Published on Nov 30, 2023
Drilling for oil is one of the greatest technological breakthroughs in recent decades, once oil or gas is discovered, a production platform will replace the offshore drilling rig, which is assembled at the location using heavy lift cranes situated on a barge.
These production platforms come in a variety of sizes, shapes, and types, which depend on the dimensions of field, the depth of the water and how far the oil and/or gas field is from shore. Usually the platforms are typically located in 150 to 350 feet of water, up to 10 miles offshore. These platforms are generally made of steel and fixed in the seabed with steel "post". These platforms harbor the entire well and can house a maximum of 80 workers. In addition platforms have concrete structure large enough to store oil, with gravity keeping them situated on the seabed.
Weather is a major factor in platform operation. When high sea and hurricanes approach a platform the crew must evacuate to prevent injury or loss of life. Thus safety is an important factor as far as an oilrig is concerned. Oil and gas production must continue on an uninterrupted manner to ensure profitability. To further this end, labor-intensive, repetitive task such as opening and closing valves, monitoring levels, and shut down of platform to prevent equipment damage are handed over to programmable logic controllers (PLC's).
To know about the control action of PLC the knowledge of its hard ware, its operation and ladder diagram is important.
A PLC is a special form of microprocessor-based controller that uses a programmable memory to store instructions and to implement functions such as logic, sequencing, timing, counting, and arithmetic in order to control machines and processes. When the PLC is programmed, the controller then monitors the inputs and outputs according to this program by turning on\off its output.
PLC's have the great advantage that the same basic controllers can be used with the wide range of control systems. To modify a control system and the rules that are to be used, all that is necessary is to change the program. There is no need to rewire. PLC's are simlar to computers but whereas computers are optimized for calculation and display task, PLC's are optimized for control tasks and industrial environment.
The primary reason for using PLC's is to eliminating the large cost involved in replacing the complicated relay based machine control systems. Such a control system is called a relay sequencer or relay logic panel.
In earlier days of industrial control process, discrete state control where provided by physical relays to put together a circuit that satisfies the requirement of a ladder diagram. The ladder diagram technique of describing discrete state control systems originated from relay logic system that is why the diagram contains so many relay related terms and symbols. The ladder diagram is still used because it has evolved into an efficient method of defining the event sequences required in a discrete state control system. It is important to realize that, relay control each rung of the ladder is evaluated simultaneously and continuously, because the switches and relays are all hard wired to ac power. If any switch in ladder change state the consequence is immediate this is not true in the case of computer based programmable controllers.
The main reason for eliminating relays where due to its increase in cost involved in replacing the complicated relay based machine control systems. Since relays are mechanical devices they had fewer lifetimes. Troubleshooting is also difficult when so many relays where used. When a relay panel has been wired to implement a ladder diagram for a special purpose, if the event sequence is to be changed it is necessary to rewire all, or part of the panel.
The design of the CPU mainly depends on the size of the process to be controlled and the internal memory required. The CPU accepts input data from various sensing devices, executes the stored program from memory and sends output command to control devices. Here input commands, device status and instructions are converted to logic signals “1” for input present and “0” for no input signal in positive logic. The CPU then processes these logic signals and then PLC activates the output.
Memory in a PLC is where the central program is stored. Memory size in PLC varies with the size of the control function to be performed. There are several memory elements in a PLC system.
a) System read only memory (ROM) to give permanent storage for the operating system and fixed data used by the CPU.
b) Random access memory (RAM) for the user’s program.
c) Random access memory (RAM) for data. This is where information is stored on the status of nput and output devices and the values of timers and the counters and other internal devices. The data RAM is some times referred to as a data table or register table. Part of this memory is set aside for input and output addresses and the states of those input and output. Part will be set aside for preset data and part for storing counter values, timers, etc.
d) Erasable and programmable read only memory (EPROM) for ROMs that can be programmed.
To prevent the loss of program when the power supply is switched off, a battery is maintained in PLC to maintain the RAM contents for a period of time.
The power supply unit converts the main a.c. voltage to low voltage d.c necessary for the processor and the input and output interface modules.
The input\output unit provides the interface with outside world, allowing for connections to be made through input\output channels to input devices and output devices. Every input\output points have a unique address, which can be used by the CPU. The input\output channels provide isolation and signal conditioning functions so that sensors and actuators can often be directly connected to them without the need for other circuitry. Electrical isolation from the external world is usually by means of opto isolators. Both input and output are optically isolated and fused .The input devices such as push buttons, limit switches, sensors, and select switches are hardwired to the input module. Output devices such motors, solenoid valves, indicator lights are hardwired to the output modules.
The programming device transforms the control scheme into useful PLC logic. The program logic is then stored in memory, where it is available to the CPU for logic operations. Various kinds of programming devices are available from PLC manufacturers, these range from a CRT programming panel, hand held calculators like devices, thump wheel input system or a hook up to a central computer or programmer through a telephone interface.
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