Starting Air Line System – Explosion in a 2-Stroke Engine

An explosion in the starting air line occurs when three essential elements are present together: fuel (oil), air (oxygen), and an աղբ ignition source. The main causes are:

• Accumulation of Lubricating Oil: Excess lubrication from the air compressor or oil carry-over due to faulty separators and filters can deposit oil inside the starting air manifold and pipelines.
• Leaking Starting Air Valves: If a starting air valve does not seat properly or remains slightly open, high-temperature combustion gases (above 1000°C) can flow back into the starting air line.
• Carbon Deposits: High compressor discharge temperatures may carbonize lubricating oil, forming deposits inside the pipeline. These deposits can glow or act as a combustible material.
• Formation of Oil Mist: When hot combustion gases come in contact with the oil film inside the pipe, the oil may vaporize into a fine, highly flammable mist.
• Adiabatic Compression (“Diesel Effect”): Sudden opening of valves can create pressure waves, causing localized temperature rise sufficient to ignite oil vapours present in the line.

Early detection of warning signs is critical to prevent serious damage. The following indications may be observed:

• High Temperature of Branch Pipes: A branch pipe that feels unusually hot during normal operation usually indicates leakage through a starting air valve.
• Discoloration of Paint: Burnt or blistered paint on branch pipes or the main manifold suggests overheating due to hot gas leakage.
• Pulsating Air Pressure: Fluctuations or “kicking” of the starting air pressure gauge while the engine is running may indicate backflow of gases.
• Abnormal Noise: Hissing sounds near the starting air valve during engine operation can point to leakage.
• High Compressor Discharge Temperature: Elevated temperatures at the compressor or after-cooler may indicate oil carry-over into the air system.
• Oil in Drain Lines: Excessive oil or oil-water emulsion observed during draining of air receivers or manifolds indicates contamination of the air system.

Immediate Actions (if leakage is suspected):

1. Isolate the Starting Air System: Close the main starting air valve and release pressure from the manifold.
2. Isolate the Affected Cylinder: Cut off fuel supply to the concerned cylinder if the leakage is significant, to reduce temperature and pressure.
3. Cooling Measures: Improve ventilation around the area, but avoid applying water directly on hot components to prevent thermal shock.

Operational Precautions:

• Regular Draining: Drain air receivers and starting air manifolds frequently (at least once per watch) to remove oil and moisture.
• Routine Temperature Checks (“Feel Test”): Physically check branch pipe temperatures after the engine reaches normal operating conditions to detect leaking valves early.
• Safety Devices: Ensure flame traps, bursting discs, or relief devices are in proper working condition and have not been tampered with.

Maintenance Requirements:

• Starting Air Valves: Periodically overhaul valves, lap the seating surfaces, and ensure a proper gas-tight seal. Test for leakage using soap solution or by checking for backflow.
• Air Compressors: Maintain piston rings, valves, and separators to prevent oil carry-over. Ensure after-coolers are clean and functioning efficiently.
• Non-Return Valves (NRV): Regularly inspect and overhaul NRVs to ensure they effectively prevent reverse flow of hot gases into the air system.
• Internal Cleaning of Manifold: Periodically clean and degrease the inside of the starting air manifold to remove accumulated oil deposits.