The bumping clearance must be adjusted properly otherwise there is risk of damage and loss of efficiency. If the bumping clearance is 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.
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.
On the other hand to play safe, the engineer gives few millimeters of extra clearance, the volumetric efficiency of the compressor would decrease, the free air delivery will fall and there will be a fall in pressure. 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.
Effects Due to Less Clearance
Small clearance volume may result in piston banging or colliding to the cylinder head.
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.