CONTROL AND INSTRUMENTATION OF THERMAL POWER STATION - PART -II

Since last post considerable time was lapsed I could not deliver any post due to illness. I hope i will resume to share my experience of professional life with all of you.

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. 

CRT chart is given and other chart will be given as soon as it is available with me.

continued to PART -III

TRAGIC INCIDENCE AT UNCHAHAR THERMAL POWER STATION. LIKELY CAUSE

Shocking news of Boiler explosion at Unchahar power station was very painful to all power engineers who are actively associated with Power station. Death of more than 30 persons is extremely deplorable and I do not remember any such deadly incident recently with all modern instruments and control at place. 

I have seen most of the videos ,interviews already published in you tube and other channel and all are blaming ash pipe choking of boiler bottom ash , air heater ash hopper /eco bottom hopper. In a video now available in you tube shows flames are coming out from 

second pass just below the economiser coil . 

Formation of clinker or ash pipe line choking is very common in any coal based thermal power station and it got cleaned through clinker grinder or any other mechanical means. This will not create disturbance at this magnitude. As it was told furnace pressure went to + 350 mm water column. This is also unlikely to shoot upto +350 mm in a zip . It will increase slowly . The furnace is balanced draft furnace therefore positive pressure is not desirable otherwise hot flue gas will come out through peep hole in the furnace. There is alarm and trip with high furnace pressure . It also 

trip the boiler means all fuel input will be stopped however fan will

be running for purging . But Operator will get time to react and save boiler. 

The Boiler is new and been commissioned very recently. It is likely full oil support was there during explosion. Some body mentioned that both boiler was tripped and operators were trying to bring the boiler in operation. Number of workers present means there are small pending jobs which may not be required for operation but required to be completed before final handing over to customer.

It is unlikely during tripping fuel valve any corner was  in stay put and pumping oil in the furnace and oil is pooled inside the furnace as bottom ash hopper is choked there is no escape . Once the boiler is again lighted up after purging and coal fired then this oil was get burn causing rapid disturbance at the furnace . Nature of smoke , flame indicates the same . It is understood that disturbance was very rapid otherwise NTPC engineers and operators are competent  enough to save the situation. It is evident from the incident that none got time to react and it is only possible if there is oil pool inside the furnace. 

Truth will come out after through inspection and analysis. I like to

mention there was an explosion in Kahalgaon boiler due oil pool as during a power failure oil valves are in stay put condition and put oil inside the furnace without fire. Therefore after statutory purging also oil vapour will be in the furnace  and some of them will get deposited in eco coil /airheater busket etc. Then after fire un control combustion will take place  in the furnace and causing explosion. 

Once such type explosion happens buck stay will fail and it will rip open the water walls , a portion of the duct wall etc. The boiler furnace is supported from the top so the mass will be  vibrating violently as it is supported from top and all other pressure parts supports are from the top.

We will waiting for the analysis and pray to GOD such incident should not happen.

CONTROL AND INSTRUMENTATION OF THERMAL POWER STATION - PART -I

Till date we have discussed all the aspects of  thermal power station starting from civil construction , equipment erection and commissioning of the system. From this dispatch we will touch up on the importance of instrumentation and controls in thermal power station.

Instrumentation can be divided in two major Groups.

A ) Primary Instruments ( like pressure gauge , Pressure Transmitters , Temperature gauges and temperature Transmitters , RTDs , Thermocouples , Vibration measurement , Analysers , position transmitters , Limit switches , Level switches and transmitters ,Pressure Switches , Temperature switches , Differential Pressure Transmitters , Different type of isolating valves according to process requirement  etc. 

There are other primary instruments also but the list given above is most widely used in thermal power plant and any other process plant.

B) Secondary Instruments  ( like recorders , printers , digital indicator , Computer , Monitor , Digital control panel , Temperature Scanner , data logger , Analogue  and digital Control system , Different type of control panel of  specific use like DCS , Soot Blower , Auxiliary PRDS , Drum level measurement , Turbo visory system including speed and vibration measurement . etc. Secondary instrument gets signal from primary instruments then  processed and  presented as per our requirement.

Instruments can also be  grouped in the following manner 

i) Mechanical Instruments 

ii) Electrical Instruments 

iii) Electronic instruments

iv) Hydraulic Instruments. ( vane control etc )

v) Pneumatic  Instruments ( like Power cylinders, Control valves ,

e/p converters etc.

continued to PART _II

CONCLUDING PART STEAM BLOWING PART -IV ABC OF THERMAL POWER PLANT

PART -IV

COMPLETION CRITERIA

Stages 1a, 2a, 1c, 3a, 3b, 4a,4b

For the first few blows Target plates need not be kept. Afterwards Target plates made of Stainless steel of mirror finish are to be used.

Target plates are mounted in the temporary exhaust pipes nearer to the permanent line.

Steam blowing can be declared complete only after ensuring cleanliness of target plates. Following criteria is followed for declaring completion of steam blowing:

 Stage 1c & 2b

Minimum 8 (eight) numbers of blows are to be given.

Stages 4c to 4i

Steam blowing is declared complete based on visual observation. Minimum three nos. of blows shall be given for each sub stage.

No of impact >= 1 mm

Less than 4 impacts >= 0.5mm

less than 10 impacts >= 0.2mm

Total impact above 2mm size not exceeding 10 nos at the end of steam blow.

Impacts below 0.2mm size at the end of steam blow can be accepted provided they are well dispersed and uniform in distribution without creating a rough surface.

Minimum number of consequtive target plate is two.

The above criteria has been fixed by BHEL for  660 MW Boiler. However there are difference of opinion regarding completion  criteria. The First difference of opinion arises on metal of target plate. Earlier days we have seen use of bronze target plate and it is malleable material therefore slightest impacts are visible. Therefore steam blowing operation gets continued longer time and disturbance factor calculation was not available with us. For any 200 to 250MW plant steam blowing operation took around 30 to 40 days. After that target plate changed to Mild steel and now stainless steel. As per CEA guidelines it is written as steel. Therefore all personnel carrying out steam blowing should agree with the material.

A Few points to be noted during steam blowing :

1.  Four number Electrical operated trip valves with quick opening and closing are needed to conduct steam blowing.  To standardize the requirements 12”/1500 class for 250 MW and below and 14”/1500 class for 500MW and above are selected. Material of valve body selected is WC9.

2. Special precaution to be taken in cleaning of CRH lines . As it is seen that Reheater tube failures happened due to debris deposited in reheater tubes. Lines are to be checked thoroughly and some time it may happen a small portion of pipe not cleaned during steam blowing operation. Therefore pre cleaning and to maintain a log on that is necessary to avoid complication at later dates.

3. The steam blowing operation produces tremendous noise and certainly does noise pollution to the maximum effect. In earlier days people are not bothered about such pollution but now all are aware therefore steps to be taken to reuce this noise pollution . Earlier days only ear caps were provided but now a days silencer has been designed to reduce the noise.      

 The Steam Blowing Silencers are used to reduce the noise produced by the valve (Structure born noise and Air born noise) , when venting of pressurized and high velocity gas to the atmosphere, especially , steam, air and nitrogen.

4. The low point drain will be getting chocked at the time of initial operation. Therefore the same to be cut and debris to be cleared immediately and again re –weld the same after ensuring that steam flows through the drain.

5. Tools and other materials should be kept ready for cleaning of BFP and CEP strainers. As initially there will be frequent choking of strainer one after another 

with  minimum    time gap. If you are not prepared you can not keep the boiler running.

6. As we will running the boiler through oil firing then Air heater soot blower is must.

7. Drum level should be on manual to take care of swelling during blow off.

Disturbance Factor   =               Qb² x Vb

or

clearing force ratio               Q² MCR x VMCR

Qb                   -           Steam flow during blowing

QMCR            -           Steam flow at MCR

Vb                    -           Sp. Volume of steam during blowing

VMCR              -           Sp. Volume of steam at MCR

                         Qb = 1.13 x A x sq.rt of P/V

                        A = Area of Blow out pipe at the end ( cm2)

                        P= Pressure at blow at blow out piping end.

                        V= steam specific volume at blow out piping end.

STEAM BLOWING STAGES PART-III (BOILER COMMISSIONING) ABC OF THERMAL POWER PLANT

STEAM BLOWING  PART -III

Stage 3a -- Steam inlet line from COLD RE HEAT to TURBINE DRIVEN BOILER FEED PUMP  A ESV. Steam valve of PRDS line will be kept closed. Electro hydraulic control valve will not be erected and steam blowing arrangement (Blanking device) shall be used. This stage shall be concluded by observing the indents on target plates.

Stage 3b -- Steam inlet line from CRH to TDBFP B ESV. Steam valve of PRDS line will be kept closed. Electro hydraulic control valve will not be erected and steam blowing arrangement (Blanking device) shall be used. This stage shall be concluded by observing the indents on target plates.

Operation  during Stage 3a & 3b

Boiler is lighted up and Separator pressure raised to 20 Kg/cm2 and steam lines charged from Boiler outlet up to dummies provided in the temporary piping of stages 2a and 2b. (Alternatively, after restoration of ESV, IV, HPBP, LPBP, 3a & 3b stages blowing can be taken up at a later date prior to commg of TDBFP turbine)

Temporary steam valve or Dummy provided in the Temporary exhaust lines connected to permanent steam lines to TDBFPs is kept open.

CRH-1 & CRH-3 are gradually opened to full. Steam blowing is carried out for 20-30 minutes at about 50% of system operating pressure. Blowing is continued with a gap of two hours between the blows.

After few blows, Target Plates are put and further blowing continued. Termination of stage 3a and 3b are as per Target plate results as given in Section 9.0.

Stage 4 -- Auxiliary Steam Lines

4a – From PRDS to TDBFP A ESV

4b --From PRDS to TDBFP B ESV

4c -- From CRH to Turbine extraction lines

4d -- From CRH to extraction line to Deaerator

4e -- From CRH to Deaerator / FST

4f -- From CRH to HP Heaters

4g -- From CRH to Turbine gland sealing header

4h -- From CRH to PRDS to Aux Steam Header

4i -- From Main Steam line to PRDS to Aux Steam Header

Steam blowing of sub-stages 4h & 4i shall be done prior to blowing of sub-stages 4a & 4b.

Operation Stage 4

Temporary steam valve or Dummy provided in the Temporary exhaust line connected to permanent steam line to BFP Turbine is kept open.

Steam blowing for sub-stages 4a & 4b is carried out for 20-30 minutes. Blowing is continued with a gap of two hours between the blows.

After few blows, Target Plates are put and further blowing continued. Termination of stage 4a and 4b are as per Target plate results as given in Section 9.0.

Minimum three nos. of blows are given for each of sub stag 4c to 4i. Blows are given at intervals of one hour.

continued to part-iv

STEAM BLOWING STAGES PART-II (BOILER COMMISSIONING) ABC OF THERMAL POWER PLANT

STEAM BLOWING -II

Stage 1b --  Super heater, Main Steam Lines up to ESVs, temporary lines from ESVs to EOTVs, temporary loop from EOTV exit temporary line to overload lines and from there to exhaust. Tap off lines from CRH for Deaerator, TDBFPs, PRDS, HP Heaters 6A & 6B, Gland sealing etc., shall remain closed / isolated. Blowing of this stage shall be concluded by observing the indents on the target plates.

Stage 1c -- Superheaters, Main Steam Lines, HP bypass lines and CRH lines upto Reheater inlet and temporary lines upto exhaust. EOTV provided in temporary loop in HPBP lines will be used for steam blowing. 8 (eight) nos. of blows shall be given through HP Bypass lines to ensure cleanliness of the limbs.

Stage 2a -- Superheaters, Main Steam Lines upto ESVs, temporary lines from ESVs to EOTVs, temporary lines from EOTVs to CRH lines, CRH lines, Reheater, HRH Lines to IVs, temporary lines from IVs to exhaust. RH Desuperheater shall be replaced by temporary spool piece before start of stage 2a  blowing. LP bypass lines shall remain blanked. Blowing of this stage shall be concluded by observing the indents on the target plates.

Stage 2b -- Superheaters, Main Steam Lines upto ESVs, temporary lines from ESVs to EOTVs, temporary lines from EOTVs to CRH lines, CRH lines, Reheater, HRH Lines, LP bypass lines with temporary exhaust pipe. RH Desuperheater shall be welded prior to this stage. 8 (eight) nos. of blows shall be given through LP Bypass lines to ensure cleanliness of the limbs.

The Boiler is started in a normal manner following start up procedure.

Stage  1a, 1b, 1c, 2a & 2b

First steam blow for each of stage 1a, 1b, 1c, 2a & 2b is given at a lower pressure (Drum pressure 25 kg / cm2) and the pipe lines including temporary piping are surveyed for proper expansion and supports. Subsequent blows are given at 50 kg / cm2 Drum pressure.

Boiler pressure is raised to 50 kg / cm2.  The steam lines to be steam blown are warmed up prior to blowing. Firing is shut off and at the same time EOTVs are opened. The opening of temporary valve allows steam to escape to atmosphere at high velocity carrying with it loose and dislodged debris. To prevent thermal stresses in the thick walled shell like Separator and headers the saturation temperature change in Separator is limited to maximum 400C. Hence, for the purpose of steam blowing of all critical piping, the valve is opened at 50 kg / cm2 and closed at 25 kg / cm2 Separator pressure.

The water level in the Separator swings beyond visible limits at the start, during and at the end of steam blowing. There are chances of water droplets getting carried over to superheaters. Hence optimum water level is maintained in the Separator.

The furnace exit gas temperature should be limited to 560 deg. C to safeguard superheaters and reheaters.

Boiler water treatment is carried out with liquid ammonia and hydrazine hydrate to maintain pH above 9 in Boiler water and 5 to 25 ppm of N2H4 in feed water.

Hot tightening of flanges, studs / bolts is done wherever such connections have been made.

During steam blowing MS, CRH & HRH line drains are kept open to atmosphere without any valve for achieving free and uninterrupted flow.

While carrying out stage 2a blowing, suitable dummies are put at LPBP outlet temporary line. Similarly, during stage 2b blowing, suitable dummies are put at IVs outlet temporary lines. Drains of suitable size are provided at the upstream of flange local blanks for the purpose of warming up.

CONTINUED PART-III

STEAM BLOWING PART -/I ABC OF THERMAL POWER PLANT

Steam blowing of MS lines, CRH,HRH,SH,RH,HP & LP bypass pipe lines of turbine is carried out in order to remove welding slag, loose foreign materials, iron pieces, rust etc. from the system, generated during manufacturing, transportation & erection.

EFFECT OF BLOWING DEPENDS ON

1)     Thermal shock

2)     Removal force of steam

3)     Cleaning force of steam

In earlier days steam blowing was done in puffing method . Boiler is being pressurized up to 40 kg/cm2 then blowing valve was opened and blowing starts . It gets completed when pressure drops down to 20 kg /cm2. Target plate was kept after some initial blows. Steam blowing operation starts in early morning to late evening. Night is being kept for cooling. In  this operation fire was killed during blowing and lighted up after one cycle of blowing is over. Drum level  has to be watched very minutely during blowing.

Around four year back before my retirement we conducted steam blowing on a 660MW boiler I will describe the same. Steam blowing procedure will be more or less same for an utility boiler. Now a days engineering software is available with the consultant to calculate disturbance factor etc of the blowing.

The Steam Generator is a once through supercritical plus Radiant Reheat Boiler. Major auxiliaries of the Boiler are Pulverisers, Feeders, Fans and miscellaneous valves. Boiler Recirculation Pump is provided to ensure proper circulation of water in the furnace wall circuits during initial subcritical mode operation.

The circuits which are subjected to steam blowing are Superheat sections of Boiler, Main steam lines up to HP Turbine inlet, Cold Reheat lines up to Reheater inlet, Reheater sections, Hot Reheat lines upto IP Turbine inlet, HP and LP bypass sections and all auxiliary steam lines.

Line Sizes

1.  SH Outlet Pipes                            : ID 350 x 60

2.  MSL                                              : ID 460 x 85

3.  CRH                                              : 864 x 29

4.  CRH to Reheater inlet                   : 660 x 23

5.  RH to HRH                                    : 711 x 42

6.  HRH                                               :  864 x 50

7.  HP BP line                                     : 323.9 x 48 / 508 X 27

8.  LP BP line                                            : 711 x 42 

To remove scales, loose material, iron cuttings etc that might have remained entrapped in Super heaters , Reheater & Steam pipings during manufacture, storage and erection. Failure to remove the debris may result in damage to Turbine blades, valves etc.

Steam blowing shall be done in eight stages viz Stage 1a, 1b, 1c, 2a, 2b, 3a, 3b, 4.

Electrically Operated Temporary Valves (EOTVs) shall be located as shown in the scheme. Loop pipes from ESV and IV (if any) to Turbine shall not be steam blown. Hence proper care shall be taken for cleanliness of these pipes before erection.

All critical pipelines covered in stages 1a, 1b, 1c, 2a, 2b shall be steam blown using EOTVs.

Puffing method shall be adopted for carrying out Steam blowing of stages 1a, 1b, 1c, 2a, 2b  in which Boiler Separator pressure shall be raised to 50 Kg/cm2 and released through a quick opening valve and steam blowing done till the Separator pressure drops from 50 Kg/cm2 to 25 Kg/cm2.

Steam blowing for stages 3a, 3b and 4 shall be carried out by continuous blowing method. Steam blowing by continuous method shall be done at 50% of normal operating pressure of the respective system. .Duration of each blow shall be 20 to 30 minutes. The time gap between two blows shall be  1 ½ to 2 hours.

The effects of steam blowing shall be evaluated and steam blowing concluded as per the completion criteria  given .

Stage 1a --  Superheaters, Main Steam Lines upto ESVs, temporary lines from ESVs to EOTVs, temporary lines from EOTVs to CRH lines, CRH lines upto Reheater inlet and temporary lines upto exhaust. Tap off lines from CRH for Deaerator, TDBFPs, PRDS, HP Heaters 6A & 6B, Gland sealing etc., shall remain closed / isolated. Blowing of this stage shall be concluded by observing the indents on the target plates.

CONTINUED PART-ii