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Difference between a safety valve and a relief valve

• Relief valve:

The higher the pressure, the more the valve disc lifts off its seat. Relieving capacity increases with over-pressurization increase.  

• Safety valve:

Designed to stay completely closed until the popping pressure is reached. At popping point the valve lifts sharply and stay wide open until the required blowdown or reduction in pressure (10%) is reached.

 

Setting 

1. Safety valve to be overhauled and reset at every survey
2. At least 2 pressure gauge required 
3. Loosen lock nut, screw down the compression screw few more turns then previous setting. Measure the height make marking
4. With all mounting in place, shut the main steam stop valve.
5. Fire the boiler on auto
6. Once boiler auto cut off, switch to manual. Rise the boiler manually till require set pressure
7. At this setting, unscrew the compression till safety valve lift
8. Fire the boiler continuously and check the safety valve lift at set pressure
9. Stop the boiler and check safety valve sit firmly at seating and no leakage
10. Safety valve now has been adjusted, tighten lock nut. This valve then had to be gagged
11. Set the other safety valve in same procedure
12. Remove the gagging tool.
13. Life the safety valve with easing tool and confirm it sit firmly and no leakage.

Drawing of Boiler safety valve

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