Month: December 2016

Q. Why Cylinder Lubrication Is Required?
Ans:  Cylinder Lubrication Is Required To Develop An Oil Film Between Piston Rings And Cylinder Liner To Avoid Metal To Metal Contact. In Latest Engines, With Higher Peak Pressure And Cycle Temperature Good Quality Of Cylinder Oil Is Required To Maintain An Oil Film Between The Rings And The Liner For A Long Period Of Time. Apart From Lubrication Its Other Properties Enhance Engine Performance.

Q. State, With Reasons, Five Desirable Properties Of Cylinder Oil Used For An Engine Which Is Burning Residual Fuel.
Ans: Five Desirable Properties Of Cylinder Oil Used For An  engine Which Is Burning Residual Fuel Are;
It Must Effectively Neutralise The Corrosive Effect Of Suphuric Acids Formed During Combustion Of Fuel.
It Must Prevent The Build Up Of Deposits In The Ring Grooves And In Ports Of Port Exhausted Two Stroke Engines.
It Must Be Able To Burn Cleanly, Therefore Leaving As Little As Possible Soft Deposits.
It Must Pocess Adequate Viscosity At High Working Temperatures.
It Must Form An Effective Seal In Conjunction With Piston Rings To Prevent Blow By.

Q. How An Effective Film Can Be Obtained Over The Entire Liner Surface?
Ans: An Effective Oil Film Can Be Obtained Over The Entire Surface Of The Liner By Means Of Cylinder Lubrication.
Oil Is Injected Through A Number Of Holes Drilled In The Liner, Usually Six Or Eight Of These Displaced Circumferentially Around The Liner At A Chosen Vertical Position Within The Piston Stroke. Oil Is Supplied By Pressure Pulses From Positive Displacement Mechanical Lubricators Driven From The Engine Camshaft And Regulated To Deliver Oil At A Given Rate.
Lubricator Quills Are Connected To Each Oil Hole And Contain A Non-return Valve To Prevent Blowback. To Assist In Distribution Of Oil Around The Liner Circumference, Oil Gutters Adjacent To Lubricator Points Angled Downwards To Assist Flow By Gravity Are Fitted. Lubricating Oil Is Spread Over The Length Of The Liner By The Piston Rings During Their Stroke. 

Q. How It Can Be Ensured That The Correct Quantity And Quality Of Cylinder Lubricating Oil Is Being Used?
Ans: It Can Be Assured That The Right Quantity And Quality Of Cylinder Lubricating Oil Is Used By;
The Quantity Is Controlled By Linking The Pump Drive To The Engine Throttle Setting Levers, The Oil Quantity Then Being Regulated To The Power Produced. To Ensure The Quality Of The Oil Used Is Maintained For Lubrication And Not Burnt By Hot Gases, An Accumulator Can Be Fitted To The Cylinder Lubricator Quills So That A Quantity Of Oil Will Enter Each Time The Cylinder Pressure Drops Below That In The Accumulator, Thus Ensuring Oil Does Not Enter The Cylinder When Exposed To Hot Gases.

Q. How A Representative Sample Of Lubricating Oil Is Obtained From The Main Engine System For Analysis?
Ans: To Obtain A Representative Sample Of Lubricating Oil From The Main Engine System; A Clean Sample Bottle Must Be Used, The Sample Must Be Taken From An Area On The System Where There Is A Good Flow, Not From Filters Or The Sump.
The Best Place For The Sample To Be Taken, Would Be From The Lubricating Oil Rail / Supply Line, Ideally The Sample Point Should Be A Dedicated Sample Point To Avoid Accidents. 

Q. Describe Three On Board Tests For Lubricating Oil?
Ans:
The Flow Stick Test: This Is Where The Viscosity Of The Lubricating Oil Is Tested. Two Samples Of Oil Are Used For This Test, One From The System, And The Other, Fresh New Oil. The Flow Stick Contains Two Reservoirs, One For The System Sample And One For The New Fresh Oil, These Reservoirs Are Filled And The Samples Are Allowed To Reach The Same Temperature.The Flow Stick Is Then Tilted From The Horizontal And The Oil Flows Down Two Parallel Chanels, When The Fresh Oil Reaches A Reference Point, The Flow Stick Is Returned To The Horizontal.The Distance Traveled By The Used System Oil Is Compared To That Of The New, Fresh Oil And If The Used Oil Is Between The Two Used Oil Comparison Points, The Used System Oil Is Still In Good Condition.

Water In Oil Test: The % Water In Oil Test Is Used To Give An Indication If Any Of Water In The Lubricating Oil, Here A Fixed Amount Of Oil Is Sampled From The System, Which Is 5ml And Placed In A Capsule Where There Is A 15ml Solution Of White And Reagent. A Lid Incorporating A Pressure Gauge That Is Calibrated To Read % Water In Oil Is Then Screwed Onto The Capsule To Form An Air Tight Seal. The Capsule Is Then Shaken Vigorously For 1 Minute, Then Left To Stand, Any Water In The Lubricating Oil Will React With The White Spirit And Reagent Causing A Rise In Pressure That Will Be Read On The Pressure Gauge As % Water In The Lubricating Oil

Microbiological Degradation Of Oil Test:  This Test Will Give An Indication Of An Infestation Of Microbiological Organisms In The Lubricating Oil. The Test Makes Use Of Slides That Are Coated With A Special Gel, Which Is Dipped In The Sample Of Lubricating Oil, The Slide Is Then Placed In A Test Tube, Sealed And Left To Incubate, Any Growth That Comes About In The Incubation Period Is Then Compared To A Chart To Indicate Any Infestation By Microbiological Organisms.

Q. If No Testing Equipment Was Available, Explain How The Condition Of Lubricating Oil Could Be Assessed On Board?
Ans: If No Test Equipment Is Available On Board, Lubricating Oil Condition Can Be Determined By A Few Simple Tests:
Crackle Test For Water, This Is Where A Sample Of  lubricating Oil Is Take And Placed In A Test Tube, The Test Tube Is Then Heated, Any Water Droplets Will Make A Crackling Noise Due To The Formation Of Bubbles, This Will Only Detect Small Amounts Of Water, For Larger Amounts, A Simple Settling Test Can Be Used.

TBN Can Be Tested By Placing A Drop Of Indicator Solution On To Blotting Paper, This Being Followed By A Drop Sampled Lubricating Oil In The Centre Of The Blotting Paper. A Colour Change Should Take Place Around The Surrounding Area Of The Drop I.e. If The Colour Is Red, This Shows Acid Is Present, Green / Blue Show S Alkaline Presense.
Note: Filters Will Also Give A Good Indication Of The Lubricating Oil Quality.

Q. What is volumetric efficiency of air compressors?
Ans: Actual volume of air drawn in / swept volume.

Q. What is compression ratio?
Ans: (Swept volume + clearance volume)/clearance volume

Q. What is free air delivery?
Ans: Volume of delivered air measured at 1 atmospheric pressure and 15*C is called free air delivery.

Q. What is bumping clearance? How do you measure it?
Ans: Clearance volume or bumping clearance is the space between the top of the piston and the cylinder head of an air compressor. This clearance is an important aspect of the compressors and should be as less as practically possible to improve the volumetric efficiency of the compressor. The clearance volume should not be too less or too more. Moreover, it affects the efficiency of the machinery and thus should be checked at regular intervals of time.
Significance and Effects of Bumping Clearance
In an air compressor, when the discharge valve closes in the end of the compression cycle, a small amount of high pressure air is trapped in the clearance volume.
Before again taking suction, the air trapped in the clearance volume must expand below the suction pressure i.e. below the atmospheric pressure.
The expansion of this trapped air in the clearance volume causes effective loss of stroke due to which the volumetric efficiency of compressor drops. Therefore, the clearance volume has a significant effect on the efficiency of the compressor.

Effects Due to Less Clearance
Small clearance volume may result in piston banging or colliding to the cylinder head.
This is dangerous when the compressor when is running in unloaded condition without any resistance to the movement of the piston.

Effects Due to Large Clearance
Large bumping clearance retards the formation of vacuum on the suction stroke and thus less air is drawn inside for compression and accordingly the weight of the air delivered is reduced proportional to the clearance volume. Compressor has to run for a longer period to provide the necessary compression pressure.

Reasons for Change in Clearance Volume
During overhauls of the air compressor, if the gasket fitted between the cylinder head joints is of the wrong type, then the bumping clearance will increase, resulting in wear down of bottom bearings or wrong bearings are put in place.
How Bumping Clearance is checked?
Bumping clearance is checked by putting a lead ball or plastic gauges over the piston and then turning the compressor one revolution by hand.
By doing this the lead ball will compress and the thickness obtained is the clearance volume.
This thickness is measured with Vernier caliper or micrometer and is then compared with the manufacturer’s value. Adjustments are made in case there is an offset in the value.

Adjustment of bumping clearance
Bumping clearance can be adjusted with the help of inserting shims (thin metallic plates) in the bottom bearings. Inserting shims will move the connecting rod and the piston which will change the clearance.
What should be the Bumping Clearance?
Generally bumping clearance depends on the manufacturer but as a thumb rule it should be between 0.5% to 1% of the bore of the cylinder.

Q. What are the reasons of HP stage relief v/v lifting?
Ans: 
a. Delivery v/v shut.
b. Setting error of pressure switch.
c. Delivery v/v which is SDNR type is fitted inverted.

Q. What are the reasons for LP stage relief v/v lifting?
Ans: 
a. HP stage suction v/v or discharge v/v leaking.
b. 2nd stage suction n discharge v/v interchanged after overhaul.

Q. How lubrication takes place in main bearing, crankpin bearing, upper & lower cylinder in case of tandem type air compressor?
Ans: 
a. For main brg & crankpin brg- pressure lubrication provided by shaft driven gear p/p.
b. For lower cyl. In tandem type- it’s by splash.
c. For upper cylinder in tandem type- there is a supply from drilled hole in con. Rod which is scraped down by scraper ring into the lube oil sump.

Q. What are the reasons for low L.O pressure in an air compressor?
Ans: 
a. Motor rotation direction reversed.
b. Oil line blocked due to rag/dirt.
c. No oil in sump.
d. Filter clogged.
e. Worn metal of the bearing=> more clearance=> low oil pressure.

Q. What are the reasons for increased lube oil consumption in an air compressor?
Ans: 
a. Excess oil in sump (above level H).
b. Oil scraper ring worn out.
c. Worn piston and liner. 

Q. What checks do you make if a compressor trips on low lube oil pressure?
Ans: In case of reciprocating compressor
a. Direction of rotation, in case the motor is overhauled or some maintenance carried on the motor.
b. Suction filter should be cleaned.
c. Check the pressure switch.
d. Inspect the lube oil pump
e. Check if any lube oil pipe is holed
f. Check for excessive foaming

In case of rotary compressor,
a. Check whether the lube oil pump is rotating with the compressor
b. Check the condition of vanes and the elliptical casing for scoring damage
c. Check the compressor is free to turn, no seizure of rotor, no wear on the vanes and the casing.
   
Q. What is the purpose of breather in an air compressor?    
Ans: Breather- it is provided so that pressure or vacuum is not created inside crankcase. It has filter to prevent moisture and dirt from going inside the crankcase and also has metallic plate on the inner side which doesn’t allow lube Oil to come out.

Q. Why intercooling is provided in an air compressor? And why is the compression distributed into stages?
Ans: 
a. By employing the inter stage cooling we are trying to achieve an isothermal compression cycle in order to minimize the work done in the process.
b. The air outlet temperature after compression is lowered by intercooling. So oxidation of the lube oil is prevented. Also good lubrication is achieved.
c. Lesser deposits in the air system.
d. Intercooling increases the air density and hence reduced volume of the HP compression chamber is possible.
e. It facilitates removal of moisture by condensation at the intercoolers.
f. To facilitate intercooling the compression is distributed into stages. Also even load distribution is achieved over the cycle.
 
What would be the consequences of too high valve lift in an air compressor?
Ans: 
a. Chances of damage due to impact.
b.Decrease in vol. efficiency.
 
Q. What would be the consequences of a high spring constant of spring plates in an air compressor?
Ans: 
a. Valve will open late.
b. It will close with higher impact which may cause damage.
c. It also reduces volumetric efficiency.
 
Q. Can we interchange spring of suction & delivery v/v?
Ans: No.Delivery valve’s spring stiffness is more.

Q. What are the necessary checks before manual starting of an air compressor?
Ans: 
1.  Lubrication: check the oil level in the sump ( between H & L )and in the lubricator tank(if provided) .
2. Unloading: the valves in the unloading lines must be open.
3. Pressure gauge cocks: the cocks must be slightly open to avoid excessive pressure fluctuations which can damage the pressure gauges.
4. Air filter: check that the air intake filter is clean.
5. Cooling: check that the Cooling water pressure is correct (0.2-2bar) and in the case of attached pump a visual check of the pump to be carried out and all the valves in the line must be open.
6. Relief valves: some relief valves have hand testing levers given for testing, the same can be done.
7. Air-line piping: check that all the valves in the discharge piping are open and correctly lined up.
8. Final check: the compressor can be turned a few revolutions with a turning bar to check for the free movement.

Q. What if the compressor motor connections are reversed after overhauling?
Ans: 
a. The lube oil pressure will not be sufficient for lubrication so the compressor may trip on low lube oil pressure. The moving parts may get damaged due to the failure of the lubrication.
b. The cooling water pump which is also shaft driven usually does not build up enough flow/pressure hence the compressor may trip because of the cooling water non flow or high air temperature.

Q. What are the reasons of piston seizure in a reciprocating air compressor?
Ans: 
a. Insufficient lubrication.
b. Lack of cooling water.
c. Valve part broken and fallen into the chamber.
d. Piston ring broken.