3p04 tutorial 6 transmission 2008
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Chemical Engineering 3P04
Process Control
Tutorial # 6
Learning goals
1. Learn basic principles of equipment in a controlloop
2. Build understanding of feedback loop
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Loop Elements: Sensor Computer ValveWhy must we transmit these signals?
What is wrong with this picture?
Central control room
controller
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Loop Elements: Sensor Computer ValveWhy must we transmit these signals?
Transmitted to/fromCentral control room
Displayed locally
Manual valves
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Loop Elements: Sensor Computer ValveWhy must we transmit these signals?
Transmitted to/fromCentral control room
Safety related ortime critical
Used for control
Important for
quality, reliability,performance
Trouble shoot andmonitor longer-term
behavior
Displayed locally
Manual valves
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Loop Elements: Sensor Computer ValveWhy must we transmit these signals?
Transmitted to/fromCentral control room
Safety related ortime critical
Used for control
Important for
quality, reliability,performance
Trouble shoot andmonitor longer-term
behavior
Displayed locally
Used for localmaintenance/operation
Not safety or timecritical
Manual valves
Infrequently adjusted
Not safety or time
critical
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Central control
room
Loop Transmission: Why learn about it?
We need to understandthe closed-loop
We select equipment to
achieve requiredperformance
We trouble-shootproblems
These are our sensesand our handles
?
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Central controlroom
Class workshop: What are general features that weseek for the transmission of signals from thesensor computer and from the computer valve?
Hint: We have lists of features for sensors and for valves already
Loop Transmission: Why learn about it?
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Loop Transmission: What features do we seek?
Accuracy and reproducibility
Noise sensitivity
Reliability
Dynamics
Distance
Interoperability
Safety
Diagnostics
Cost
Class Workshop: Explain thesefeatures
Typically much betterthan sensors and valves
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Dynamics:Transmission delays are in the feedbackloop. Delays in transmission are as bad as delays
in the process.
Good news: Electronic transmission is very fastcompared with other elements in the loop.
Caution: Old transmission systems using airpressure (pneumatic signals) can be slow for
distance over 50 meters.
Loop Transmission: What features do we seek?
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Distance:Process plants can extend over 1000s ofmeters. The transmission must be capable of thesedistances.
Good news: Electronic transmission via hard wirehas a large enough range.
Caution: Pneumatic signals have limited range.
Note: Telemetry is not now used for process control.It is used for monitoring remote equipment (wells)
Loop Transmission: What features do we seek?
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Interoperability
When you purchase one loop element from acompany, do you want to buy all other elementsfrom the same company for the life of the plant?
NO!
Standards are recognized so that equipment fromvarious manufacturers can be used
interchangeably. This was easy for older, analogtechnology.
Standards are available for digital technology.
Loop Transmission: What features do we seek?
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Loop Transmission: Two typical designs.
Life is exciting during a revolution!
Analog transmission
Continuous electronic signal
Digital transmission
Digital numeric representation
Older technology, but widely
employed and will be in use for
decades
Newer technology, generally used
in new facilities and when
replacing analog technology
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Loop Elements: A Typical Analog Loop
It
CVCVTtE
TEKMV
CVSPE
ni
i n
nn
dii
I
nCn
nnn
0
1)(1
Heating medium
fc i/p
Digital controller
Digital number
Thermocouple temperature
sensor, mV signal
transmitter
Analog signal transmission
(4-20 mA)
Digital number
Analog signal transmission
(4-20 mA)
Pneumatic signal
transmission
(3-15 psig)
Valve stem
position
0-100%)
D/A
A/D
Analog to digital
conversion
Digital to analog
conversion
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Loop Elements: A Typical Analog Loop
It
CVCVTtE
TEKMV
CVSPE
ni
i n
nn
dii
I
nCn
nnn
0
1)(1
Heating medium
fc i/p
Digital controller
Digital number
Thermocouple temperature
sensor, mV signal
transmitter
Analog signal transmission
(4-20 mA)
Digital number
Analog signal transmission
(4-20 mA)
Pneumatic signal
transmission
(3-15 psig)
Valve stem
position
0-100%)
145 C
7.734 mV 11.2 mA
14.08 mA11.56 psig63% open D/A
A/D
Analog to digital
conversion
Digital to analog
conversion
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Loop Elements: All digital transmission
Sensor/transmitter
Special
purpose
controllers:safety, PLC,
etc.
History,
diagnosis,
optimization,etc. data storage
and calculations
Digital controllers
(PID, etc.)
Process Process
Operators
consoles
-Processorat everysensor andvalve
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Loop Elements: Life is exciting during a revolution!
Why have a micro-processor at every sensor and
valve?
ValveFlow Sensor
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Loop Elements: Life is exciting during a revolution!
Why have a micro-processor at every sensor and valve?
ValveFlow Sensor
Improve accuracy
Correct for densitychanges
Diagnoseperformance and
warn whendegradation begins
Calibrate quickly
Power supply error
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Loop Elements: Life is exciting during a revolution!
Why have a micro-processor at every sensor and valve?
Valve
Diagnoseperformance and
warn whendegradation begins
Valve sticking
Air pressure low
Signal not received
Flow Sensor
Improve accuracy
Correct for densitychanges
Diagnoseperformance and
warn whendegradation begins
Calibrate quickly
Power supply error
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Loop Elements: Life is exciting during a revolution!
Table 4.3.1 Typical communication for analog and digital transmission.
Loop elements
involved
Traditional, analog Enhanced, digital fieldbus
Sensor to controller Signal representing themeasured value sent to the
controller
To controller
Measured value Diagnostic from sensor
To sensor
Configuration of sensors (e.g., zero and spanvalues)
Calculations at sensor
Filtering measurement Linearization Correction for process environment (e.g.,
orifice for fluid temperature and pressure) which
can require the use of several sensors
Controller to valve Output of controller
calculation sent to the
valve (i/p converter)
To valve (to the i/p converter)
Output of controller Configuration of valve (max/min openings,
characteristic, etc.)
To controller
position of stem position of valve diagnostic from valve
Calculations at valve
Modification of relationship between controlsignal and stem position to modify characteristic
Note that both
have two-waycommunication
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Loop Transmission: Two typical designs.
Life is exciting during a revolution!
Analog transmission
Continuous electronic signal
Digital transmission
Digital numeric representation
Older technology, but widely
employed and will be in use for
decades
Newer technology, generally used
in new facilities and when
replacing analog technology
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Chemical Engineering 3P04
Process Control
Tutorial # 6
Learning goals
1. Learn basic principles of equipment in a controlloop
2. Build understanding of feedback loop
Lets look at
some examples
from Tutorial #7
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FC
Flow Control:
Centrifugal pump with
constant speed (rpm) Orifice plate sensor
Globe valve
(a)
a) The centrifugal pump increases the pressure of the fluid, i.e., it provides head.
The pump can operate at low or no flow, at least for a short time; the speed of the rotordoes not determine the flow through the pump. Thus, the fluid flow rate is determined b
the driving force (pressure) and the resistances to flow. The pump provides the driving
force and the valve provides an adjustable resistance. Opening the valve increases the
flow rate.
Yes, feedback control is possible. There is a causal relationship between the valve(resistance) and the flow rate
The orifice plate is a good sensor for clean fluids, and the globe valve is the workhorse
control valve body in the process industries.
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FCFlow Control:
Positive displacement
pump
Orifice plate sensor Butterfly valve
(b)
b) The positive displacement pump has moving components that define the liquidflow rate by the speed of rotation or by the linear movement distance and speed.
Therefore the valve resistance does not affect the flow rate, and if the valve is closed too
far could result in damage to the pump.
No, feedback control is not possible in this situation. The operation of the pump
could be adjusted to influence the flow rate; in this case the control valve should be
removed.
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FC
Flow Control:
Centrifugal pump with
variable speed driver
Orifice plate sensor
(c)
c) The pressure increase from a centrifugal pump dependson the rotor speedthe fast the rotation, the higher the pressure.
A variable speed motor can be adjusted to achieve the desired
flow rate, which is more energy efficient than adjusting a variable
pressure drop (valve) in the pipe. Increasing the speed increases
the flow rate.
Yes, feedback control is possible.
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PC
Flows into the pipe
Flows exiting the pipe
Pressure Control:
Manipulate one exiting
flow Flexible diaphragm
Globe valve
(h)
h) The pressure in a pipe can be controlled by adjusting one of the
flows. We can prove this by formulating a dynamic material balance.
Naturally, successful control can only be achieved over a range of flows;
when the valve is either fully opened or closed, control is no longer possible.
Yes, feedback control is possible.
A pressure sensor that deflected because of pressure and converted the
deflection to an electronic signal is used in such circumstances. A globe
valve is acceptable here.
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PC
Pressure Control:
Manipulate exiting flow
from vessel
Piezoelectric
Globe valve
(i)
i) The pressure in a vessel can be controlled using the exit
(or inlet) flow. The principles are identical to the previous design.
Yes, feedback control is possible.
A piezoelectric sensor generates a small electronic signal when a
pressure is applied; it can be used in this application.
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LC
Composition Control in isothermal CSTR
Manipulate the inlet flow
Control CB
Ball valve
Level maintained constant by LC
AC
CB
Reaction: A B C
(k)
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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
volume / flow
ConcentrationofB
CB can be
controlled;
increase the flowrate to increase
CB
CB cannot be controlled by
adjusting F
CB can be
controlled;
decrease the flowrate to increase
CB
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k) The conversion
(or extent of reaction) depends
on the space time in the reactor.
Clearly, the flow rate affects
the space time.
However, this process is more complex, some might say, Tricky. For
control to be successful, we need to have a controller gain that has a non-
zero gain. The gain can be either positive or negative, but it should notchange sign! What happens in this example? The figure below shows that
the gain changes sign, because of the two reactions. In two regions,
control is possible, but would only function within the region. At the
maximum CB point, control is not possible by adjusting the feed flow rate.
While control is possible, great care would have to be employed when
implementing. A different manipulated variable, such as feed
concentration should be investigated.
A ball valve would be an acceptable choice
LC
AC
CB
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