Control And Instrumentation Of Thermal Power Station 

There are four instruments generally used any process plant as basic instrument since invent of such instruments. Therefore basic specification is given below for procuring instrument at site if required. The instruments are Pressure gauge and Differential pressure gauge , Temperature gauges , Resistance temperature detector, Thermocouples. Different and details of their principle can be available in a book therefore need not to be discussed here. I request reader to scan through the leaflet of the manufacturer and understand the applicability. I like to mention please be careful about the connection ( i.e thread available in the instrument and process). Most of the time it is observed mismatch between the two and land into problem during installation which may require one more piece to match both.

Specification  for Pressure Gauge and  Differential Pressure Gauges

Type                                    :Bourdon /Bellow /Diaphragm

Sensing Element material   : AISI 316  SS

Movement Material            : AISI 304  SS

Case Material                      : Die cast Aluminium, stove enameled ,black finish

                                             Threaded bezel Ring ,clear glass cover as per 

                                             NEMA 4x

Dial Size                              : Generally 150 mm, other sizes  are 100mm,      

                                              250 mm as specified.

Scale Size                            : Black lettering on white background in 270  Degree arc.

Range Selection                   : Normal process pressure 70 per cent of Range.

Over range Protection          :12% of maximum range by internal stop. External

                                              Stop at Zero.

Adjustment                           : External micrometer screw for zero adjustment .

                                              Internal micrometer screw for range adjustment

Element connection               Brazing for range below 40 kg/ cm2

                                              Argon welding for range above 40kg/cm2.

Process Connection               M20x1.5 (M)  { bottom or back}

Performance                          Accuracy :+/- 1.0 % or less of span. 80deg ambient

Safety feature                         Blow out disc at the back.

Applicable standard               IS:3624-1966 

Specification for local Temperature gauges (Capillary)

Type                                      Mercury filled

Sensing Element material            Bourdon- AISI 316 ss/ chrome moly

Bulb and capillary material ,dia  AISI 316 ss /12 mm

Capillary armouring                    Solid drawn 316SS with SS armour

Movement material                      304 SS

Thermowell connection               M33x2

Case material                                Die cast aluminium , black finished.

Dial Size                                       150 mm

Scale                                             Black lettering with white back ground

Mounting                                       Surface

Over   range protection                125% FSD

Instrument Connection                Bottom /M20X1.5

Accuracy                                      +/- 1% of full scale ,Zero adjustment

Enclosure class                             IP 55

Capillary Length                            5 M

Accessories                                    Thermowell ( weldable or screwed as per

                                                        ASMEPTC  code.

Specification for Resistance  Temperature Detector

Platinum / Copper

Type :                                    Platinum / Copper.

Resistance                            100 ohm at 0 deg C / 53 ohm at 0 deg C

Base                                      wound on ceramic

Wiring                                   3 wire system

Protecting  Tube                   8mm/316 Ss seamless , filling MgO ( purity 99.4%)

Head                                     NEMA 4X/IP55, Die cast aluminium, Brass Terminal 

                                              Silver plated , cable gland  with grommet , Terminal

                                              Head cover with SS chain and suitable gasket

Accessories                          Connection , flanges , thermowell , conduit etc.

Simplex / Duplex                 Duplex.

Standard                               DIN 43760

Response  Time                   30 secs

Accuracy                              0.1 at 0 deg C

Insulation Resistance         10M ohm at 250V Dc at 27deg C and 600deg C

Specification for thermocouples

Type                                       Cr. Al  ( popularly known as  K-type.)

Wire gauge                          16 AWG

Standard                               ANSI-C96-1- 1975

Protecting Tube                  6 mm  OD, 316 SS Seamless , MgO filling

Loading                                 Spring loaded.

Head                                      IP 55 , Die –cast aluminium , Brass Silver plated

                                                Connector , cable gland, head cover with  SS

                                                Chain and suitable gasket.

Accessories                          Thermowell , flanges ,fittings etc as per requirement

Type of grounding                         Ungrounded/grounded as per requirement

Response time                     within six seconds

Accuracy                               Standard ISA

All the specification given above is typical can be changed according to process requirement. RTD generally used are platinum RTD or copper RTD. In BHEL sets copper RTD are used mostly in generator stator and stator water circuit and blance all the places it is platinum RTD.