7/27/2019 Marine Reciprocating Compressors

1/3

Marine Reciprocating Compressors: Correcting Piston to Head

Clearancewritten by: Chief Engineer Mohit Sanguri edited by: Swagatam updated: 8/9/2011

The adjustment of Bumping Clearance is a very critical adjustment of the clearance volume. If more the volumetric

efficiency of the compressor suffers and if less the unloaded piston may hit the cylinder head and damage both. In this article

we discuss the need of this clearance and its adjustment.

What is Bumping Clearance

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. In new compressors this clearance is adjusted by the manufacturers and the marine

engineers are blissfully unaware of its importance. However the ship does not remain new for ever and every machine

demands overhauling and that is where the problems start. Even routine jobs like lifting the cylinderhead to change the low

pressure or first stage valves can change the bumping clearance if the correct thickness gaskets are not used or if the head is

overtightened thus squeezing out the gaskets. Many engineers miss this vital adjustment during overhaul of the compressors

and efficiency and free air delivery of the compressor suffers.

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:

1. 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.

2. 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.

3. 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.

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 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.

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.

7/27/2019 Marine Reciprocating Compressors

2/3

How to Check Bumping Clearance

The bumping clearance can be checked by the following methods:

1. 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.

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 dead centre 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 bumping clearance once found to be incorrect would have to be adjusted. The methods of adjusting the bumpingclearances are as follows:

1. The cylinder head gaskets can be changed to a different thickness thus altering the bumping clearance.

2. 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

Why CORRECT TOP, BOTTOM, And MAIN BEARING CLEARANCE SHOULD BEMAINTAINED. These three bearing clearances effect the lift of the workingposition of the piston in thecylinders. The lifting of working position of the piston reducethe bumping clearance at top, alsoincrease the clearance at the lower part of the stroke. Thisalters the volumetric efficiency of thecompressor, sometimes efficient LP anddecrease efficiency of HP and vice versa. Decrease involumetric efficiency, increasein running hours of the compressor also in extreme cases contactpiston with head surfaceso subsequent damage.Lubricating oil pressure is affected, as these are high speed

machines, so the clearance iscrit ica l as they will loose the hydro dynamiclubrication. As the bearing surface area is small

compared to the piston area and the forc acting is quite high due to high pressure on thebearing,where if the clearance is not maintained will cuase destruction OD loadcarrying capaci ty of lubricating film. Also clearance is required to be maintained,otherwise impact loading will existonce the bearing and journal which may lead to a fatiguefailure or in severe cases a crack on thebearing and or journal.Why THE CLEARANCEIS NOT ADJUSTED AT ONE STAGE ALONE. The clearance in hp and lp stages areindependent individually. Hence adjusting one stagesimultaneously changes the relativeposition of the piston with respect to liner. Hence consideringonly one stage may cause

piston to touch the liner causing a mechanical damage, or increase the clearance volume, so

7/27/2019 Marine Reciprocating Compressors

3/3

decrease the volume efficiency. Hence both stages are taken into account while consideringthe adjustments of bumping clearance