You have heard the term “Air starting system”; but don’t know what it is? Well, Starting air system use the compressed air to start the main engine. Compressed air is inserted into various cylinders in a proper sequence to provide the initial torque; required to overcome the inertia of reciprocating parts.
Normally, compressed air at 20 to 30 bar pressure is required to start an engine in a certain direction. The compressed air which is inserted to the cylinder is also termed as “starting air”. This starting air is inserted to a cylinder when it is just past the TDC; and end when the exhaust opens. Pilot valve along with the starting air valve is responsible for proper injection of starting air timed on camshaft movement.
Important Parts of Starting Air System
It takes air from the atmosphere, compress it and then fill up the air bottles up to 30 bar pressure. Normally a two stage air compressor is used on board for this purpose. (We will discuss in details in upcoming posts)
There are two main air bottles in the engine room which stores the compressed air for sudden demand. By law, the air bottles must be of sufficient capacity. Which require it to store minimum compressed air for twelve consecutive starts for reversible engine and six starts for non reversible engine.
Master starting air valve
Master starting air valve / automatic valve is used to connect; or disconnect the supply of compressed air to the engine or starting air line. It ensures that air is available to the starting air line; only for a brief period of time during starting. It also works as a safety device by preventing blow-back to reach the air bottles; Under situation such as cylinder starting air valve stuck in open position.
Starting air valve
It is a valve fitted on the head of the cylinder to allow admission of starting air into the cylinder. It operates on the pilot air timed after the camshaft position; To admit air according to the engine’s fire order.
Oil particles are always available in the starting air system; thanks to the lube oil of compressors which carried over time to deposit on the walls. Under such condition’s, a leaky starting air valve not only reduce the combustion pressure; but also increases the risk for blow back into the starting air system.
Hot combustion gas (blow back) along with oil deposits and compressed air can lead to starting air line explosion. So to protect the pipelines from damage; a relief valve is provided to release the extra pressure.
The work of air distributor’s in starting air system is to open the starting air valve by providing pilot air in accordance with the engine’s firing order.
Control valve operates the master starting valve by passing control air through it. This allows the main starting air valve to open after checking various interlocks.
A compressed air of 20 to 30 bar is required to start an engine by providing enough force to turn the engine at speed suitable to reach the combustion temperature and pressure. There is no set rule for the size of the compressed air cylinder; they should only comply to the rule which states: “There should be enough compressed air to start a reversible engine twelve times in a row and six times for non reversible type”.
In modern uniflow marine engine the compressed air is injected when the piston is just pass the TDC ( 10 degree before TDC) and close while the exhaust valves going to open ( 5 Degree before the exhaust valve opening). Steps should be taken too avoid leaky starting air valve; as this can not only increase the temperature of starting air line but also cause staring air line explosion. To check for any leakage, simply open starting air automatic valve and air distributor shut.
To start the main engine, control air is given to the control valve which then sends air signal to automatic valve to open. This leads compress air to pilot and starting air valve. Now the pilot valve is under equilibrium by control and starting air at different sides of piston in pilot valve. Now the movement of cam profile lead to the flow of high pressure air to operating piston of starting air valve.
Which then opens to admit air in cylinder according to the firing order. It will turn engine on air until it reach certain RPM; after that the control air is released from the air control valve. Which then vent the system leading to closure of automatic valve. Then fuel admits the engine and ignition starts.
The above long process would not have taken place if the control valve had detected the turning gear interlock active. A slow turning gear valve is also provided on the system to avoid any water hammering.
Understanding Importance of Overlap in Starting Air System
To ensure starting of marine diesel engine at any position in either direction; starting air is injected in more than one cylinder at a time. This brief period when more than one cylinder is going through starting air operation is called “Overlap”. An overlap in starting air system is required to maintain engine momentum by keeping engine under condition when an cylinder is at the end of starting air injection while the other is just at the start.
The condition is achieved by controlling the operation of pilot valve under starting air distributor. Typically pilot valve are set such that the overlap is about 15 degree of crank rotation for more than three cylinder engine. It is so because, there is no need for overlap in engine upto three cylinder as it covers 120 degree of starting interval for each cylinder. Overlap is only required when starting interval is less than 105 degree of rotation.
- Regular maintenance should be carried out on the starting air system.
- Be careful, alert and watchful while operating the system.
- Drain’s should be checked at regular interval.
- Engine should never be run while starting air valve stuck or leaking; unless there is an emergency.
- Take immediate actions in case of a fault.
- Do periodic inspection of starting air pipe’s to avoid oil deposition.
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