safety valve setting in a boiler
A.) Procedure of Safety valve setting in Boiler
The safety valve setting can be carried out on this type of valve to give the designed discharge and blow down characteristic.
1. Safety valve pressure setting can be done from high to low pressure or vice-versa.
2. Take necessary personal safety valve precaution and arrange tools i.e. gagging tool and master gauge.
3. Slowly raise the boiler pressure and blow off the safety valve manually few times for thermal expansion and to reduce the thermal stress on the valves.
4. The screw down all the safety valve higher than the setting pressure at which you are going to set.
5. Gain the boiler steam pressure 2- 3 % more than the designed pressure of the boiler then stop fixing and on screw the first valve safety valve slowly when it blow off at 2- 3 % more than the designed pressure then note this opening and closing pressure of the valve and finally gag it.
6. Raise the boiler pressure at the designed pressure of the boiler and unscrew the 2nd valve when it blow off at designed pressure than note this opening pressure and check the closing pressure also. Feed check the setting and gag the valve. 7. Then set the super heater safety valve lower than the designed pressure of the boiler in same procedure.
8. Finally take out the gagging tools pressure setting should be done in presence of surveyor.
B.) Safety precautions taken during setting
Following safety precautions to be taken during safety valve setting :
1. Use all adequate PPEs(Hand glove, Gagging tolls, rope, helmet, safety shoes etc.
2. Follow SOP(standard operating procedures) during safety valve setting.
3. Keep safe distance away from live safety valve during setting.
4. Gradual raising of boiler pressure.
5. Quick pressure drop during safety puffs.
6. Blowdown of safety valve within 2.5 to 4 % range.
C.) Safety Valve Blow down, Set Pressure and Reset Pressure
Blowdown :
Blowdown is the pressure difference between set pressure and pressure at which the valve reseats expressed as a % of the set pressure.
Blow-down% =[Set pressure-Reset Pressure]x100/Set pressure
Usual range of blowdown % setting of safety valve is 2.5 to 4%.
Set pressure :
It is the pressure at which the safety valves starts to open measured at valve inlet normally 1.1 x working pressure. Higher set pressure lead to invite accident & lower setting is lead to frequent puffing of safety valve which may hamper process.
Reset Pressure:
It is pressure at which safety valve is fully closed. Setting limit within blowdown range 2.5 to4% is the best safety operating of any system to avoid any disturbance during normal operation.
D.) safety valve of super-heater set at lower pressure than the safety valve of the boiler drum?
Super-heater safety valve is always set at lower pressure than boiler drum safety valve pressure because there will be chances of starvation of super-heater tubes .
In case of any excess pressure in boiler super-heater safety valve must blow before drum safety valve otherwise steam flow will not be establish through super-heater tubes & at that case super-heater tubes will get overheated. To avoid such occurrence this provision has to be made during steam test of any boiler.
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