PROCESSINSTRUMENTATION

BY:P. Dhana Lakshmi

Recording Instruments Indicating and signaling instruments Transmission equipment Controls center Instrumentation diagram Process analysis

RECORDING INSTRUMENTS

A recording instrument should be employed when a permanent record of the variable is desirable.

More expensive and require more maintenance than indicating types.

ELEMENTS OF RECORDING INSTRUMENTS:o Indicating pointer o Calibrated scaleo Recording charto Recording pen

TYPES OF CHARTS:

1.CIRCULAR CHART:

2. STRIP CHART:

3. Z-FOLD CHART:

4. ROLL CHART:

MULTIPLE POINT RECORDERS:

They are used to record several variables on one chart.

This can be done only when the variables do not vary greatly with time and are not close together in value.

INDICATING INSTRUMENTS

An indicating instrument is used when only the present value of the variable is to be known and the past record is of no use.

Indicating scales are of three types:o Eccentric scaleo Concentric scaleo Linear scale.

Eccentric and concentric scales:

SIGNALING INSTRUMENTS They are used effectively when it is only necessary to indicate that the

variable is within certain limits.

Common signaling systems are:o “Hi-low” systemo “hi-low-neutral” systemo “5-position” system.

Transmission of readings

Pneumatic transmission:It is generally operated on transmitted pressures of about 0 to 20

psig. The receiver here is simply a pressure gage of 0 to 20psi calibration. Actual calibration can then be made in terms of desired variable.

lag is due to resistance and volume of the tubing.

Electric transmission:It appears in the inductance- bridge systems.

As the distance increases the resistance increases and there will be an increasing dead zone caused by the reduced current in the bridge.

CONTROL CENTER Instrumentation equipment are grouped at one location near the processing unit or manufacturing operation which they serve. This group may be termed a control center

It controls or aids an operator in controlling the process operation.

The control center must have:o Clean dry atmosphereo Relatively constant temperature and humidityo No vibrationo Adequate lighto Unfailing source of electric power free of surges in voltage and frequencyo Unfailing source of clean dry air of constant pressure and required capacity.

Types of instrumentation layout:o Central layouto Unit layout

Central layout:

Unit layout:

INSTRUMENTATION DIAGRAM

Its purpose is to provide information quickly for use in process analysis, production control, specification of equipment, and preparation of equipment requisitions..

Constructed on the basis of a process flow diagram.

It should indicate:o The variable being measuredo Whether indication, recording, or other service is requiredo Whether control or alarm functions are requiredo The auxiliary features of instrument or controllero Type of connecting lineso Approximate location of the point of measurement or point of controlo Which instrumentation is to be included in the control center and

which is to at the processing unit.

SYMBOLS FOR INSTRUMENTATION DIAGRAMS

IDENTIFICATIONS ON INSTRUMENTATION DIAGRAMS

First letter second letter third letter

D- density A- alarm A- alarmF- flow C- control C- controlH- hand actuated E- element(primary) V- valveL- level G- glass M- moisture I- indicatingP- pressure R- recorderT- temperature S- safety

W- well

PROCESS ANALYSISProcedure for analyzing a process and applying instrumentation:

Divide the plant function into the smallest operation elements or operation units.

List all variables that may affect each process element

Add to this list ambient temperature , ambient humidity, barometric pressure, sun and wind conditions.

Divide this list of variables into four groupso Variables to be controlled automaticallyo Variables to be measured continuouslyo Variables to be measured periodicallyo Variables neither measured nor controlled

For each variable of the automatic control group , select:o A method of measurement that will provide data most indicative of the desired process performance. o A location for the controller- at the control center or on the unit.o A style of controller- non indicating, indicating or recording.

For each variable of continuous measurement group, decide:o What method of measurement will provide data most indicativeo Whether signaling, recording, or indication is desirableo Whether the element should be placed at control center or on the unit.

For each variable of the periodic measurement group, decide on a method of performing the measurement and the frequency with which the measurement has to be made.

Construct the instrumentation diagram.

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