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Air Compressor

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Effects of leaking valves in Air Compressor:

First Stage Suction:

• Reduce air delivery
• Reduce 2nd stage suction pressure
• Unload the compressor
• Increase running time.

First Stage Delivery:

1. Reduce air delivery
2. Increase discharge temperature
3. Less air drawn in, due to high-pressure air leaking back into cylinder.

Second Stage Suction:

1. Reduce air delivery
2. High temperature & pressure in 2nd stage suction line
3. Increase running time

Second Stage Delivery:

1. Reduce air delivery
2. Increase suction pressure in 2nd stage
3. Increase delivery pressure from 1st stage.
4. Back pressure from air bottle.

How to check Air Compressor Efficiency?

1. Regular overhauling of valves done or not.
2. Check Air Bottle filling time.
3. Compare test results and records.

How to check Air Compressor capacity is sufficient?

• Total no. of Air Compressors must be sufficient to fill the empty Air Bottle to maximum pressure within 1 hour.
• Must be sufficient to start at least 12 times for Reversible Engine, and at least 6 times for Non-Reversible Engine.

Bursting Disc:

1. Fitted on the shell of Intercooler at waterside.
2. Relieves pressure if the tubes burst.
3. Rolled Copper Alloy and relief pressure is set while the disc is at softest condition.
4. Material tends to harden due to time and surrounding temperature, and set pressure also increased.
5. Bursting Disc needed to be annealed, to regain correct relief pressure.

Compound Valves, why used in Air Compressor?

1. Give large area of opening and small amount of valve lift.
2. Improve Volumetric Efficiency, as valve open and close in minimum time.
3. Reduce bumping clearance.
4. Reduce wear and tear.

LP Relief Valve opening:

Causes:

1. HP suction valve
2. Intercooler choked.
3. Relief valve, jammed by foreign particles, in open position.

HP Relief Valve opening:

Causes:

1. HP discharge valve, in closed position.
2. After cooler choked.
3. Relief valve, jammed by foreign particles, or spring sticking in open position.

(Relief Valves opening pressure are set at  ≯ 10% above stage pressure.)

Why Intercooler is fitted?

1. Reduce air temperature and volume, and increase air density for next stage.
2. Increase Compressor Capacity and Volumetric Efficiency.
3. Better lubrication for cylinder and rings.
4. Water and excess oil can be drained out, preventing fouling of Intercooler and pipes, Air Bottle corrosion, and starting airline explosion.
5. Work done is saved.
6. Metal stresses reduced, due to control of temperature.

Volumetric Efficiency:

VE  = (Volume of Air drawn into Cylinder) / (Stroke Volume of LP Piston)

and

VE = (Volume of Air discharged as ‘free air’) / (Stroke Volume of LP Piston)

Why Multistage Air Compressor is built?

1. To obtain near to ideal isothermal compression, compressor is to be constructed of multistage with inter-stage cooling.
2. Inter-stage cooling reduces air temperature and volume after 1st stage compression, thus increase mass of air for 2nd
3. Workdone is saved and air compressor efficiency increased.

Other advantages are:

1. Easy to get high final air pressure.
2. Easy to control air temperature.
3. Easy to maintain correct lubrication.
4. Better compressor balancing.
5. Reduction in size.
6. reduction in clearance volume loss.

Capacity of air compressor:

1. Capacity is checked upon how much filling time is lowered.
2. Compressor should have enough filling capacity so that sufficient stopping timeshould be provided between fillings.

• 12 consecutive starts in reversible engine  and
• 6 consecutive starts in non-reversible engine.

What is Free Air Delivery, FAD?

1. Capacity of Air Compressor is stated in terms of [ m³/ hr ].
2. Volume of air actually discharged in 1 hour, that would occupy if expanded down to atmospheric pressure and cooled to atmospheric temperature.

Safety devices on Main Air Compressor:

1. Bursting Disc on Intercooler: (At water side)
2. Bursting Disc and Fusible Plug (121°C) on Aftercooler
3. Relief valves on LP and HP stages. (Set to lift at 10% rise above normal stage pressure.)
4. Automatic Moisture Drain Valve.
5. Cooling water supply failure alarm.
6. Low LO pressure alarm.
7. Relief valve on crankcase LO pump.
8. Delivery air HT cut out and alarm on Aftercooler outlet. (Max. 93°C)

•   LP discharge pressure 4 bars and HP discharge pressure 30 bar
• Intercooler inlet air 130°C and intercooler outlet air 35°C
• Aftercooler inlet air 130°C and Aftercooler outlet air 35°C:

Intercooler is single pass type:  Aftercooler, double pass U-tube type:}

Uses of Compressed Air: 

• Engine Starting 20 to 25 bar
• Boiler Soot Blowing 20 to 25 bar
• General Service (Whistle, Pneumatic Tools, Lifeboat , service air for cleaning and Pilot ladder) 7 to 10 bar
• Instrumentation and Control 5 bar

Air Filter: 

1. Material: Felt, Metal gauze or Nylon strands
2. Removes contaminants from air. Dirt and dust act as abrasives and increase wear.
3. Contaminant deposits on valves can become hot and source of ignition.

Hazard of Dirty Filter: 

1. Very dirty filter or muffling a filter results in large pressure drop.
2. Air has to be compressed over higher range.
3. In extreme case, discharge air temperature may exceed flash point, or auto-ignition temperature resulting in an
4. As a safety against explosion, air temperature is limited to 93° Fusible Plug(121°C) or a High Temperature cut out (93°C) is provided on Compressor.

Pressure Test on Air Compressor:

• Cylinders, cylinder cover, Inter & After coolers are hydraulically tested to:

Air Side:  1.5 x max. Working Pressure.
Water Side: 4 bar or 1.5 x max. WP (whichever is greater)

Air receiver:

• Total capacity of air receivers must be sufficient to give at least 12 starts for reversible engine, and at least 6 starts for non-reversible engine, without refilling the receivers.
• There must be two identical main air receivers and one emergency bottle for every vessel.

 Mountings of Air receiver:  

1. Fusible plug: 

• composition – Bismuth 50%, Tin 30%, Lead 20%,
• Melting point: 220΄F (104.4΄C).    Fitted at the reservoir’s bottom or on reservoir at ship side, when relief valve (safety valve) is not directly fitted on the reservoir.

2. Atmospheric relief valve: provided for back-up of fusible plug. In case of ER fire when CO₂ flooding is required, this valve is opened before evacuating ER.
3. Spring loaded safety valve: setting pressure: 32 bar (for 30 bar working pressure), with ≯ 10% rise in accumulation of pressure. May be fitted directly or with extension.
4. Compensation ring: when a hole is cut or machined in pressure vessel, higher stresses will subject to the material around the hole, and to reduce this, compensation rings are fitted. It is a flange on which a valve or fitting is usually mounted.
5. Manual Drain valve or Automatic Drain valve.
6. Pressure gauges.
7. Access doors.
8. Main starting air valve, auxiliary starting air valve, filling valve, service air or whistle air valve.

Internal surface coating:

Graphite suspension in water, Linseed oil, Copal vanish or Epoxy coating having basic properties of anti-corrosive, anti-toxic or anti-oxidation.

Safety devices on Main Air Bottle:

1. Fusible plug.
2. Pressure Relief Valve
3. Atmospheric Relief Valve.
4. Low Air Pressure alarm.
5. Automatic or remote control Moisture Drain Valve.

how to power up from a dead ship/starting first time?

First Start Arrangement:

Emergency Air Compressor is:
a) Battery started b) Hand crank type
Note: [ If prime mover is motor, it is associated with Emergency Generator and not a first start arrangement]

1. Emergency air bottle is filled by Emergency hand air compressor.
2. During this time, one of the generators should be standing-by such as priming the LO, fuel oil, turning the flywheel, etc.
3. Start this generator and check running condition. If satisfactory, close the Main Circuit Breaker of concerned Switchboard.
4. Run cooling service pump for generator.
5. Run main Air Compressor and fill-up Air Reservoir.
6. Prepare the remaining generator, and start, equalized and load-shared.
7. Auxiliary boiler should be started.
8. ME warmed-up for operation.

Marine Air Compressor Used on a Ship

Different Parts of a Marine Air Compressor Used on a Ship

A marine air compressor provides compressed air for various purposes on a ship. An air compressor is one of the most important equipment's on a ship which needs special care and routine maintenance. In this article we will know the basics of the marine air compressor by first getting familiarized with the different parts of the same.

The main parts of the marine air compressor are:

1) Cylinder liner:

It is made of graded cast iron and is accompanied with water jacket around it to absorb heat produced during compression process. It is designed so as to give a streamline passage to the pressurized air resulting in minimum pressure drop.

2) Piston:

For a non-lubricating type compressor, light weight aluminum alloy piston are used and for lubricating type graded, cast iron piston are used with piston rings for sealing and scrapping off excess oil.

3) Piston Rod:

In high capacity compressor which is normally big in size, piston is attached to piston rod made up of alloy steel. They are fitted with anti friction packing ring to avoid chances of compressed air leakage.

4) Connecting rod:

Connecting rod plays its role to minimize thrust to the bearing surface. It is made up of forged alloy steel.

5) Big end bearing and Main bearing:

They are constructed to give rigidity to the running rotational mechanism. They are made up of copper lead alloy and have a long operational life if proper lube oil and lubrication is provided.

6) Crank shaft:

It is a one piece designed part, using counterweights for dynamic balancing during high speed of rotation to avoid twisting due to torsion forces. Connecting rod big end bearing and main bearing are connected to crank shaft at crank pin and journal pin which are polished to ensure long working life of bearings.

7) Frame and crankcase:

Normally they have rectangular shape and accommodate all the moving parts and that’s why are made up of rigid cast iron. Main bearing housing is fitted on a bore in crank case and is made with highest precision to avoid eccentricity or misalignment.

8 ) Oil pump:

A lubricating oil pump is fitted to supply lube oil to all the bearings, which can be chain or gear driven, through crank shaft. Pressure of oil can be regulated by means of regulating screw provided in the pump. A filter in the inlet of the pump is also attached to supply clean and particle free oil to the bearings.

9) Water pump:

Some compressor may have attached water cooling pump driven by crankshaft through chain or gear. Some system does not use attached pump as they use water supply from main or auxiliary system for cooling.

10) Suction and Discharge valve:

These are multi-plate valves made up of stainless steel and are used to suck and to discharge air from one stage to another and to the air bottle. Proper assembling of valves is very important for efficient operation of the compressor.

11) Suction Filter:

It is a air filter made up of copper or soft steel with a paper material to absorb oil and wire mesh to avoid any metal or dust particle to go inside compression chamber.

12) Inter-coolers:

Inter-coolers are normally fitted in between two stages to cool down the air temperature and to increase the volumetric efficiency of compressor. Some compressor have inbuilt attached copper tubes for cooling and some have outside assembly of copper tube inter-coolers.

13) Driving Motor:

An Electrical motor is attached to the compressor for making it operational and is connected to the compressor through the flywheel.

There are the main parts of a marine air compressor. The parts may vary according to the requirement of the system or ship.

ROLE OF COMPRESSED AIR IN ENGINE ROOM

ROLE OF COMPRESSED AIR IN ENGINE ROOM

As you know that a ship is a mobile power plant or a moving mini-city. Air comp these moving giants have a pre-designed and erected compressed air system, which facilitates many activities onboard a ship.

There are mostly 4 to 8 and sometimes 10 air compressors found onboard. These air compressors take suction from the engine room atmosphere which is already under a slight positive pressure.

These air compressors compress the air in stages and fill up the huge air bottles, which acts as accumulator. They compress usually up to 30 bar and keep the air bottles filled up all the time.

The number of air bottles and its volume (capacity) depends on the power (size) of the main propulsion engine. Air compressors are of varying capacity and used as per the requirement onboard.

Role of Compressed Air on a Ship control air from reducer and de-humidifier the role of compressed air onboard is of a very vast nature.

Every ship will have a “DEAD-START” or “The FIRST START” arrangement. This is nothing but when the ship is totally “Dead” i.e, with out any power and no machineries running and no compressed air in the air bottle to start the generator engine(auxiliary diesel engine), then a provision is given for every ship to get the air bottle filled up to bring back the ship to safe, normal, working condition. This is provided either by a “Emergency Air Compressor” driven by a small diesel engine or electric motor getting its power supply from “Emergency Generator”.

The various uses of compressed air onboard ship are listed below:
1. To start main propulsion engine
2. To start Auxiliary diesel engine (power generation)
3. To blow ship’s whistle
4. For Engine Room general service and cleaning,

    5. For the operation of pneumatic tools
6. For deck services, to carry out chipping.
7. For Automation & Instrumentation of various machineries,
8. For fresh & sea water hydrophores,
9. Fire alarms & operation of Quick closing Valves,
10. For Soot Blowing Exhaust Gas Economizer
11. And more!

Compressed Air System Layout

The compressed air system onboard typically has a set of 4 to 6 compressors, out of which 3 will be main are compressors, 2 service air compressors & one topping up air compressor. The Emergency air compressor is not to be counted as normally used machinery.

General layout Main air compressors: These are used only when a ship enters or leaves a port, mostly during maneuvering only. They are of higher capacity of all. During maneuvering, these compressors when put in use, fills up the air bottle faster than other compressors, thus enabling the ship to berth herself in the port.

Topping-Up air compressors: These are comparatively of lesser capacity than the main air compressors. They are just used when the ship is sailing in the mid-seas where the consumption of air is very less (only for engine & deck services).

Service Air compressors: These are general service air compressors, which are used either for engine or deck service. They are of lesser capacity and they are designed for their pure strict oil free air quality. These compressors are used for pressing up the control air bottle and thus the control air of high purity is used for various automation purposes in the engine room and the pump room for oil tankers.

There may be set of main air bottles, service air bottle and a control air bottle for their respective purposes. The system also incorporates many pressure reducing valves and driers (de-humidifiers) to get rid of the moisture present in the compressed air.

Starting of an Auxiliary Diesel Engine

The auxiliary diesel engine is mostly started with the help of compressed air, depending upon the size of the engine. Other means of starting includes Electric start (battery) and air motor (engaged in the flywheel). The most common method is the use of compressed air.

Referring to the above starting diagram of an auxiliary engine, the” main air” from the main air bottle arrives at the air starting valve. There is a tapping from the main air starting line, “pilot air” going to the starting air distributor. When the engine rotates, the camshaft also rotates which in turn rotates “the starting air distributor cam”.

This cam is designed as per the firing order of the engine such that, the distributor rotates and lets the pilot air to the particular unit. The pilot air reaches on top of the air starting valve, opening it, in turn making the long awaited main air to let inside the combustion chamber.

The main air which is at 30 bar, pushes the piston down making the crankshaft to rotate. This leads to continuous rotation of the crankshaft making the engine to achieve the minimum r.p.m at which firing of the injected fuel takes place. When the engine picks up on fuel, the air is cut off and drained. Thus the auxiliary diesel engine is started with the help of compressed air.