Basics of Main Air compressor on board Ship

Main Air compressor on ship is used to produce service, control and starting air. In the following article we explain the introduction, working and maintenance done on Main Air compressor.

What is a compressor?

A compressor is a machine capable of compressing and delivering the air at desires pressure and is driven by a prime mover(Engine or a electric motor). A compressor takes in atmospheric air, compresses it and delivers the high pressure air to a receiver(storage vessel) from which it may be conveyed by a pipeline.

Applications of compressed air on Ship:

• For control air and service air application on board ship.
• For various automation and controls, general service applications.
• For starting main and auxiliary engines
• To supercharge IC Engines
• painting and spraying.
• For operating pneumatic hand tools such as rivet sets, drills..
• To operate air brakes in automobiles , ram lifts and pneumatic conveyors.
• To blow air.
• To transmit power for operation of machines.
• To clean machines and work shop floor

Types of Compressors: 

Types of air compressors in use:

1. Reciprocating  Compressor
2. Centrifugal Compressor
3. Screw Compressor
4. Rotary compressor
5. Diaphragm compressor

Classification of Air Compressors -

1.According to working: Reciprocating compressors, and (b) Rotary compressors.

2.According to action: Single acting compressors, and (b) Double acting compressors.

3.  According to number of stages: (a) Single stage compressors, and (b) Multi-stage  compressors.

Main Components in Compressed Air Systems:

• Intake air filters
• Inter-stage coolers
• After coolers
• Air dryers
• Moisture drain traps
• Receivers

Reciprocating Compressor:

The first commercial piston compressors were built in the middle of the last century, and evolved from the steam engines which provided the prime mover. Construction at first was double acting, but there was difficulty in maintaining gas-tightness at the piston rod, so the design evolved further into a single-acting machine.

The stroke/ bore ratio has diminished to the point of becoming fractional because of improvements in valve design and manufacture. Machines of four, six and eight cylinders arranged in either V or W formation are common. These are arranged in a multi bank configuration with two, three or four connecting rods on the same throw of the crankshaft to give a short, rigid machine.

This construction gives a large number of common parts – pistons, connecting rods, loose liners and valves through a range of compressors, and such parts can be replaced if worn or damaged without removing the compressor body from its installation.

Working Cycle

The general form of positive displacement compressor is the piston type, being adaptable in size, number of cylinders, speed and method of drive.

It works on the two-stroke cycle, as the piston descends on the suction stroke, the internal pressure falls until it is lower than that in the suction inlet pipe, And the suction valve opens to admit air from atmosphere. At the bottom of the stroke, this valve closes again and the compression stroke begins. When the cylinder pressure is higher than that in the discharge pipe, the discharge valve opens and the compressed air passes to the air bottle. Clearance air left at the top of the stroke must re-expand before a fresh charge can enter the cylinder.

Single stage Compression

1. It suffers the following drawbacks:
2. The size of the cylinder will be too large.
3. Due to compression, there is a rise in temperature of the air. It is difficult to reject heat from the air in the small time available during compression.
4. Sometimes, the temperature of air, at the end of compression, is too high. It may heat up the cylinder head or burn the lubricating oil.

In order to overcome the above mentioned difficulties, two or more cylinders are provided in series with inter-cooling arrangement between them. Such an arrangement is known as multistage compression.

Advantages of Multistage Compression

Following are the main advantages of multistage compression over single stage compression.

1. The work done per kg of air is reduced in multistage compression with intercooler as compared to single stage compression for the same delivery pressure.
2. It improves the volumetric efficiency for the given pressure ratio.
3. The sizes of the two cylinders (i.e. high pressure and low pressure) may be adjusted to suit the volume and pressure of the air.
4. It reduces the leakage loss considerably.
5. It gives more uniform torque, and hence a smaller size flywheel is required.
6. It provides effective lubrication because of lower temperature range.
7. It reduces the cost of compressor.
8. In commercial shipping industry, highest air pressure requirement is about 30 bar (M/E starting air).

It is a standard practice to use two stage reciprocating compressors on board ship to satisfy all compressed air requirements on board ship.

Operation and maintenance:

Compressors must always be started in the unloaded condition otherwise pressures build up rapidly producing very high starting torques. During running there is an accumulation of oil carried over from the cylinders and water from moisture, precipitated in the coolers.

The emulsion is collected in separators at cooler outlets and these must be drained off regularly, to reduce carry over. This is extremely important, first to prevent any large quantity of water and oil emulsion reaching a subsequent compression stage and causing damage to a further stage and secondly to reduce the amount carried over to the air receivers and starting air lines.

Moisture in air receivers can give rise to corrosion and despite the proper operation of compressor cooler drains, a large amount tends to collect, particularly in humid conditions or wet engine rooms. It is good practice to check air reservoir drains regularly to access the quantity of liquid present.

In extreme conditions, drains may have to be used daily to remove accumulated emulsion. This is very important if air for control systems is derived from the main receivers, to prevent problems with the reducing valve, moisture traps and filters.

Moisture traps for the control air system also require regular checking and possibly daily draining. A compressor is unloaded before stopping by opening first and second stage drains.

Effect of Choked Inlet Filter

Safety Devices:

1. Relief valve fitted to every stage
2. Fusible plug   (melts at 120°C)
3. Bursting disc or relief valve for cooling jacket
4. Alarms and cut outs for:
• High air temperature
• High cooling temperature
• Low lubricating oil pressure

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Author Amit                                                                            Article Requested By: Abhishek Verma

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Marine I.C engines Cycles and Timing diagrams.

The IC engines or “Diesel Engines” as they are referred to, are named after Rudolf Diesel, who invented these engines in 1893.

The first working engine was built in 1897. It weighed 5 tonnes and produced 20 hp at 172 rpm with an efficiency of about 26%. (Modern low speed diesel engines can have thermal efficiency that exceeds 50%).

In 1912 the first ocean going ship to have installed diesel engines was “Selandia” which was powered by twin, 1010 BHP B&W, 8 cylinder, 4 stroke engines, which were direct coupled.

The worlds largest diesel engine is Wartsilla Sulzer RTA96-C common rail marine diesel of about 113,210 hp @ 102 rpm output.

Modern Diesel engines operate on Dual Combustion Cycle which is a combination of Constant Volume (Otto) and Constant Pressure (Diesel) cycle is shown in the sketch.
The area of the diagram represents the work done on the piston during one cycle.
Starting from point C, the air is compressed adiabatically to a point D. Fuel injection begins at D, and heat is added to the cycle partly at constant volume (shown by vertical line DP), and partly at constant pressure ( shown by horizontal line PE). At point E expansion begins. This proceeds adiabatically to point F when the heat is rejected to exhaust at constant volume.
The exhaust and suction strokes are shown by horizontal line at C, and this as no effect on cycle.

This theoretical cycle deviates from practical cycle due to :

• Unavoidable thermal, hydraulic and mechanical losses.
• The manner in which, and the rate at which, heat is added to the compressed air is a complex function of fuel injection equipment.
• The compression and expansion strokes are not truly adiabatic. Heat is lost to the cylinder walls to an extent which is influenced by coolant temperature and by design of the heat paths to the coolant.
• Some of the useful work is expended to perform the induction and exhaust process. Greater losses are involved during exhaust as the unused energy is lost by compressed hot gases when the exhaust valve opens before the piston arrives.
• Action arising out of reciprocating, rotating and rubbing components also contribute to losses.
• Some energy is used to drive auxiliaries like lub oil pumps, jacket water pumps, etc.

 4 STROKE CYCLE

Nickolaus Otto invented the 4 stroke cycle in 1862.In 1893 Rudolph Diesel invented the compression ignition engine, now named after him.

The four stroke cycle is so called because it takes four strokes of the piston to complete the process needed to convert the energy in the fuel to work. Because the engine is reciprocating, this means the piston has to move up and down the cylinder twice, and therefore the crankshaft must revolve twice.

The four stroke of the piston are known as induction stroke, compression stroke, power stroke and the exhaust stroke.

Induction Stroke : The crankshaft is rotating clockwise and the piston is moving down the cylinder. The inlet valve is open and the fresh charge of air is drawn inside the cylinder at a pressure existing in the intake manifold. The inlet valve closes at the end of the stroke.

Compression Stroke : Both inlet and exhaust valves are closed and air is compressed by the piston as it moves up in the cylinder. Because energy is transferred into air the pressure and temperature of the air increases.

Power stroke : Just before piston is reaching the TDC ( top dead centre) fuel is injected into the cylinder by fuel injector. Fuel is ignited by the high temperature produced at the end of compression and the expanding gases forces the piston down the cylinder. The gases expand until at the end of the stroke when exhaust valve opens.

Exhaust Stroke : As the piston is approaching the BDC ( bottom dead centre) , the exhaust valve opens. As the piston moves up the cylinder the exhaust gases are expelled from the cylinder.

As the piston approached the TDC again the inlet valve starts to open and the cycle repeats itself.

Timing Diagram : Position of crank at which each operation during the cycle is commenced and completed.

1-2       : Completion of aspiration. Air inlet valve closed. 145-155 BTDC
2-3       :  Compression.
3-4-5    : Fuel Injection. From 10-20 BTDC to 10-20 ATDC
5-6       : Expansion.
6-7-8    : Exhaust. Exhaust valve opens 120-150 ATDC
8-9-10  : Overlap. Air inlet valve opens 70-80 BTDC
10-1     : Aspiration. Exhaust valve closed. 50-60 ATDC
1-etc     : Aspiration continues for next cycle.

4 and 9 are TDC positions and 1 and 7 are BDC positions.

2 STROKE CYCLE

Two stroke cycle was invented by Sir Dugald Clerk in 1881.
The two stroke cycle is so called because it takes two strokes of the piston to complete the process needed to convert the energy in the fuel to work.

The working of two stroke cycle engine differs from that of four stroke cycle engine because of complete absence of two distinct strokes of induction and exhaust. A part of each of compression and expansion strokes in a two stroke engine is utilized for the process of induction and exhaust.

Induction and Compression Stroke : Air is admitted as the scavenge ports are uncovered by the piston. The cylinder is filled up with the charge of fresh air at the beginning of compression. The air is compressed by the piston moving upwards in the cylinder and pressure and temperature of air is increased.

Expansion and Exhaust : Fuel is injected towards end of compression and heat is added to the mass of air and fuel starts to burn as piston is passing over TDC. The expanding gases push the piston downwards.
 At the end of the expansion stroke the exhaust is opened first , the pressure drops below the scavenge manifold pressure and fresh air is admitted through scavenge ports. The cylinder is cleansed of residual products of combustion by incoming air and the cycle is repeated.

1-2    : Completion of scavenge. Scavenge ports close 130-150 BTDC
2-3     : Post scavenge . Exhaust closes 110-150 BTDC
3-4     : Compression
4-5-6    : Fuel injection. Begins at 10-20 BTDC and ends at 10-20 ATDC
6-7    : Expansion
7-8    : Exhaust blowdown. Exhaust opens 110-120 ATDC
8-1    : Scavenge. Scavenge ports open 130-150 ATDC
1-etc    : Scavenging then continues for next cycle.

Position 1 represents BDC and position 5 represents TDC

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Author ABHINAV KAUSHAL

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Irregularities in Diagram | Indicator diagram Part-2

EARLY  IGNITION :

IT WILL CAUSE AN ABNORMALLY HIGH PEAK PRESSURE

POWER GENERATION AND SFC WILL IMPROVE AND DROP IN EXHAUST TEMPERATURE.

A HEAVY SHOCK LOAD WILL BE TRANSMITTED TO THE RUNNING GEAR AND BEARINGS WITH A KNOCKING SOUND.

CAUSES OF EARLY IGNITION MAY BE INCORRECT FUEL PUMP TIMING, BROKEN OR WRONGLY SET INJECTOR SPRINGS, INCORRECT FUEL CONDITION, OVERHEATED PART SUCH AS HOT PISTON.
THE SHOCK LOAD AND VIBRATIONS MAY CAUSE DAMAGE

LATE IGNITION :

CAUSES A LOW PEAK PRESSURE WHICH OCCURS WELL AFTER TOP CENTRE OF THE PISTON.

LOSS OF POWER AS FUEL IS NOT BURNED COMPLETELY.

COMBUSTION INCOMPLETE, LOSS OF ENERGY, HIGH EXHAUST TEMP AND SMOKE.

CAUSES- EXCESSIVE INJECTOR SPRING SETTING, POOR ATOMIZATION, HIGH VISCOSITY OR POOR QUALITY OF FUEL, FUEL PUMP LEAKING OR INCORRECTLY TIME, LOW COMPRESSION, INSUFFICIENT SUPPLY OF COMBUSTION AIR, OR UNDER COOLING OF PARTS WITHIN THE CYLINDER.

LOW COMPRESSION

POWER GENERATION WILL BE LESS.

FAULTY COMBUSTION, HIGH EXHAUST TEMPERATURE.

LOW PRESSURES DUE TO LACK OF AIR (TURBOCHARGER FAULTS, CHOKED SCAVENGE PORTS OR INLET VALVE, CHOKED AIR COOLER, CHOKED AIR FILTER).

BLOW PAST FROM WORN OUT OR BROKEN PISTON RINGS, WORN LINER, LACK OF CYLINDER LUBRICATION, STICKING PISTON RINGS.
BLOWPAST CAN CAUSE SCAVENGE FIRE IN 2 STROKE ENGINES AND THERE IS A RISK OF CRANKCASE EXPLOSION IN 4 STROKE ENGINES.

 AFTERBURNING

SLOW OR LATE COMBUSTION OF FUEL DURING EXPANSION STROKE OF PISTON.

SHOWN BY A RISE IN THE EXPANSION LINE DURING THE LATER PART OF THE STROKE.

EXHAUST TEMP AND PRESSURE WILL BE HIGH, WITH BURNING FUEL AND CARBON PASSING TO EXHAUST.

THIS MAY DAMAGE THE EXHAUST VALVES AND FOUL THE EXHAUST SYSTEM, CAUSING TURBOCHARGER SURGING OR UPTAKE FIRES.

HIGH TEMP OF LINER CAUSES DETERIORATION OF LUBRICATION AND DAMAGE TO LINER AND PISTON RINGS.

MAY CAUSE BURNING OF PISTON CROWN.
CAUSES OF AFTER BURNING ARE : INCORRECT FUEL PUMP TIMING, FAULTY FUEL INJECTOR, FO TEMP TOO LOW, LACK OF SCAVENGE AIR, POOR COMPRESSION.

eAnd  f. CHOCKED INTAKE

LIGHT SPRING DIAG WILL SHOW LOW PRESSURE AT INTAKE
LOSS OF POWER,LOW COMPRESSION,HIGH EXHAUST TEMP.
INCOMPLETE COMBUSTION DUE TO INSUFFICIENT AIR.
LACK OF AIR  DUE TO TURBOCHARGER FAULTS, CHOKED SCAVENGE PORTS OR INLET VALVE, CHOKED AIR COOLER, CHOKED AIR FILTER.


LEAKING FUEL INJECTOR :

 
Leaking Fuel Injector will be detected by loss in power, smoke at Exhaust, high exhaust temperature.
Indicator diagram will show fluctuation of pressure during the expansion process due to secondary burning of fuel leaking from injector.
It can lead to loss in power,  there may also be a knock or pressure wave.
There can be blow back of hot gases into injector tip, resulting in forming carbon and choking of the atomizer.
To reduce this possibility proper maintenance on fuel pumps and injectors should be carried out. Fuel must be purified, filtered and maintained at correct temperature.


EXHAUST VALVE OPENING

Can be checked by means of light spring indicator card diagram.
This will not give accurate timing check but can be compared with normal diagram or one taken during sea trials.
Early opening :Will cause a loss in power since pressure is released too soon. Results in high exhaust temperature with risk of overheating.
Late Opening  : Will reduce the scavenge efficiency by reducing blow down.
Chocked Exhaust – Prevents blow down and recharge of the cylinder with air. Will result in Loss of power, High Exhaust temperature, smoke and surging of turbocharger.

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Author AMIT

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Indicator Diagram and Indicating Instrument | Explained

AN INDICATOR DIAGRAM IS A PRESSURE/VOLUME DIAGRAM TAKEN FROM THE PRESSURES IN THE CYLINDER OF A WORKING ENGINE DURING ONE COMPLETE ENGINE CYCLE.

POWER GENERATED IN ENGINE CYLINDER CAN BE EVALUATED BY INDICATOR DIAGRAM. IT INDICATES THE PERFORMANCE OF THE ENGINE AND IRREGULARITIES, IF ANY, CAN BE DETECTED.

INDICATOR DIAGRAMS ARE RECORDED BY INDICATOR INSTRUMENT. THREE TYPES OF INDICATOR DIAGRAMS CAN BE OBTAINED. THEY ARE POWER CARD, DRAW CARD AND LIGHT SPRING DIAGRAM.

INDICATOR INSTRUMENT

An engine indicator is shown in Figure. It is used for recording the engine cycle. It consists of a small piston of known size which operates in a cylinder against a specially calibrated spring. The engine cylinder pressure is applied against indicator piston which reciprocates an amount depending on calibration of spring. The vertical motion of piston is recorded by a stylus on a recording device on a paper wrapped around the oscillating drum.

The drum oscillates i.e swing from front to back due to the cord pull. The cord is moved by a reciprocating (up and down) mechanism which is proportional to the movements of the piston in the cylinder. At different points of the stroke the stylus draws out indicator diagram representing the gas pressure on the engine piston , and the area of the indicator diagram represents the power developed in the cylinder.

PEAK PRESSURE INDICATOR

This instrument will measure the maximum pressure within the cylinder. It does not identify the position within the cycle at which it occurs. It is similar in principle to the engine indicator but uses beam type spring which is less susceptible to vibration.

The indicator is attached to indicator cock and when the cock is opened the pressure raises the indicator piston which records maximum beam deflection on a dial gauge.

If the fuel is shut off from that cylinder, the maximum compression pressure can be recorded and the “ignition jump” can be calculated (difference between peak pressure and compression pressure).

INDICATOR DIAGRAMS

1. POWER CARD IS TAKEN WITH THE INDICATOR DRUM ROTATION IN PHASE WITH PISTON MOVEMENT.

THE AREA WITHIN THIS DIAGRAM REPRESENTS THE WORK DONE DURING THE CYCLE TO SCALE.

THIS IS USED TO CALCULATE THE POWER PRODUCED OR THE MEAN INDICATED PRESSURE (MIP) FOR THE CYLINDER.

IRREGULARITIES IN THE SHAPE OF THE DIAGRAM WILL SHOW OPERATIONAL FAULTS.

MAXIMUM OR PEAK PRESSURE MAY BE MEASURED TO SCALE BETWEEN THE ATMOSPHERIC LINE AND THE HIGHEST POINT ON THE DIAGRAM.

2. DRAW CARD OR OUT OF PHASE DIAGRAM-

IT IS TAKEN IN A SIMILAR MANNER TO THE POWER CARD BUT THE INDICATOR DRUM 90 DEG OUT OF PHASE WITH PISTON STROKE.

THIS ILLUSTRATES MORE CLEARLY THE PRESSURE CHANGES DURING FUEL COMBUSTION OR INJECTOR FAULTS MAY BE DETECTED FROM ITS SHAPE.
COMPRESSION PRESSURE AND MAX PRESSURE CAN BE MEASURED FROM THIS DIAGRAM. 

3. LIGHT OR WEAK SPRING DIAGRAM

IT IS AGAIN SIMILAR TO THE POWER CARD AND IN PHASE WITH THE ENGINE, BUT TAKEN WITH A LIGHT COMPRESSION SPRING FITTED TO THE INDICATOR SHOWING PRESSURE CHANGES DURING EXHAUST AND SCAVENGE TO AN ENLARGED SCALE.

IT CAN BE USED TO DETECT FAULTS IN THESE OPERATIONS.

THE PRESSURE BEING LOW, LIGHT SPRING IS USED. THE EXHAUST VALVE OPENS AT ‘A’ AND THERE IS INSTANTANEOUS DROP IN PRESSURE. PRESSURE DROPS TO Pb TILL SCAVENGE PORTS OPEN (AT ‘B’) AND SCAVENGING TAKES PLACE. THE DIAGRAM SHOWS SCAVENGE AIR PRESSURE Pa AT CYLINDER INTAKE.

#For Irregularities in diagrams and counter actions Refer to our next Post.

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Author AMIT

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