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Main Air Compressor ( safeties, unloader, bumping clearances )

Discharge Unloader And  Its Operatiom

  •  One Of The Most Important Component, An Unloader Is Used For Unloaded Starting Of Compressor And Draining At Continuous Intervals. 
  • An Unloader Is Fitted At The End Of Drain Line From Inter And Aftercooler. It Is Basically A Solenoid Operated Spring Loaded Valve Arrangement. 
  • Unloader Has Two Lines Drain From Above And Delivery Air Pressure Of 1st Stage From Below. When Compressor Is Shut And Delivery Air Pressure Is Zero, The Drain Opens The Unloader Valve Against Spring Pressure, Thus It Is Always Open. When Started, Slowly The Delivery Pressure Rises And Tries To Shut
  •  The Valve, During This Time The Actuated Solenoid Valve Overcomes The Air Pressure And Keeps The Valve Open. 
  • The Solenoid Valve Is Operated By A Time Delay Circuit As The Time Span Of 10-15 Sec Gets Over The Solenoid Is Deactivated And The Unloader Valve Shuts. 
  • After Every 20-30 Mins., It Is Activated Again And Clears The Drain For 10-15 Secs, Then Deactivates. If The Unloaderdoes'nt Function Properly, There Is A Manually Operated Valve Just Before Unloader, Which Should Bebe Open Before Starting The Unloader.
  • Now, Why Is It So Important To Start The Compressor In Unloaded Condition?
    It Must Be Unloaded Because When Started The Compressor Draws A Very High Current, If It Will Be Loaded I.e Valve Not Open. The Motor Which Is Driving The Machine May Get Overloaded

Safeties In A Reciprocating Compressor

1. Lubricating Oil Pressure Low Cut Out: It Is Provided Such That If Lub Oil Pressure Falls Down The Parts Which Are Lubricated Like Liner, Piston, Main Bearings, Bottom End Bearings Might Get Worn Out.

2. Bursting DiscIt Is Given In Intercooler And Aftercooler In The Water Side So That If Any Highly Pressurized Air Tube Bursts, The Cooler Shell Will Not Be Pressurized , Bursting Disc Will Burst And Liberate All The Water , Indicating Air-tube Burst. Generally Made Of Copper.

3. Non- Return Valve In Delivery Line:
 It Is Provided, So That The Air Once Delivered Does'nt Return Back To The Compressor In Case A Low Pressure Develops In Compressor Side.

4. Discharge Unloader: Already Explained Above.

5. Relief Valve On Intercooler:
 A Relief Valve With Setting 10% Above The 1st Stage Pressure Is Provided To Release Air If High Pressure Is Generated In 1st Stage Generally Due To Valve Malfunction.

6. Fusible Plug:
 Generally Made Of Tin, Antimony And Bismuth, Is Fitted In Inter And Aftercooler To Release Excess Air When Temperature Rises Upto 121 Degree Celcius Due To Rise In Pressure.

7. High Temperature Alarm: At Around 90 Degree Celcius, The Alarm Sounds Denoting The Rise In Air Temperature.

Multistaging In Compressor .why
Multi-staging Is Conducting The Process Of Compression In More Than One Stagesi.e Air Is Compressed By Two Or More Pistons Before Delivery. Generally, Two Stage Reciprocating Compressors Are Used On Board. The Purpose Of Multistatgingis :-

1.If We Increase The Pressure Of Air Upto 30bar (pressure Of Air At 2nd Stage) In One Stage, The L.o Will Start Burning Due To Rise In Temperature. As Per The Thermodynamic Equation For A Polytropic Process,
T2/T1= (P2/P1)^(n-1/n )
Where N=1.35 For Air.
P2/P1= 30/1
T2= 450 Degcelcius.
At This Temperature L.o Will Burn As Flash Point Of L.o Is 200 Deg. Celcius. So We Keep The Pressure Ratio Limited To 5:1.

2. To Reduce The Work Done By Compressor In The Whole Process. The Compression Of Air To Higher Pressure And Lesser Volume Is A Reversible Adiabatic Process. I.e Change In Temperature Takes Place, If Change In Temperature Is Kept Minimum Or Zero , The Work Done By Compressor Will Be Minimum.

Bumping Clearance In Compressor And Measurement

 Bumping Clearance As The Name Signifies Is A Clearance Given So That The Piston Of The Marine Reciprocating Compressor Would Not Bump Into Its Cylinder Head.

How To Check Bumping Clearance:-
1. In Case A Suitable Opening Is Available The Piston Can Be Barred To The Top Dead Centre And Then Feeler Gauge Can Be Put Inside And The Clearances Checked At Two Three Points.

2. The More Convenient Method Is To Take Lead Wire From The Engine Store And Make A Small Ball Based On The Expected Clearance And Put It Between The Piston And The Head From The Valve Opening. Then The Piston Is Slowly Turned To The Top Dead Centre With The Help Of A Tommy Bar. After That The Piston Is Again Turned Down And The Lead Wire Ball Is Extracted And The Thickness Measured With The Help Of A Micrometer. This Measurement Would Give The Bumping Clearance. The Caution Which Must Be Observed In These Methods Is That The Clearances Of The Main And The Crank Pin Bearing Have Not Been Taken Into Account. The Correct Method Is Thus That After Turning The Piston To Top Deadcentre The Piston Connecting Rod Must Be Jacked Up With The Help Of A Crow Bar. It Is Only After This Hidden Clearance Has Been Accounted For, Will The Correct Bumping Clearance Be Found.

How To Adjust The Bumping Clearance

  • The Cylinder Head Gaskets Can Be Changed To A Different Thickness Thus Altering The Bumping Clearance.
  • The Shims Between The Foot Of The Connecting Rod And The Bottom End Bearing Can Be Changed Thus Changing The Bumping Clearance. However After Adjusting The Bumping Clearance The Clearance Should Be Checked Once Again To Make Sure That There Is No Error And The Clearance Is Within The Range As Specified By The Manufacturers.

Change In Bumping Clearance
  • Wear At The Crankpin Bearing. The Crankpin Bearing Wears Down Due To Use And This Clearance Can Travel Right Up To The Piston And An Unloaded Piston Can Hit The Cylinder Head. This Type Of Wear Can Be Recognized When The Compressor Makes Impact Sounds Running Unloaded At The Starting And Stopping Operations. This Type Of Wear Would Also Be Accompanied By A Slow Decrease In Oil Pressure Over A Period Of Time.
  • Opening Up Of Cylinder Heads. In Certain Types Of Reciprocating Compressors The Cylinder Head Have To Beremoved For The Changing Of The First Stage Suction And Discharge Valves. When The Cylinder Head Is Put Back Thecorrect Thickness Of The Cylinder Head Gaskets Should Be Used Otherwise It Would Change The Bumping Clearance.
  • Wear On The Main Bearings. Over All Wear On The Main Bearings Would Lower The Crank Shaft And Would Thus Lower The Piston And Increase The Bumping Clearances.


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