TYPES OF PROTECTIONS

-PROTECTIONS CAUSING UNIT

SHUTDOWN

-PROTECTIONS CAUSING LOAD

REDUCTION

-PROTECTIONS CAUSING ANNUNCIATION

ONLY

REASONS FOR PROTECTIONS

-Deviation from normal condition,

which can not be easily

controlled automatically

-Hence, it calls for human

intervention.

-Can cause loss of generation,

property and / or life

Protections causing unit shutdown

-Failure of all BFPs.

-Boiler shutdown due to both ID fan tripping, both FD fans tripping, loss of all fuel etc.

-Reheater protection to ensure continuous steam flow through reheater.

Protections causing load reduction

-One ID Fan tripping

-One FD fan tripping

-One PA fan tripping( tripping can be avoided only when air gates close quickly.)

-Mills or feeders tripping.

Furnace safeguard supervisory system

-This system takes care of interlocks required for starting, supervision and safe shutdown of the equipments.

Basically, FSSS prevents any firing till safe purging of boiler is complete.

It prevents ignition of fuel firing instruments till certain interlocks are satisfied.

It effects master fuel trip in certain conditions.

Conditions for purging(FSSS)

1. No Boiler trip conditions2. HOTV and LOTV closed.3. HONVs and LONVs closed.4. All PA fans off.5. All feeders off.6. All Hot air gates closed.7. All aux.air dampers modulating.8. Air flow >30% and <40%9.All scanners no flame..

ID fan start permissives

1. IGV in minimum position.

2. I/L and O/L gates closed.

3. Bearing temperatures not high.

4. One SAH running with its i/l and o/l gates open.

5. One of the two VFD channels ready.

6. Lube oil supply healthy.

-Once one of the two channels is taken i/s, i/l and o/l gates are opened in auto. Then the other channel can be taken i/s.

ID fan tripping

ID fan can trip on protections like bearing temp. very high, fan/motor vibration very high or fault from VFD channels( only the faulty channel trips).

-when ID fan-A trips and ID fan-B is not i/s-: 1.I/D fan-B i/l and o/l gates open. 2.Auto control system is disconnected. 3.Both IGVs open to wide open position. 4.Working FD fans trip. 5. Boiler trips through FSSS. 6.ID Fan A i/l and o/l gates remain wide open.

-When both ID fans are in service and one of them trips.

1. Corresponding FD fan trips. 2. Runback operates and load is reduced to 60

% of full load. 3.IGV and VFD get disconnected from auto

control loop and come to minimum position. 4.I/L and O/L gates closed.

FD fan start permissives

Blade pitch in minimum position.O/L damper closed.Bearing temperatures not high.One ID fan i/s.Lube oil system healthy.

-For the second FD fan to start, two ID fans should be i/s; other conditions will remain as above.

FD fan trippings--When one FD fan is in service-

1.Dampers of both the fans open to wide open position.

2.Blade pitch of both the fans go to maximum positions.

3. Emergency scanner air damper opens.

4. Boiler trips.

When both FD fans are in service and one of them trips.

Apart from the conditions like high bearing temp., high winding temp., high vibration etc., one FD fan will trip also when one ID fan trips.

1. Blade pitch control of running fan comes to manual.

2. Blade pitch of tripped fan comes to minimum and o/l damper gets closed

3. Runback operates and load is reduced to 60 % of BMCR.

One PA Fan trips and other is in service.

REASONS:-

-Fan/motor bearing temp. is high.

- Winding temp. very high.

-Lube oil flow low.

-Fan vibration very high.

Effect of one PA Fan tripping

- Runback will operate It is very difficult to save the unit because- There is sudden drop in PA header

pressure. (Alarm at 625 mm which starts top mills’ tripping).

At 500mm, mills trip on PA header pressure very low(2/3 logic).

Time Delay is very less (Approx. 6 sec) Air gates closing and PA fan o/l gate

closing takes more than 10 sec.

RUNBACK OPERATION1. Top mills will trip with 2sec delay till

4 mills from bottom remain.2.Turbine comes in pressure control

mode.3.Fuel master will take command from

unit capability.4. Load will start reducing after 30-45

sec.5. Fuel will not reduce below 150 TPH.

Other effects-

PA header pressure will increase due to mill trippings.

Furnace pressure fluctuation. Drum level (+/-100mm)

In case of BFP tripping our aim should be to reduce steam flow. Pressure set point should be on the higher side. Oil gun should not be taken i/s.

MASTER FUEL TRIP

-There are 3 channels for MFT. Boiler trips on 2/3 logic.

1. All ID fans off.

2. All FD fans off.

3. Flame failure

4. Loss of all fuel

5. Less than Fireball and loss of 110 V AC.

6. Waterwall circulation incorrect

7. Reheater protection.

8. Furnace pressure very high>150mm

9. Furnace pressure very low <-175mm

10. Drum level very high>250mm11. Drum level very low <-375mm12. Loss of FSSS 220V AC.13.Low Air flow (<30 %).14. All BFPs off.15. Both SAH off.16.2/3 MFT both networks fail.17.2/3MFT DPU outputs frozen

Water wall circulation incorrect-: 1. All 3 BCW on AND each

dP<10psi(2/3) 2. Any two BCW pps. On AND both

dP <10 psi 3.Any one BCW on AND its dP<10

psi 4.Boiler on AND all BCW pps off

(TD=5s)

Loss of all fuel trip

Loss of all fuel trip arming AND All feeders off AND[LOTV not open OR elevation AB all LONVs closed ]

AND [HOTV not open OR {In all HO elevations all HONVs

closed } Loss of all fuel trip arming:- Any elev. any nozzle valve not closed.

Unit Flame failure trip

Unit flame failure trip arming AND all elevation A-K voted no flame.

Unit flame failure trip arming:- Any feeder proven for 2s.

Ee

No flame voting (For elev. A)1.Feeder A not proven for more than 2s.

2. Elevation AA ¾ FB scanner no flame AND

[{Elev. AB started AND loss of 110 V AC (TD=2s)} OR {Elev. AB 2/4 nozzle v/v not proven AND elev. AB ¾ FB scanner no flame}]

Loss of reheat protection:

1.Reheater protection enabled ANDGenerator CB opened AND[Both HPBP closed(<2%) OR { Either

LPBP stop v/v OR LPBP CV in both LPBP lines closed}]

2. Reheater Protection enabled AND [{Turbine HP v/vs and HPBP closed}OR{ Turbine IP v/vs and LPBP closed}]

Reheater Protection enabling

Drum pressure > 8Ksc AND {Any feeder ON OR Boiler firing ON}

AND{ Any LPBP stop v/v open AND

corresponding LPBP CV>2% open}AND [Any HPBP v/v >2% open] AND No MFT.

Turbine HP v/v and HPBP closed

[Either HP stop v/v OR HP control v/v in both HP lines closed]

ANDBoth HPBP closed(<2%)Similarly for IP v/v & LPBP closed[Either IP stop v/v OR IP control v/v in both IP lines closed]

AND[Either LPBP stop v/v OR LPBP control

closed(<2%) in both LPBP lines closed.]

BOILER TRIPWhen the boiler trips the following events takes place

Boiler trip red lamp comes on. MFT A & B trip lamp comes on and reset lamp goes off. Cause of trip memory can not be reset till the furnace purge is

completed. F D Fans Control is transferred to manual. I D fans vane position is transferred to manual. Pulverizers are tripped. Coal feeders are tripped. P A Fans are tripped. Cold air dampers opens 100 %. HFO trip valve closes. All HFO Nozzle valves closes. Upper and Lower Fuel air damper opens. Auxiliary air damper opens and control is transferred to manual. Loss of all fuel trip protection disarms. S/H and R/H spray block valves S-82 & R-31 closes and can not be

opened unless furnace purge is completed. Turbine Trips