PLC AND TURBINE SYSTEM QUESTION AND ANSWERS

This blog is wrote about PLC AND TURBINE SYSTEM QUESTION and focuses on instrument engineers, supervisor and technicians of petrchemical industry, plants and fertilizers companies.

  1. WHAT IS A PLC? WHAT ARE ITS DISTINCT ADVANTAGES OVER CONVENTIONAL CIRCUITS?

ANS:    A PLC STANDS FOR PROGRAMMABLE LOGIC CONTROLLER. ALL TYPE OF LOGIC FUNCTIONS CAN BE PERFORMED BY IT.                SUCH AS SEQUENTIAL START UP OF SYSTEMS, FAIL SAFE RUNNING PLANT AND SEQUENTIAL SHUTDOWNS. THE                          CONVENTIONAL CIRCUITS USING RELAY DIGITISED GATES AND DIFFICULT TO CHANGE OR BYPASS AND BULKY. WHILE                PLC IS SMALLER AND CHANGES  CAN BE MADE BY PROGRAMMING WITHOUT CHANGING HARDWARE, MEANT FOR                          INDUSTRIAL TMOSPHERE, CAN BE INSTALLED IN PLANT CONVENIENT, NO MOVING OR MECHANICAL PARTS SO,                              CHANCES OF FAILURE IS MINIMISED.

  1. WHAT ARE DIFFERENT TYPES OF I/O IN TYPICAL PLC SYSTEM USED FOR SAFETY SHUTDOWN SYSTEM IN OIL INDUSTRIES?            

ANS: INPUTS

  • RELAY AND SWITCHES CONTACTS. WITHOUT VOLTAGE (DRY CONTACT)
  • RELAY AND SWITCHES CONTACTS WITH 120 V AC OR 24 V DC.
  • LOGIC LEVELS FROM TRANSISTORISED CIRCUITS CAN ALSO BE INPUTS.

OUTPUTS

  • LOW VOLTAGE CONTACT FOR OTHER INTERLOCK FUNCTION
  • HEAVY RATED CONTACTS FOR SOV OR OTHER DEVICES
  • THYRISTER OUTPUTS.
  1. DEFINE THE FOLLOWING TERMS USED IN DIGITAL SYSTEMS.

ANS: A) CPU – CENTRAL PROCESSION UNIT. THE HEART OF COMPUTER. B) ALU – ARITHMETIC LOGIC UNIT.  PERFORMS ALL MATHEMATICAL FUNCTIONS. C) EPROM  – ERASABLE PROGRAMMABLE READ ONLY MEMORY D) RAM  –  RANDOM ACCESS MEMORY E) FLOPPY DISK –  FLEXIBLE MAGNETIC DISK FOR DATA/ PROGRAM STORAGE F) MODEM  –  MODULATOR / DEMODULATOR FOR COMMUNICATION OVER LONG DISTANCE G) RS 232  – MOST COMMONLY USED SERIAL INTERFACE FOR PERIPHERALS. H) HIWAY – MAIN CABLE FOR COLLECTION AND DISTRIBUTION OF DATA AT HIGH FREQ TO ALL THE DCS SUBSYSTEMS. I) WATCH DOG TIMER – TIMER TO KEEP OVER MICROPROCESSOR CYCLE OPERATION. J) PARITY – DIGITAL SIGNALS ARE CODED TO HAVE A DEFINE PATTERN (EVEN/ODD/OTHER) FAILING WHICH. K)  A/D CONVERTER – ANALOGUE TO DIGITAL CONVERTER.  1 TO 5V.  0 TO 12 BITS OF DIGITAL SIGNAL BY A/D CONVERTER.

  1. GIVE MOST COMMONLY USED ENGINEERING UNITS FOR FOLLOWING IN MKS AND FPS SYSTEMS.

 

PARAMETERS                              MKS                                              FPS

 

TEMPERATURE                   DEG .C, DEG .K                            DEG .F, DEG. R

GAS FLOW                           MMSCF, ST. M3/D                         SCFM, SCFH, M3/HR

OIL FLOW                            BBL/D, METRIC TONS/D              GPM, LPM, LB/HR

TANK LEVEL                      METER, CM, M3                            FT, INCH, FT3

HEAT                                   CALORIE, K .
CALORIE                                                                                              B.T.U

DENSITY                            GM/CM3, KG/M3                              POUND/IN3

PRESSURE                          KG/CM2, BAR, KP                        PSI, IN. OF H2O, MM
H2O, IN.OF HG.

VIBRATION                       G/SEC, GM/SEC, CM/SEC                MILS, THOU.

  1. GIVE NAMES OF TWO MOST COMMONLY USED FLAME DETECTORS USED FOR GAS FIRED HEATERS AND BRIEFLY DESCRIBE THE WORKING.

ANS:  A UV FLAME DETECTOR – UV RAYS SENSITIVE MATERIAL COATED ON THE TUBE ELECTRODE. WHEN IT SENSE RAYS IT STARTS CONDUCTING ELECTRONS ARE SENSED BY AMPLIFIER AND CIRCUIT IS OPERATED. FLAME ROD – ITS MADE OF HEAT SENSITIVE MATERIAL WHICH CHANGE IS SMALL AMOUNT OF CURRENT FLOW WHEN IT IS HEATED. THIS CURRENT AMPLIFY AND OPERATE THE CIRCUIT.

  1. WHAT SHOULD BE THE SETTING OF A PRESSURE SWITCH, IF IT IS REQUIRED TO TRIP A PUMP AT 30 % OF THE FULL SCALE FLOW? ( SWITCH IS CONNECTED TO OUTPUT OF A 3 – 15 PSI DP CELL)

ANS:      12 X (30%) SQUARED + MIN. (3) = 12 X 0.09 + 3 = 4.08 PSI.

Please check also : COMPLETER OVERVIEW OF PROGRAMMABLE LOGIC CONTROLLER PLC

  1. WHAT WILL BE THE OUT PUT OF A SQUARE ROOT EXTRACTOR, IF IT IS INPUT 8 MA IN A STANDARD 2 WIRE TRANSMITTER SYSTEM?

ANS: 12 MA.

  1. WHAT IS THE EFFECT OF TEMPERATURE AND PRESSURE ON GAS FLOW MEASUREMENT BY ORIFICE /DP CELL COMBINATION?

ANS:          MASS FLOW = VOLUME FLOW X (PRESSURE/ TEMPERATURE).

  1. GIVE DEFINITION OF PROPORTIONAL BAND, INTEGRAL ACTION AND DERIVATIVE ACTION TIME.

ANS:        P.B – AMOUNT OF % ERROR REQUIRED TO CHANGE OUTPUT BY 100%.

I.A.T- IT IS THE TIME TAKEN BY INTEGRATION TO CHANGE OUTPUT BY SAME COMMAND AS PROPORTIONAL BAND.

D.A.T – IT IS THE TIME WHICH WOULD HAVE BEEN REQUIRED TO OPEN AND CLOSE THE VALVE, BEFORE THE ERROR OCCURS.

  1. WHAT IS THE DISADVANTAGES OF PROPORTIONAL ONLY CONTROL AND HOW WILL RESET ACTION OVERCOMES IT?

ANS: PROPORTIONAL ONLY CONTROL HAS OFFSET WHENEVER CHANGES OCCUR. THE OFFSET VARIES WITH THE AMOUNT OF PROCESS DEVIATION.  RESET ACTION INTEGRATES THIS ERROR AND KEEPS CHANGING THE OUTPUTS TILL THE OFFSET IS REDUCED TO ZERO.

  1.   WHAT ARE THE MOST IMPORTANT PARTS OF A GAS TURBINE?

ANS:  A) STARTOR B) AIR COMPRESSOR AND BLEED VALVES C) COMBUSTION CHAMBERS –  AIR INLET FOR GAS CONTROL VALVES, FLAME DETECTION AND TEMPERATURE CONTROL. D)   HP TURBINE – SPEED CONTROL GUIDE VANES, NOZZLE CONTROL. E) LP TURBINE OR POWER TURBINE – SPEED CONTROL, TEMP TRIPPING

  1. WRITE DOWN THE STEPS IN STARTING UP OF A GAS TURBINE SEQUENTIALLY.

ANS: A) LUBE OIL SYSTEMS ARE O.K… B) STARTOR ON C) REACH CERTAIN SPEED. PCD O.K. D) IGNITION ON. FLAME DETECTOR E) REACH CERTAIN SPEED F) ACCELERATE TO A HIGHER SPEED.  MAIN BURNERS ARE ON.  ACCELERATION CONTROL G) MAIN LUBE OIL SYSTEM ENHANCES.  AND SPEED CONTROL O.K. H) SPEED CONTROL ACTIVATES I) AUXILLARY LUBE OIL SYSTEM OFF.

  1. WHAT IS A CDP OR PCD OR P2 PRESSURE AS REFERRED TO GAS TURBINE? WHAT IS ITS IMPORTANTCE?

ANS: IT IS A COMPRESSOR DISCHARGE PRESSURE AND IT IS IMPORTANT FOR MONITORING/CONTROL OF THE FOLLOWING: A) COMPRESSOR SURGE – TO OPERATE BLOW OFF VALVES. B) CONTROL OF SPEED – DURING START UP AND TIMING RATE OF CHANGE IN SPEED

  1. WHAT PARAMETERS LIMIT THE TURBINE SPEED CONTROL?

ANS:  A) TEMP B) COMPRESSOR DISCHARGE PRESSURE C) LOAD

  1. HOW THE SURGE IS IS AVOIDED IN A GAS TURBINE AIR COMPRESSOR?

ANS: BY THE CONTROL OF ‘BLOW OFF’ VALVES.

  1. WHAT IS SURGE IN COMPRESSORS?  HOW IT IS PREVENTED?

ANS: IF THERE IS NOT ENOUGH CHARGE GAS TO COMPRESS (SPEED BEING CHANGED) THEN THE COMPRESSORS DISCHARGE PRESSURE RISES WHICH IN TURN NEEDS MORE FLOW OF GAS.
THIS WILL END UP IN AVALANCHE WHICH IS CALLED ‘SURGE’ IN COMPRESSORS.

 

 

 

 

 

 

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