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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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.
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.
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.
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