Tag: Process automation
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What is the Safety Instrumented Function (SIF)?
The safety instrumented function (SIF) is used to monitor a selected process parameter and takes an executive action to make the process safe, if certain limits are exceeded. An illustration of safety instrumented function in a process plant is shown below: The figure above illustrates a gas pipeline which supplies a feed to a power…
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Combustion & Temperature Control for a Direct-Fired Process Heater
The figure below shows the control schemes applied to a direct-fired process heater. From the diagram above, the heat from the burning fuel is applied directly to the tubes carrying the process fluid in similar way as the water tubes in a steam generator. This type of heater is usually used as a reboiler in…
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How PLC Systems are Protected from Plant Voltages
Internally a PLC typically operates at 5 V dc. The external devices such as solenoids, limit switches, motor starters, etc. operate at voltages up to 110 V ac. The mixing of these voltages will cause irreparable damage to the PLC electronics. A less noticeable problem may occur from electrical noise introduced into the PLC from…
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Key Requirements for Industrial Control Computers
A computer is often used as part of a control system. The input data is typically from the operator’s commands and signals from the plant (flows, pressure, temperatures, limit switches, etc.). The output data comes in form of control actions to the plant and status displays to the operator. The instructions fed into the computer…
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Analog vs. Digital Controllers
Analog signals pass continuously through a full range of values for example a measuring device converts the process variable (PV), say temperature, into a signal corresponding to this temperature. Each temperature value corresponds to a value of the electrical signal. Digital signals belong to the group of discrete signals; here the individual signals are represented…
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Steam-heated Reboiler Control System
When steam-heated reboilers are used in distillation columns and the heating medium is steam, a flow control loop for the steam will typically be incorporated. The control valve can then be placed either in the steam or condensate line. Steam pressure compensation is always recommended in these cases, and the controls are illustrated in the…
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Furnace Control System for a Rapidly Changing Demand
Let’s consider the figure below: In the system above, the output of temperature controller TC represents the process demand, and this signal is applied as one of the inputs, each to a high (>) and a low (<) signal selector block (module). The second input to the high (>) selector is obtained from the measurement…
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The Instrumentation for Furnace Control
The figure below shows a typical basic furnace used to raise the temperature of a heat-transfer medium that is used on other equipment in a plant process. From the figure above, the temperature of the heat-transfer medium is the parameter that sets the demand on the heating system. The amount of heat required is manually…
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The Operation of a Temperature Control System for lubricating oil
Lubricating oil reduces friction between moving parts and also removes heat from the components. As a result, the oil becomes hot. This heat is removed from the lubricating oil by a cooler to prevent both the breakdown of the oil and damage to the mechanical components it serves. The lubricating oil cooler consists of a…
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Principles of Control Systems
A control system is a system of integrated elements whose function is to maintain a process variable at a desired value or within a desired range of values. The control system monitors a process variable or variables, and then causes some action to occur to maintain the desired system parameter. A good example in a…