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Q. How to take air compressor bumping clearance without opening the head ?
Ans.) Open c/case remove bottom end bearings put magnetic type dial guage on connecting rod n lift it note that before lifting the connecting rod piston must be at T.D.C then lift conn. rod and check the reading in dial gauge that will give the bumping clearance.

Q. Bumping clearance is alright but compressor still bumping, why?
Ans.) Total clearance of gudgeon pin, main bearing and bottom end bearing > bumping clearance
Q. State material of compressor valves and valve seat?
Ans.) valve seat 0.4% carbon steel.
valve – stainless steel
spring- hardened steel.

Q. In compressors how to reduce the clearance volume?
Ans.) 1. Reduce the thickness of cylinder cover gasket
2.Add shims between the foot of the connecting rod and the bottom end bearing

Q. Define volumetric efficiency for a 2stage air compressor.
Ans.) The volumetric efficiency of an air compressor is the relationship between the quantity of air discharged when brought to standard atmospheric conditions and the swept volume of the low pressure piston.
Volumetric efficiency = volume of air discharged as ‘free’ air / swept volume of L.P. piston
Free air is air at atmospheric pressure and a temperature of 15°C.
In the case of two or three stage compressors the volumetric efficiency always refers to the low pressure piston.

Q. what is minimum & maximum range of value of “n” in polytropic process?
Ans.)n=0 to infinite
If n=0, isobaric
n=1, isothermal
n=Y, isoentropic or adiabatic
n=infinite, isochoric

Q. In a 2 stage air compressor, what should be the intermediate stage pressure for minimum work done?
Ans.)   Pressure=sq. root (P1xP2)
Q. How to test compressor 2nd stage relief v/v?

Ans.) Compressor 2nd stage relief valve can be tested in hydraulic in test bed in dry dock. it can also be tested by starting the compressor and closing the inlet valve of air bottle for small time and note the pressure at discharge pressure.
Q. If the bursting disc of air compressor has bursted, what will be your immediate action? How will you run the air compressor?
Ans.) Stop the water first and then stop the compressor.
Q. Why piston rings of a compressor are fitted at much above position of piston as compare to D/E piston?
Ans.) In air compressor, piston ring is higher to reduce the clearance volume and thus increase volumetric efficiency
In engine, piston rings are comparatively lower on piston to protect them from immediate high temp. zone

Q. If air compressor is running long (performance has deteriorated), what can be the causes? What action will you take?
Ans.) 1.suction filter chock
2. valves dirty, sluggish, damaged

3. leaky, worn out piston
4. rings piston leaking

5. piston rings stuck in grooves
6. safety valve leaking
7. defective cylinder head gasket
8. leakages in unloaders

9. auto drains leakages in system, pipes etc.
10.bumping clearance increased

Q. How to decide on capacity of main air compressor? What is the compressor capacity as per regulation?
Ans.) The capacity of an air compressor is stated in terms of cubic metre per hour (m3/h), this being the volume that the air actually discharged in one hour would occupy if it were expanded down to atmospheric pressure and cooled to atmospheric temperature. Air at atmospheric pressure and temperature is referred to as normal air. Normal air has an absolute pressure of 1.013 bar and a temperature of 15°C.
At least 2 compressors of equal capacity out of which one is to be driven independently
Total capacity of compressors is to suffice filling of air receivers within 1 hr. from atmospheric pr. to the pr. needed to satisfy minimum starts requirements
Q. What modifications will you do to increase compressor efficiency as per SEEMP?

Ans.) 1. a duct can be bought from veg room to the suction of the compressor.

2. filters and valves should be cleaned

3. intercooler should be cleaned and flow of water can be increased by adding a                     cooling pump.
Q. What is valve loss in compressor?
Ans.) When the piston is making a suction stroke, the pressure within the cylinder space is lowered to the point where the air pressure behind the suction valve overcomes the spring load and causes the valve to open. When the suction valve opens, the external air flows into the cylinder by virtue of the greater air pressure outside the cylinder. Once the air commences to flow, the pressure in the cylinder rises slightly but remains slightly lower than the pressure outside. This pressure difference is necessary to get air to flow into the cylinder and is called the suction valve loss. Similarly on the discharge stroke of the piston the pressure in the cylinder rises to some point at which the discharge valve opens. After the valve opens the ‘ pressure in the cylinder falls slightly, then remains
approximately constant, and falls again just before the discharge valve closes. This approximately constant pressure would be slightly higher than the pressure into which the compressor is discharging. This pressure difference comes about due to flow resistance set up within the valve and is called as delivery valve loss. These valve losses lower the mechanical efficiency of the air compressor and so careful attention must be
given to suction and delivery valves when they are overhauled and that it is essential that the correct type and weight of springs be fitted.

valve loss is actually the loss in energy in motor of compressor due to above.

Q. How air compressor is started in unloaded conditions.
Ans.) Many methods of unloading
1. throttling of suction
2. speed variation
3. depressors to hold suction valve plate on their seats
4. discharge bypassed to suction or vented to air

Q. Discuss construction and material of air bottle. Is thickness same overall? How is the welding between different section?
Ans.) Good quality, mildsteel plate having similar specifications to boiler plate material
UTS 360 MN/m2 to 500 MN/m2 elongation of not less than 23% to 25% cylindrical part
one or two welded longitudinal seam (doublewelded butt joint) thickness less than end plates End plates semiellipsoidal made by spinning process (seamless) edge left by the spinning is machined and tapered down to the thickness of the cylindrical shell welded
to cylindrical shell by fullpenetration one end has an opening formed with a lip to take an elliptical manhole door

Q. Explain air bottle survey in detail. How to inspect auxiliary air bottle where you can’t go inside?
Ans.) Entry:
Inspectionis to be carried out when the receiver capacity is not required for maneuvering

Receiver properly isolated, depressurized and notice pasted
The internal pressure is to be checked by the pressure gauge and by opening the drain valve
Open the manhole door carefully and ventilate the air bottle
Obtain an enclosed space permit and comply with it
Ventilation is to be maintained during the entire time of enclosed space entry
Inspection:
Check for corrosion both externally and internally
Inside coating,COPAL varnish condition to be checked to ensure that it is unbroken and no peeling is taking place
Welding seam to be checked, carry out dye penetrant test at suspected areas
The tell tale hole between the lap welding seams of the doubler plate / ring to be checked for leaks in service to ensure that the internal welding is intact
Face of the manhole is to be checked for pitting marks
Drain should be clear
All points of high stress and corrosion are to be thoroughly checked viz. welds, supports braces, the bottom of the bottle where water/oil is collected. Pay attention to the zone of water line.
Check for leaks at the mounted valve glands/flanges, carry out inspection of the valve face and seat for wire drawing effect
A timing check with the air bottle completely charged and isolated will give a good idea of the intactness of the pressure vessel
The safety/relief valve setting to be inspected as per the class requirement (to be set not more than 10% of max. working pr.
Q. Why does air bottle drain appear milky?
Ans.) When pressurized water comes out, sudden drop in pressure results in vaporisation of dissolved gases in the emulsified mixture, causing foaming which gives a milky appearance. Similar thing happens in c/case of refrig. compr. when it is started, due to sudden drop in pr. dissolved refrigerant causes foaming in oil.