What is emergency air compressor ?

• It is a small compressor independently driven by a prime mover having power supply from emergency switch board. They are also driven by diesel engines.
• It must be fitted to press up the emergency air bottle and to start auxiliary engine of a dead ship.
• It has no connection between the main air bottle.

What are the safety devices in air compressors on ships ?

1. Bursting discs are fitted on the cooler shells (At water side).
2. Relief valves are fitted to discharge side for every stages.
3. Moisture drain valve (unloader) are fitted at each cooler side.
4. Cooling water failure alarm.
5. Low L. O pressure alarm and trip.
6. Delivery air high temperature alarm on after cooler outlet (Max 93° C)

 What are the normal parameters of air compressor ?

• LP discharge pressure: 4 bars
• HP discharge pressure: 30 bars.
• Intercooler inlet air temperature: 130 ° C
• Intercooler outlet air temperature: 35 °  C
• After cooler inlet air temperature: 130 ° C
• After cooler outlet air temperature: 35 °  C

Type of intercooler and after cooler ?

• Intercooler is single pass type
• After cooler is double pass U-tube type

Purpose of unloder valve (moisture drain valve) in air compressor ?

• At starting this valve must be opened, this reduced the starting torque for the machine and clear out any accumulated moisture and oil in the system.

What would be effect of suction valves of an air compressor having too much lift ?

• The valve will be late in closing and this would reduce the volumetric efficiency of the machine.
• The valve experience greater force and therefore are more liable to break.

Effects of reduced volumetric efficiency of air compressor ?

1. Greater bumping clearance.
2. Sluggish opening and closing of suction and delivery valves.
3. Insufficient cooling water that effect of high air temperature.
4. Dirty or partially chocked suction air fitter.

Difference between relief valve, bursting disc and fusible plug ?

Pressure relief valve

• Excess pressure is released by opening the valve.
• It opens at 10% over working pressure.
• Valve lift is proportional to excess pressure build up.
• Valve setting pressure can be altered by spring tension.

Bursting disc

• Pressure is released by bursting the disc.
• It permanently damaged.
• It burst at setting pressure.
• Setting pressure cannot be altered in place.

Fusible plug

• When the air temperature from compressor is high (above 105 ° C) pressure is released by melting (fusing) the metal.
• It cannot be used next time. ( permanent damage)
• Release all content or pressure to empty.

 Why multistage compressors are mostly used than single stage compressor ?

1. More stages are needed to increase the required final pressure.
2. Easier to control the air temperature.
3. Reducing in air compressor size.
4. Lubrication problem does not exit.
5. Reduced the thermal stress.
6. Lower work done to compressing air.
7. Improve compressor efficiency

Advantages of inter cooling of air compressor ?

• To avoid excessive temperature rise associated with higher compression ratios, and to approach isothermal compression.
• Saving in power.
• Volumetric efficiency is increased.
• Reduced the volume of air delivered and also reduced the compressor size.
• It can reduce the air temperature.
• Due to less temperature suction & delivery valves remain cleaner without being fouled with carbonized oil.
• It can avoid a danger of an explosion takes place in compressor cylinder.
• It allows good lubrication of the compressor piston.
• Moisture separation is easier through inter cooler drains.
• It also enables to deal with a greater weight of air for the same energy expended.

Why intercooler is fitted in main air compressor on ships ?

1. Reduced air temperature, volume and increased air density for next stage
2. So increased volumetric efficiency and compressor efficiency.
3. Due to reduced temperature give better lubrication for cylinder and piston rings
4. Drain are fitted from which water and excessive oil can be drained out, to prevent air bottle corrosion and starting air explosion and fouling of inter coolers and pipe.
5. Save the work done.

Advantages of after cooler in air compressor ?

• To reduce final discharge air temperature thus air bottle size can be reduced.
• To reduce air volume after it has been compressed to the final pressure.
• So greater amount of air could be stored in air bottle.
• Increase volumetric efficiency

 How to start the air compressor manually on ships ?

1. Change the switch to manual position on the switch board.
2. Check the L.O sump level and condition.
3. Open the moisture drain valve.
4. Open the compressor discharge valve & charging valve of air bottle.
5. Open cooling water system valves.
6. Turn the compressor flywheel by hand (one turn).
7. Start the motor, after draining the moisture shut the drain valve.
8. Check the motor ampere consumed.
9. Check the pressure gauge readings.
10. Frequently drain the moisture.
11. When charging full, open drain valve and stop the compressor

How do you check compressor efficiency during running ?

1. Checked by  filling time with the previous record and also check the first stage discharge pressure. If compressor efficiency is lower, compressor will run longer and compressor temperature will rise.
2. First stage and second stage pressure gauge must be correct and stable.
3. No escape of air from suction filter.
4. Intercooler and after cooler outlet air temperature should not be high.
5. If open drain valve nothing can be found.
6. Low L.O consumption.
7. Oily air mixture must not blow out from breather pipe.

Safety devices on main air bottle ?

1. Pressure relief valve.
2. Fusible plug.(if safety valve is not directly fitted on the bottle)
3. Pressure gauge.
4. Low air pressure alarm.
5. Moisture drains valve.

Where is the fusible plug fitted and its purpose ?

• Fusible plug is fitted under side of the pipe between relief valve and air bottle.
• It is fitted to release the compressed air in the event of abnormally high compressed air temperature.
• Fusible plug melt at 105 °C and release all content of air.
• It is made by 50% bismut, 30% tin, 20% lead.
• A fusible plug is fitted after the second stage cooler to limit the delivered air temperature and thus protect the compressed air reservoirs and pipe work.

Can you fit the relief valve at the space of bursting disc and why ?

• At the compressor, relief valve can not be fitted at the bursting disc, because of their difference operation.
• Bursting disc is fitted for totally release and stop operation circuit.
• Release valve opens excess pressure at compressor running and reset when pressure reduce or when the compressor is stopped. Thus cooling water can enter to compressed air space; it can cause water hammer when the next start of compressor.

 

Reference

Oral Guide by – MIN ZAR TAR