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  #1  
قديم 01-15-2011, 02:31 PM
الصورة الرمزية Khaled Hamza
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Lightbulb Elements of a Process C**trol System

Elements of a Process C**trol System


INTRODUCTI**
-The term automatic process c**trol came into wide use when people learned to adapt automatic regulatory procedures to manufacture products or process material **** efficiently. Such procedures are called automatic because no human (manual) interventi** is required to regulate them.
-All process systems c**sist of three main factors or terms: the manipulated variables, disturbances, and the c**trolled variables. Typical manipulated variables are valve positi**, motor speed, damper positi**, or blade pitch. The c**trolled variables are those c**diti**s, such as temperature, level, positi**, pressure, pH, density, moisture c**tent, weight, and speed, that must be maintained at some desired value. For each c**trolled variable there is an associated manipulated variable. The c**trol System must adjust the manipulated variables so the desired value or “set point” of the c**trolled variable is maintained despite any disturbances.





Elements of a Process C**trol System
-The following figure illustrates the essential Elements of a process c**trol system. In the system shown, a level transmitter (LT), a level c**troller (LC), and a c**trol valve (LV) are used to c**trol the liquid level in a process tank. The purpose of this c**trol system is to maintain the liquid level at some prescribed height (H) above the bottom of the tank. It is assumed that the rate of flow into the tank is random. The level transmitter is a device that measures the fluid level in the tank and c**verts it into a useful measurement signal, which is sent to a level c**troller. The level c**troller evaluates the measurement, compares it with a desired set point (SP), and produces a series of corrective acti**s that are sent to the c**trol valve. The valve c**trols the flow of fluid in the outlet pipe to maintain a level in the tank.


-Thus, a process c**trol system c**sists of four essential elements: process, measurement, evaluati**, and c**trol. A block diagram of these elements is shown in the following figure. The diagram also shows the disturbances that enter or affect the process. If there were no upsets to a process, there would be no need for the c**trol system. The figure also shows the input and output of the process and the set point used for c**trol.




PROCESS
In general, a process c**sists of an assembly of equipment and material that is related to some manufacturing operati** or sequence. In the example presented, the process whose liquid level is placed under c**trol includes such comp**ents as a tank, the liquid in the tank, and the flow of liquid into and out of the tank, and the inlet and outlet piping. Any given process can involve many dynamic variables, and it may be desirable to c**trol all of them. In most cases, however, c**trolling **ly **e variable will be sufficient to c**trol the process to ****** acceptable limits. **e occasi**ally encounters a multivariable process in which many variables, some interrelated, require regulati**.

MEASUREMENT
- To c**trol a dynamic variable in a process, you must have informati** about the entity or variable itself. This informati** is obtained by measuring the variable.
- Measurement refers to the c**versi** of the process variable into an analog or digital signal that can be used by the c**trol system. The device that performs the initial measurement is called a sensor or instrument. Typical measurements are pressure, level, temperature, flow, positi**, and speed. The result of any measurement is the c**versi** of a dynamic variable into some proporti**al informati** that is required by the other elements in the process c**trol loop or sequence.


EVALUATI**
- In the evaluati** step of the process c**trol sequence, the measurement value is examined, compared with the desired value or set point, and the amount of corrective acti** needed to maintain proper c**trol is determined. A device called a c**troller performs this evaluati**. The c**troller can be a pneumatic, electr**ic, or mechanical device mounted in a c**trol panel or ** the process equipment. It can also be part of a computer c**trol system, in which case the c**trol functi** is performed by software.

C**TROL
The c**trol element in a c**trol loop is the device that exerts a direct influence ** the process or manufacturing sequence. This final c**trol element accepts an input from the c**troller and transforms it into some proporti**al operati** that is performed ** the process. In most cases, this final c**trol element will be a c**trol valve that adjusts the flow of fluid in a process. Devices such as electrical motors, *****, and dampers are also used as c**trol elements.

------------------------------------------------------------------------------


Process and Instrumentati** Drawings
-In standard P&IDs, the process flow lines, such as process fluid and steam, are indicated with heavier solid lines than the lines that are used to represent the instrument. The instrument signal lines use special markings to indicate whether the signal is pneumatic, electric, hydraulic, and so **. two types of instrument signals are used: double cross-hatched lines denote the pneumatic signals to the steam c**trol valve and the process outlet flow c**trol valve, and a dashed line is used for the electrical c**trol lines between various instruments. In process c**trol applicati**s, pneumatic signals are almost always 3 to 15 psig (i.e., pounds per square inch, gauge pressure), and the electric signals are normally 4 to 20 mA (milliamperes) DC (direct current).




- A ballo** symbol with an enclosed letter and number code is used to represent the instrumentati** associated with the process c**trol loop. This letter and number combinati** is called an instrument identificati** or instrument tag number.
- The first letter of the tag number is normally chosen so that it indicates the measured variable of the c**trol loop. In the sample P&ID shown, T is the first letter in the tag number that is used for the instruments in the temperature c**trol loop. The succeeding letters are used to represent readout or passive functi** or an output functi**, or the letter can be used as a modifier. For example, the ballo** in Figure marked TE represents a temperature element and that marked TIC is a temperature-indicating c**troller. The line across the center of the TIC ballo** symbol indicates that the c**troller is mounted ** the fr**t of a main c**trol panel. No line indicates a field-mounted instrument, and two lines means that the instrument is mounted in a local or field-mounted panel. Dashed lines indicate that the instrument is mounted inside the panel.
- Normally, sequences of three- or four-digit numbers are used to identify each loop. In our process example, we used loop numbers 100 and 101. Smaller processes use three-digit loop numbers; larger processes or complex manufacturing plants may require four or **** digits to identify all the c**trol loops.
- Special marks or graphics are used to represent process equipment and instruments. For example, in our P&ID example in Figure two parallel lines represent the orifice plate that is used to detect the discharge flow from the process heater.


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  #2  
قديم 07-26-2011, 04:24 AM
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thnx


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  #3  
قديم 08-10-2011, 03:50 PM
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قديم 07-15-2012, 07:54 PM
الصورة الرمزية suliman_eng
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Thank you


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