Bumping Clearance In Air Compressor, How to messure It And Adjust It?----- Video Included

Bumping Clearance or Clearance volume
  • It is the distance between the cylinder head of air compressor and top dead center (TDC)
  • If the clearance is not given piston of the marine reciprocating compressor bump into its cylinder head so it is called as bumping clearance
 How Bumping Clearance Changes over Time
               The bumping clearance in a new machine is set properly by the manufacturers during construction but over a period of time the clearance changes because of the following reasons:

  • Wear at the crank pin bearing: The crank pin 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 be removed for the changing of the first stage suction and discharge valves. When the cylinder head is put back the correct 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 crankshaft and would thus lower the piston and increase the bumping clearances.
Significance of Bumping Clearance: -
               The bumping clearance must be adjusted properly otherwise there is risk of damage and loss of efficiency.  
  • If the bumping clearance were less the volumetric efficiency would increase but there is risk of the piston hitting the cylinder head, especially when the compressor is unloaded during start and stopping. 
  • If the bumping clearance is large the extra clearance would result in a small volume of air being re-expanded every time causing increase in air temperature, fall in efficiency and overheating of the compressors. This would endanger the ship during maneuvering by sudden loss of propulsion.
How to Check Bumping Clearance: -
  • In case a suitable opening is available the piston can be barred to the top dead centre and then feeler gauges can be put inside and the clearances checked at two three points.
  • 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.
  • Now a days very accurate depth calipers are using to check the Dumpling clearance
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. It must be stressed that compressors are unforgiving and incorrectly maintained compressors have claimed many a lives

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.

Once you see the video you will exactly know

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