Process Control
Transcript of Process Control
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Process Control
Prof. Cesar de Prada Dpt. Systems Engineering and Automatic
Control University of Valladolid, Spain
[email protected] http://www.isa.cie.uva.es/~prada/
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Valladolid
•Capital of Castilla-León •Medium size town •Car industry, Renault
Madrid
Spain
France
Miguel de Cervantes
“El Quijote”
Cristobal Colombus
Valladolid-Madrid 55min.
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University of Valladolid Second oldest in Spain ( XIII century ) All branches: Humanities, Law, Engineering,
Medicine, … 26000 students
Santa Cruz Palace XV century
Vice-Chancellor offices
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Dpt. of Systems Engineering and Automatic Control
Founded in 1973 School of Industrial Engineering
Three locations: – Mergelina Building – Paseo del Cauce – Mendizabal
Two Technology Centres – CARTIF (Automation and Robotics) – CTA (Centre for Sugar Technology)
Master/PhD Course: Process and Systems Engineering (in cooperation with the Chemical Eng. Dpt.) Award of Excellence of the MEC
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“Process control and supervision” Research group
The group 2 Professors 5 lecturers 3 doctoral contracts 12 research grants 2 technicians Research topics: Advanced Control, MPC Process Optimization Modelling and Simulation Fault detection and diagnosis
Web: www.isa.cie.uva.es
Develop new ideas and theory
Develop software tools
Industrial applications
Sede Mergelina
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Process control deals with the problem of maintaining the main process variables close to its desired values, in spite of disturbances, by means of an automatic system
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Process Operation Manual operation
Observe Compare Decide Act
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Automatic operation
LT LC
Measure Compare Decide Act
Continuous Control
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On/Off Control
Min/max detector
ON/OFF valve
Relay
Variables take a discrete number of values or states and change only at certain time instants
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Automatic operation
Process Measure Act
Changes Responses
Regulator
Desired values
Closed loop operation
Block diagram
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Components of a Control loop
Process
Variables to be controlled, y, CV, PV
Regulator
Desired Values w, SP
Actuator
Transmitter
Measured values
Manipulated Variables u, MV, OP
y (EU)
x
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Temperature Control
We will focus on continuous control
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Index: Instrumentation Control Systems: Terminology Continuous / Discrete Control Transmiters
– Definitions – Level, Pressure, Flow, Temperature...
Actuators: – Valves – Pumps, compressors
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P&I Diagrams
Control and measurement instruments are represented by circles with letters and figures
Schematics where process units and instruments are represented using special symbols
Connection lines
LT 102
LC 102
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Instruments
Indicators Transmitters Registers Converters Controllers Actuators Transducers
Connected by : •Pneumatic •Electric •Digital lines
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Instruments in P&I Diagrams
LRC
128 PT 014
Field
Process connection
Pneumatic signal Panel
Electric signal
Same number in all instruments of a control loop
FT 12
FC 12
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Digital Instruments LRC
128
PT 014
DCS controller, microprocessor,...
PLC, logic or secuential control represented by rhombus
Accessible to the operator (Configuration, display…)
Not accessible to the operator
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Digital Instruments LRC
128
Computer Different from a DCS controller Several functions: DDC, register, alarms,etc. Access by network
Software or digital network connection
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1ª letter: measured variable 2ª letter: may qualify the first one D differential F proportion S safety Q integration 3ª y sig: Function of the Instrument I indicator R register C control T transmitter V valve Y computation H high L low
A analysis D density E voltage F flow I current J power L level M moisture P pressure S speed T temperature V viscosity W weight Z position
1ª letter
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Instruments
PDT LRC PIC
TDT
DT
FY FFC ST
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Heat exchanger
MV
CV
DV
TT 12
TC 12 P / I
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Transmitters Sensor: Primary element with properties
sensitive to the physical variable Transmitter: Converts, amplifies, conditions
and normalise the sensor signal in order to send it to other instruments
Indicator: Shows the measured variable Transmitter
Sensor
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Pressure transmitter
Electronic circuit
Piezoelectric Sensor
Amplifier Filter Calibration Power Normalisation
Pressure
Normalised signal
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Transmitters (Signals) Pneumatic: 0.2 - 1 Kg/cm2
3 - 15 psi Electric: 4 - 20 mA 1 - 5 V cc, .... Frecuency: pulses/time Others: RTD, Contacts,... Digital: HART, Fieldbus,
RS-232...
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Normalised signals
Process Controller Transmitter
Actuator w u y
4-20 mA 4-20 mA
SP 45 PV 45.5
4-20 mA from the transmitter
4-20 mA to the actuator
MV 38
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Controller
Process w u e
+ -
Transmitter
+= ∫ edt
T1eKu
ip
y
Actuator y
Controller
Panel mounting
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Control room (DCS)
4 – 20 mA
Field
Operation
Control cabinet, Enclosure
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Operation
Typical PID face
Typical operator screen
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Computer control
Process Microprocessor AO
AI T
y(kT)
u(kT)
T sampling period
Power supply, Ethernet AI AO Controller DI DO
Actuator
Transmitter
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4-20 mA
•Current is the same at any point of the line •A broken line can be identified as different from a measurement in the bottom of the range • A limited number of devices are allowed in the line
Transmitter
mA
FC
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Pulses/Frecuency
Transmitter FC Pulse counter
The number of voltage pulses per time unit is proportional to the value of the variable
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Power supply
Transmitter
mA
Transmitter
mA
220 V ac
24 V dc
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Conecting instruments
XT Protection Shielding Filter
Other devices Conditioning XC
SP
CV
MV
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Shields
Transmitter
mA
FC
Metal envelop
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Conditioning / protecting
Transmitter
mA
AI card
Filter Optocoupler
Zener diode Breaker
I / R
Safety Noise Conversion
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Wiring,...
Control room TT
FT
DT
Wiring costs Noises Calibration Maintenance,...
Distance
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Field buses
PLC Computer
Digital Bus 1101...
Microprocessor A/D converter Communications
TT FT
DT
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DCS
4-20 mA
H1 AS-i
DeviceNet/Profibus
Control room
Room behind
Field
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Smart Instrumentation
Incorporates a microprocessor and digital communications
This provides computer power and data storage capability: – Data of the instrument – Dynamic data
It is based in a two-way digital communication system
Gives new functionalities
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Fieldbus
•Less wires •Less noise •New functions: range adjustment, self-test, documentation,.... •Better information •Different architectures and protocols
PLC Computer
Digital bus
1101...
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Fieldbuses
Fieldbus Foundation (H1 and H2 levels) Profibus DP, PA WorldFIP CAN DeviceNet
.....
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FIELDBUS • Foundation Fieldbus • WorldFIP • Profibus PA
Type of Control
Logic Control
Process Control
Simple Devices Powerful Devices
Device Functionality / Cost
Networks- Fieldbus
Sensor Busses • AS-i • LonWorks • Seriplex
Device Busses • CAN • ControlNet • DeviceNet • LonWorks • Profibus DP • Interbus
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HART
HART Unit RS-232
LT
PT
FT
Digital signals on top of a 4-20mA line It allows having both systems at the same time
4-20 mA
1011..
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Architectures
HART I/O
H1 AS-i
DeviceNet/Profibus
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Diagnosis, configuration
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Configuration⇒Download
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Control in the instruments
HART I/O
H1 AS-i
DeviceNet/Profibus
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Wireless Instrumentation
•Less wires •Automatic routing •Battery •Today they are reliable enough
PLC Computer
1101...
Base station
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Terminology (SAMA)
Range Span Dynamic error Precision Sensibility Repetitiveness Dead band / Histeresis
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Transmitters
Range: [ 20 , 80 ] ºC Span: 80 - 20 = 60 ºC
20 mA
4 mA
20 ºC 80ªC
Calibration: reading = f ( real value ) Zero and span
mA = 0.2667 ºC - 1.3333
TT ºC mA
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Transmitters / Calibration
Transmisor
mA In order to calibrate an instrument it is necessary to compare its output signal with the one of a reference instrument under the same conditions
There are instruments (calibrators) that provide measurements with high precision, and are suitable for this task
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Calibration
20 mA
4 mA
20 ºC 80ªC
mA = 0.2667 ºC - 1.3333
Cero
Span
Calibration: reading = f ( real value ) Zero and span
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Transmitters
20 mA
4 mA
20 ºC 80ªC
Real value
Dynamic error
reading
Accuracy: Maximum error due to non-linearity, hysteresis, etc.... % of span % of reading Direct value,...
Tolerance
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Transmitters
1 unit
20 mA
4 mA
20 ºC 80ªC
Sensibility
Sensitivity: Change in the signal corresponding to a unit change in the measured variable % of span
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Transmitters
20 mA
4 mA
20 ºC 80ªC
resolution
Resolution: Minimum change in the input required to observe a change in the output % of span Direct value,...
The whole measurement chain has to be considered including the AI card of the DCS
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Transmitter
mA
Resolution
AI card
mA
Digital reading
0000 0001 0010
0011
All signals in this interval will have the same reading in the DCS
An AI card with 12 bits can distinguish
4096 = 212 different numbers
Resolution: 16/4096 mA
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Temperature Transmitters
Bulb RTD (Pt100 0ºC 100 Ω) Thermistors (Semiconductors) Thermopars E, J, K, RS, T Pirometers (High temperature, radiation)
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Pt-100 0ºC 100Ω
Electrical resistance changes with temperature
An electrical bridge converts changes of resistance in changes of voltage
Range: -200 500ºC Sensitivity: 0.4 Ω/ºC
Accuracy: 0.2%
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Bridge
V
R R
R Rt
Pt100 In a balanced bridge left and right branches have the same resistance, so V =0. If Rt changes, V ≠ 0
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3 wires
V
R R
R Rt Pt100
The length of the connecting wires influences the measurement, the third wire introduces the same resistance in each branch, compensating the unbalance due to the wires.
Many TT incorporate a head with 4-20mA output
+
-
From + to – there are two resistors and two wires in the left or right branch on the circuit
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Thermopars
T1 T2
I
In the junction of certain classes of metals, an e.m.f appears if both ends are at different temperatures. This e.m.f. depends on the temperature difference
Termopar
T
M
Measurement: A known voltage is oposed to the one generated by the termopar until a null voltage is obtained at the output of the differential ampliflier.
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Thermopars
Kind Range Accuracy
T -200 250ºC 2%
J 0 750ºC 0.5%
K 0 1300ºC 1%
R / S 0 1600ºC 0.5%
W 0 2800ºC 1%
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Pressure Transmiters
Absolute Pressure Manometric Pressure Differential Pressure
Physical Principles: •Displacement •Strain Gauges •Piezoelectricity
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Displacement sensors
Capacity
Induction
Potentiometer
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Pressure
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Piezoelectric Sensor
Quartz crystal
Force
Metal Plate
+
-
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Strain Gauges / Hall Effect
N
S
Current
Force
Hall effect
Strain gauges R changes with deformation
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Pressure Transmiters
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Level Transmiters
Displacement – Floating devices – Force: Archimedes Principle
Differential Pressure Capacitives Ultrasounds Radar
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Level: Differential Pressure
LT
(p0 + ρgh) - p0
Level is proportional to the differential pressure
Density is assumed constant
Condensation in pipes
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Electrical Capacity
A conderser is formed between the electrode and the tank wall. Its capacity depends on the fluid level
dSC ε
=
d
S
ε
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Level: Ultrasounds, radar
The elapsed time between the emission of the wave and its reception is proportional to the fluid level.
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Flow Transmiters
Differential Pressure Electromagnetic Turbine Vortex Doppler Mass Flow (Coriolis) …..
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Plates P1 P2
Dd)PP(g2
4D
1Cq 21
2
4
2
=βρ−π
β−
β=
Based on differential Pressure
D d
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S
Electromagnetic Flowmeters
N
S
B
In a conductor (liquid) flowing at a speed v within a magnetic field B, an e.m.f. appears that it is proportional to the velocity N
- +
V
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Vortex Flowmeters
When a fluid stream passes an obstacle, vortices are alternatively shed on each side
The frequency at which vortices are shed is directly proportional to the fluid velocity
v = 4.167 d frequency d = diameter of the obstacle
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Vortex flowmeters
Industrial vortex flowmeters have a bluff body (obstacle) that generates vortices.
Counting the number of vortices per unit time: disturbances in pressure sensors, capacitance, ultra sonic, etc.
Valid for gases and liquids in a wide range of conditions
Not valid for very low flows
v = 4.167 d frequency
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Coriolis Flowmeters
The pipe is forced to oscillate, but the U-shape design induces (Coriolis) opposite forces in both sides. There is a phase shift between input and output sides that depends on the mass flow. Magnetic sensors measure this phase shift.
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Choosing a transmitter
http://www.emersonprocess.com/rosemount/ http://www.yokogawa.com/fld/fld-top-en.htm
http://www.flowexpertpro.com/
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Actuators
Final control elements. They change the manipulated variable according to the signal from the controller. – Valves – Motors – Variable speed pumps – Power amplifiers – ....
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Valves Devices that allow modifying the flow of
the fluid by means of a change in the pressure drop in the line. Several types: – Manual valves – One way – Safety – On/Off – Control
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Automatic control valves Structure and operation Types Formulas Static characteristics Cavitation Installed Characteristics Valve dynamics
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Pneumatic valve (Globe)
Air
Fluid
Obturator
Seat
Flanges
Diafragm
Stem Spring
Packing bonnet
Indicator
Body
Servomotor Pneumatic Electric
3 -15 psi
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Automatic valves Air Globe
Double seated Needle Saunders Ball Butterfly Camflex II 2 -3 ways
•Sealed •Maximum pressure •Flow capacity •Kind of fluid
Fluid
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Butterfly / Ball / Camflex
Camflex II
Butterfly
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Air open/close Fail closed/ open
Air
Air
Air closes Air opens
Air closes
Air opens
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I/P Converter
Air 3-15 psi
I P 4 - 20 mA
Low accuracy in the position of the stem
Air and electricity supply
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Double seated valve
Air 3-15 psi
Force on the stem due to the fluid
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Positioner
Positioner
Air supply
Air
Control signal 4-20 mA
Stem position control system
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Intelligent valves
Positioner + Microprocessor
Aire
FieldBus
Data
Diagnosis
Alarms
Control blocks, etc.
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Digital positioner
Non contacting
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Pressure drop
∆pa C
qv =1
2 22ρ
∆p pressure drop q flow a opening C coefficient ρ density
q
∆p
a
p1 p2
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Formulas
ρ∆
= vv p16.1
aCq
)pp(p4.72
aCq 21vv +∆=
q Tm/h p bars a opening fraction
q m3/h p bars ρ relative density a opening fraction
Saturated steam
Líquids
q
∆p
a
p1 p2
Cv flow coefficient
Viscosity corrections
ρ∆
= vv
paCq
q gpm p psi
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Static Characteristics
% stem position
% of seat area 100% % of max Flow in nominal conditions under constant ∆p
Linear Equal percentage Quick opening Butterfly Camflex
0 % 100 %
0 %
Different obturator shapes
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0 % 100 %
0 % Butterfly
Quick opening
Linear Equal percentage
Static Characteristics
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Rangeability
0 % 100 %
0 %
máx. controlable flow R = ------------------------------- mín. controlable flow
% steam position
Non controlable flow
R= 100, 50...20
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Cavitation / Flashing
lenght
pressure p1
p2
lenght
presión p1
p2
Saturation pressure Saturation pressure
The liquid will boil if its pressure is below the saturation one
lenght
p1
p2
Saturation pressure
Flashing
Cavitation
p1 p2
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Flashing q aC pv v=
116.∆ρ
Incipient Cavitation
∆pv
q
Máximum admisible ∆p for controlling flow
Critical flow (Choked flow)
As ∆pv increased q increases until flashing appears which will choke the flow
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Flashing / Cavitation q aC pv v=
116.∆ρ
Incipient Cavitation
q
∆pM
Critical flow ∆p K p pv c v≤ −( )1
Kc incipient cavitation Coefficient
drop pressure admisible m Maximup
)pp28.096.0(ppCp
M
c
vv1f
2M
∆
−−=∆
Cf Critical flow Factor pc critical point pressure
∆pv
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More precise formulas for gases
qa C C p y y
yC
pp
f v
f
v
=−
= ≤
13
1
014854 5
163 15
ρ( . ).
. .
gas
y∆
q Tm/h p bars
flow critical 7.83
pCCaq
steam saturated 7.83
)y148.0y(pCCaq
1vf
31vf
=
−=
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Installed Characteristics
q aC pv v=116.
∆ρ
q
∆pv
a
p1 p2
h
q p gh
Ka CL
v
=−
+
1116 1
0
2 2
.∆ ρ
ρ
∆ ∆p p K q ghv L02= + +ρ ρ
∆p0
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Installed Characteristics q p gh
Ka CL
v
=−
+
1116 1
0
2 2
.∆ ρ
ρ
% stem position
% q
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Valve sizing
Critical for many control loops Find the adequate Cv and type of valve Commercial Software available
Fisher Masoneilan
http://www.masoneilan.com/
http://www.emersonprocess.com/fisher/
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Pumps
Positive displacement Centrifugals Installation Power and efficiency Characteristic curve Cavitation
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Positive Displacement
Shaft,. Membrane,…
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Centrifugal pumps
Impeller
Electrical energy
Mechanical energy
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Centrifugal pump
( )∆p aw bqb = −ρ 2 2
power Absorved W WPpower Supplied P qp022.36P b
η=∆= P kw
q m3/h p bars
Energy increment = supplied energy - losses
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Characteristic curves
∆pb
q
ω1
ω2 ω2 > ω1
η
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Operating point
∆pb
q
q
∆pv
a h
∆ ∆ ∆
∆ ∆
p p p K q gh
pa C
K q gh p
b v L
bv
L
02
2 22
01
+ = + + =
= + + −
ρ ρ
ρ ρ( )
∆p0 ∆pb
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Variable speed pumps
M Frecuency converter
∼
4 - 20 mA
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Centrifugal Compressors
∆pb
q
ω1
ω2 ω2 > ω1
Surge line
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Select commercial instruments for the implementation of the following control loop: And fill in the following form: Transmitter Valve Type of measurement
Kind of valve
Output signal Air open / close Range Cv Precision Diameter Sensibility Kc Linearity Rangeability Max. temperature Max. Pressure Process connection Process Connection Manufacturer Manufacturer Reference Reference
Maximum flow: 120 m3/h Nominal flow: 60 m3/h Max height: 4 m Max Temperature: 80 ºC Nominal temperarure: 50 ºC Pipe diameter: 10 cm. Pressure in the pipe: 2 bar Fluid: water
LC
LT
Exercise
Select the instrumentation from a commercial supplier and fill in the form.