Lecture 1

Question 1

Importance of Process Control

Answer 1

• Required for stable and safe operation
• Should influence process design

Question 2

Hierachy of Control

Answer 2

1 - Intrumentation (hardware)
2 - Regulatory Control (control schemes that hold the process at SP)
3 - Constraint Control (control schemes that drive towards profitability)
4 - Optimisation (process simulations)

Question 3

Types of Flow Transmitter

Answer 3

Orifice type - Correlates pressure difference on each side of place to flow rate

Question 4

Type of Pressure Transmitter

Answer 4

Piezoresistive Strain Gauge Type - Gauge changes electrical conductivity when pressure is applied - change in conductivity is measured and is proportional to the strain or force applied
Diffrential Capacitance Type - Pressure causes the diaphragm to move, changes capacitance which is measured and is proportional to pressure difference

Question 5

Types of Temperature Transmitter

Answer 5

Thermocouple Type - Tells temperature by the varying current detected in a wire

Question 6

Type of Level Transmitter

Answer 6

Float Type
Diffrential Pressure Type - Measures the pressure difference between two points in a tank which is proportional to the liqiuid level

Question 7

Process Dynamics

Answer 7

Change with respect to time
Characterised using parameters such as gain, deadtime, lag
Understanding process dynamics is the essential first step to achieving good process control

Question 8

Direct (vs Cascade)

Answer 8

Slower reaction to disturbances, corrections are only made when deviation from set point has already occured
Less precise
Simpler design

Question 9

Cascade (vs Direct)

Answer 9

Faster reaction to disturbances affecting the secondary variable, improving the overall stability of the system
More precise
More complex, requires additional sensors and tuning

Question 10

Cascade Control

Answer 10

A control system where a secondary controller acts as the set point for the primary controller
eg - a flow control valve will open more to reduce the high level detected by a level transmitter

Question 11

Process Gain

Answer 11

= change in process variable / change in manipulated variable
A measure of how much the output reacts to a change made to the input

Question 12

Deadtime

Answer 12

The time delay between the change made to the manipulated variable and the first observed change in the process variable

Question 13

Lag

Answer 13

The time delay between a change made to the manipulated variable and the process becoming steady state
Arises from inertia of the process

Question 14

Order

Answer 14

Most processes have an order much greater than 1 - in effect a mixture of a large number of first order processes
We can almost always assume that a process is first order - although there are exceptions.