Chemical Engineering 3P04

Process Control

Tutorial # 6

Learning goals

1. Learn basic principles of equipment in a control loop

2. Build understanding of feedback loop

Loop Elements: Sensor Computer Valve

Why must we transmit these signals?

What is wrong with this picture?

Central control room

controller

Loop Elements: Sensor Computer Valve

Why must we transmit these signals?

Transmitted to/from

Central control room

Displayed locally

Manual valves

Loop Elements: Sensor Computer Valve

Why must we transmit these signals?

Transmitted to/from

Central control room

•Safety related or time critical

•Used for control

• Important for quality, reliability, performance

•Trouble shoot and monitor longer-term behavior

Displayed locally

Manual valves

Loop Elements: Sensor Computer Valve

Why must we transmit these signals?

Transmitted to/from

Central control room

•Safety related or time critical

•Used for control

• Important for quality, reliability, performance

•Trouble shoot and monitor longer-term behavior

Displayed locally

•Used for local maintenance/ operation

•Not safety or time critical

Manual valves

• Infrequently adjusted

•Not safety or time critical

Central control room

Loop Transmission: Why learn about it?

•We need to understand the “closed-loop”

•We select equipment to achieve required performance

•We “trouble-shoot” problems

•These are our “senses” and our “handles”

?

Central control room

Class workshop: What are general features that we seek for the transmission of signals from the sensor computer and from the computer valve?

Hint: We have lists of features for sensors and for valves already

Loop Transmission: Why learn about it?

Loop Transmission: What features do we seek?

• Accuracy and reproducibility

• Noise sensitivity

• Reliability

• Dynamics

• Distance

• Interoperability

• Safety

• Diagnostics

• Cost

Class Workshop: Explain these features

Typically much better than sensors and valves

Dynamics: Transmission delays are “in the feedback loop”. Delays in transmission are as bad as delays in the process.

Good news: Electronic transmission is very fast compared with other elements in the loop.

Caution: Old transmission systems using air pressure (pneumatic signals) can be slow for distance over 50 meters.

Loop Transmission: What features do we seek?

Distance: Process plants can extend over 1000’s of meters. The transmission must be capable of these distances.

Good news: Electronic transmission via “hard wire” has 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?

Interoperability

When you purchase one loop element from a company, do you want to buy all other elements from the same company for the life of the plant?

NO!

Standards are recognized so that equipment from various manufacturers can be used interchangeably. This was easy for older, analog technology.

Standards are available for digital technology.

Loop Transmission: What features do we seek?

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

Loop Elements: A Typical Analog Loop

It

CVCVTtE

TEKMV

CVSPE

ni

i n

nndii

InCn

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 position0-100%)

D/A

A/D

Analog to digital conversion

Digital to analog conversion

Loop Elements: A Typical Analog Loop

It

CVCVTtE

TEKMV

CVSPE

ni

i n

nndii

InCn

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 position0-100%)

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

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

-Processor at every sensor and valve

Loop Elements: Life is exciting during a revolution!

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

ValveFlow Sensor

Loop Elements: Life is exciting during a revolution!

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

ValveFlow Sensor

Improve accuracy

•Correct for density changes

Diagnose performance and warn when degradation begins

•Calibrate quickly

•Power supply error

Loop Elements: Life is exciting during a revolution!

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

Valve

Diagnose performance and warn when degradation begins

•Valve sticking

•Air pressure low

•Signal not received

Flow Sensor

Improve accuracy

•Correct for density changes

Diagnose performance and warn when degradation begins

•Calibrate quickly

•Power supply error

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 the measured value sent to the controller

To controller Measured value Diagnostic from sensor To sensor Configuration of sensors (e.g., zero and span

values) 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 control

signal and stem position to modify characteristic

Note that both have two-way communication

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

Chemical Engineering 3P04

Process Control

Tutorial # 6

Learning goals

1. Learn basic principles of equipment in a control loop

2. Build understanding of feedback loop

Let’s look at some examples from Tutorial #7

FC

Flow Control:• Centrifugal pump with

constant speed (rpm)• Orifice plate sensor• Globe valve

FCFlow Control:• Positive displacement

pump• Orifice plate sensor• Butterfly valve

FC

Flow Control:• Centrifugal pump with

variable speed driver• Orifice plate sensor

(a)

(b)

(c)

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 rotor does not determine the flow through the pump. Thus, the fluid flow rate is determined by 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.

FCFlow Control:• Positive displacement

pump• Orifice plate sensor• Butterfly valve

(b)

b) The positive displacement pump has moving components that define the liquid flow 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.

FC

Flow Control:• Centrifugal pump with

variable speed driver• Orifice plate sensor

(c)

c) The pressure increase from a centrifugal pump depends on the rotor speed – the 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.

PC

Flows into the pipe

Flows exiting the pipe

Pressure Control:• Manipulate one exiting

flow• Flexible diaphragm• Globe valve

PC

Pressure Control:• Manipulate exiting flow

from vessel• Piezoelectric• Globe valve

(h)

(i)

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.

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.

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)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

volume / flow

Con

cent

ratio

n of

B

CB can be controlled; increase the flow rate to increase CB

CB cannot be controlled by adjusting F

CB can be controlled; decrease the flow rate to increase CB

k) The conversion (or extent of reaction) dependson 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 not change 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