25 most frequently asked viva questions & answers on Boiler safety valves

Boiler safety valves question & answers:
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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