Piston Striking Head-Reciprocating Compressor

When the piston is travelling towards the head, the piston rod is in compression, so this is not your problem.

As for calculating the load it is simply discharge pressure minus suction pressure times the area of the piston (minus the area of the rod on the under-side).

Give some more clues on the type of compressor and operating conditions. What was your TDC clearance?

hi,

the compressor tdc clearance is .090 inch. the temperature at inside cylinder is 180 farrenheit. the thermal expansion of rod(as per formula) is 0.030 inch but during operation when the temperature reaches to 183 degree farrenheit, the expansion goes to .095 inch.

Make and model would still go a long way to helping us help you.

People are answering your question based on assumptions, some of which are certainly wrong as you state that this is a double acting compressor. This confusion could be easily solved with a little bit more info.

What is the length of the piston rod from the crown of the piston to the crosshead?

There is no amount of cyclic loading that will change the thermal coefficient of a metal by a factor of three. Perhaps you have mis-identified the metal. Was the rod replaced at some point, with aluminum replacing steel? Are you sure that the rod length actually increases by .095" when going from room temperature to 183 degrees, (a difference of only slightly over 100 degrees) Is this rod on the order of 50" long (aluminum) or 100" long (steel)?

The cylinder should be expanding by a similar amount, if the materials are similar.

It seems that you can rule out a change in the coefficient of expansion, and that you should look for other, more likely, causes of head/piston contact.

GA - some logic to the problem. Kind of rules out thermal expansion, which is why I asked the OP how long his piston rod is. A lot of posters are looking at excess clearances as the problem and they might well be right, but if the problem occurs during normal operation then the entire system and all the clearances are in compression when the piston reaches TDC (when it apparently strikes the head) so all the clearances that could cause this are at their smallest.

It would be possible for this phenomena, (excess clearances in the piston rod to Xhead attachment, Xhead pin, small end and or big end bearing) to occur only with the compressor unloaded. ie No compression.

Until the OP gives more info on the machine and under what operating conditions the problem occurs, I give up.

the problem arises when we go for run test. initially there is no striking of piston head or crown to head. but after some time when the temperature reaches about 180 degree farrenheit, it starts striking. the piston and line is of cast iron. the od of piston is 5 ", length of rod is 70".

play in the bearing is fine and doesnt seem to pose a problem

no the rod wasnt replace. rod is 70" length.

piston material- cast iron

cylinder material- cast iron

rod material- aisi 410 ss

I think the prblem is not due to the expansion of rod nor the piston, it may be due to the excess clearence of big end bearings, small end bushes, due rectify the same I think your problem may be solved.

Please also explain if it is non-lub or a lub Compressor.

Its a non lube machine.

1.Explain if the problem arose after maintenance or while running?

2.The kind of rings used Glass filled PTFE, Carbon filled PTFE, Bronze filled PTFE.

3. Check the Piston to cylinder clearance.

4. Check the pressure in both the stages (related Pressure)

5. As you say the piston strikes after some time please ensure the tightness of valves. May be due to loose valves, as pressure increases it may oscillate in pockets.

Hmmm seems like, less calculation and more inspection and measurement is required.
Del

How much play do you have in your bearings and wrist pin?

i think my answer lies in your assumption. the reason is that our compressor is two cylinder, 180 degree apart, and the striking of piston and head occurs on one side only. i think i will have to change or check the bearing.

You may have a stretched connecting rod for some reason but also check the bearings on the crank even though there should be lots of noise if it is the crank components. Some connecting rods have an adjustment similar to gas compressors and maybe it loosened. Check with the manufacturer.

i.e. liquid, rather than gas.

Maybe you shouldn't have self-voted that as OT. If it's a refrigeration compressor, the presence of liquid may be the most important thing to check.

Whatever the problem is, it is. I see it as a question of how to deal with it. .005+" is a lot of stacked excess wear clearance unless the crank, rod and wrist pin bearings/bushings are so loose/ovaled that they are really knocking. A stretched rod makes more sense. If that's the case, nothing short of a rebuild can correct it AND make it reliable without risk of major destruction if something lets go. If you want a quick and dirty patch job you could have the head fly-cut for more clearance but you risk rod failure if it isn't beefy enough. Questions you have to answer are: What will it cost to rebuild. After rebuilding, how long do you think it will give good service. What is the cost of a replacement compressor section that could even be of greater capacity for your needs. It's a cost/benefit/reliability issue.

If I understand the problem, fly-cutting the head would make the symptoms worse. for a quick fix I would use a thicker head gasket to increase the piston crown/head clearance. I would also disassemble the whole compressor and carefully measure relevent components to determine the source of the problem.

If you have a double acting compressor then you are compressing on both sides the stroke. Check clearances at both ends of the cylinder. The outer end (away from the crankcase ) should be two thirds of the total, while the inner end (at the crankcase end) should be one third of the total. I have worked on these types for almost 40 years and never had a rod stretch so bad that I could not maintain proper clearance. The other problems could be as others suggested, loose bearings at the connecting rod.

From Tim Hawley Master Mech.

Hello mtrived,

Can you tell me; Are you using the Reciprocating Compressor to compress air or using to pump other fluid or gas materials?

Best Regards,

Tim

From Tim Hawley Master Mech.

Hello mtrived,

Still waiting for response to first question sent this morning.

Please provide the following additional information:

1. How many Pistons are in this reciprocating compressor?

2. What is the shape of the top of the piston? Provide pic if possible.

3. When did the piston start making this noise? Was it before or after the head was removed?

4. Can the head be rotated and mounted 180 degrees?

5. Who took it apart? Who put it together?

If you answer all 5 questions to me and to your self, and still can not see the problem.

I will tell what the problem is...

Best Regards,

Tim

Waiting your response

we are using compressor to compress the mixture of methane, ethane, propane and butane gas
1.. How many Pistons are in this reciprocating compressor?- there are two piston 180 degress apart

2. What is the shape of the top of the piston? Provide pic if possible.- the piston is cylindrical but it is slice to 45 degrees at the top to facilitate flow of compressed air through valve.

3. When did the piston start making this noise? Was it before or after the head was removed?- when piston goes to outer dead centre it make noice. because it is striking head.

4. Can the head be rotated and mounted 180 degrees?- no not possible.

5. Who took it apart? Who put it together?- couldnt got your point.

The suspense is killing me, too.

From Tim Hawley Master Mech.

You didn't answer my 5-6 questions and I'm going out of town the rest of the week so..

Drain oil, remove lower rod journal bearing caps on both connection rods. Install plastic gauge stock and torque to specifications. Remove gauge stock, and gauge thickness with micrometers. If it's more then .001/.0015", and if it's more then manufactures specs- replace crankshaft journal bearings. Torque to spec's.

Replace both head gaskets and cylinder head bolts.

Since you have the rod disconnected it would be a bad idea to check piston rings and skirt for burnish marks. You may have had lower piston to cylinder wall clearance issues causing your noise problem, (PISTON SKIRT SLAP), after warm up time. Most of the time piston slap is apparent at cold start then the clearance between the cylinder and piston is minimal and piston ring clearance is is very high, which will indicated low compression.

Check both cylinders, they have the same amount of work hours, Then you will only have to tear it down once.

Best Regards,

Tim

Have the two cylinders or cylinder heads become swapped over at some point?

If the head and piston are contacting an obvious mark will be left behind at the points of contact.

Possibly a little die grinder work is all thats needed to correct an improperly cast head or surface defect to open up the clearances where the striking occurs. If it starts to happen again in another location then something else is wrong and needs to be corrected.

From Tim Hawley Mater Mech.

Hello tcmtech,

Good point and good idea, as I mentioned in response #22 check for burnish marks on piston....

If he removes the piston he will visually see where it is hitting.

Good response

Tim

Thanks all for your support. the final decision taaken was to grind the head.

I am eternally grateful that I do not work on your plant.

To grind away at the head of a reciprocating compressor because you can not work out why you are striking it even though you measure the correct TDC clearance is very poor engineering practice bordering on criminal negligence.

From Tim Hawley Master Mech.

Thank you for the answers to my questions.

How many hours are on this compressor?

I believe your rod bearings are the problem.

Let me know what you found.

Thanks once again,

Tim