Boiler safety valves question &
answers:
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1-Which type of
safety valves are used in modern high pressure Boilers?
Open yoke, spring loaded, Semi Nozzle & full lift type
2-What is the over
pressure range for safety valves?
It is around 6 to 7% of operating pressure of particular
line/vessel
3-What is the recommended
% of blow down for Boiler safety valves?
It is around 2 to 3%
4-How do you
calculate safety valves blow down?
Blow down = SV (Set pressure – Reseat pressure) X 100 / Set
pressure
5-What is the MOC of
safety valve disc & spindle?
Disc: Stain less steel with 13% chromium & Spindle: SS
316
6-What happens if
drum safety valve set at lower pressure than super heater safety valves?
Upon blowing of drum safety valve first, all the steam
formed will blows out through the drum safety valves causing less or no steam
flow to the super heater coils. This situation will lead to the failure of
super heater coils.
7-When should be gags
for safety valves applied & removed?
Safety valve gags should be applied & removed at 80-85%
of the pressure of the low set valve, even at safety valve setting & during
hydro. Test.
8-Why safety valve
gags should not be applied in cold condition?
Spindle of the safety valves
expands considerably with increase in temperature as pressure is raised. Due to
constraint to the expansion may cause spindle bend & also cause damage to
the seating surface.
9-How do you adjust
the set pressure of a safety valve?
Safety valve is set by adjusting the compression of spring
10-How do you adjust
the blow down of a safety valve?
By turning up/down of upper ring.
To reduce the blow down: Upper ring is taken down & to
increase the blow down upper wring is turned up.
11-How do you reduce the
simmering sound of safety valve?
By adjusting the lower ring
12-What will happen
if a safety valve has higher blow down than required?
Higher blow down of safety valve causes
1-Fluctuation steam demand & drum level
2-Increase in super heater steam temperature
3-Sudden & fast dropping of boiler pressure
4-Insufficient steam flow to super heater coils (In case of
drum safety valve)
5-Loss of high temperature steam eventually leads loss of
fuel
13-What will happen
if a safety valve has lower blow down than required?
Lower blow down setting of safety valve leads to the pressurizing
of boiler eventually damages to the pressure parts.
14-What are the
potential reasons for failure/damage of safety valves?
Potential reasons for safety valve failure are:
- Crossing the quantity of steam to be discharged
- Quality of the steam is not up to the mark
- Higher back pressure
- Stress in discharge piping
- Improper gagging & bolting
- Foreign materials in steam
- Damages during disassembly & assembly in field
15-Why drum safety
valves are set first instead of super heater safety valve?
Possibilities of valve part
damage because of “girl blasting” are greater on the super heated valves in
contrast to the drum valves. If a super heater valve is gagged after seat
damage while testing of other valves continues, the total valve damage will
most likely be increased.
And also steam temperature is
more while testing the drum safety valves; hence more temperature for setting
super heater valve will be available producing accurate results.
16-Why the drum level should be maintained lower in the range of 30-40%
during SV setting?
Higher drum level will cause
stugging of valves with water causing long blow down & damages to the disc
& seat.
17-What is the pressure rising rate during safety valve setting?
Usually 2 to 3 kg/cm2 per minute.
Slow rising may cause simmering & valve to lift very erratically
18-What precautions shall be taken for lifting the safety valves
several times?
A cooling off period around 25-30
minutes should be given between every lift to get accurate results & to
avoid damages to the valve
19-What are the reasons for simmering in safety valves?
- Improperly set lower ring
- Holding the boiler pressure at or nearer to the set pressure of valve
- Maintaining higher drum level during lifting of SV
20-Why it is necessary to drop the pressure of boiler after setting a safety
valve before removing gags?
Boiler pressure should be dropped
20% of valve set pressure to avoid valve lifting prematurely
21-Why it is been recommended that SH safety valve to be set at higher
blow down rate if the steam temperature available is less?
Yes, as the steam temperature
increase SH safety valve’s blow down decreases, hence at lower temperature it
is always set at higher blow down.
22-What are the criteria for selecting the safety valves?
- Super heater safety valve should have blowing (relieving capacity) 30-35% Boiler MCR
- Drum safety valves (Including 1 &2) should have relieving capacity 90 to 95% of Boiler MCR
- All safety valves (SH & Drum) should have relieving capacity 125 to 130% of Boiler MCR
- Solenoid operated electromatic safety valves are generally deigned to relieve steam 12-15% of Boiler MCR.
23-On which side of steam drum higher pressure set safety valve is to
be fitted?
It depends on the inclination of
the drum & CBD line layout. Actually HP boiler drums are made inclined at
an angle 2-3 for enabling collection of internal treated sludge. So in order to
not to lift that side SV , that SV is set at higher pressure
24-What is meant by back pressure in safety valves? & how does it
affect SV performance?
Back pressure is the static
pressure existing at the outlet of a pressure relief device due to pressure in
the discharge system.
If pressure in the valve outlet
varies while the valve is closed, the valve set pressure will change. If back
pressure varies while the valve is open and flowing, valve lift and flow rate
through the valve can be affected.
25-What is meant by
accumulating test pressure?
The accumulation test is done on
boilers to limit the excessive pressure rising while the safety valve is in
open. The test is carried on new boilers or new safety valves with full firing
condition with MSSV and feed water valves closed. It is conducted as long as
water in drum permits generally 7 minutes for water tube boilers.
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