Thermocouples Types, Range and Metal used

( J )
Iron & White
Constantan & Red
-300°C to 1400 °C

( K )
Chromel & Yellow
Alumel & Red
-300°C to 2300°C

( E )
Chromel & Purple
Constantan & Red
-300°C to 1600°C

( T )
Copper & Blue
Constantan & Red
-300°C to 650°C

( R )
Platinum and 10%Rhodium & Black
Platinum & Red
32°C to 2700°C

( S )
Platinum and 13%Rhodium & Black
Platinum & Red
32°C to 2700°C

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BASICS OF TRIPS, INTERLOCKS, PERMISSIVES & SEQUENCES

■ TRIP : The term trip refers to an action that is initiated by the control system and which forces a device or devices to a pre-determined state. Example of Trip Signals: Close Valve, Open Valve, Stop motor, etc. The Safety Instrument System (SIS) or a Hardwired systems normally initiate trips, however the PLCs or DCS may also initiate trips provided the necessary independence and SIL ratings are met.Once a device or devices have been forced to a pre-determined state by the action of a Trip they will remain in that state until the Trip is manually reset by a conscious operator action. For example: High level in a vessel initiates a trip system which stops the pump feeding that vessel, the pump will remain stopped even if the level in the vessel falls to a safe level. The Trip must be ‘reset’ by the operator before the pump can be re-started. The Trip can only be ‘reset’ if the level in the tank has fallen to a safe level. Resetting the Trip will no...

Instrument Range Selection

Instrument range shall be selected based on operating range of process to be measured. In common practice, it is recommended to select instrument with full scale range that can give operating process reading between 30% and 70% especially for pressure gauge. The following is an example of selecting pressure gauge range Given Process data: Minimum operating pressure: 5 barg Maximum operating pressure: 9.5 barg Maximum Design pressure: 25 barg Available standard Pressure Gauge range: 0 – 10 barg, 0 – 16 barg, 0 – 25 barg [X] Selecting 0-10 barg will give 95% reading during maximum operating condition. Instrument has only narrow margin above maximum operating pressure. [√] Selecting 0-16 barg range will give best reading during operating condition. However, the maximum overpressure of pressure gauge is commonly 1.3x of the maximum scale which is only 16×1.3 = 20.8 barg. In such case, pressure gauge shall be equipped with overrange protector to withstand ...

What is difference gap control ?

Differential gap control is similar to on off control except that a band or gap exists around the control point. Use: In industry differential gap control is often found in non critical level control applications where it is desirable only to prevent a tank from flooding or drying. When a measured variable exceeds the upper gap the control valve will open fully or be closed fully. Similarly when it exceeds the lower gap it will open or close fully.

INSTRUMENTATION AND CONTROL ENGINEERING

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