /4 x π D 2 /4

Continuity Equation

A1v 1 = A2v 2A = area of pipe cross section v = velocity

⌫ d/D= β

v 1 = β2 x v 2


FlowFlow





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Bernoulli’s Equation

⌫ cancel - off for level pipe

v 1 v 2

P1 P2

D d

Three energies:Kinetic (1/2 ρv2)Potential ( ρgh)Static Pressure (P)

Flow

The total energy before the restriction in the pipemust equal the total energy after the restriction.


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P 1 P 2..1

2ρ v 2

2 ..1

2ρ v 1

2 common

P1..1

2ρ v 1

2 ..ρ g h 1 P2..1

2ρ v 2

2 ..ρ g h 2

Before restriction After restriction

dP = ½ ρ (v 22 - v 1

2 )

2 / ρ x dP = v 22 - v 1

2 V 1

2 = ( β 2 x V 2 ) 2

2 / ρ x dP = v 22 - β4 x v 2

2

2 / ρ x dP = (1- β4) v 22

common subject

v 22 = (2 / ρ x dP) / (1- β4) Re-arranged


FlowFlow





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v 2 = [(2 / ρ x dP) / (1- β4)] ½

v 2 = (2) ½ x (1/ ρ) ½ x 1/ (1- β4)½ x (dP) ½

Q v2 = ( πd 2 /4) x (2) ½ x (1/ ρ) ½ x 1/ (1- β4)½ x (dP) ½

Q v2 = A 2 x v 2

constant constant assumed constant

velocity of approach constant - “E”

Q v2 = k (dP/ ρ) ½Volumetric FlowQ m2 = k (dP x ρ) ½Mass Flow k (dP/ρ)½ x ρ


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(i) What would be thedifferential at 10m³/s?

Quiz:If an orif ice plate creates a differential of 50 kPa at 30m³/s

DP2 = 5.6kPa

(ii) What would be the flowrate at 30kPa differential?

30/Qv2 = √50/ √30

Qv = K √DP

Qv1 √DP1--- = ----

Qv2 √DP230/10 = √50/ √DP2

Qv2 = 23.26m³/s

Qv = K √DP

Qv1 √DP1--- = ----

Qv2 √DP2


FlowFlow





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H

P P P P

D

Liquid

Hydrostatic Pressure - The liquid will rise to the same levelin each vessel regardless of its diameter & shape.

Which shape gives higher pressure at the bottom of the

vessel?

Unit Area (eg. per cm 2 )

Similar height of column will have same mass acting on the same unit area

SAMEPRESSURE

Inferring nonInferring non --pressure variablespressure variablesLevelLevel

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The hydrostatic pressure exerted by the column of li quiddepends on the S.G. (or density) of the liquid and itsvertical height.

Density of liquid = D Average cross-section area of vessel = AVertical height of liquid = HVolume of liquid, V =Total weight of liquid, M =

=

Pressure at the bottom of liquid = weight of liquidcross-section area==

H x AD x V

A x H

D x H With reference to inches or mm WATER S.G x H

D x

(D x A x H) / A


LevelLevel





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P = r x g x height x area / area

mass x g

r x volume Density = mass/volume = r

P= force / area g = gravitational acceleration

height x area

P head = r x g x h Pascal


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LevelLevel

XMTR

HL

Ullage or Vapor

S.G

Phead

P head = S.G x Height 0%

100%

H e i g h t

DP Transmitter at thebottom of the tankmeasures HEAD.HEAD = pressure at thebottom of a column of liquid with known

relative density (S.G)

Height = P head / S.G

Cancelled off sinc e both L& H sides of transmitt er experience i t.





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Quiz: Open TankWhat is the level if P max = 120inH2O, s.g.= 1.2?

XMTR

HL

?Height = P head / S.G

Height = 120 / 1.2

Height = 100 inches


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Quiz: Closed TankDry leg: no fluid in lowside impulse piping, or legP h = 105 psiP l = 100 psiWhat is level if s .g. = 1.0?

P top = Ullage

XMTR

HL

dP = 5 psi = 5 x 28 inH 2OHeight = 140 / 1.0

Height = 140 inches

Phead


LevelLevel





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P bottom =

P top =

P bottom - P to p =

Hence,

S.G =

Ptop

Phead(top)

P bottom

Ptop

Phead(bottom)

h1

h2

Liquid level must be above the Top transmitter tap.

H

H

S.G X h 2

S.G X h 1

S.G (h 2 - h 1 )

diff. Pressure / dist. betw. taps

Inferring nonInferring non --pressure variablespressure variablesDensityDensity

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Ullage

P bottom

Ptop

50”

H

H

Quiz:Determined the S.G of the processfluid if P top = 20 psiP bottom = 22 psiDistance between taps = 50 inches

Assuming 1 psi = 28”H 2O

S.Gprocess = DP / dist. betw. Taps= 56 / 50= 1.12

DP = (22 -20) = 2 psi = 56”H 2 O


DensityDensity





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At 0% Liquid Interface (4mA)

DP = H side - Lside

= (SG 1*h1) - [(SG f *(h1-h2)) + (SG 1*h2)]

Indirectly measures liquid Interface

P bottom

Ptop

L H

RemoteSeal

Vapor

0%

100%

SG 1

SG 2

Dist. Betw.Taps

(h1 - h2)

Total Liquid level must always be above theTop transmitter tap.

SG f

Inferring nonInferring non --pressure variablespressure variablesInterfaceInterface

h1

h2

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Total Liquid level must always be above theTop transmitter tap.

P bottom

Ptop

L H

RemoteSeal

Vapor

0%

100%

SG 1

SG 2

Dist. Betw.Taps

(h1 - h2)

At 100% Liquid Interface (20mA)

DP = H side - Lside

= [SG 2*(h1-h2) + SG 1*h2)] - [(SG f *(h1-h2)) + (SG 1*h2)]

Indirectly measures liquid Interface


InterfaceInterface

h1

h2SG f





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Application Example:

• Transmitter calibrated from120”H 2Oto 132”H 2O

• Determine % of interface of Liquid A with respect to Liquid B

Vapor

0%

100%

SG 1= 1.0

SG 2= 1.1

P bottom

Ptop

L H

RemoteSeal

10 ft

Liquid A

Liquid B

123 inH 2O

If transmit ter reads 123 inH 2O

% interface= (3/12) * 100%= 25%

Inferring nonInferring non --pressure variablespressure variablesInterfaceInterface

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Barometer Used to measure Barometric Pressure

Reference is 0 psia, due to low vapor pressure of Hg.General operating principle:

Phead Patm

Barometric Pressure = Atmospheric Pressure

29.9 inHgWhat i s the barometric Pressure?

• Tube completely filled with mercury & Invert intothe container filled with mercury.

• The mercury level in the tube will drop unt il itreaches an equilibriu m.

• This equilibrium h eight is a measure of atmospheric pressure. P head = P atm

Pressure measurement technologyPressure measurement technology

Pressure GaugesPressure Gauges





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dP = H (SGfill fluid - SGprocess fluid ) – Reference side can be:

• Sealed (AP reference)• Open to atmosphere(GP reference)• Connected to reference pressure(DP reference)

– Typically used for low pressures, non process control

Manometers

U-tube wi th one side reference, one side measured pressure

H

How to check for dP ?

Pressure measurement technologyPressure measurement technologyPressure GaugesPressure Gauges

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Mechanical

The mechanical elementtechniques convert appliedpressure into displacement .

The displacement may beconverted into electricalsignal with help of Linear

Variable DisplacementTransformer (LVDT).


Pressure GaugesPressure Gauges





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Output to Actuator (or Relay)Constant flowrate maintained

(Compressed air)

NozzleFlapper

Bourdon Tube

Process Pressure

Pressure measurement technologyPressure measurement technologyPneumatic Pressure CellsPneumatic Pressure Cells

Pneumatic Controller

Relay’s modulated output is the controller output which isusually a pneumatic signal that adjusts the final controlelement (Control valve)

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Disadvantages – Reconfiguration costly

– Losses occur over longpiping runs

– Performance levels are notcomparable to electronicinstrumentation

Pressure Transmitter Produce a linear output proportional to input pressure

Zero Scale:Full Scale:

3 psig15 psig


Pneumatic Pressure CellsPneumatic Pressure Cells





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– Made up of 2 main elements:• Transducer - Electronic sensor module

that registers processvariable and outputs acorresponding usableelectrical signaleg. resistance, millivolts,capacitance, etc.

• Electronics - Convert transducer output toa standard output signal

eg. 4 - 20 mA, 1 - 5 V dc,digital signal, etc.

Pressure measurement technologyPressure measurement technologyElectronic Pressure Transmit tersElectronic Pressure Transmitters

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Transmitter

Signal fromsensor module(Transducer)

Signal To Controller

Process Variable

(Standard signals)

SensingDiaphragm

(Line / Static Pressure)

Example of Application

Transmitter configured tooperate from:

0 to 50 psiElectronic Output:

4 to 20 mAThis mean 0% reading (0 psi)represents 4 mA and 100%reading (50 psi) represents 20

mA.

What will be the output current at 25 psi reading?4 + (25/50)*16 = 12 mA


Electronic Pressure Transmit tersElectronic Pressure Transmitters





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Characterized by the type of sensing element:

– Variable capacitance – Variable Resistance (Wheatstone bridge)

• Strain gauge» Thin -film strain gauge» Diffused, strain gauge

– Variable inductance – Variable reluctance – Vibrating wire – Piezoelectric

Pressure measurement technologyPressure measurement technologyElectronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

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Variable Capacitance

• Process pressure transmitted thruisolating diaphragm

• Distortion of sensing diaphragmproportional to the differentialpressure

• Position of sensing diaphragmdetected by capacitor plates

• Differential capacitance translated to4-20mA or 10-50mA output dcsignal.


Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules





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Variable Resistance / Piezo-Resist ive

Thin FilmStrain Gauge

DiffusedStrain Gauge

• Process pressure transmitted thru isolating diaphragm• Very small distortion in sensing diaphragm• Applies strain to a wheatstone bridge circuit• Change in resistance translated to 4-20mA or 1-5V dc signal• GP XMTRs - ref. side of sensor exposed to atm. Pressure• AP XMTRs - sealed vacuum reference.


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• Piezoelectric crystal is anatural or a synthetic crystalthat produces a voltage whenpressure is applied to it.

• Voltage produce by crystalincreases with increases inpressure and vice-versa.

• The produced small voltage is

then amplified to a standardcontrol signal.

Piezoelectric

Ampl if ier &electronics

Control Signal

PiezoelectricCrystal

Diaphragm

ProcessPressure







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• Inductance is the opposition toa change in current flow

• Alternating current passthrough the coil

• Elastic element connected tocore

• Applied pressure deflectselastic element

• Position of core changesrelative to coil resulting inchange in inductance

• Resistor connected in serieswith inductor to measurechange in voltage.

Variable Inductance


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• Reluctance is a property of magnetic circuit

• A moving magnetic elementlocated between two coils

• Coil turn electromagnet whenexcited by AC source

• Position of element with respect tothe coils determines differentialmagnetic reluctance

• Thus differential inductance withinthe coils

• A bridge is used to measurechanges in a circuit

Variable Reluctance







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• Wire located in magnetic fieldvibrate when current pass throughit

• Wire movement within fieldinduces current into it

• Induced voltage amplified asoutput signal

• Vibration frequency depends onwire tension

• Elastic element connected towire.

• Frequency of wire vibrationbecome a function of measuredpressure

• Direct digital output signal

Vibrating Wire


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– Sensor (transducer) module is part of the transmitter. – Sensor w ill become active only when the transmitter is

powered. (Attenuation) – Output Electronics in the transmitter translates the

userable electrical signal from the sensor into astandard output signal.

Output Electronics

Sensor Module

Output Electronics

Sensor Module

DiaphragmSeal







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ISO Require calibration device to be 4 times more accurate thanthe accuracy of the instr ument being calibrated.If the reference accur acy of a 3051C transm itter is 0.075% of span,

– What should the accuracy of the C/V pressure sourcebe?

– the equipment for calibrating the pressure source?

If the diameter of the ball on a dead weight tester is 0.75 inches. Theweight o f a plate is 723g.

– What is the pressure required to freely float that plate on thedead weight tester (g/cm 2)?

Pressure calibratorsPressure calibratorsISO RequirementISO Requirement

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ExerciseExercise

1. If the atmospheric pressure drop by 0.1 % and the linepressure remains unchanged, what changes will occur in the

readings?(A) AP reading will change.(B) GP reading will change.(C) Both reading will change.(D) Both reading will not change.

[ ]

2. If a customer wants to measure vacuum, what type of transmitter should be used?(A) AP(B) DP(C) GP [ ]

Liquid flow

Line pressure = 80 psig

94.7psi 80.psi GPTransmitter

APTransmitter




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ExerciseExercise

Write down the readings in (psi) that are recorded by the transmittersin the above application (Atmosphere = 14.7 psi).

3. Differential Pressure Transmitter (a): [ ]

4. Gage Pressure Transmitter (b): [ ]

5. Absolute Pressure Transmitter (c): [ ]

50 psig80 psig

c a b

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ExerciseExercise

6. What is the differential pressure (P1 - P2) in kPa being applied to themanometer in the the above application ?

S.G of Process Fluid @Temp + Pressure = 1.0

P2P1

S.G. = 13.6200mm

(Note 1 mm H 2O = 9.8 Pa)

Training - Pressure1

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