STARTING AIR VALVE

AIR STARTING VALVE:The valve is fitted into the cylinder head. It is opened by control air from the starting air distributor. The valve shown is from a slow speed MAN-B&W two stroke engine but a lot of modern engines have valves working on similar principles and design.

Materials The body of the valve could be of mild steel, the spindle of high tensile or stainless steel, and the valve and seat could have the contact faces hardened.

How it works

Main starting air at about 30 bar from the manifold enters the chamber above the valve via the circumferential ports in the valve body. The air pressure will not open the valve because a spring is holding the valve shut, an the area of the balance piston is the same as that of the valve lid so the valve is pneumatically balanced.

When the valve is required to open, air at 30 bar from the air start distributor enters the the top of the valve body and acts on a piston which is bigger in size, This pressure overcomes the spring force holding the valve shut and pressure of trapped air from the main starting air valve, and the valve opens. When the air signal from the air start distributor is vented, the spring closes the valve

When the start sequence is finished the main air start pressure is vented through holes in the main start air manifold.

The Sulzer air start valve uses air on both sides of the operating piston to maintain positive closing. The piston is stepped. The reason for this is so the starting air valve will not open when the gas pressure in the cylinder is higher than the starting air pressure; i.e. when the cylinder is firing. Once the valve starts to open then the opening is accelerated when the larger diameter piston has the opening air acting on it.

MAINTENANCE

After certain periods of service starting air valves are changed and overhauled. If piston rings are fitted, care must be taken to ensure that they are free in their grooves. Should it be necessary to fit new rings, the butt clearances of the rings must be carefully checked by placing the ring into the operating cylinder and measuring the clearance. This is especially important if they are usually made of brass which has a larger coefficient of expansion than the other parts of the valve. The valve and valve seat are ground with grinding paste and finished to a fine surface with lapping paste. It is essential to ensure that all parts of the valve are scrupulously clean before reassembly. Lubricate all sliding surfaces sparingly with a molybdenum disulphide grease.

HOW TO TELL IF AN AIR START VALVE IS LEAKING OR HAS JAMMED OPEN?

When an engine is in operation leakage of starting air valves is shown by overheating of the branch pipe connecting the starting air valve to the starting air rail. The heating occurs due to the leakage of hot gases from the engine cylinder into the starting air line connected to the starting air rail.

STARTING AIR LINE EXPLOSION

Lubricating oil from the compressor will under normal operation pass along the air lines and deposit on them. In the event of a cylinder air starting valve leaking, hot gases would pass into the air pipes and ignite the lubricating oil. If starting air is supplied to the engine this would further feed the fire and could lead to an explosion in the pipelines. In order to prevent such an occurrence, cylinder starting valves should be properly maintained and the pipelines regularly drained. Also oil discharged from compressors should be kept to a minimum, by careful maintenance.

SAFETY DEVICES IN STARTING AIR LINE

Flame Arrestors

The flame 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 air line 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.

                             

FLAME ARRESTOR

Bursting Disks:

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 air line, 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.

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.

NON RETURN VALVE:
Non return valve is provided after the Automatic air starting valve to prevent any flow of air/ hot gas to the air reservoir.
PRECAUTIONS:
Starting Air System Precautions

Great care is to be exercised in the operation and maintenance of starting air systems. The hazard of compressed air and lubricating oil forming an explosive mixture must be avoided. Oil from any source must be excluded from the starting air system.

Air compressor, starting air reservoir blow down drains are to be operated at regular intervals, and if automatic, their function verified.

Periodic inspection of air starting system pipelines is to be carried out to ensure that no build up of oil is occurring. Highly flammable cleaning fluids must never be used in any part of the starting air system. Any residue of liquid or vapours could result in an explosion.

Routine duties must include the manual checking of the main air starting valve pipes for any increase in temperature, which would indicate leakage of combustion gasses into the system. It is particularly relevant during manoeuvring when the main air starting reservoirs are open to the system despite the existence of non-return valves and other devices.

It is absolutely essential that if an air starting valve is in any way suspect that immediate action is taken i.e. shutting the fuel off the unit in accordance with the manufacturer's instructions, "gagging" the valve shut, and replacing the valve at the first opportunity.

AIR STARTING SYSTEM:

Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air. The starting air system usually has interlocks to prevent starting if everything is not in order. Compressed air is supplied by air compressors to the air receivers. The compressed air is then supplied by a large bore pipe to a remote operating non-return or automatic valve and then to the cylinder air start valve. Opening of the cylinder air start valve will admit compressed air into the cylinder. The opening of the cylinder valve and the remote operating valve is controlled by a pilot air system. The pilot air is drawn from the large pipe and passes to a pilot air control valve which is operated by the engine air start lever.

When the air start lever is operated, a supply of pilot air enables the remote/ automatic starting  valve to open. Pilot air for the appropriate direction of operation is also supplied to an air distributor. Distributor is usually driven by the engine camshaft and supplies pilot air to the control cylinders of the cylinder air start valves. The pilot air is then supplied in the appropriate sequence for the direction of operation required. The cylinder air start valves are held closed by springs when not in use and opened by the pilot air enabling the compressed air direct from the receivers to enter the engine cylinder.

An interlock(Turning Gear interlock) is shown in the remote operating valve line which stops the valve opening when the engine turning gear is engaged.

This is a simplified diagram. Engine starting and reversing system has numerous small components and interlocks for safe and positive starting and reversing and fuel admission, which is given in Engine Instruction manual and is commonly known as Manoeuvering Diagram.