Process Control Flashcards
Reasons for control
- Prevent injury to plant personnel, protect environment and prevent damage to equipment
- Maintain product quality on a continuous basis with minimum cost
- Maintain plant production rate at minimum cost
Manual Control
Highlights an error and then decides what action to complete in order to correct this error
Basic components of all control systems
Sensor-transmitter: often called primary and secondary elements
Controller: brain of control system
Final Control Element: often a control valve, other common elements are variable-speed pumps, conveyors, electric motors and electric heater
Basic Operations
Measurement(M): measuring variable to be controlled, usually done by combination of sensor and transmitter
Decision(D): on basis of measurement, decides what to do to maintain variable at desired level
Action(A): as a result of decision, system takes action, usually done by final control element
M, D and A are present in all control systems and it must be a closed loop (action taken comes back to affect the measurement)
Control Terms
Controlled variable = variable that must be maintained at a certain desired value
Set Point = desired value of controlled variable
Manipulated variable = variable used to maintain cv at its set point
Any variable that makes controlled variable deviate from set point is known as, disturbance or upset
Control Conditions
Manual control: condition in which controller disconnected from process (not making decision)
Closed-loop control: condition in which controller is connected to process, comparing set point to cv and determining and taking action
Regulatory control: systems designed to compensate for distrubances
Servo control: control systems designed where set point acts as its own disturbance. Set point may be changed as func of time, so controlled variable must follow set point
Signals
Three types:
1. pneumatic signal/air pressure
range 3-15 psig
2. electrical signal
range 4-20 mA
3. digital/discrete/signal (zeros and ones)
Usually referred to as %
Signals are used by devices to communicate, signal is NOT a measurement but is a signal that’s proportional to the measurement
Relationship to measurement depends on calibration of sensor-transmitter
Often need to change one type of signal to another; done by transducer or converter.
Feedback
When a disturbance impacts the controlled variable and deviates it from the set point feedback control kicks in and initially overcompensates for the change. The level then oscillates until it stabilises back at the set point
Feedforward Control
Objective is to measure the disturbances and compensate for them before the controlled variable deviates from the set point
Advanced Control
Combination of feedforward control and feedback compensation, to deal with disturbances arising later in the process
Usually more expensive than feedback control, expenses must be justified before implementation. Best option is to implement a simple control strategy, with an option to become more advanced if it is needed.
Transmitter calibration
relation between transmitter’s output and the physical variable to control, T(t)
Process
Anything between the controller’s input and the controller’s output
Mathematical Models
Start from a balance on a conserved quantity (mass or energy)
Balance can be written as:
(rate of mass/energy into control volume) - (rate of mass/energy out of control volume) = (rate of change of mass/energy accumulated in control volume)
Steady-state = algebraic eqs Unsteady-state = differentials with time as independent variable
Thermal Processes
Equations from page 37
Laplace transforms table on page 46
T (tau)
= time constant
the slower a process responds to an input, the larger the value of tau
+ vice versa
Time constant composed of different physical properties and operating parameters of the process
Time constant depends on the volume of liquid in the tank, the heat capacities and the process flow
if any of these change, behavior of the process changes and change is reflected in speed of response of the process, or the time constant
Pure dead time (t0)
time it takes liquid to move from exit of tank to point 1 e.g.
only significant for temp, composition, and other fluid and solid properties propagated through space by the moving fluid or solid
Eq = pg67+68
Transfer Functions
Fundamental to study of process dynamics and automatic process control
Eqs from pg69
- completely defines steady-state and dynamic characteristics of a system described by a linear diff. equation
- terms determine whether system is stable or unstable + whether its response to a non-oscillatory input is oscillatory
System = stable when output remains bound (finite) for all times for a bound input
In process dynamics + control the independent variable is time, so unit of s is 1/time.
Block Diagrams
Useful to visualise processes
All formed by a combination of: arrows, summing points, blocks and branch points
Arrows: indicate flow of info, represent process variables/control signals
Summing points: represent algebraic summation of input arrows E(s) = R(s) - C(s)
Blocks: represent mathematical operation, in transfer function form such as G(s), which is performed on input to produce output
Branch point: position of an arrow at which info branches out + goes concurrently to other summing points or blocks
Sensors: Bourdon tube
Piece of tubing in horseshoe shape, one end sealed, other end connected to a pressure source
Cross section = elliptical or flat, so tubing straightens at pressure is applied
Amount of straightening = proportional to applied pressure
If open end of tubing fixed, closed end connected to pointer to indicate pressure
Sensors: Bellows and diaphragm
Bellows: like corrugated capsule of somewhat elastic material (ss or brass). Increasing pressure = bellows expand. Amount of expansion/contraction = proportional to applied pressure
Diaphragm: typical circular diaphragm clamped between pair of flanges, pressure on diaphragm causes deformation. Size of deformation proportional to pressure to be measured, coupled to pointer mechanism.
Sensors: Orifice meters
Flow is one of most commonly sensed variables
Most installations use equation from pg81
Output signal from orifice is pressure drop across orifice, not the flow. If flow desired, use square root of pressure drop
Not the entire pressure drop measured is lost by process fluid, certain amount recovered in next few pipe diameters as flow regime re-established.
Rangeability of the orifice meter, ratio of max measurable flow to min flow, is roughly 3:1
Use prevented by:
- not enough available pressure to provide pressure drop
- flow of corrosive fluids
- fluids with suspended solids that may plug orifice
- fluids close to saturated vapour pressure that may flash when subjected to pressure drop
^^^ require other sensors to measure flow
Sensors: Magnetic flow meters
Faraday’s Law = operating principle: conductive material (fluid) moves at right angles through magnetic field, induces voltage
Voltage created proportional to intensity of magnetic field and to velocity of fluid.
If intensity of mag field constant, voltage proportional to velocity of fluid. Velocity measured is average velocity, so sensor can be used for laminar or turbulent
Because does not restrict flow, its a zero pressure drop device, suitable for measuring gravity flow, slurry flows, and flow of fluids close to vapor pressure
Fluid needs min required conductivity of about 10 ohms/cm^2, makes meter unsuitable for measuring gases and hydrocarbon liquids
Important to maintain clean electrode coating
Rangeability = 30:1 (much higher than orifice, but more expensive)
Sensors: Turbine meters
One of most accurate commercially available flowmeters
Working principle: consists of rotor that fluid velocity causes to spin. Rotation of blades detected by magnetic pickup coil that emits pulses with frequency proportional to volumetric flow rate
Rangeability = 14:1
Main problems: with bearings, require clean fluids with some lubricating properties
Flow meter selelction
Key factors: accuracy, cost, legal constraints, flow rate range, head loss, operating requirements, maintenance, life time
Table of characteristics pg 90
