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.
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
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.
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.
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.
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) 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……
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.