Boiler Safety
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
Necessity of Safety valve
It prevents the boiler pressure from going above a safe predetermined 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.
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.
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.
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.
ASME markings required on
High Pressure Safety Valves:
 Name or identifying trade mark of manufacturer
 Manufacturer’s 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.
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 boiler’s 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.
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.
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 reheater 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.
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….
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 ……
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 ……
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.
CASE STUDY OF A MAJOR DISASTER IN THERMAL STATION
Suggested remedial measures:
i)
ii)
Boiler local BMS panel should be made fully operational.
iii)
Practice of operating the fuel trip valves by shorting of the contacts should be
dispensed with.
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……
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) O&M staff should be augmented and adequate number of trained personnel should be
posted for operation and maintenance.