PLC is defined as a digital electronic device that uses programmable memory to store instructions and to implement specific functions such as logic, sequence, timing, counting, and arithmetic to control various machines and processes. The PLC consists of:

  • Input modules
  • Output modules
  • Central processing unit (CPU)
  • Memory
  • Power supply
  • Programming devices
    The block schematic of the PLC is as shown picture

Hydrocarbon vapors, generated from truck loading, enter into one of two carbon adsorbers. The hydrocarbon and air vapors flow through the adsorber where the bulk of the hydrocarbons are adsorbed. The air continues through the carbon adsorber and is vented to the atmosphere. While this carbon adsorber is on-line processing the hydrocarbon vapors, the second carbon adsorber is off-line being vacuum regenerated (i.e. cleaned). The purpose of regeneration is to restore the carbon to a level where it will effectively adsorb hydrocarbons again. The two carbon adsorbers alternate between adsorption and regeneration at @15- 20 minute intervals.

The main purpose of an alarm system (annunciator) is to bring attention to an abnormal or unsafe operating condition in the plant. Traditional annunciators used discrete alarm modules for this purpose. These dedicated hardware units are diminishing in numbers yet are still used in installations where simplicity is desired or where separation from the basic process control system is required for safety reasons.In some installations where traditional units have been replaced by PLC- or DCS-based annunciators, the recognition of and response to alarm conditions have deteriorated because on computer screens they are not very visible and can go unnoticed.

open-loop control systems, the output signal has no effect upon the controlled variable(s). In other words, in an open–loop control system the output is neither measured nor “fed back” for comparison with the input.

Operation of a washing machine, where the duration of soaking, washing, and rinsing are set in advance and on the basis of experience with how these settings affect the output signal, which is the cleanness of the clothes. In such an open-loop system, the output signal is not automatically measured. Example :- clothes drier , Electric bulb