Boiler Safety

1

• Boiler Safety

2
Safety Appurtenances In Boiler

• Pressure gauge Test connection
• Safety Valves (Drum Super-heater)
• Blow down valve
• Drum Level Water Gauge Glasses
• Stop Valve in Steam line
• Stop Check Valves in Feed Line

3
Necessity of Safety valve

• It prevents the boiler pressure from going
  above a safe pre-determined pressure, by opening,
  to allow the excess steam to escape into the
  atmosphere when the set point is reached. This
  guards against a possible explosion from
  excessive pressure.
• Each Boiler must have one S.V, the minimum
  size being a ½ inch valve for miniature boilers
  a ¾ inch valve for others. A Boiler with more
  than 500 Sq.ft of heating surface an electric
  boiler with a power input of over 500 KW should
  have two or more Safety valves.

4
Capacity of Safety valve

• In any case , SV capacity must be such as to
  discharge all the steam the boiler can generate
  without allowing pressure to rise more than 6
  above the highest pressure at which any valve is
  set in no case more than 6 above the maximum
  allowable working pressure.
• All the Safety valves used on Steam boilers
  must be of Direct Spring Loaded type.

5
Definition of Safety Relief valve

• It is an automatic pressure-relieving device
  actuated by the pressure upstream of the valve
  which opens by pop action, with further increase
  in lift of the valve, when pressure increases
  over popping pressure.
• It thus combines the feature of pop action
  further lift with pressure increase.

6
Terminology of Safety Relief valves

• Blow down
• It is the difference between the opening
  closing pressure of
• the Safety valve.Ex- A safety valve that pops
  at 200psi seats
• at 195 psi has a blow down of 5 psi.
• Code rule for determining SV relieving capacity
  
• SV capacity on a boiler must be such that the
  SV or valves will
• discharge all the steam that can be generated
  by the boiler
• (this is assumed to be the max. firing rate)
  without allowing the
• pressure to rise more than 6 above the
  highest pressure at which any
• valve is set in no case more than 6 above
  the max allowable
• pressure.

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ASME markings required on High Pressure Safety
Valves

• Name or identifying trade mark of manufacturer
• Manufacturers design or type number
• Size in inches, Seat Diameter in inches( i.e the
  pipe size of
• valve inlet )
• Pressure in lbs/ sq.ft or Kg / sq.cm (Steam
  pressure at
• which it is to blow)
• Blow down in Kgs or lbs ( BD is the difference
  between the
• opening closing pressure )
• Capacity in lbs / hr or kg / hr
• Capacity lift in inches. It is the distance the
  valve disc rises
• under the action of steam when the valve is
  blowing under a
• pressure of 3 above the set pressure.

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Number of Safety Valves requirement

• Each Boiler requires at least one SV, but if the
  heating surface exceeds 500 Sq.ft, the boiler
  must have two or more Safety relief valves.
• When not more than two valves of different sizes
  are mounted singly on the boiler, the smaller
  valve must not be les than 50 in relieving
  capacity of the larger valve.
• Every super-heater attached to a boiler with no
  intervening valves between the super-heater
  boiler requires one or more safety valves on the
  super-heater outlet header.With no intervening
  stop valves between the super-heater boiler,
  the capacity of the safety valves on the
  super-heater may be included in the total
  required for the boiler, provided the safety
  valve capacity in the boiler is at least 75 of
  the aggregate SV capacity required for the
  Boiler.
• Ex- If a boilers steaming capacity is 100
  tons, a minimum of two valves are required
• on Boiler with a total relieving
  capacity of 75 of (100 Tons) I.e 75 tons. The
• super-heater would then require a SV
  with a capacity of (100 75) tons
• i.e 25 Tons.

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Important Points

• Heating Surface
• That side of the boiler surface exposed to
  the products of combustion, exclusive of
  super-heating surface.The areas to be considered
  for this purpose are tubes, fire-boxes, shells,
  tube sheets the projected area of the headers.
• Sequence of Safety valves Blowing
• The super-heater safety valves should always
  be set at a lower pressure than the drum safety
  valve ,so as to ensure steam flow through the
  super-heater at all times. If the drum safety
  valve blows first, the super-heater could be
  starved of cooling steam, leading to possible
  super-heater tube overheating rupture.

10
Important Points

• Re-heater Safety valves
• The capacity of re-heater safety valves can
  not be included in the total safety valve
  capacity required for the boiler
  super-heater.The relieving capacity of re-heater
  safety valve must not be less than 15 of the
  required total on the header. The total capacity
  on the re-heater must be at least equal to the
  maximum steam flow for which the re-heater is
  designed . One SV must be on the re-heater
  outlet.
• Installation of Safety valves
• Every safety valve shall be connected so as
  to stand in an up-right
• position with spindle vertical.

11
CASE STUDY OF A MAJOR DISASTER IN THERMAL STATION
Explosion in Boiler of Unit No.1 of Bongaigaon
TPS ( 4 x 60 MW) on 23.6.91 Bongaigaon TPS
consists of 4 units of 60 MW each commissioned
during the period 1981-1986. A severe explosion
was reported on the Boiler of Unit No.1 on
23.6.1991 at 5.15AM a voltage dip was observed
at that time and it was learnt that the Unit No.1
had tripped. It was noticed that there was no AC
or DC supply available in the Unit 1 Control
Room. The reserve AC supply did not come
automatically. The Boiler Attendant was
instructed to close the oil supply valves to cut
off the oil supply to the boiler. At the time
of the incidence , one oil gun at CD level and
three at AB level were in operation. The
attendant put closed the AB level oil guns and
while going to close the CD level oil gun the
boiler exploded. The Operator ran under a state
of shock and fell down from 12 metre to 8 metre
level Another Boiler Operator was found
unconscious laden with dust and coal on the 12
metre level near the boiler . DG set was started
manually , in spite of the fact that station AC
supply was available . The 6.6 Kv switchgear was
reported to be under water making it difficult
to approach However all the 6.6 Kv breakers
were manually tripped and the reserve supply was
established.


contd.
12
CASE STUDY OF A MAJOR DISASTER IN THERMAL STATION
Observations During the investigation it was
found     Explosion residue taken out from the
furnace bottom was very rich in un-burnt
oil and pulverized coal.     Furnace wall
from inside had oil sticking to the surface.
    Boiler operating floor was full of coal /
ash/ dust and even the local BMS panel
was fully covered with dust.     BMS panel
was never used for operating the trip valves
    Flame scanners were inoperative for many
years and also the igniters were
Non-functional.     Oil used to be ignited
manually by Flame Torch (Mashal ).    
Whenever the unit tripped the cause of tripping
was never analyzed.

Contd
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SOME MAJOR DISASTER IN THERMAL STATIONS
Following damages had taken place
i)        Boiler furnace corners 1 4 got
completely ripped open above CD elevation up to
the furnace top. ii)       Front and
left water walls got bulged outward and the
buck-stays were damaged. iii)     Economiser
duct from the boiler bank outlet up to the
economiser opened out and had blown
. Contd
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CASE STUDY OF A MAJOR DISASTER IN THERMAL STATION
Causes of the explosion i)        Flame failure
took place prior to boiler trip. ii)       Flame
failure was not detected since the flame scanners
were not in operation. iii)     Due to sudden
grid fluctuation, the boiler tripped on loss of
air flow protection. iv)     The generator
breaker and UAT breaker tripped. v)      There
was no AC or DC supply available on the generator
board. The automatic change over to
6.6 Kv reserve supply from station did not take
place. This resulted in failure of
supply in 6.6 Kv 1A bus. Since the battery was
not available, the DC supply had failed
resulting in non-operation of all the
controls. vi)     Situation further aggravated
inside the furnace since the draft fans were also
not available. vii)   The fuel oil
supply continued to go inside the furnace since
the fuel oil pumps were on station
supply and the oil trip valves could not be
closed manually causing boiler explosion.

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CASE STUDY OF A MAJOR DISASTER IN THERMAL STATION
Suggested remedial measures i)        Boiler
local BMS panel should be made fully
operational. ii)       Practice of operating the
fuel trip valves by shorting of the contacts
should be dispensed with. iii)     All
the boiler auto controls should be made
functional and must be maintained. iv)     The
DC battery and other connected system should be
maintained in perfect condition. v)      All the
interlocks of auxiliaries like FD fans, ID fans,
etc should be made operative. The flame
scanners and igniters are also to be made
operational.

contd
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CASE STUDY OF A MAJOR DISASTER IN THERMAL STATION
Suggested remedial measures vi)     The use of
oil should be discontinued beyond 40 unit load
and stable operation should be achieved
with coal firing after 40 load. vii)   A
systematic preventive maintenance procedure for
various equipment should be
evolved. viii)  Procedure of Annual inspection
and overhaul of boiler as per Indian Boiler
Regulation should be regularly
followed. ix)    OM staff should be augmented
and adequate number of trained personnel should
be posted for operation and maintenance.