CHAPTER 6 PROCESS AND INSTRUMENTATION DIAGRAMS

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6.1 Role of Process Instrumentation and Control

The important feature common to all processes is that process is never in a state of static

equilibrium for more than a very short period of time. A process is dynamic and subjected to

deviate from the desired state of equilibrium. Thus, the process must then be manipulated

upon or corrected to drive it back towards the desired state and thus to maintain the efficiency

of the process. Instruments are used to measure the variable such as temperature, pressure,

composition, level, flow rate etc. In chemical industry it can be operated automatically, semi

automatically or manually.

Instrument applied to plant involves the use of level controls, flow rate controllers,

temperature controller, pH and automatic control of process variable. Electronic or pneumatic

controller systems are used mostly. The automatic controller are much more efficient and

accurate then manual controller because it is not possible all the time to controls the variable

manually and hence it is necessary to fix the limit to give the optimum economic operation

some of the operation equipment give alarm with light on the panel. The subsequent control

is to be exercised when temperature, level, pressure, and flow deviates from its operating sate

value.

During the start up and shutdown of the plant and during abnormal and emergency conditions

the plant is operated under manual control when plant becomes steady state and under normal

operation it is operated under auto control.

Automatic control is the norm throughout the chemical industry, and the resultant savings in

labour combined with improved ease and efficiency of operations has more than offset the

added expense of instrumentation.

All the operation in a chemical plant depends on the measurement and control of the process

variables. Instruments are used in the chemical industry to measure process variables, such as

temperature, pressure, density, viscosity, humidity, pH, liquid level, flowrate, chemical

composition, specific heat, conductivity and dew point. By use of instruments having varying

degrees of complexity, the values of these variables can be recorded continuously and

controlled within narrow limits.

6.2 Instrumentation and control objectives:

The primary objectives of the designer when specifying instrumentation & control schemes

are:-

(a) Safe plant design:

- To keep the process valuable within known safe operating limit.

- To dictate dangerous situation as they develop in to provide alarms and automatic shutdown

systems.

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- To provide inter locks and alarms to prevent dangerous operating procedures.

(b) Production rate:

- To achieve the desired production output.

(c) Production quantity:

- To maintain the product consumption within the specified quality standards.

(d) Cost:

- To operate at the lower production cost, commensurate with the objectives. But sometimes

it may be better strategy to product a better quality at a higher cost.

(e) Labour:

- The process can operate with less labour power and hence lower the operating cost.In a

typical chemical processing plant, these objectives are achieved by a combination of control,

manual monitoring & laboratory analysis.

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6.3 PROCESS & INSTRUMENTATION DIAGRAM FOR EQUIPMENTS

In this section P & ID of major equipment’s used in the process are discussed in brief.

6.3.1 P & ID on Dehydrogenator (reactor) Figure 6

Notations:-

NUMBER DESCRIPTION

2 Conduit

4 Vaporizer

6 Conduit

8 Conduit

10 Fired Tube Reactor

12 Catalyst Filled Tubes

14 Furnace Coil

16 Convection Section

18 Radiant Section

20 Transducer

24 Process Controller (PI) Temperature

26 Set Point Signal

28 Scaling Signal

30 Control Valve

32 Conduit

34 Conduit

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System description

The output signal here can be representative of a desired change in flowrate,to make

actual temperature equal to desired temperature,or signal can be pressure.

Transducer 20 in combination with a sensing device such as thermocouple,which is

operably located at conduit 6 establishes an output signal 22 which is provided as as a

process variable input to temperature controller 24.

Temperature controller 24 is also provided with a set point signal 26 representative of

desired temperature of effluent flowing in conduit 6.

Here set point signal 26 is set between 600-650°C and pressure about 0-1 atm.

In response of signal 22 & 26,the temperature controller 24 establishes an output

signal 28 responsive to the difference between signals 22 & 26.Signal 28 is scaled so

as to be representative of position of valve 30 required to maintain the actual effluent

temperature represented by signal 22 equal to desired effluent temperature represented

by 26.

Signal 28 is provided from temperature controller 24 to control valve 30 & control

valve 30 is manipulated in response of signal 28.

6.3.2 P & ID on distillation column Figure 7

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Notations:-

NUMBER DESCRIPTION

1 Fractionating Column

2 Rectification Zone

3 Stripping Zone

4 Line

5 Line

6 Overhead Condenser

7 Line

8 Receiver

9 Line

10 Line

11 Orifice

12 Valve

13 Line

14 Flow Controller

15 Line

16 Line

17 Chromatographic Analyser

18 Line

19 Bottom Zone

20 Line

21 Line

22 Valve

23 Level Controller

24 Level Sensor

25 Line

26 Orifice

27 Valve

28 Flow Controller

29 Line

30 Line

31 Line

32 Line

33 Line

34 Line

35 Orifice

36 Line

37 Flow Controller

38 Line

39 Valve

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System description

Typically fractionating column 1 containing rectification zone 2 & stripping zone 3 is

shown in figure.

Here the fractionation column 1 is maintained under condition of thermal equilibrium

by passing bottom liquid to reboiler 31 & flow controller 37 to adjust heat input to

reboiler 31 in a manner sufficient to generate vapour within the reboiler in an amount

sufficient to hold a constant flow signal passing from orifice 35 to flow controller 37

via line 36.

6.3.3 P & ID on storage tank Figure 8

System description

Here process control instruments are basically required for level indicating and

temperature indicating.An alarm system is installed to notify if there is non-ideal

variation in level and temperature.

tank consists of a regulating valve to control inert gas flow into the tank,a low

temperature indicator (LT 1),a level indicator (LI 1),an low level alarm indicator

(LAH 1).Here 1,2 denote loop numbers for a system.

some portion of process control is shown in detail in storage and handling section,

which discusses topic storage tank.

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6.3.4 P & ID on centrifugal pump Figure 9

System description

Centrifugal pump cosists of valves,check valves,control valves(FCV),flow temperature

detector (FT 2),flow indicating controller (FIC 2) pressure indicator (PI 1) to control proper

operation of pump. Here 1,2 denote loop numbers for a system.