Introduction
A chemical plant
is an industrial production site, where chemicals are processed on a large
scale.
The plant
consists of a set of equipment also present in other types of plants and
industrial sectors, such as Oil Refining, Pharmaceutical, Food, Power Plants
for the production of Electricity, Water Treatment, etc.
The equipment
mainly used in the sectors mentioned are the following:
● Absorbers ●
Distillation Columns ● Compressors ● Evaporators ● Filter Presses ● Mixers ●
Reactors ● Heat Exchangers ● Blowers ● Strippers….
The necessary
reactions to obtain the desired product take place within these equipment, i.e.
the raw material is transformed into a final product by passing through a
series of intermediate products.
Other auxiliary
components necessary for the operation and optimization of the system can be:
● Pumps ● Valves
● Meters of physical quantities ● Signal transducers
● Pipes ●
Fittings ● Filters ● Lubricators ● Pressure reducer ……
The automation
of the process control is essential for the safe operation of the plants, and
to guarantee an economic return.
The operation of
the plant, through the equipment described, is then conditioned by variables,
events internal to the process, or external causes that may vary over time.
This implies
that the management of the plant, aimed at obtaining the best performance, must
comply with the project operating conditions, while conducting the plant in
alternative conditions, dictated by process requirements, must only take place
after a careful technical evaluation, that highlights any adjustments and / or
changes to the process and related equipment.
Some fundamental parameters to be respected are:
Stable process . Often the
processes are unstable, and therefore it is necessary to intervene with
feedback controls, or with special loops that make the process stable, through
feedbacks that intervene on the regulation devices of the process variables.
Low environmental impact .
The control system must be designed with criteria that minimize damage to the
environment, optimizing the processing of raw materials, avoiding the
production of harmful waste.
Qualitative product. The
process control also has the task of obtaining the right quality of the final
product, and that the same is homogeneous on production.
Production capacity. The
process control must allow to regulate the production flow according to the
production needs of the specific product.
The complexity
of the systems makes a control system and adequate automation indispensable
today.
In rare cases, a
manual process can be thought of, which would inevitably be economically not
convenient both for the quality of the final product and for the large number
of operators required, with high labor costs.
PROCESS VARIABLES (*)
With the term
process, in chemical plant engineering, we mean a set of operations performed
on a certain quantity of material in order to modify some of its properties in
whole or in part (physical characteristics, chemical composition, energy
content, etc.).
Each process can
be described by physical quantities, some of which remain constant because they
refer to the geometry or construction methods of the equipment used, or
represent characteristics of the material used that are not affected by changes
due to the process itself.
Other
quantities, on the other hand, can change their value during the process and
therefore constitute the so-called process variables.
Process variables are divided into three groups:
Disturbance
variables : these are those quantities whose value can change for reasons
beyond the control of the person who manages the process and therefore in an
unpredictable way;
Controlled
Variables : these are the quantities whose variation (due to the change
undergone by the disturbance variables) must be carefully monitored.
The
effectiveness of the process itself in terms of achieving the set objectives
depends on the value assumed by these variables;
Manipulated
variables : they represent the quantities on which it is possible to operate
and which allow (if appropriately modified) to restore the controlled variables
to the original value they had before the disturbance occurred.
This value,
fixed on the basis of the needs that the process must satisfy, or the limits of
the machines used, or even on considerations of economic convenience, is called
Set-Point.
An automatic
process control system is therefore a set of equipment which has the purpose of
maintaining a controlled variable on its Set-Point value even in the presence
of disturbances.
It is
synthetically formed by three elements:
Measuring
instrument: it is used to define the value assumed by the controlled variable
at any time;
Controller: compares
the value read by the measuring instrument with the set point value and then
calculates the error.
If the error is
different from zero, the controller acts according to a certain law on the
third element of the control system;
Final control
element (final regulation organ). It usually consists of two parts, the first
of which (actuator or servomotor) receives the signal from the controller, thus
modifying the status of the second (almost always a control valve).
However, note
that even if the feedback control is the most widely used, in some cases it can
have limitations.
This occurs
especially when the measurement of the controlled variable requires a certain
time interval for which the response of the system occurs with delay
with respect to
the disorder.
In this case it
may be convenient to use open loop control
(open loop) in
which this time to be measured and the disturbance variable while the
controller acts (through the final element) always on the manipulated variable.
PROCESS INSTRUMENTATION
The
"Control" in industrial processes represents a set of techniques and
solutions, to give the plant to perform functions automatically.
The process
instrumentation (or control instrumentation ) now makes it possible to realize
the techniques and solutions mentioned, allowing complex adjustments that would
otherwise not be possible.
The
instrumentation is mainly represented by the set of Regulators, Measurement
Sensors, Actuators and Control of the process itself.
The combination
of several devices allows the creation of regulation and control chains also
known as “Regulation Loops” .
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Reading thermometer
Process
instrumentation is mainly used in the fields of Chemistry, Petrochemicals,
Pharmaceuticals, Food, Energy Production, Water Treatment, etc.
It is these
environments that require the adjustment of the values of physical and / or
chemical parameters , to allow the plants to be efficient and produce what they
were designed for.
Control loops
comprise several components, each with its own specific function.
Several interacting
tools form chains of regulation , each with its own specific function.
For indicative
purposes only, among the main tools we find:
Controllers
(Regulators)
Transmitters
(measuring instruments)
Actuators (Final
control bodies)
The Controller
The controller
or regulator determines the behavior of the control variables.
To act
appropriately on the process, the controller must have information on the
reference signal ;
The objective of
the controller, in exercising the control action, is to ensure that the trend
of the controlled variable does not deviate too much from the trend of the
reference signal itself.
When the
controller has information only on the reference signal or possibly also on the
noise , it is called "open loop" (in English feedforward ).
If the
controller also has information on the controlled variable
(or possibly on
variables dependent on the controlled one) we say "closed loop" or
feedback (in English feedback ).
If this
information is partial, a state observer is used who produces an estimate of
the controlled variables moment by moment.
The Transmitter
The transmitter
is an instrument installed on the plant ( in the field ).
It has a sensor
that is in physical contact with the process and of which it measures the
instantaneous value of the quantity concerned.
For example, it
can be a thermocouple for measuring temperature, or a piezoresistive
sensor for measuring pressure.
The parameters
that need to be measured can be multiple according to the process needs.
The most common are
Temperature, Pressure, Flow, Level transmitters etc.
The signal
measured by these sensors is transduced in a proportional way, inside the
transmitter, into another standardized signal which is transmitted to the
control room towards its own regulator.
The transmitted
signal is generally a continuous electric current between 4 and 20 mA, or
through suitable digital protocols (HART, Profibus, Foundation FIeldbus)
The transmitters
have their own measuring range, and the value of the transmitted signal is proportional
to the measured value.
The Actuator
A typical final
regulating organ is constituted by the regulating valve whose opening
influences the flow rate of the fluid, and indirectly the value of the measured
quantity.
In this way, the
value of the measured quantity is forced to approach the one fixed by the
controller ( set point )
The control
valve has a body inserted into the process fluid.
Inside the body
there is a shutter that moves within a seat .
The displacement
of the shutter varies the fluid passage area and therefore its flow rate .
The stem is
mechanically connected externally, and with watertight seal, to the servomotor
associated with a membrane of suitable material, on which the control pressure
is applied.
The diaphragm is
contrasted by a spring which gives the proportionality ratio between the
command signal value and the position of the stem.
Valves can be:
Normally Closed
(NC) and Normally Open (NO).
The word
"normally" means no control air on the membrane.
Thus the NC ones
open when they receive command air on the membrane;
The NO ones
close when they receive the command air on the membrane.
So for example,
a valve with air-open function will be
completely closed with a 4 mA signal, and the valve will be completely open for
a 20 mA signal.
For intermediate
values there will be proportional intermediate positions.
(for example
with 12 mA we will have a run at 50%).
To send the
control pressure to the pneumatic servomotor , an electro-pneumatic device
called a positioner is used and mounted
on the valve itself.
The command
signal enters the positioner, and the air exits from these to command the
valve.
The positioner is
mechanically connected to the stem to measure its position and force it to
assume the value set by the command signal.
The control
valves are generally two-way (one inlet and one outlet).
However, there
are other models such as the three-way ones (two inputs and one output). These
are used to continuously mix two fluids, such as sink mixers work in domestic
bathrooms.
The Safety of
the Plant requires that all possible precautions be taken to prevent accidental
events that can cause damage to the plant personnel.
It is very
frequent that process plants are characterized by areas with the presence of
flammable and explosive substances, which can cause major accidents.
With the advent
of technology, and with the definition of construction methods for electrical
instruments suitable for working in classified areas, electronic
instrumentation has spread , initially analog, and then increasingly digital.
This made it
possible to operate even very complex systems, with a high degree of safety for
operators in the field and in the control room, and for the population of the
surrounding area.
Over time, new
legal constraints related to the impact on the environment have been added,
such as emissions, be they liquid or gaseous, production waste (wastewater),
and finally noise pollution.
In addition to
the prevention of major accidents described above, there is also the prevention
of other types of accidents (e.g. slippery or particularly hot surfaces)
Product safety,
i.e. from its creation to safe disposal.
Industrial
hygiene , i.e. the prevention of phenomena that can cause damage to humans over
time.
