3P04 Tutorial 6 Transmission 2008

8/23/2019 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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Transmitted to/fromCentral control room

Displayed locally

Manual valves


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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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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.



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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)



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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.



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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


(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


(4-20 mA)

Digital number


(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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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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Why have a micro-processor at every sensor and valve?

Valve



Valve sticking

Air pressure low

Signal not received

Flow Sensor

Improve accuracy

Correct for densitychanges



Calibrate quickly

Power supply error


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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


20/31

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


21/31

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.


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.


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

3P04 Tutorial 6 Transmission 2008

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