Overhauling and Setting of Boiler Safety Valves

Boiler safety valves are critical protective devices designed to automatically release excess steam pressure and prevent boiler overpressure. Most auxiliary boilers are fitted with full-lift or pop-type double spring safety valves, which open rapidly and fully once the set pressure is reached, ensuring effective pressure relief.

A typical boiler safety valve consists of the following main components:

• Valve and Seat: Usually made of high-grade materials such as stainless steel or Monel metal to resist erosion (wire drawing) caused by high-velocity steam.
• Compression Springs: Helical springs that hold the valve tightly closed against steam pressure until the set pressure is reached.
• Valve Lip / Shroud (Waste Steam Piston): A specially designed projection that increases the effective area when the valve begins to lift, producing a rapid “pop” action and ensuring full opening.
• Spindle and Guides: Maintain alignment and ensure smooth vertical movement of the valve.
• Waste Steam Pipe: A large-diameter pipe that safely discharges steam to the atmosphere.
• Easing Gear: A mechanical arrangement that allows manual lifting of the valve for testing or emergency purposes.
• Drain Arrangement: Prevents accumulation of condensate in the valve body, which could otherwise affect operation.

1. Isolation and Removal: Isolate the boiler, ensure zero pressure, and remove the safety valve carefully from its seating.
2. Dismantling: Mark all parts for correct reassembly. Carefully dismantle the valve, including removal of springs, spindle, and valve disc.
3. Cleaning: Clean all components thoroughly to remove deposits, scale, and corrosion products.
4. Inspection of Components: Each component must be examined for wear, damage, or distortion (details given below).
5. Repair and Refurbishment: Carry out necessary repairs such as lapping of valve and seat, replacement of worn parts, or renewal of springs if required.
6. Reassembly: Reassemble the valve carefully, ensuring correct alignment and clearances. Avoid over-tightening or misalignment during assembly.

Parts Requiring Close Attention During Overhaul

• Valve and Seat Surfaces: These must be perfectly smooth and free from pitting, scale, or wire drawing. They should be lapped to a fine finish to ensure a steam-tight seal and prevent leakage or “simmering.”
• Springs: Check for cracks, corrosion, and loss of elasticity (permanent set). Defective springs will affect the lifting pressure and proper reseating of the valve.
• Spindle and Guides: Ensure the spindle is straight and moves freely. Guides should be clean and free from deposits, as any restriction may cause sticking or improper operation.
• Lip/Shroud Clearance: The clearance between the valve lip and seat ring is critical for correct “pop” action. Incorrect clearance may result in delayed opening or poor reseating.
• Drain Passage: Ensure that drain holes are clear. Blockage can allow condensate to accumulate, which may interfere with valve operation and cause corrosion.

After overhaul, safety valves must be reset and tested, usually in the presence of a classification society surveyor.

1. Preparation: Ensure that the boiler pressure gauge is calibrated and accurate. One safety valve is temporarily gagged (held closed) while the other is being set.
2. Raising Boiler Pressure: Gradually raise the boiler pressure up to the Maximum Allowable Working Pressure (MAWP).
3. Adjustment of Set Pressure: Adjust the compression of the spring using the adjusting nut until the valve lifts (“pops”) at the required pressure.
   • For boilers with two valves, typically one is set at the working pressure and the other slightly higher (e.g., about 3% above), as per class or manufacturer requirements.
4. Verification of Operation: Allow the valve to lift and reseat several times to confirm consistent operation. Check the blowdown, which is the difference between opening and closing pressure, typically around 3–5% of the set pressure.
5. Accumulation Test (if required): With the main steam stop valve closed and boiler firing at full capacity, verify that the pressure does not rise more than 10% above MAWP, ensuring adequate relieving capacity.
6. Sealing and Locking: Once the correct setting is confirmed, fit locking arrangements such as split collars or distance pieces. Apply a lead seal to prevent unauthorized adjustment.