Answer EKM Question 39
Q39.
Why does an explosion occur in the starting air line of an internal
combustion engine and how can the possibility of such an occurrence be
reduced? Sketch and describe devices, which may be fitted to reduce the
severity of such an explosion. State the attention which air starting
valves should be given before manoeuvring system.
Answer:
- Air Start explosions occur during a start sequence, when oll, which
can accumulate in the air mart receivers or on the surface of the start
air lines, becomes entrained with high pressure air in the air start
manifold and is ignited.
- The source of ignition for these explosions can be attributed to one of the following:
- A leaking air start valve. Whilst the engine is running, the hot gases produced as the fuel burns in the cylinder (at above 1200°C) leak past a valve which has not re-seated correctly. The branch pipe to the ale start manifold heats tip to red hot if the engine is stopped and restarted before the pipe has time to cool, any oil vapour in the air can be ignited and an explosion can result if the mixture of oil/air is correct.
- Fuel leaking into the cylinder whilst the engine is stopped. The engine then undergoes a start sequence, and builds up speed, the fuel which has leaked into the cylinder vaporises and the heat from the compression of the air in the cylinder, as the piston rises, ignites the fuel. When the air start valve opens as the piston comes over TDC, the pressure in the cylinder is higher than the air mart pressure, and the burning combustion gases pass to the air start manifold, igniting the oil entrained in the air.
- The principal cause of explosions in starting air manifolds of marine engines is probably the auto ignition of all deposited on the inner surface of the manifold, not backfire from cylinders as previously thought.
- Auto-Ignition conditions occur because of the high temperature generated by the rapid inflow of high-pressure air, says the research. This incoming air compresses air downstream of the main starting Valve, causing its temperature to reach as high as 400°C which In some cases causes oil deposits in the manifold to self-ignite leading to an explosion.
To minimise the risk of explosions:
- To minimise the risk of explosions, the oil carry over from the compressor should be reduced to a minimum. Class regulations require that the air compressor's air intakes are located in an oil-free atmosphere, and it drain/filter for intercepting oil/water mist is fitted between compressor discharge and air receiver.
- There most be complete separation of compressor discharge and starting air supply to engines at the receiver which is fitted with a drain and a relief valve.
- The air start system must be protected with a non-return valve at the starting air supply to each engine. This is normally part of the automatic valve which opens when an air start is initiated.
ln addition to this IACS require that:
- For direct reversing main engines >230 mm bore flame arresters or bursting discs are required for each cylinder fitted between the cylinder start air valve and the manifold.
- For non-reversing and auxiliary engines >230 mm bore a single flame arrester or bursting disc is acceptable fitted at the supply inlet to the starting air manifold.
- Although not part of IACS regulations, a relief valve may be fitted to the manifold where flame arrestors are used Instead of bursting discs.
Unsatisfactory practices which have led to explosions in the air start system Include;
- Tell-tales / drains at each end of the starting air manifold found to have been blanked off with screwed plugs.
- Failure to drain starting air receivers and starting air pipes at regular intervals or before manoeuvring.
- Failure to check for leaking die start valves.
- Failure to maintain starting air valves and systems strictly in accontance with manufacturers recommended practices.
- Failure to maintain fuel valves correctly.
Safety Devices:
1. The flame trap:
- The flam trap is manufactured from brass or aluminium alloy which both have a high specific heat capacity. A number of holes are bored through the thick circular form to allow the air to pass through. They are fitted in the main airline immediately before the air start valve to restrict the risk of a flame in the cylinder propagating back to the main air start manifold, by dissipating the heat energy in the flame.
2. Bursting disc:
- The safety cap consists of a bursting disk enclosed by a perforated cylinder and a perforated cover in order to protect any bystanders, in the event of a burst. The cover is fitted with a tell-tale, which shows if the bursting disc has been damaged.
- If the bursting disc of the safety cap is damaged due to excessive pressure in the starting airline, overhaul or replace the starting valve which caused the burst, and mount a new disk if a new disk is not available, or cannot be fitted immediately, then the cover can be turned in relation to the perforated cylinder, In order to reduce the leakage of starting air.
3. Relief Valve:
- The sketch shows a relief valve as fitted to the air start manifold of Sulzer RTA 2 stroke engines. Its Purpose is to relieve excess pressure in the air start manifold.
- It consists of a spring loaded valve disk which locates on a mating seat which is bolted to the end of the air start manifold. When the force exerted on the disk due to excessive pressure is greater than the spring force holding the valve closed, the valve will open.
- The source of ignition for these explosions can be attributed to one of the following:
- A leaking air start valve. Whilst the engine is running, the hot gases produced as the fuel burns in the cylinder (at above 1200°C) leak past a valve which has not re-seated correctly. The branch pipe to the ale start manifold heats tip to red hot if the engine is stopped and restarted before the pipe has time to cool, any oil vapour in the air can be ignited and an explosion can result if the mixture of oil/air is correct.
- Fuel leaking into the cylinder whilst the engine is stopped. The engine then undergoes a start sequence, and builds up speed, the fuel which has leaked into the cylinder vaporises and the heat from the compression of the air in the cylinder, as the piston rises, ignites the fuel. When the air start valve opens as the piston comes over TDC, the pressure in the cylinder is higher than the air mart pressure, and the burning combustion gases pass to the air start manifold, igniting the oil entrained in the air.
- The principal cause of explosions in starting air manifolds of marine engines is probably the auto ignition of all deposited on the inner surface of the manifold, not backfire from cylinders as previously thought.
- Auto-Ignition conditions occur because of the high temperature generated by the rapid inflow of high-pressure air, says the research. This incoming air compresses air downstream of the main starting Valve, causing its temperature to reach as high as 400°C which In some cases causes oil deposits in the manifold to self-ignite leading to an explosion.
To minimise the risk of explosions:
- To minimise the risk of explosions, the oil carry over from the compressor should be reduced to a minimum. Class regulations require that the air compressor's air intakes are located in an oil-free atmosphere, and it drain/filter for intercepting oil/water mist is fitted between compressor discharge and air receiver.
- There most be complete separation of compressor discharge and starting air supply to engines at the receiver which is fitted with a drain and a relief valve.
- The air start system must be protected with a non-return valve at the starting air supply to each engine. This is normally part of the automatic valve which opens when an air start is initiated.
ln addition to this IACS require that:
- For direct reversing main engines >230 mm bore flame arresters or bursting discs are required for each cylinder fitted between the cylinder start air valve and the manifold.
- For non-reversing and auxiliary engines >230 mm bore a single flame arrester or bursting disc is acceptable fitted at the supply inlet to the starting air manifold.
- Although not part of IACS regulations, a relief valve may be fitted to the manifold where flame arrestors are used Instead of bursting discs.
Unsatisfactory practices which have led to explosions in the air start system Include;
- Tell-tales / drains at each end of the starting air manifold found to have been blanked off with screwed plugs.
- Failure to drain starting air receivers and starting air pipes at regular intervals or before manoeuvring.
- Failure to check for leaking die start valves.
- Failure to maintain starting air valves and systems strictly in accontance with manufacturers recommended practices.
- Failure to maintain fuel valves correctly.
Safety Devices:
1. The flame trap:
- The flam trap is manufactured from brass or aluminium alloy which both have a high specific heat capacity. A number of holes are bored through the thick circular form to allow the air to pass through. They are fitted in the main airline immediately before the air start valve to restrict the risk of a flame in the cylinder propagating back to the main air start manifold, by dissipating the heat energy in the flame.
2. Bursting disc:
- The safety cap consists of a bursting disk enclosed by a perforated cylinder and a perforated cover in order to protect any bystanders, in the event of a burst. The cover is fitted with a tell-tale, which shows if the bursting disc has been damaged.
- If the bursting disc of the safety cap is damaged due to excessive pressure in the starting airline, overhaul or replace the starting valve which caused the burst, and mount a new disk if a new disk is not available, or cannot be fitted immediately, then the cover can be turned in relation to the perforated cylinder, In order to reduce the leakage of starting air.
3. Relief Valve:
- The sketch shows a relief valve as fitted to the air start manifold of Sulzer RTA 2 stroke engines. Its Purpose is to relieve excess pressure in the air start manifold.
- It consists of a spring loaded valve disk which locates on a mating seat which is bolted to the end of the air start manifold. When the force exerted on the disk due to excessive pressure is greater than the spring force holding the valve closed, the valve will open.
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