basics of i&cs

8/12/2019 Basics of I&Cs

1/24

Introduction to Instrument Functions

Instrument symbols define what variable shall be measured andwhat action is required from the instrument.

1. First letter defines what variable is to be measured.

2. The next letters define either a function or a modifier regardless ofthe variable to be measured and regardless of the way it shall berealized. Main functions:

a- Indicating b- Recordingc- Switching d- Alarminge- Controlling f- Mathematical functionsg- Transmitting h- Measuring

Main modifiers: L (Low), H (High), LL (very low), HH (very high).


2/24

Introduction to Instrument Functions(contd)

a) Indication

Is the process in which the instrument, (indicator),satisfies the operator needs to know about some

process variables in one of two ways: the qualitative wayor the quantitative way.


3/24


b) Recording

Usually operators write the values read on indicators inlog books once per hour. To avoid the drawbacks of

losing data, or to keep information for further studyand/or evaluation, important process variables areautomatically recorded by various types of recorders.


4/24


Recording (contd) 1. Continuous Line RecordersIn this type of recorders a pen is assigned to the pointer of

each required process variable, this pen is marking on amoving chart.

100 %

ProcessVariableScaleTimeScale

TimeScale

100 %

ProcessVariableScale


5/24


Recording (contd) 2. Dotted Line Recorders These are time shared equipment

Rotaryswitch

Measuringsystem

Printingmechanism

Chartpaper

Measuredvariables


6/24


c) Switching

SPSTSwitch

B B

A A

Position (2) Position (1)

A

B

A

B

C C

Position (2) Position (1)

SPDTSwitch


7/24


A2

B 2

A1

B 1

C2 C1

DPDT

Switch

B 2 B 1

A2 A 1

DPSTSwitch

Switching (contd)


8/24


(Low current)

Tip

High frequencycurrent

Metallicbody

(High current)

Switching (contd)

A

CB

C

Glass tube

Mercurydrop

A

B

Mercury Switch

Proximity Switch


9/24


Examples

1) 2 port /2 position valve

2) 3 port / 2 position valve

a a

b b

c b c b

a a

Switching (contd)


10/24


3) 4 port and 3 position valve

Example

3/2 valve with pressure / spring actuators

c b

d a

Switching (contd)


11/24


d) Alarming

Alarms are needed to alert the operator in case of one ormore process variable value crosses a predefinedthreshold value, (increased above a high limit ordecreased below a low limit), or a machine motorchanges its status, (motor stops or motor starts).

Alarms are divided into two groups; Normal Alarms which do not cause shutdown of plant and stop theoperation, and Shut down Alarms which cause shutdown and stop the operation.


12/24


Alarming (contd) Examples of alarm sequences

1. Normal alarm sequence

Condition Lamp hornNormal off off

Alert flashing on Acknowledge on offReturn to normal off offReturn to normal flashing onbefore acknowledge

Acknowledge off off

2. Motor alarm sequence

Condition Lamp hornMotor starts flashing on

Acknowledge on offMotor stops flashing on

Acknowledge off off


13/24


e) Controlling

1. Compare the requirements to the existing values/status and find thedifference.

2. Use this difference and a pre-built in instructions to do certainmathematical/logical operations.

3. Feed the results of these mathematical logical operations tomanipulating devices (not part of the controller).

4. Manipulating devices changes the manipulated variables of theprocess to make the process tend to have the controlled variablevalue / status equal to the required value/status.


14/24


Controlling (contd)

Common nomenclature:

1. The process variable values required are commonly called setpoints.

2. The difference between the set point and the existing value iscommonly called error.

3. The pre-built in instruction are called control algorithm or controllogic.

4. The result of mathematical/logical operation of the controller iscalled controller output.5. The manipulating devices is called final control element.


15/24


Controlling (contd)

Control algorithm/logic

Manipulatingdevice

Process

The controller

Manipulated

Variable

Disturbance (z)

(r) (y)

ControlledVariable

Final control element


16/24


f) Mathematical Units

In many cases some mathematical treatment of signalsare needed before indicating, recording or using them incontrol.

The square root extraction is one of the most commonlyused operations in flow measurements. Multiplying anddivision is also used in state correcting. Furthermore,adding and subtracting is used for biasing andcorrection.


17/24


g) Transmitting

When it is necessary to concentrate all data in one placeeither to a local panel or control room, for indicating,recording or any other type of processing.

The method adopted to do this is to transfer themeasured data into standard signals and send them viatubes (in pneumatic systems) or cables (in electricsystems) to control or local panels, there they can beprocessed and scaled back to original values, thisoperation is done by a transmitter at one end by areceiver at the other end.


18/24


h) Measurement

The measurement of the process variables is the main

function of instruments.

Any instrument connected to process directly contains ameasuring system for the process variable denoted in its

tag number, other instruments not connected to theprocess directly contain one or more signal measuringsystem and scaling factors.


19/24

Instruments in Hazardous Areas

Ignition hazards occur when the following threeelements co-exist:

1. Flammable material (gas / liquid / dust)

2. Air or oxygen or other oxidant support of combustion

3. Starting source of energy


20/24

Instruments in Hazardous Areas(contd)

Several techniques are used to prevent explosions whenflammable material is present in the atmosphere, each ofthese techniques separates one of the three elements

mentioned above from the other two elements, whichcannot cause explosion to happen alone.


21/24


The following are the techniques used toprevent explosions:

1. Oil immersion, and designated Ex "o"2. Pressurization, and designated Ex "p"3. Powder filling, and designated Ex "q"4. Flame proof, and designated Ex "d"5. Increased safety, and designated Ex "e"

6. Intrinsic safety, and designated Ex "i"7. Non incendive, and designated Ex "n"8. Encapsulation, and designated Ex "m"9. Special, and designated Ex "s"


22/24


Areas are grouped according to the time probability ofhazardous gas existence Div 1 and Div2 in America andzone 0 to zone 2 in Europe where zone and Div2 areleast probability for gas existence.

Gases are grouped according to the energy they need tostart ignition (IIA to IIC) where IIC is the lowest energyneeded.

Apparatus are classified according to the maximumtemperature that can occur on its outer surface due tofaults (T1 to T6) where accordingly T6 is the lowest.


23/24

Safety Valves and other ReliefingDevices

These reliefing devices are the last physical resource inthe (ILOPPA), Independent Levels of Plant Protection

Arrangement, they open and relief process to flare.

They can be replaceable like rupture discs, or reusablelike safety relief valves, breathing valves etc.

They should be simple mechanical devices with no

system to control them; they only depend on mechanicalproperties of materials.


24/24

Safety Valves and other ReliefingDevices (contd)

Process engineers are responsible of sizing these devicesand choosing their type, instrumentation engineers areresponsible only of requisioning them, testing and

calibration if required.

basics of i&cs

Documents