Air Starting of main engine

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An interlock blocking valve will operate, for instance if the turning gear is left in, and this will stop high pressure air from reaching the air start control valve and thus either the automatic valve or the pilot valve.

A slow turning valve is fitted. This will open instead of the main automatic valve if the engine has been stopped for more than 30 minutes during maneuvering. It will only supply enough air to turn the engine over very slowly;

This is a precaution in case a cylinder has had oil or water leak into it which would cause damage to the engine when starting. If the engine completes a full revolution on the slow turn, then the main automatic valve opens and the engine will start. (note: The operating system for the slow turning has been omitted for simplicity).

International Association of Classification Society rules state:

In order to protect starting air mains against explosion arising from improper functioning of starting valves, the following devices must be fitted:

An isolation non-return valve or equivalent at the starting air supply connection to each engine.

A bursting disc or flame arrester in way of the starting valve of each cylinder for direct reversing engines having a main starting manifold. OR

At the supply inlet to the starting air manifold for non-reversing engines

Devices under (ii) above may be omitted for engines having a bore not exceeding 230 mm.

The system may also be provided with a relief valve.

2. This is very much related to engine balancing and no hard and fast rules can be laid down about crank firing sequences, as each case must be treated on its merits.

It may be useful to note that for 6-cylinder, 2 stroke engines a very common firing sequence is 1 5 3 6 2 4 and similarly for 7 and 8 cylinders 1 7 2 5 4 3 6 and 1 6 4 2 8 3 5 7 respectively are often used

The cam on no. 1 cylinder is shown for illustration, as it would probably be for operating say cam operated valves, obviously the other profiles could be shown for the remaining three cylinders in a similar way. The air period for nos. 1,4,3and 2 cylinders are shown respectively in full, chain dotted, short dotted and long dotted lines and the overlap are shown shaded.

Angle between cranks = Number of degrees in engine cycle

Number of cylinder.

= 360* (Two stroke) or 720*(Four stroke)

Number of cylinders.

= Firing interval.

Two Stroke engine Air admission Period

= Angle between cranks + Over Lap

Four Stroke engine Air admission Period

= 2 x Angle between cranks + Over Lap

Some overlap of the timing of starting air valves must be provided so that as one cylinder valve is closing another one is opening to ensure a positive start at all angular position of crankshaft. In practice air starting valve are timed to open at 45 deg ATDC and number of cylinders is kept sufficient to permit air shut off at 45 deg BBDC. 20 deg to 90 deg is good valve overlap. Minimum amount of overlap is 15 deg.

Starting air is admitted on the working stroke and the period of opening governed by three main factors.

1. Admission period should be slightly more than firing interval.

The valve must close before exhaust commences.

The valve should open after firing dead centre to give a positive turning moment in the correct direction.

Air starting gears for marine diesel engines

The starting system of an engine will consist of:

Relay valve or master valve or automatic valve.

Pilot valve

Distributor

Air starting valve

Various types of air starting system

Various types of air starting system are classified as follows according to method of actuating air starting valve.

Mechanical

Mechanical with air check valve

Pilot operated a. individual b. rotary distributor

Distributor ( engines with one direction of rotation )

The mechanical system has a serious disadvantage. As starting air valve is kept physically open by the rocker arm, there is explosion risk if fuel is injected in the cylinder at this time. To avoid this, a check valve is incorporated in the system. Air starting valve, which is operated is not situated on cylinder head but is situated in the airline before a check valve. Which is situated on the cylinder head and opens into combustion chamber. This prevents backflow of air in case a cylinder fires while the main air-starting valve is open. The arrangement is shown in the figure

Starting air interlock

A number of interlocks are provided in connection with the air starting system to safeguard the engine and personnel.

These are:

1. The turning gear interlock valves which are arranged to shut off the starting air pilot line when the turning gear is engaged, thus preventing the engine being started.

2. To prevent operation of the starting air system whilst the engine is running, a pilot air interlock valve is operated by the main control lever. The valve remains open during the starting sequence, i.e. until the main control lever is moved just beyond the starting position, but remains closed after this point and does not open again until the main lever is moved back to the stop position.

3. A mechanical interlock, is provided in the control box to prevent the main control lever being moved beyond the starting position while the starting air lever is in either the ahead or astern starting position.

4. Another mechanical interlock is provided to prevent the main control lever being moved until starting air is admitted to the cylinders. This prevents fuel being admitted to the cylinders when the engine is at rest.

Master Air Starling Valve:

The automatic starting air stop valve serves to open or close the admission of air to engine starting line. The duty of this valve is to pressurise the starting air line adjacent to cylinders only during a very brief period of air starting. For the most of thb time the pipe line ahead of the stop valve remains vented. The incorportation of a stop valve prevents a blow back of hot gases from the cylinder into the air bottle in case of a cylinder starting valve is jammed in the open position.

The figure (134) shows the construction of such a valve. There is an auxiliary spring loaded non-return valve with flame trap which prevents a blow back in the reverse direction. The valve is balanced by air pressure acting at the annular space and by leak off at the underside of the valve. The opening of the valve coincides with the venting of the space beneath the valve. Air from the main bottle forces the main valve to open against the spring pressure. A hand wheel is provided which can be used for opening and closing in case the automatic operation is affected.

The Sulzer Automatic valve New type

1. Description

The automatic starting air shut-off valve blocks off or releases the starting air to the engine. It can be put in the following positions by means of a hand wheel:

— manually closed

— automatic

— manually opened.

With the engine shut down, the valve spindle is screwed up into the closed position, holding the valve closed against the seat.

When the engine is prepared for starting, the valve spindle is moved to the automatic position. The spring holds the valve shut.

When the start air isolator at the air receivers is opened, air enters the valve body and flows to the underside of the valve through the balancing bores. The air pressure on the underside of the valve prevents the valve opening.

When an air start signal is given, the control valve opens and the space under the valve is vented. The air pressure acting on the top surfaces of the valve overcomes the spring force and the valve opens.

The valve can be tested by opening the test valve. In an emergency it can be manually operated.

The valve outlet incorporates a non return valve. A pressure gauge connection and a manual venting valve are fitted to the valve body (not shown).

Individual pilot operated (Axial)

The pilot operated system is illustrated in the figure above. The camshaft actuates the pilot valve rather than the large air-starting valve in the engine head. A branch line leads from the starting air valve to the pilot valve; another line leads from the starting air valve to the air header.

The air valve in the piston is a balanced valve, as shown. Pressure, rather than the rocker arm, opens the valve.

THE MAN B&W AIR START 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

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 top of the valve body and acts on a piston. This force overcomes the spring force holding the valve shut, 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 RTA AIR START VALVE

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

The stepped piston also means that closing of the valve is damped as air gets trapped in the annular space formed when the smaller diameter piston enters the upper part of the cylinder.

The air to operate the valve comes from the main air start supply. The distributor pilot air operates the pneumatic change over valve.

WHAT CAUSES AN AIR START VALVE TO STICK?

Leakage of a starting air valve is usually caused by sluggish valve action preventing fast closure of the valve, or by dirt or foreign particles from the starting air supply lodging on the valve seat and so preventing the valve from closing fully. Sluggish valve action may be caused by dirty pistons or valve spindle guides and the like. In newly overhauled valves sluggish valve action may be caused by parts fitted with inadequate clearances

HOW CAN I 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. During periods of manoeuvring the temperature of each supply pipe from the air rail to the starting air valve should be checked by feeling the pipe as close to the valve as possible

WHAT SHOULD I DO IF AN AIR START VALVE JAMS OPEN WHILST MANOEUVRING?

The fuel pump should be lifted (fuel rack zeroed, puncture valve operated or whatever) on the affected unit and the bridge informed. The load should be kept at a minimum, as one unit is now out of operation. As soon as safe to do so, the engine should be stopped and the air start valve replaced

THE AIR START PILOT VALVE

The start sequence is underway. Pilot air flows through bores in the shuttle valve plunger and escapes through the pulse pipe. Due to the difference in air pressure the shuttle valve plunger is in the lower position and the air start valve operating cylinder is vented.

As the cam turns the exit from the pulse pipe is restricted by the cam profile. The pressure under the shuttle valve plunger increases and due to the difference in areas the plunger now lifts allowing pilot air to operate the air start valve.

At the end of the opening angle for the air start valve, the underside of the shuttle valve plunger is vented through the pulse pipe and the valve closes allowing the air start valve pilot air to vent

The direction lever in the control room is set to AHEAD. This activates solenoid valve 2 which sends a signal to operate valve 14 which will allow 30 bar control air to the distributor when valve 16 is operated.

It also operates valve 11 which allows air to the fuel pump cam follower servo motor.

When the lever is moved into the start position, it activates solenoid valve 4. This operates valve 6 which in turn operates valve 9.

This allows air to operate the automatic valve.

Should solenoid valve 15 be activated, then only the slow turning valve will open.

Valve 16 is also activated allowing 30 bar air to the start air distributor.

When the lever is moved to the run position, solenoid valves 1 and 4 are deactivated and vented.

This causes valve 8 to vent, shutting the fuel pump puncture valve.

Valve 13 also vents, as does valve 16, venting thw air start distributor.

Valve 16 moves across allowing control air to shut the automatic and slow turning valves.

Points to Note:

Valve 6 has a 1 second delay timer fitted.

This allows the main starting valve to remain open, so as to supply air to those cylinders that are in the start position.

There is a manual shut off valve fitted between the air start manifold and valve 16 for maitenance puposes.

The fuel pump reversing servo motor is fitted with relief valves and restrictors to dampen operation.

Valve 7 is vented by engaging the turning gear. This prevents start sequence taking place.

The Sulzer RTA Air Start System

Illustrated is a Sulzer RTA Air Start System. These do vary depending on the type of engine, so if you've had experience of one, it might be slightly different to the one illustrated here.

Start at the air receiver and open the valves. Air at 30 bar flows to the automatic valve which is in the automatic position. air flows via the turning gear interlock to the control valves for engine starting. If the turning gear is engaged, then the control air won't get past the interlock.

The captain wants to start the engine ahead. As he moves his little lever on the bridge, an electrical signal operates the start solenoid valve and the ahead solenoid valve. This allows control air to open the respective pneumatic control valves. Air flows to the control valve on the automatic valve allowing air under the piston to vent and thus open the valve. Main start air then flows through the non return valve to the air start manifold.

At the same time air from the direction control valve flows to the air start distributor. This moves the distributor servo piston into the ahead position, flows through the start cut off valve and operates the control valve to allow air from the air start manifold to operate the pilot valves.

Dictated by the distributor cam, air flows from the pilot valves in sequence to the control valves for the individual air start valves mounted in the engine cylinder.

After a set period of time (10 seconds), or when the engine has reached firing revs (whichever comes first), the start air is cut off and the governor puts fuel on the engine.

Air Start Explosions and Safety Devices

Air Start explosions occur during a start sequence, when oil, which can accumulate in the air start receivers or on the surface of the start air lines, becomes entrained with high pressure air in the air start manifold and is ignited.

As normal with large marine diesel engines, compressed air was used to start the engines by admitting it into the ‘ cylinders in sequence using main air start valves, the timing being controlled by camshaft operated pilot valves

After starting the system was vented through 3/8 “ (10mm) copper drain lines.

Because some of the air start valves were defective and not seating properly, products of combustion and unburnt carbon leaked past the valves and used to choke the drains on the air start venting system.

This had led to a routine being established to clear the choked drains before arrival in port.

However, no instructions had been issued on how this work was to have been carried out, and was generally left to the junior engineers.

A practice had grown up to connect the hydraulic pump used for the tie bolt jacks to the drain lines and force

lubricating oil up the drain lines to clear the blockage.

lf the drain lines had been disconnected, then no harm would have been done, but on the occasion in question some oil must have been forced into the air start line.

The ChiefEngineer was at the controls of the starboard engine and the 2nd Engineer at the controls of the port engine. (This ship was built in the days before control rooms; the engines were manoeuvered locally, and extra engineers were present in the engine room to operate the “handamatic” equipment).

After slowing down, “Stop” was rung at 04:48, followed by “Half Astern” at 04:49. Seconds later an explosion

occurred and a sheet of flame swept through the engine room killing the Chief, First, Snr and Jnr 2nd, two Junior Engineers and a greaser.

At the formal investigation which followed it was established that the explosion started in the air start line in the port engine.

For an explosion to occur, that together with the air, there must have been oil and a source of ignition.

Expert witnesses agreed that the initial explosion was due to the presence of about 4 fluid oz (110cc) of oil - about

half a teacup.

It was agreed that this oil probably came from the hydraulic pump used to clear the drain lines.

The Air start pipe lines were coated with a film of oil carried over from the compressors.

It was agreed that this film of oil did not cause the initial explosion, but was contributory to the escalation of the incident as described below.

Oil, in quantity of at least four fluid ounces from the force pump that was used for clearing drain pipes was retained in the air starting system of the port engine, in or near to an air starting valve.

When the port maneuvering lever was moved to “start’ the air start valve opened and highly turbulent air entrained the oil, sweeping it into the cylinder, acting to some extent like an air blast atomizer.

The mixture of oil and air ignited as a result of high

temperatures in the cylinder at some point remote from the starting air valve.

This ignition first expelled a mixture of UN- burnt oil and air into the open line through the valve, then the explosion flame propagated from the source of ignition through the cylinder and out through the starting valve into the pipeline, there consuming the explosive mixture that had just been expelled.

The flame accelerated and initiated a film detonation involving compressor oil in the main air pipelines.

The detonation waves when reflected at the extremities of the system, or at T junctions, produced very high instantaneous pressures (in some way akin to a “water hammer” effect) and caused severe , damage at these places.

Rupture of the connections to the port aft air receiver caused the air in the receiver to be discharged directly at the back of the port engine, passing between the cylinders and swirling down over the maneuvering platform.

Flames associated with this discharge caused the casualties in the engine room.

Fires in the generator room were likewise caused when the port forward air receiver connections were ruptured.” (report ofcourtNo 8022)

The fires were eventually extinguished by the use ofCO2 gas.

Following the formal investigation an M notice was issued (No M474) which recommended that:

Oil force pumps should not be used to clear drains on starting air pipe lines.

2. Oil from any source should, as far as practicable and reasonable, be excluded from air pipe lines. in particular, air compressor discharge lines should be provided with means for effective interception and draining of oil and water.

If necessary, filters or separators should be fitted for this purpose and drains of adequate size and number should be fitted to air pipes, receivers and other fittings to avoid any accumulation of oil at low points in the system.

3. Periodic inspections should, where practicable, include examination of air pipe lines to ensure that measures taken are effective.

In 1999, a large container ship, built in 1981 and fitted with a large bore two-stroke engine, suffered damage when

the starting air manifold was blown apart by an internal explosion.

This occurred during manoeuvring when berthing. Fortunately therewere no casualties.

Reference to Lloyds Register database has shown that this was not an isolated incident — between 1987 and 1999, 11 incidents of explosions in air start systems have been reported and most have been attributable to

unsatisfactory shipboard practices by ships staff, resulting in the presence of oil or explosive vapour in the manifold.

The source of ignition for these explosions can be attributed to one of the following:

1. 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 air start manifold heats up to red heat.

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.

2. Fuel leaking into the cylinder whilst the engine is stopped.

When 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 startvalve opens as the piston comes over TDC, the pressure in the cylinder is higher than the air start pressure, and the burning combustion gases pass to the air start manifold, igniting the oil entrained in the air.

3. A recent theory by ClassNK has concluded that the principal cause of explosions in starting air manifolds of

marine engines is probably the auto ignition of oil 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.

ClassNK has adapted its safely requirements for a starting system to account for the findings.

It now requires the fitting of rupture discs to the manifold on engines with a flame arrester in each branch pipe leading to the cylinders.

This is beyond IACS unified requirements, which account for cylinder backfire as the cause of starting air manifold 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 a drain/filter for intercepting oil/water mist is fitted between compressor discharge and air receiver.

There must 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 he automatic valve which opens when an air start is initiated.

In 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 oft with screwed plugs.

. Failure to drain starting air receivers and starting air pipes at regular intervals or before maneuvering.

Failure to check for leaking air start valves.

Failure to maintain starting air valves and systems strictly in accordance with manufacturers recommended practices.

Failure to maintain fuel valves correctly.

SAFETY DEVICES

Flame Arrestors

The flame trap is manufactured from brass or aluminum 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.

Bursting Disks

The safely 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 can not be fitted 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 two stroke engines.

Its purpose is to release 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 start air manifold.

When the force exerted on the disk due to excessive pressure is grater than the spring force holding the valve closed, the valve will open.