The exhaust valve in a large two-stroke marine engine operates under extremely high temperature and pressure conditions, making it one of the most highly stressed components. Renewal of the exhaust valve assembly, or its individual parts, becomes necessary under the following circumstances:

• Gas Leakage (Blow-by): When the sealing surfaces of the valve spindle and seat become burnt or “wire-drawn” due to high-temperature gas flow, proper sealing is lost. This results in leakage of combustion gases and loss of compression.
• Cold Corrosion: If the temperature of the valve housing falls below the dew point of exhaust gases, sulfuric acid may form and attack the metal surfaces, leading to corrosion damage.
• High-Temperature Corrosion (Vanadium Attack): When operating on low-quality heavy fuel oil, vanadium and sodium compounds can form molten deposits. These aggressively corrode the valve spindle and seat, reducing their service life.
• Cracks or Mechanical Damage: Cracks in the valve spindle, valve disc, or seat—detected through visual inspection or non-destructive testing (NDT)—necessitate immediate replacement to prevent catastrophic failure.
• Excessive Wear: If wear in the valve spindle stem or guide bush exceeds the manufacturer’s permissible limits, proper alignment and sealing cannot be maintained.
• Operational Abnormalities: Indicators such as unusually high exhaust gas temperature in a particular cylinder, or alarms indicating failure of valve rotation, suggest malfunction and may require valve renewal.

The following procedure outlines the safe removal and refitting of a modern hydraulically operated exhaust valve assembly.

Removal Procedure

1. Preparation: Stop the engine and engage the turning gear. Allow the engine to cool down completely. Isolate the engine - close starting air, cooling water, and hydraulic oil supply.
2. Disconnection of Services: Disconnect the hydraulic high-pressure pipe (actuator line), air spring connection, and cooling water inlet and outlet pipes connected to the exhaust valve.
3. Hydraulic Jacking: Mount hydraulic jacks on the valve housing studs. Apply the specified pressure to stretch the studs, allowing safe removal of the securing nuts.
4. Lifting the Valve Assembly: Attach the designated lifting tool to the exhaust valve housing. Using the engine room crane, carefully lift the complete valve assembly, ensuring it is raised steadily without swinging and clears the cylinder cover safely.
5. Protection of Opening: Immediately cover the exposed opening in the cylinder cover to prevent ingress of dirt or foreign particles into the combustion space.

Refitting Procedure

1. Cleaning and Preparation: Thoroughly clean the landing surface on the cylinder cover. Lapping of the seating surface to be carried out by special tool provided by the maker.
2. Positioning the Valve: Carefully lower the serviced or new valve assembly into position. Ensure proper alignment, particularly of the cooling water passages.
3. Securing the Assembly: Fit the securing nuts and use hydraulic jacks to tension the studs to the specified pressure as per manufacturer’s instructions.
4. Reconnection of Systems: Reconnect all hydraulic, air, and cooling water lines. Properly bleed the hydraulic oil system and cooling water circuit to remove any trapped air.

Checks Before Fitting

• Inspection of Landing Surfaces: Ensure that the cylinder cover seating area and valve housing contact surfaces are clean, smooth, and free from carbon deposits or pitting.
• Spindle Movement: Check that the valve spindle moves freely within the guide, either manually or by applying air pressure.
• Clearance Measurements: Measure the clearance between the spindle and guide bush to confirm it is within permissible limits. Verify that the valve seat and spindle have been properly ground or lapped.
• Condition of Seals: Ensure all O-rings and sealing elements are new, correctly sized, and lightly lubricated with suitable grease before installation.

Checks After Fitting (and During Initial Operation)

• Leak Testing: After restoring cooling water and hydraulic systems, check for any external leakage from connections or sealing areas.
• Air Bleeding: Ensure complete removal of air from the hydraulic actuator system to prevent erratic operation, such as hammering or delayed valve response.
• Functional Testing: While the engine is on turning gear, verify that the air spring maintains proper closing force. If possible, test the hydraulic opening and closing operation of the valve.
• Monitoring During Running-In: During the initial hours of operation, closely observe exhaust gas temperatures, valve rotation (pumping action), and overall performance. Listen for abnormal noises and check for unusual vibrations, particularly in the hydraulic piping.