1.What is emergency air compressor ?
It is a small compressor independently driven by diesel engine or hand operated.
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.
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| Two Stage Air Compressor |
2.Air Compressor Safety Device.
>Bursting discs are fitted on the cooler shells (At water side).>Relief valves are fitted to discharge side for every stages.
>Moisture drain valve (unloader) are fitted at each cooler side
>Cooling water failure alarm.
>Low L. O pressure alarm.
> Delivery air high temperature alarm on after cooler outlet (Max 93°C)
LP discharge pressure 4 bars HP discharge pressure 30 bars.
Intercooler inlet air 130°C Intercooler outlet air 35 °C
After cooler inlet air 130°C After cooler outlet air 35°C
Intercooler is single pass type After cooler, double pass U-tube type.
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| Three Stage Air Compressor |
3. Purpose of unloder valve (moisture drain valve) ?
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.
4. What would be affect 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 too much lift reach at the end of their travel will grater force and therefore are morel iable to break.
5. Affects of reduced volumetric efficiency of air compressor ?
1.Grater 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.
6. Different between relief valve and bursting disc ?
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:
1. Pressure is released by bursting the disc.
2. It permanently damaged.
3.It burst at setting pressure.
4.Setting pressure cannot be altered in place.
Fusible plug:
- When the temperature high (above 105 °C) Pressure is released by melting (fusing) the metal.
- It cannot be used next time. ( permanently damage)
- Release all content or pressure to empty.
7. Why multistage compressors are mostly used than single stage compressor ?
@ More stages are needed to increase the required final pressure.
@ Easier to control the air temperature.
@ Reducing in air compressor size.
@ Lubrication problem does not exit.
@ Reduced the thermal stress.
@ Lower work done to compressing air.
@ Improve compressor efficiency
8. 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 wt of air for the same energy expended.
Why intercooler will fitted in Main air compressor?
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 purpose
•To reduce final discharge air temperature thus air bottle seize can be reduce.
•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
9. How to start the air compressor manually ?
@ Change the switch to manual position on the switch board.
@ Check the L.O sump level and condition.
@ Open the moisture drain valve.
@ Open the compressor discharge valve & charging valve of air bottle.
@ Open cooling water system valves.
@ Turn the compressor flywheel by hand (one turn).
@ Start the motor, after draining the moisture shut the drain valve.
@ Check the motor ampere consumed.
@ Check the pressure gauge readings.
@ Frequently drain the moisture.
@ When charging full, open drain valve and stop the compressor
10. How do you check compressor efficiency @ running ?
▪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.
▪First stage and second stage pressure gauge must be correct and stabled
▪No escape air from suction filter
▪Intercooler and after cooler outlet air temperature should not be high
▪If open drain valve nothing can found
▪Low L.O consumption
▪Oily air mixture must not blow out from breather pipe.
11. Safety devices on main air bottle ?
@ Pressure relief valve.
@ Fusible plug.(if safety valve is not directly fitted on the bottle)
@ Pressure gauge.
@ Low air pressure alarm.
@ Moisture drains valve.
12. Where is the fusible plug fitted ? 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.
13. 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 reseat at 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.
14. What is volumetric efficiency ?
It is the ratio of the actual volume of air drawn in each suction stroke to the stoke volume.
Volumetric efficiency =
Actual volume of air drawn in suction stoke/Volume of air for stoke volume.
FACTORS AFFECTING VOLUMETRIC EFFICIENCY
1) The bumping clearance (the larger the bumping clearance the less air is discharged per stroke)
2) Sluggish opening and closing of suction and delivery valves
3) Leakage past compressor piston rings.
4) L.P inlet air temperature too high
5) L.P inlet filter dirty and choked
6) Inlet cooling water temperature too high
7) Insufficient cooling water owing to fouling of coolers
15. Difference between intercooler and after cooler ?
Intercooler
@ Fitted intermediate stages.
@ Saving in power.
@ Increase volumetric efficiency.
@ Reduce temperature for next stage and it can avoid a danger of explosion in compressor cylinder.
@ Can provide good lubrication of the compressor piston.
@ No carbonized material form of discharge valve.
@ Moisture separation is easier through intercooler drain.
After cooler
@ Fitted at the final stage discharge side.
@ Reduce final discharge air temperature to room temperature.
@ Increase volumetric efficiency.
@ Air bottle size is smaller.
@ Moisture separation is easier through after cooler drain.
16. Why need bumping clearance ?
This is a clearance must be provided between the piston top and the cylinder head when the piston is top dead centre. it must be kept as small as possible for achieving the best compressor efficiency.
@ To prevent mechanical damage to the compressor.
@ To provide for thermal expansion and
@ To provide necessary space for valve operation.
17. How to adjust the bumping clearance ?
It can be adjusted by two ways.
@ By altering the head gaskets (cover joint ) thickness,
@ By adding and subtracting shims between the connecting rod food and bottom and bearing.
18. Main air compressor bumping clearance taking procedure ?
▪Stop the compressor and (lock off) take out fuse.
▪Drain the cooling water.
▪Remove the cylinder cover.
▪Clean the cylinder head face and piston crown.
▪Place the lead wire ball on the top face of the piston. That is larger size than expected by
clearance.
▪Cylinder cover is placed on the cylinder with correct joint thickness and tightens the head bolts.
▪Turn the compressor slowly by hand over top centre so that lead wire ball is pressed.
▪Then remove cylinder cover again.
▪Remove the compressed lead wire ball.
▪Take the measurement of lead wire thickness by using micrometer.
19. At what condition fusible plug is fitted main air bottle ?
Fusible plug is fitted at main air bottle, when relief valve is fitted indirectly to the air bottle. It can cause any obstruction by human error.
20. Purpose of bursting disc? Where is fitted ?
It is fitted on each cooler shell of air compressor, to give ample relief of pressure when cooler tubes burst.
21. Why bursting disc need to be annealed ?
Due to heating and time expanded bursting disc is harden so to get back the normal relief pressure . It must be annealed.
22. First start arrangement ?
- Firstly start the emergency air compressor to fill up the air to the emergency air bottle up to
desired pressure obtained.
- Drain out oil and water.
- Open emergency air bottle outlet valve and pressed up desired pressure.
- Then start the generator with correct procedure.
- Close the breaker and switch on the lighting.
- Start the required machinery ( especially G/E FO booster pump, S, W cooling pump, E/ R
- blower).
- Start the main air compressor and press up to main air bottle.
- After that, made M/E warming by using generator jacket water and start the L.O pump.
23. Why provide air compressor on board ?
(1) Starting Diesel engines,
(2) Instrumentation and control systems,
(3) Various pneumatic tools and cleaning equipments,
(4) Boiler soot blowing and
(5) Ship's whistle.
24. Why need minimum lubrication ?
@ To prevent carbonizing of the valves
@ To avoid loss of compressor efficiency due to sluggish action of valve
@ To avoid loss of cooler efficiency due to deposition of oil on the cooling surface
@ To prevent air line explosion, to prevent air bottle corrosion
25. VALVES LEAKING
Valves can leak and finally fail. Valves may fail owing to wear and fatigue, insufficient
of over-lubrication, presence of foreign particles, oil decomposition and excess valve lift.
EFFECTS OF VALVES LEAKING
First stage suction
1) Reduce air delivery
2) Reduce second stage suction pressure
3) Increase running time
4) Unload the compressor
First stage delivery
1) Reduce delivery
2) Increase discharge temperature
3) Less air drawn in due to high pressure air leaking back into the cylinder
Second stage suction
1) High pressure in suction line of 2 nd stage
2) High temperature in suction line of 2 nd stage
3) Reduce delivery
4) Increase running time
Second stage delivery
1) Increase suction pressure in2nd stage
2) Reduce delivery in 2nd stage
3) Increase delivery pressure from 1st stage
26. FALLS OF COMPRESSOR PERFORMANCE
(Causes of poor performance)
(01) Choked air suction strainer
(02) Leaky or sticky valves
(03) Too much L.P bumping clearance
(04) Leaky piston rings
(05) Leaky cover and gaskets
(06) Leaky drains
(07) Leaky relief valves
(08) Leaky coolers
(09) Worn crankpin or bearings
(10) Faulty capacity controller
How to maintenance air compressor efficiency ?
1. Cleaning of suction air filter
2. Adjusting bumping clearance
3. Regular overhaul the suction and delivery valve and checking bearing
4. Check the piston rings and liner
5. Clear the cooler drain free from deposits
6. Clean the cooler
Air bottle pressure test
01) Gag the relief valve
02) Close all openings
03) Fit test pressure gauge
04) Remove filling valve and fill F.W completely
05) Air purge and connect hydraulic pump at filling line.
06) Apply pressure 1.5 times of working pressure and maintain at least 30 min
07) Check leaking points.
Purpose of drain valve (Compressor and Air bottle)
It is process to remove the accumulation of oil water and sludge from the air bottle, scavenge space, compressor intercooler and settle and service tank.
01) To drain the water and surplus oil in the air bottle.
02) To avoid air bottle corrosion.
03) To prevent staring air line explosion and sluggish & corrosions system valve.