Setting auxiliary boiler safety valves to lift at the desired pressure under steam is a critical procedure that ensures the safe operation of the boiler. The process typically involves the following steps:

1. Preparation:

   • Ensure the boiler is under steam and can be brought up to the desired set pressure. All other safety valves on the boiler (if multiple) should be gagged or secured to prevent them from lifting prematurely, allowing the focus to be on the valve being set.
   • The easing gear of the valve to be set should be checked for freedom of movement and then disengaged.
   • Ensure accurate pressure gauges are fitted and calibrated.

2. Initial Adjustment:

   • The compression screw (adjusting nut) on top of the safety valve, which controls the spring tension, is initially adjusted to a point where the valve is expected to lift below the desired set pressure.

3. Raising Pressure and Observation:

   • The boiler pressure is slowly and steadily raised by increasing the firing rate while monitoring the boiler pressure gauge.
   • As the pressure approaches the desired set pressure, observe the valve carefully. The valve should begin to 'feather' (emit a slight hiss of steam) just before it fully lifts.

4. Fine Adjustment of Lifting Pressure:

   • If the valve lifts below the desired pressure, the compression screw is tightened (turned clockwise) to increase the spring compression and thus the lifting pressure.
   • If the valve does not lift at the desired pressure, the compression screw is loosened (turned anti-clockwise) to decrease spring compression.
   • This adjustment is done incrementally, allowing the boiler pressure to fluctuate slightly and observing the valve's response. The aim is for the valve to lift cleanly and fully at the exact desired set pressure.

5. Blowdown Adjustment (if applicable):

   • Once the lifting pressure is set, the blowdown (the pressure drop before the valve reseats) is checked. This is typically adjusted by a blowdown ring or adjusting ring located around the valve seat. Adjusting this ring changes the escape area for the steam, influencing the pressure at which the valve reseats.
   • The blowdown should be within the manufacturer's specified limits or classification society requirements (e.g., 3-5% of set pressure).

6. Final Check and Securing:

   • After the lifting pressure and blowdown are correctly set, the lock nuts on the compression screw and blowdown ring (if adjustable externally) are securely tightened to prevent accidental movement.
   • The process is usually repeated for all safety valves, ensuring they lift sequentially at their designated pressures (e.g., one at maximum working pressure, the other slightly higher).

To ensure the integrity of the safety valve settings and prevent unauthorized tampering after completion, the following precautions are essential:

1. Lead Sealing: This is the primary method. Lead seals are applied to:

   • The compression screw (adjusting nut) and its lock nut, securing them to the valve spindle or yoke. This prevents alteration of the spring tension.
   • The blowdown adjusting ring (if externally adjustable) and its locking mechanism, to prevent changes to the blowdown pressure.
   • The easing gear mechanism, to ensure it cannot be used to manually lift the valve without breaking the seal.
   • The seals are typically stamped with the classification society's mark or the ship's official stamp.

2. Witnessing and Documentation:

   • The setting procedure is usually witnessed by a qualified classification society surveyor and/or the Chief Engineer.
   • Detailed records of the set pressures, blowdown, date of setting, and names of personnel involved are entered into the boiler logbook and official records.

3. Restricted Access:

   • Safety valves are typically located in the boiler room, which is a restricted access area on board, limiting opportunities for unauthorized personnel to interfere.

4. Regular Inspections:

   • During routine rounds and surveys, engineers and surveyors visually inspect the safety valves to ensure all seals are intact and show no signs of tampering.

The accumulation of pressure test, also known as the overpressure test or full capacity test, is a crucial test performed on boilers to verify the adequacy of their safety valve discharge capacity. Its primary purpose is to ensure that, under the most severe operating conditions, the safety valves can release steam quickly enough to prevent the boiler pressure from rising to a dangerous level.

Procedure:

1. The boiler is fired at its maximum continuous evaporation rate (maximum firing capacity).
2. All steam outlets from the boiler, including the main stop valve and auxiliary steam lines, are closed.
3. All safety valves on the boiler are allowed to lift and discharge steam simultaneously.
4. The test is continued for a specified duration, typically 7 minutes for water-tube boilers and 15 minutes for fire-tube boilers, or as per classification society requirements.

Acceptance Criteria: During the entire duration of the test, the boiler pressure must not rise by more than 10% above the maximum permissible working pressure (or the highest set pressure of any safety valve, whichever is greater). If the pressure rise exceeds this limit, it indicates that the safety valves do not have sufficient discharge capacity, and corrective action (e.g., increasing valve size, adding more valves, or reducing boiler firing rate) would be required.

Significance: This test is vital for confirming the design and operational integrity of the boiler and its safety system. It demonstrates that even if all steam demand ceases while the boiler is firing at full capacity, the safety valves can prevent catastrophic overpressure. It is typically performed during commissioning, after major repairs, and periodically during classification surveys, usually witnessed by a classification society surveyor.