Safety Valve Inspection / Survey

Valve & Seat

• Check visually for any sign of scoring (scratches), pitting etc
• Carry out a ‘mating check’ between valve and seat using Prussian blue or engineers’ blue
• A continuous ring indicates there will be no leak between valve and seat

Spindle

• The spindle is always under compression during service. As such, it is prone to get bent.
• Checks must be done to ensure its ‘straightness’ or ‘true-ness’

Spring

• Visual inspection carried out for excessive pitting due to rusting. The common spring material is carbon steel which gives the right stiffness but is prone to rusting. (FYI, stainless steel has very high stiffness)
• Due to ‘skewering effect’, springs tend to slant to one side over time. Check for perpendicularness.
• Due to prolonged compression, spring tends get ‘shortened’. Free length (after usage) > 95% of natural length (when new)

Valve Body

• Look for signs of corrosion inside/outside of valve body
• Check for scale deposits build-up inside body

Drain Pipe

• Ensure drain pipe is clear by blowing with air

FAQ

What should be the capacity of each safety valve? (For guidance)

Answer: Each safety valve should match the maximum steam generation rate of the boiler

Why do you set the second safety valve slightly higher than the first valve?

Answer: If the first safety valve fails to lift, then the second safety can act as a back-up.

Then why can’t you set both the safety valves equally?

Answer: If both the safety valves lift together, then the blowdown will double which is not economical due to excessive loss of steam pressure.

What is the difference between Design Pressure (DP) and Maximum Allowable Working Pressure (MAWP)?

Answer: In most cases, they are of the same value. However, during newbuilding, if you have sourced for a boiler with DP 12 bars but ship’s heating coils designed to withstand 10 bars, then the ship’s boiler can only operate to a maximum of 10 bars which is the MAWP. DP is no longer valid on the ship.

Your boiler has a design pressure of 10.0 bars. You would like to operate your boiler at maximum working pressure. What can be the settings of your safety valves?

Answer: Maximum settings of safety valves are about 9.7 bars & 10.0 bars maximum. The Maximum Working Pressure is usually 10% lower than the safety valve setting. MWP is 9.0 bars.

State the operational problems encountered in service for the boiler safety valves.

• Due to wear between valve and seat, lip clearance reduces and causes increase in blowdown
• Drain passage getting choked causing water accumulation and hence prevent smooth valve operation
• Spring tends to rust and cause valve seizure
• Valve spindle bending due to constant compression (can cause valve to have erratic/sluggish operation)