• To protect systems or vessels
from excessive pressure
According to construction
1) Dead weight
2) Lever type
According to operation
To provide immediate relief of over
pressure by making rapid full opening for
compressive fluids.( air, gases and
There should not be any isolation
valve before safety valve.
To relieve excessive
pressure in system containing
Generally have gradual lift
proportional to the increase in
( oil & water)
Safety relief valve
- It is an automatic pressure-relieving
device whose open and close movements
are fully controlled by source of power –
such as air,steam and hydraulic.
- It may be operated manually as desired
at any pressure.
- It is designed to remain open after
Vacuum relief valve
•Designed to admit fluid to prevent
an excessive internal vacuum.
•It is designed to re-close and prevent
further flow of fluid after normal
conditions have been restored.
REQUIREMENT OF SAFETY
- Proper functioning
- Opening at correct set pressure
- Correct blow down
- Proper mechanical operation &
Clean closure without leakage.
As per boiler regulations, every boiler
shall have the following mountings
1. Two safety valves
2. Two water level indicator
3. A steam pressure gauge
4. A steam stop valve
5. A feed check valve
6. Two feed pump
7. A Blow down valve
- Rapid opening and closing of the valve is
known as chattering.
- Insufficient blow down
- More back pressure due to under size
- Insufficient steam flow to the valve
- lifting with water
The valve is just about to open
and has indefinite contact.
An audible sound or visible
escape of fluid between seat and disc
at an inlet static pressure below the
popping pressure and at no measurable
• Flutter is an abnormal, rapid
reciprocating motion of the safety
valve stem, in which the disc
does not contact the seat.
Opening pressure – Closing pressure
----------------------------------------------- X 100
Important codes governing
1) IBR – 1950
2) ASME Sec- I Power
3) ASME Sec – VIII
4) BS - 756
General conditions for safety
- Drum safety valves should be 75% of
- Super heater safety valves should be
50% of evaporating capacity
- Both drum & super heater safety valves
discharge capacity should be more than
100% evaporating capacity.
- Each Reheater safety valves shall
discharge more than 20% of evaporation.
- All reheater safety valves combined
should be more than 100% evaporating
- Inlet connection should be welded
for high pressure safety valves.
- Safety valves should be full lift
- Blow down should be 3 to 5 %.
- Easy maintenance
- No seat tightness problems
- No Chatter problems
PARTS OF SAFETY VALVE
Body , Yoke
Stem , Seat bushing, Spring,
Disc, spring retainer plates,
upper & lower rings
Ensure availability of tools
Availability of spares
Check condition of stem,seat,
spring & disc.
Cleaning of all parts
Lapping of seat & disc
Spindle run out
Replace defective parts
Use Molykote lubricant
Assemble as per sequence
Maintain recommended readings
Tightening of all bolts & Nuts.
UPPER RING ADJUSTMENT
- To shorten the blow down raise
- To increase the blow down
lower upper ring
LOWER RING ADJUSTMENT
- To obtain clean popping action
- To cushion the closing action of the
- Reduces simmering
• Use of hydro set unit
• Full system pressure
• Valve should be set with in 1% of set
• Valve should be hand popped at
75 to 80% of set pressure
- Arresting the stem mechanically from
- Do not gag in cold condition
- Gag at 80% of low set value pressure.
- Calibrated gauges are to be used.
- Test and achieve within minimum
number of pops.
- Apply gag load finger tight only
- Remove gags after floating
- Bring Blow down between 3 and 5 %
• The recommended spares and tools
are to be made available.
• Flat lapping plate.
• High temperature lubricant.
• Lapping compound of grit size
• 320 – general, 500- finishing &
1000 for polishing
- Step height shall be 0.635 MM, if it is
less than 0.254 MM, seat cutting is
to be done.
- Spindle run out should be with in
- Don’t go near discharge side
- Body drain and vent must be piped to safe
- Gag the valve during ring adjustment
- Hand pop with 5 to 6 Mts safe rope)
- Use ear muffs / hand gloves
- Adequate access around the valve for
PROCUREMENT OF SAFETY
- Inlet & outlet size
- Inlet & outlet construction
- Medium details
- Blow down
- Set pressure
- Body material
- IBR requirement