RLI Sipat Case Study Booklet 4

MODIFICATION-3 FOR TELESCOPIC CHUTE

There was a continuous problem of telescopic chute breakage in the
silo area. The chute was made of rigid material and was getting
damaged when there was truck movement during ash filling.
The chute has been converted into flexible type by modification. As a
result, now during the movement of truck, either chute slightly bends
or detaches from the truck without breakage. Therefore, the incidence
of telescopic chute failure is now minimized and trucks for ash filling
need not to wait due to telescopic chute breakage.

Before Modification- After Modification-
Rigid Type Flexible Type

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The modification has been completed in both silo of stg-III in 3rd
quarter of 2021. After that the breakage in telescopic got minimized as
shown in chart above.
MODIFICATION-4 FOR TAC LINE BRANCHING STATION(CROSS OVER)

Before Modification

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There was frequent choking in the TAC lines. The location of the knife
gate valve in the previous arrangement was quiet far away from the y
joint (almost 6 metre) of Y-lateral. So in case of closure of that valve
also, a long length behind the valve was getting filled with the ash &
causing choking in the line which had to be cleared out.
The maintenance space available was also very congested.

After Modification

The location of the knife gate is changed nearest to the y joint (almost
1.5 to 2 metre) of Y-lateral. The problem of the ash filling in the back of
KGV has been eliminated. The line choking got minimized. All TAC lines
are available. Hence the ash utilization got improved.The maintenance
space also shall be improved.

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Modification done

The modification done in the starting of fourth quarter 2021. After that,
the choking in TAC lines of U#09 got minimized.

Major learning

The above modifications have resulted into a higher ash utilization.
The dust emission also got reduced.
There was significant reduction in water consumption required for
clearing in silo area.
The man power which was getting consumed in clearing silo
choking & TAC line, which was very frequent, also got eliminated.
This has also made a positive impact on the health of the workers
who were engaged in clearing the silo & TAC lines, Because a huge
quantity of ash was going inside the lungs while clearing the
choking which was very harmful for their health. Even in the rainy
season, no silo choking incident was reported.

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BCSTEOAAMGRPIENR-E1GS(M3SXOO2DR0ISF0IYMCSAWTTE)IOMNNE-MWOTOR

Name of the authors -

G.Baskar, Emp. No:- 032587
DGM (Operation), NTPC KAHALGAON
Divya Pande, Emp. No. 101459
Mgr (EM), NTPC RAMAGUNDAM

Background of the case

R&M of Old Compressor system (4 Instrument Air compressors and 5
Plant Air Compressors) in Stage-I (3X200MW) at NTPC Ramagundam
was carried out in 2015 in which this system was replaced with Atlas
Copco make Compressor system (3 Instrument Air compressors & 3
Plant Air Compressors). These compressors supplied in new R&M
package have Marathon make motors of rating 6.6kV, 350kW,
3000RPM.

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These motors were having frequent DE side bearing failures after rise
in motor DE bearing temperature as the manufacturer had set the
alarm point for bearing temperature as 110°C and trip value as 120°C.
As these motors are flange mounted and are coupled to compressor
with 4 nos. of Rubber Coupling Bushes, due to failure of these rubber
coupling bushes, DE side ball bearing 6220-C3 was failing frequently
due to uneven loading (wobbling effect) and motor was reported to
have high vibration. Failure of DE side bearing was leading to
bending/deformation of rotor shaft at DE side and was damaging of DE
side bearing seating area, grease cup and end-shield. Also, rotor core
rubbing with stator on DE side was noticed.

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What was done

Above cited failures were noticed in 4 such motors out of total 6
motors of new Atlas Copco Compressor System. To avoid recurrence of
such failures, DE side Ball bearing 6220-C3 was replaced with Roller
bearing NU-220 (SKF make) as these bearings can accommodate heavy
radial loads and are designed to operate at high speeds (3000 RPM).

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SFHITARRZINAAGTREDINGAYBSTOSOIOLMECRIIATTIGEADTWE IFTIHRELDO

Name of the authors 

Arnab Bhattacharya, Emp no -007876
DGM, FH-Operation, NTPC Sipat

Vikram Kejriwal, Emp no –103536
Manager, Main Plant Operation, NTPC Sipat

Background of the case

HFO contains very high levels of Sulphur which is the key pollutant in
fuel. When high Sulphur fuel is burnt, it contributes to emission of
particulates directly and to emissions of SO2 gas. The high combustion
temperature also produces NOx. Therefore, to reduce adverse impact
on environment, in Sipat plant, use of HFO has been gradually phased
out and presently, only LDO is used for start-up and low load operations
/ emergency handling. However, compared to HFO, LDO is less viscous
and do not require preheating and steam atomization for combustion
and with a flash point of 660C has resulted in increased fire hazard in
boiler. One such case study is presented here which unfolds potential
risks associated with LDO system and necessary modifications required
to mitigate fire hazard.

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During morning shift on 10.12.2018 at 11:10 AM, fire incident occurred in
U#5 near mill-C hot air duct. Boiler Operator of Unit#5 observed fire
near mill-C hot air gate duct area and informed control room about fire
incident. Immediately HOTV closed, control engineer rushed to the site
and assessing the fire, instructed boiler desk engineer to trip the boiler.
Boiler tripped from desk by pressing EPB and Boiler DVs operated. Fire
quenched by CISF personnel.

Fig1: Location of accumulators in Boiler Fig2: Flange gasket causing
LDO leakage

Root cause: LDO leakage occurred from one damaged gasket in LDO
station. Same fallen over hot air duct caught fire.

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What was done

Boiler tripped from desk EPB, all running LDO pumps stopped, isolating
MOV and manual valves in LDO station closed. In the meantime, flame
reached up to firing floor and caught fire over some power & control
cables. SAPH-B, PAPH-B, AC and DC scanner air fans tripped. All OWS LVS
supply interrupted due to UPS supply changeover delay problem after
unit shutdown, the same was restored within 5 minutes. Safe shutdown
of unit was done from programmer room without damage to equipment
and turbine put on barring. Leakage points arrested and burnt cable
replaced. LDO station main isolating MOV is kept closed during normal
running of unit. CCTV camera display monitor provided at unit Desk for
surveillance of critical area. Proper housekeeping is done to prevent coal
dust accumulation. All the Power cables & Control cables proximity to
the hot air duct re-routed. Hot air duct below oil station provided with
canopy.

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Major learning

1.In case of fire in Boiler, if fire reaches firing floor, then there is a
possibility of coupling failure of coal pipes resulting in explosion due to
release of pulverized coal. Therefore, boiler is to be tripped with no
time delay.

2.In case of fire in Boiler due to leakage in LDO system, LDO pumps are
to be stopped immediately and LDO station to be isolated by closing
MOV, pneumatic valves and manual valves (as early as possible). This
will prevent rapid propagation of flame due to unavailability of fuel.

3.Start stop operation of all LDO pumps are to be given from OWS of at
least two units so that in case of LVS supply failure due to damage of
power and / or control cables in one of the units, running LDO pumps
can be stopped from OWS of other unit.

4.Accumulators installed at LDO stations in boiler are to be kept
functional. This will prevent leakage of LDO due to pressure surge
during start / stop of pumps and opening and closing operations /
malfunctions of any valves.

5.Inspection of gaskets in LDO system are to be done during every
overhauling.

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GDUENCETRFAATUOLRTTRIPPING ON BUS

Name of the author

G C Mohanta (AGM, Elect), Emp No
Saurabh Nema (Sr. Manager, Elect), Emp No
Ganeswar Sethi, (Sr. Manager, Elect), Emp No
G Rukmangadha Reddy (Sr. Manager, Elect), Emp No
NTPC Sipat

Background of the case

In our turbo-generators, output is taken through a thick hollow
conductor (called bus) up to generator transformer (GT) as current
rating is very high. This bus is supported by insulators and enclosed by
a metallic duct. For expansion due to heating, expansion bellows are
provided in certain locations of the duct. Duct is filled with dry air of
about 10 millibars. Conductors are passing through the seal off
bushings.
Bus duct pressurization system avoids ingress of moisture during rain
or any water/steam leak nearby, helps keep dust/contaminants out
and eliminates any faults caused by moisture in the bus duct.

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Improper bus duct condition may result a failure or fault leading to
unplanned outages, expensive equipment replacement and safety
risks to personnel. Bus duct design must fulfill the ability to shed
rainwater. safeguard the duct even in geographies with wide
temperature swings and protect the bus while peaking / cycling
loading conditions.

Cross-sectional view of bus Bus Ducts external view
duct

On date 20.08.2020 incessant rain fall was there. At 13:20 hrs, Unit
#1 tripped on earth fault. At Sipat, earth fault is measured by derived
voltage unbalance scheme*. As a tracking occurred in R-ph conductor
near bus PT, generator terminal voltage collapsed and unit tripped
on earth fault as broken delta voltage is sensed high.

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What was done

Upon physical inspection, water found in the duct near PT and UT-1B
seal off bushing area.

Water was removed from the duct.
Bushings were cleaned.
Also, a dent was found on bust duct outside the TG building. So, the
dent was filled with putty.
Sealant applied to each support insulators, expansion bellows and
all joints.
Polythene covering done on expansion bellow and dent area.
Bus duct pressurization system was taken into service.

Major learning

Bus duct pressurization system should always be kept in-service. Put
dry air and record dew points weekly (-200°C). Monitor pressure
(10mbar) for effectiveness of the system.
Check any water accumulation in duct during short shutdown.
Do thorough physical inspection of duct during overhauling.
No material to be thrown from TG building towards bus duct.
Inspect expansion bellow, support insulators, and seal off bushings
are healthy and sealed properly during overhauling. Plan for
replacement after its life or if any crack found.

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*Note: On a healthy three-phase power system, the addition of each of
the three-phase to earth voltages is nominally zero, as it is the vector
sum of three balanced vectors at 120° to one another. However, when
an earth fault occurs on the primary system, this balance is upset and a
residual voltage is produced. This could be measured, for example, at
the secondary terminals of a voltage transformer (VT) having a “broken
delta” secondary connection. Hence, a residual voltage measuring can
be used to offer earth fault protection on such a system.
When the residual voltage, Vo exceeds the set value (6V) for 1 second
the relay will operate and trips the generator.

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CASE STUDY PUBLICATION

Compiled and Published by

REGIONAL LEARNING INSTITUTE(RLI)
NTPC SIPAT
Ujjwal Nagar, Bilaspur, Chhattisgarh
Pin Code:- 495555
Email:- [email protected]