pressure measurement in Gas technology

1. 1
Level 1 - Pressure 1RMT Training - 05 /98
Fundamental TrainingFundamental Training
Level 1

2. 2
Level 1 - Pressure 1RMT Training - 05 /98
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

3. 3
Level 1 - Pressure 1RMT Training - 05 /98
Why measure pressure?Why measure pressure?
4 Common Reasons4 Common Reasons
Safety
• prevent pressurized pipes & vessels from bursting
Process Efficiency
• variation of pressure below or above a set-point will result in
scrap rather than useable product in some manufacturing
process
Cost Saving
• preventing unnecessary 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 more liquids in a tank interface

4. 4
Level 1 - Pressure 1RMT Training - 05 /98
What is pressure?What is pressure?
The Same Weight, Different PressureThe Same Weight, Different Pressure
1 sq ins 100 sq ins
1 sq ins100 sq ins
Weight = 100lb
Pressure = Pressure =1lb/in² 100 lb/in²

5. 5
Level 1 - Pressure 1RMT Training - 05 /98
What is pressure?What is pressure?
Liquid & Gas PressuresLiquid & Gas Pressures
LIQUIDS
The 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.
GASES
The pressure exerted by a gas is influenced by 2 main factors.
1. Volume of the gas container.
2. Temperature of the gas
Note. Gases are compressible whereas liquids are not

6. 6
Level 1 - Pressure 1RMT Training - 05 /98
I/P
PT
PIC • Pressure Loop Issues:
– May be a Fast Process
» Liquid
» Small Volume
– May Require Fast Equipment
Pressure terminologyPressure terminology
Pressure Control LoopPressure Control Loop

7. 7
Level 1 - Pressure 1RMT Training - 05 /98
Pressure terminologyPressure terminology
Engineering UnitsEngineering Units
Pressure is defined as FORCE applied over a unit AREA.
P = F/A
Examples of pressure units:
Units of force per unit area
Pascals Pa N / m2
(Newtons / square metre)
psi lbs/in2
(Pounds / square inch)
Bar Bar = 100,000 Pa
Units referenced to columns of liquids
ins. water gauge in H2O
mm water gauge mm H2O
ins. mercury in Hg
mm mercury mm Hg
Atmosphere atm
Pressure applied by a 1 inch column of mercury with
a 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.

8. 8
Level 1 - Pressure 1RMT Training - 05 /98
Gage(psig) - Level of pressure relative to atmospheric
– Positive or negative in magnitude
Atmospheric Pressure
Approx. 14.7 psia
Absolute
Gage Compound
Range
Barometric
Range
Pressure
Total Vacuum
(Zero Absolute)
Absolute(psia) - based from zero absolute pressure - no mass
Typical atm reference: 14.73 psia
Compound Range (psig) - Gage reading vacuum as negative value
Differential(psid) - difference in pressure between two points
Pressure terminologyPressure terminology
Reference PressureReference Pressure

9. 9
Level 1 - Pressure 1RMT Training - 05 /98
Absolute
Zero
Total Vacuum
Atm. Pressure 14.7 psia
5 psig
?
Psia19.7
5 psi
vacuum
?
Psia
?
Psig-5 9.7
Assume: Patm = 14.7psia; 28 inches H2O per psi
1000 in H2O = ___________ psi35.71
Pressure terminologyPressure terminology
QuizQuiz

10. 10
Level 1 - Pressure 1RMT Training - 05 /98
Pressure terminologyPressure terminology
Measurable PressuresMeasurable Pressures
The four most common types of measurable pressures
used in the process control industries are:
1. Head Pressure or Hydrostatic Pressure.Head Pressure or Hydrostatic Pressure.
Pressure exerted by a column of liquid in a tank open to
atmosphere, HEAD PRESSURE = HEIGHT x DENSITY
2. Static Pressure, Line Pressure, or Working pressureStatic Pressure, Line Pressure, or Working pressure
Pressure exerted in a closed system
3. Vapor PressureVapor Pressure
The temperature at which a liquid boils, or turns into a vapor
varies depending on the pressure. The higher the pressure, the
higher the boiling point.
4. VacuumVacuum
Absolute pressure below atmospheric pressure ( a compound
range gage transmitter will read a negative pressure)

11. 11
Level 1 - Pressure 1RMT Training - 05 /98
Pressure terminologyPressure terminology
Measurable PressureMeasurable Pressure
Typical Vapor Pressure Curve
Pressure(log)
Temperature
liquid
gas
Higher
Altitute
Lower
Altitute
(Sea Level)
T1 T2
Vapor pressure increases with temperature.
• Liquid boils when its vapor pressure equals
atmospheric pressure.

12. 12
Level 1 - Pressure 1RMT Training - 05 /98
Flow Restriction in Line cause a differential Pressure
Line Pressure
QV= K DP
Orifice Plate
Inferring non-pressure variablesInferring non-pressure variables
FlowFlow

13. 13
Level 1 - Pressure 1RMT Training - 05 /98
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
Qm= K DP
Inferring non-pressure variablesInferring non-pressure variables
FlowFlow

14. 14
Level 1 - Pressure 1RMT Training - 05 /98
The volume flowing into a pipe equals the volume
flowing out of pipe, assuming constant density
A1V1 A2V2Flow Flow
v1 = A2/A1 x v2
v1 = d2
/D2
x v2
πd2
/4 x πD2
/4
Continuity Equation
A1v1 = A2v2
A = area of pipe cross section
v = velocity
d/D = β
v1 = β2
x v2
Inferring non-pressure variablesInferring non-pressure variables
FlowFlow

15. 15
Level 1 - Pressure 1RMT Training - 05 /98
Bernoulli’s Equation
cancel - off for level pipe
v1 v2
P1 P2
D d
Three energies:
Kinetic (1/2ρv2
)
Potential (ρgh)
Static Pressure (P)
Flow
The total energy before the restriction in the pipe
must equal the total energy after the restriction.
Inferring non-pressure variablesInferring non-pressure variables
FlowFlow

16. 16
Level 1 - Pressure 1RMT Training - 05 /98
P 1 P 2
..1
2
ρ v 2
2 ..1
2
ρ v 1
2
common
P 1
..1
2
ρ v 1
2 ..ρ g h 1 P 2
..1
2
ρ v 2
2 ..ρ g h 2
Before restriction After restriction
dP = ½ ρ (v2
2
- v1
2
)
2 / ρ x dP = v2
2
- v1
2
V1
2
= (β2
x V2)2
2 / ρ x dP = v2
2
- β4
x v2
2
2 / ρ x dP = (1- β4
) v2
2
common
subject
v2
2
= (2 / ρ x dP) / (1- β4
) Re-arranged
Inferring non-pressure variablesInferring non-pressure variables
FlowFlow

17. 17
Level 1 - Pressure 1RMT Training - 05 /98
v2 = [(2 / ρ x dP) / (1- β4
)] ½
v2 = (2)½
x (1/ρ)½
x 1/ (1- β4
)½
x (dP)½
Qv2 = (πd2
/4) x (2)½
x (1/ρ)½
x 1/ (1- β4
)½
x (dP)½
Qv2 = A2 x v2
constant constant assumed
constant
velocity of approach
constant - “E”
Qv2 = k (dP/ρ)½Volumetric Flow
Qm2 = k (dP x ρ)½Mass Flow k (dP/ρ)½
x ρ
Inferring non-pressure variablesInferring non-pressure variables
FlowFlow

18. 18
Level 1 - Pressure 1RMT Training - 05 /98
(i) What would be the
differential at 10m³/s?
Quiz:
If an orifice plate creates a differential of 50 kPa at 30m³/s
DP2 = 5.6kPa
(ii) What would be the flow
rate at 30kPa differential?
30/Qv2 = √50/ √30
Qv = K √DP
Qv1 √DP1
--- = ----
Qv2 √DP2
30/10 = √50/ √DP2
Qv2 = 23.26m³/s
Qv = K √DP
Qv1 √DP1
--- = ----
Qv2 √DP2
Inferring non-pressure variablesInferring non-pressure variables
FlowFlow

19. 19
Level 1 - Pressure 1RMT Training - 05 /98
H
P P P P
D
Liquid
Hydrostatic Pressure - The liquid will rise to the same level
in each vessel regardless of its diameter & shape.
Which shape gives higher pressure at the bottom of the
vessel?
Unit Area (eg. per cm2
)
Similar height of column
will have same mass acting
on the same unit area
SAME
PRESSURE
Inferring non-pressure variablesInferring non-pressure variables
LevelLevel

20. 20
Level 1 - Pressure 1RMT Training - 05 /98
The hydrostatic pressure exerted by the column of liquid
depends on the S.G. (or density) of the liquid and its
vertical height.
Density of liquid = D
Average cross-section area of vessel = A
Vertical height of liquid = H
Volume of liquid, V =
Total weight of liquid, M =
=
Pressure at the bottom of liquid = weight of liquid
cross-section area
=
=
H x A
D 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
Inferring non-pressure variablesInferring non-pressure variables
LevelLevel

21. 21
Level 1 - Pressure 1RMT Training - 05 /98
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
Phead = r x g x h
Pascal
Inferring non-pressure variablesInferring non-pressure variables
LevelLevel

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Level 1 - Pressure 1RMT Training - 05 /98
Inferring non-pressure variablesInferring non-pressure variables
LevelLevel
XMTR
HL
Ullage or
Vapor
S.G
Phead
Phead = S.G x Height 0%
100%
Height
DP Transmitter at the
bottom of the tank
measures HEAD.
HEAD = pressure at the
bottom of a column of
liquid with known
relative density (S.G)
Height = Phead / S.G
Cancelled off since both L
& H sides of transmitter
experience it.

23. 23
Level 1 - Pressure 1RMT Training - 05 /98
Quiz: Open Tank
What is the level if Pmax = 120
inH2O, s.g.= 1.2?
XMTR
HL
?Height = Phead / S.G
Height = 120 / 1.2
Height = 100 inches
Inferring non-pressure variablesInferring non-pressure variables
LevelLevel

24. 24
Level 1 - Pressure 1RMT Training - 05 /98
Quiz: Closed Tank
Dry leg: no fluid in low
side impulse piping, or leg
Ph = 105 psi
Pl = 100 psi
What is level if s.g. = 1.0?
Ptop= Ullage
XMTR
HL
dP = 5 psi = 5 x 28 inH2O
Height = 140 / 1.0
Height = 140 inches
Phead
Inferring non-pressure variablesInferring non-pressure variables
LevelLevel

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Level 1 - Pressure 1RMT Training - 05 /98
Pbottom =
Ptop =
Pbottom - Ptop =
Hence,
S.G =
Ptop
Phead(top)
Pbottom
Ptop
Phead(bottom)
h1
h2
Liquid level must be above the Top transmitter tap.
H
H
S.G X h2
S.G X h1
S.G (h2 - h1)
diff. Pressure / dist. betw. taps
Inferring non-pressure variablesInferring non-pressure variables
DensityDensity

26. 26
Level 1 - Pressure 1RMT Training - 05 /98
Ullage
Pbottom
Ptop
50”
H
H
Quiz:
Determined the S.G of the process
fluid if
Ptop = 20 psi
Pbottom = 22 psi
Distance between taps = 50 inches
Assuming 1 psi = 28”H2O
S.Gprocess = DP / dist. betw. Taps
= 56 / 50
= 1.12
DP = (22 -20) = 2 psi = 56”H2O
Inferring non-pressure variablesInferring non-pressure variables
DensityDensity

27. 27
Level 1 - Pressure 1RMT Training - 05 /98
At 0% Liquid Interface (4mA)
DP = Hside - Lside
= (SG1*h1) - [(SGf*(h1-h2)) + (SG1*h2)]
Indirectly measures liquid Interface
Pbottom
Ptop
L H
Remote
Seal
Vapor
0%
100%
SG1
SG2
Dist. Betw.
Taps
(h1 - h2)
Total Liquid level must
always be above the
Top transmitter tap.
SGf
Inferring non-pressure variablesInferring non-pressure variables
InterfaceInterface
h1
h2

28. 28
Level 1 - Pressure 1RMT Training - 05 /98
Total Liquid level must
always be above the
Top transmitter tap.
Pbottom
Ptop
L H
Remote
Seal
Vapor
0%
100%
SG1
SG2
Dist. Betw.
Taps
(h1 - h2)
At 100% Liquid Interface (20mA)
DP = Hside - Lside
= [SG2*(h1-h2) + SG1*h2)] - [(SGf*(h1-h2)) + (SG1*h2)]
Indirectly measures liquid Interface
Inferring non-pressure variablesInferring non-pressure variables
InterfaceInterface
h1
h2
SGf

29. 29
Level 1 - Pressure 1RMT Training - 05 /98
Application Example:
• Transmitter calibrated from
120”H2Oto 132”H2O
• Determine % of interface of
Liquid A with respect to Liquid B
Vapor
0%
100%
SG1= 1.0
SG2= 1.1
Pbottom
Ptop
L H
Remote
Seal
10 ft
Liquid A
Liquid B
123 inH2O
If transmitter reads 123 inH2O
% interface
= (3/12) * 100%
= 25%
Inferring non-pressure variablesInferring non-pressure variables
InterfaceInterface

30. 30
Level 1 - Pressure 1RMT Training - 05 /98
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 is the barometric Pressure?
• Tube completely filled with mercury & Invert into
the container filled with mercury.
• The mercury level in the tube will drop until it
reaches an equilibrium.
• This equilibrium height is a measure of
atmospheric pressure. Phead = Patm
Pressure measurement technologyPressure measurement technology
Pressure GaugesPressure Gauges

31. 31
Level 1 - Pressure 1RMT Training - 05 /98
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 with one side reference, one side measured pressure
H
How to check for dP ?
Pressure measurement technologyPressure measurement technology
Pressure GaugesPressure Gauges

32. 32
Level 1 - Pressure 1RMT Training - 05 /98
Mechanical
The mechanical element
techniques convert applied
pressure into displacement.
The displacement may be
converted into electrical
signal with help of Linear
Variable Displacement
Transformer (LVDT).
Pressure measurement technologyPressure measurement technology
Pressure GaugesPressure Gauges

33. 33
Level 1 - Pressure 1RMT Training - 05 /98
Output to Actuator (or Relay)
Constant flowrate maintained
(Compressed air)
Nozzle
Flapper
Bourdon Tube
Process Pressure
Pressure measurement technologyPressure measurement technology
Pneumatic Pressure CellsPneumatic Pressure Cells
Pneumatic Controller
Relay’s modulated output is the controller output which is
usually a pneumatic signal that adjusts the final control
element (Control valve)

34. 34
Level 1 - Pressure 1RMT Training - 05 /98
Disadvantages
– Reconfiguration costly
– Losses occur over long
piping runs
– Performance levels are not
comparable to electronic
instrumentation
Pressure Transmitter
Produce a linear output proportional to input pressure
Zero Scale:
Full Scale:
3 psig
15 psig
Pressure measurement technologyPressure measurement technology
Pneumatic Pressure CellsPneumatic Pressure Cells

35. 35
Level 1 - Pressure 1RMT Training - 05 /98
– Made up of 2 main elements:
• Transducer - Electronic sensor module
that registers process
variable and outputs a
corresponding usable
electrical signal
eg. resistance, millivolts,
capacitance, etc.
• Electronics - Convert transducer output to
a standard output signal
eg. 4 - 20 mA, 1 - 5 V dc,
digital signal, etc.
Pressure measurement technologyPressure measurement technology
Electronic Pressure TransmittersElectronic Pressure Transmitters

36. 36
Level 1 - Pressure 1RMT Training - 05 /98
Transmitter
Signal from
sensor module
(Transducer)
Signal To Controller
Process Variable
(Standard signals)
Sensing
Diaphragm
(Line / Static Pressure)
Example of Application
Transmitter configured to
operate from:
0 to 50 psi
Electronic Output:
4 to 20 mA
This 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
Pressure measurement technologyPressure measurement technology
Electronic Pressure TransmittersElectronic Pressure Transmitters

37. 37
Level 1 - Pressure 1RMT Training - 05 /98
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 technology
Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

38. 38
Level 1 - Pressure 1RMT Training - 05 /98
Variable Capacitance
• Process pressure transmitted thru
isolating diaphragm
• Distortion of sensing diaphragm
proportional to the differential
pressure
• Position of sensing diaphragm
detected by capacitor plates
• Differential capacitance translated to
4-20mA or 10-50mA output dc
signal.
Pressure measurement technologyPressure measurement technology
Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

39. 39
Level 1 - Pressure 1RMT Training - 05 /98
Variable Resistance / Piezo-Resistive
Thin Film
Strain Gauge
Diffused
Strain 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.
Pressure measurement technologyPressure measurement technology
Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

40. 40
Level 1 - Pressure 1RMT Training - 05 /98
• Piezoelectric crystal is a natural or a synthetic
crystal that produces a voltage when pressure
is applied to it.
• Voltage produce by crystal increases with
increases in pressure and vice-versa.
• The produced small voltage is then amplified to
a standard control signal.
Piezoelectric
Amplifier &
electronics
Control Signal
Piezoelectric
Crystal
Diaphragm
Process
Pressure
Pressure measurement technologyPressure measurement technology
Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

41. 41
Level 1 - Pressure 1RMT Training - 05 /98
• Inductance is the opposition to a change in
current flow
• Alternating current pass through the coil
• Elastic element connected to core
• Applied pressure deflects elastic element
• Position of core changes relative to coil
resulting in change in inductance
• Resistor connected in series with inductor to
measure change in voltage.
Variable Inductance
Pressure measurement technologyPressure measurement technology
Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

42. 42
Level 1 - Pressure 1RMT Training - 05 /98
• Reluctance is a property of
magnetic circuit
• A moving magnetic element
located between two coils
• Coil turn electromagnet when
excited by AC source
• Position of element with respect to
the coils determines differential
magnetic reluctance
• Thus differential inductance within
the coils
• A bridge is used to measure
changes in a circuit
Variable Reluctance
Pressure measurement technologyPressure measurement technology
Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

43. 43
Level 1 - Pressure 1RMT Training - 05 /98
• Wire located in magnetic field vibrate when current
pass through it
• Wire movement within field induces current into it
• Induced voltage amplified as output signal
• Vibration frequency depends on wire tension
• Elastic element connected to wire.
• Frequency of wire vibration become a function of
measured pressure
• Direct digital output signal
Vibrating Wire
Pressure measurement technologyPressure measurement technology
Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

44. 44
Level 1 - Pressure 1RMT Training - 05 /98
– Sensor (transducer) module is part of the transmitter.
– Sensor will become active only when the transmitter is
powered. (Attenuation)
– Output Electronics in the transmitter translates the
userable electrical signal from the sensor into a
standard output signal.
Output Electronics
Sensor Module
Output Electronics
Sensor
Module
Diaphragm
Seal
Pressure measurement technologyPressure measurement technology
Electronic Pressure Sensor ModulesElectronic Pressure Sensor Modules

45. 45
Level 1 - Pressure 1RMT Training - 05 /98
ISO Require calibration device to be 4 times more accurate than
the accuracy of the instrument being calibrated.
If the reference accuracy of a 3051C transmitter is 0.075% of
span,
– What should the accuracy of the C/V pressure source
be?
– the equipment for calibrating the pressure source?
If the diameter of the ball on a dead weight tester is 0.75 inches. The
weight of a plate is 723g.
– What is the pressure required to freely float that plate on the
dead weight tester (g/cm2
)?
Pressure calibratorsPressure calibrators
ISO RequirementISO Requirement

46. 46
Level 1 - Pressure 1RMT Training - 05 /98
ExerciseExercise
1. If the atmospheric pressure drop by 0.1 % and the line
pressure 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 GP
Transmitter
AP
Transmitter

47. 47
Level 1 - Pressure 1RMT Training - 05 /98
ExerciseExercise
Write down the readings in (psi) that are recorded by the transmitters
in 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

48. 48
Level 1 - Pressure 1RMT Training - 05 /98
ExerciseExercise
6. What is the differential pressure (P1 - P2) in kPa being applied to the
manometer in the the above application ?
S.G of Process Fluid @
Temp + Pressure = 1.0
P2P1
S.G. = 13.6
200mm
(Note 1 mm H2O = 9.8 Pa)