Instrumentation Questions Part One

Introduction

When implementing and designing a controller, we must have identified a process variable we seek to regulate, be able to measure it (or something directly related to it) with a sensor which being able to transmit the measurement variable as an electrical signal back to our controller.

And have a final control element (FCE) which can receive the controller output (CO) signal, reacted in some fashion to impact the process (valve moving), and as a result cause the process variable to respond in a consistent and predictable manner.

Process Controlling

For controlling any practical process one must know the following points of the particular process:

1. Control Objective
2. Process Variable
3. Measurement Sensor
4. Measured Process Variable (PV) Signal
5. Set Point (SP)
6. Controller Output (CO)
7. Final Control Element (FCE)
8. Manipulated Variable
9. Disturbances (D)

Control Loop Components

Following are some control loop components which are normally part of the control system:

1) Recorders

Recorders provide continuous records of measured variables with respect to time. Recorder charts use the same scales that are used for indication but with an added coordinate to designate time.

Recorder types are dependent on their shape such as Circular Chart, Strip Chart etc:

• Circular Charts are normally 24-hour charts and must be replaced daily. Standard circular chart diameters include 6,8,10 and 12 inches and with speeds of 0.25 to 24 hours per revolution. Most charts are driven electrically though spring wound and pneumatic drives are also available.
• Strip Charts normally fast for 30 days at a standard speed of 0.75/1.0 inch per hour. Special purpose recorders such as X-Y recorder, event recorder, magnetic tape recorder etc. are also available.

Both Circular Chart and Strip Chart recorders are calibrated to read directly in process variable units that are familiar to operators. Strip charts may be rolled or folded and are available in standard lengths of 100 to 250 feet.

Multi point recorders are often used when more than one variable is needed on a common time and range basis. These records are needed for comparison, future use and as a checkpoint to guide operators within specified tolerance limits.

2) Switches and Alarm and/or Shutdown Functions

Alarm and shutdown functions represent abnormal situations and both are actuated normally by safety switches. An alarm condition warns that a shutdown may be eminent. A shutdown is a situation in which an alarm condition existed that was so serious that no time was available for corrective or remedial action.

The switch is a device that measures the process variable at a particular value and operates (opens or closes) when the preset value is reached. Switches may be directly connected to the process or they may be actuated by transmitted signal. Set point for the switches are normally set by screwdriver or knob and output contacts are available in SPDT (single pole double throw) contact form.

3) Solenoid Valves

A solenoid valve is a combination of two basic functional units - a valve and a solenoid (electromagnetic device). There are several types-direct action, pilot operated, 2/3/4 way operated etc. They come in several enclosures to suit various electrical classifications and there may be many variations related to electrical, mechanical and operating characteristics. 

The function of solenoid valve is to provide an on-off switching option in the system. They are frequently used in conjunction with control valves to open or close the valve at predetermined conditions or limits.

4) Actuators

An actuator is a mechanical device for moving or controlling a mechanism or system. Actuator is device, which causes type like, hand wheel or lever. With the help of manual adjustment, any position from fully open closed can be achieved. Other types of actuators are operated by compressed air, hydraulics and electricity,

5) Positioner

A positioner is a type of air relay, which acts to overcome hysteresis, packing box friction, and effects of pressure drop across the valve. It assures the exact positioning of the valve stem and provides precise control.

There are many types of positioners. Basic principles of operation for all types are similar. The instrument pressure (from an I/P, for example) acts on the input module, which controls the flapper nozzle system of relay. Supply pressure applies to the relay and the output pressure of the relay goes to the control valve actuator.

6) I to P converter

Current to Pneumatic Converter is used to convert the electronic/ electric standard signal of 4 to 20 mA to pneumatic standard signal of 0.2 to 1.0 kg/cm2. It is used in the standard control loop since majorities of the controllers are electronic and the control valve is pneumatically operated (pneumatic diaphragm type).

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