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Piston Compressor

02/02/2013 4:49 AM

Hi I am a mechanical Engineering student doing my practical (semester 2), I have written my Technical report on Piston compressor that failed and we fixed. My lecture tells me my practical considerations are superficial. I do not understand this as there is only one way of rebuilding a compressor. how do I write this section/ maybe I am missing something. secondly how do you write the literature survey.

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#1

Re: Mr

02/02/2013 5:43 AM

Post the report here and we'll offer a constructive criticism (sprinked with humour and sarcsm).
You should ask your lecturer to exlain what he means, although if he's anything like the lecturers I had he won't be much help.
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#2

Re: Mr

02/02/2013 6:38 AM

Are you sure that the "superficial" comment is related to reassembly during rebuilding. Perhaps you are expected to examine whatever failed and investigate the root cause of the failure. Did it just wear out? Was maintenance poor? Improper operation? Design problem? As a mechanical engineering student you are probably expected to go beyond turning a wrench and squirting some oil.

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#5
In reply to #2

Re: Mr

02/03/2013 11:12 AM

Also expected to do his own work, too.

21051981, do as the Cat suggested. Post it here for critique.

Don't expect us to write it for you, but someone will make suggestions as to what YOU should do.

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#8
In reply to #2

Re: Mr

02/09/2013 2:11 AM

yes that is what he said on his comments, I will post parts of the report now

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#12
In reply to #2

Re: Mr

02/09/2013 11:00 AM

I have put part on my report you can check for yourself and let me know what you think..

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#3

Re: Mr

02/02/2013 8:00 AM

Are you reporting, or doing an analysis?

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#4

Re: Mr

02/02/2013 8:57 AM

Your question is too vague....and superficial....

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#6

Re: Piston Compressor

02/03/2013 1:49 PM

After re-reading this I think you were given the task of discovering and documenting what the ACTUAL failure was rather than replacing a part to get it working again (you are doing a degree not a technician certificate after all).

As an engineer you are likely expected to perform a metallurgical analysis, discover failure mode, analyse cause, etc, to show learned knowledge and applied practical understanding in a Degree level Engineering project, not just make it work again.

Please let us know if we are on the right track.

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#7

Re: Piston Compressor

02/09/2013 1:43 AM

The report is a bit long but I will put it in, he is not complaining about the other parts of the report but literature survey and pactical part.

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#9

Re: Piston Compressor

02/09/2013 2:12 AM

here is the part of the report,

2.2 Practical Considerations

On arrival to site the compressor was re-started and the motor could not turn. The coupling was then taken off in order to try and turn the compressor shaft by hand, this was done in order to check if the problem was the motor or the compressor itself. The compressor shaft still could not turn and it was then clear that the compressor had ceased and had to be stripped. On stripping the compressor it was found, that the following parts needed to be replaced/repaired as they were worn and/or damaged (see figures 1-9):

· Crankshaft

· 6 x Cylinder liners

· 6 x Pistons

· 3 x Connecting rods

· Both the drive and non drive main bearings

· Intermediate bearing

· Oil pump

· 6 x Big end bearings

Due to the inconvenience of working on site, it was decide that the compressor be taken to workshop for stripping further. The compressor was further stripped in G workshop and all parts were inspected and material list was made.

2.2.1 Preparation of parts

As oversized bearings are not used as original replacement parts, the crankshaft was restored to its original replacement parts. This was done using the metal spraying technique; it was first ground down roughly to remove any wear. The rough ground allowed the metal to bond together, in the next step the crankshaft was rotated in a special lathe. Molten metal, with similar qualities to the original crankshaft, was sprayed onto the ground bearing journals as the crank rotates. The new bearing surface layer was built up and allowed to cool down and, then ground to its original specifications. A new steel thrust surface was also fitted to the drive side main and big end bearing.

The main bearings were renewed and press fitted into their housings and tolerances were checked. Big end bearings were fitted into the conrods to check the pre tension, conrod bolts were torqued to the specifications and big end bearings tolerances were checked. Bearing end caps, bearing shells and conrods were then put together as sets before assembly to the compressor. The intermediate bearing was pre-tensioned in its housing and tolerances were checked.

The gudgeon pins were inserted into the new small ends bearings to check for free play; gudgeon pins come with pistons as a set and should not be mixed up. The new pistons were heated up to 80 degrees Celsius to allow gudgeon pins to be fitted through the small end, which had already been press fitted into its conrod. The parts were then allowed to cool down, after which the locking bolt was put in and torqued. The piston rings were then put into the pistons and pre-checked by putting the piston into the cylinder bore and measuring the end gap with feeler gauge.

The compressor casing was checked for straightness along with bearing housings and intermediate bearing block. The casing was then cleaned and all threads of studs and fittings were inspected and some renewed. All gaskets were removed and sealing surfaces cleaned with fine sand paper.

The wear on oil pump casing was repaired by machining the oil pump and pushing in a bush, which is machined according to new gears. All hydraulic delay pistons wear inspected for wear and re-used. All springs, oil pressure regulator valves and filters were replaced. The oil pump cover was skimmed in a lathe until the marks were gone.The shaft seal housing was thoroughly cleaned and the new shaft seal was put in.

2.2.2 Assembling of the parts and commisioning

The intermediate bearing assembly was fitted into the crankshaft, cross tightened and torqued at 120Nm, with M16 stretch bolts. The drive side bearing housing was bolted to the compressor casing with 1.5mm gasket and torqued to 120Nm. The crankshaft was then fitted into the casing, guiding the nose of the crank through the drive side bearing, ensuring that the bearing does not get damaged. It is critical that, the loose brass thrust bearing is located centrally on the bearing housing, in order to get the correct end float clearance of the crankshaft.

The oil pump side bearing housing was then fitted and torqued, while the intermediate bearing housing remained loose until the end float was checked. This was done with an assistant and the clock gauge mounted on the compressor casing. The crankshaft end float depends on the compressor model and is set by shimming the bearing housings with gaskets of 1, 1.25 & 1.5mm thicknesses. The end float is normally about 1mm but depends on the compressor model.

Once the end float was satisfactory, the intermediate bearing housing was centred and fastened down. The crankshaft must turn freely by hand only, at this stage. The liners were now pressed down (with a special tool) into their bores, with the loading mechanism lever facing the loading piston locations on compressor casing. Once the depth was checked, the conrod/piston assembly was put in and torqued to 100Nm. At this stage, the crankshaft will become more difficult to be turned, due to the resistance of piston rings in the bore.

The new springs were put into the lapped valve plates and torqued down to 120Nm. The reconditioned valve gear was then pressure tested to ensure that they hold pressure in the closed position. This was done as the valve gears essentially operate as pressure relief valves. The passing valve gear causes increased operating temperatures, about the vaporisation point of any refrigerant oil, causing poor lubrication on piston rings, liners and small end bearing surfaces. The new suction rings were installed into the liner, the discharge valves were then fitted and bumper clearance checked. After the bumper clearance was satisfactory, the buffer springs were put in and heads bolted down. The loading pistons were fitted and compressor was pressure tested.

The compressor was then spray-painted and ready for a new life in refrigeration; it was transported to site. The compressor was put on its base, all piping and electrics were connected. All pressure and oil safety switches were replaced with new ones and suction + discharge strainers replaced. The safety switches were first tested and the compressor started afterwards. The plant operators were trained on operation of the compressor and its safeties to prevent the similar problem happening again. The safe works instruction was prepared and issued to the client and one copy put inside the plant room next to the machine.

3. CONCLUSIONS AND RECOMMENDATIONS

The compressor seized, this happened when the main and intermediate bearings seized onto the crankshaft (see fig. 3 & 4). It would appear that the seizure occurred as a result of poor lubrication and not as a result of localised heat in one or more cylinders (commonly known as a heat seizure). Lack of lubrication can be as a result of one or more of the following reasons:

1. Poor quality oil

2. Lack of oil pressure/flow

3. Liquid refrigerant in the oil

4. Compressor operating at too high a temperature resulting in the oil becoming too thin.

A heat seizure is normally as a result of one or more discharge valve rings failing resulting in re-compression of the discharge gas. With re-compression of the same discharge gas the heat continues to increase until the piston seize is the liner.As can be seen from the photographs the valve gear (see fig. 2) is still in a good condition and has not been exposed to excessive heat as would be the case if the compressor suffered from a heat seizure. Also the buffer springs heights are still within tolerances, when buffer springs are exposed to excessive heat the spring tension is lost and they reduce in height.

It's also unlikely that liquid refrigerant was the cause of poor lubrication as there is no damage to the valve gear. When liquid returns to a compressor the valve gear is normally the first component to be damaged. Also the damage to the cylinder liners is not indicative of liquid washing.As the compressor had recently been serviced, prior to the failure, and new oil charged in, it's unlikely that the poor quality oil caused the seizure.

As can be seen in the figures the oil pump housing and cover are both worn (see fig. 5(f) & 5(b)). This wear would have resulted in poor oil pressure. In addition to this the oil pump gears seized in their respective bearing bushes resulting in the oil pump drive gear spade breaking off after which no oil pressure would have been achieved (see fig. 5(a) & 5(b)).

It is difficult to say what the sequence of events was, as the compressor seized but all indications point to a lack of oil pressure. This implies that the oil pressure stat didn't stop the compressor from running soon enough and/or the compressor oil safety stat was reset on a few occasions. With the inbuilt time delay of the oil safety stat this means that the compressor could have run with no oil pressure being achieved after the failure of the drive gear. It is recommended that the plant operators be trained on how the compressor safeties work to eliminate these problems from happening. Also the safety stats have to be tested every month to see if they are still operational. The operators must also report any unusual noise on the machine, as this can eliminate the machine from breaking further

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#10

Re: Piston Compressor

02/09/2013 3:29 AM

At a quick skim read, it looks pretty good to me.
My only criticism is you don't justify your conclusion that the siezure was caused by..

this happened when the main and intermediate bearings seized onto the crankshaft.... Why do you reach that conclusion?
Later you add a nice section saying you can't know the actual sequence of events and some other stuff about valve seats., which rather contradicts the bit I've quoted.

E.G Are the bearings being seized the cause or a symptom?

I know I'm being rather pedantic...
Maybe it needs a separate section discussing the probable causes, then a more concise 'conclusion', even if it only states, "the exact sequance can't de assertained, but the siezure was probably caused by poor lubrication".
Maybe add a section with suggestions to prevent this happening again, precautions and remedies.

Overall it looks good to me... dunno what the lecturer wants... blood? Greasy thumbprints?
Del

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#11
In reply to #10

Re: Piston Compressor

02/09/2013 10:58 AM

this happened when the main and intermediate bearings seized onto the crankshaft.... Why do you reach that conclusion?
Later you add a nice section saying you can't know the actual sequence of events and some other stuff about valve seats., which rather contradicts the bit I've quoted. The statement above was witnessed on stripping the compressor. Due to the fact that the client had his own people working on the plant and the compressor had ceased, it was obviously difficult to conclude what really happened. This was a tough one, but thanhks for the vote of confidence...

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#13
In reply to #11

Re: Piston Compressor

02/09/2013 11:22 AM

I take your point, you are refering to the actual seizure, whereas your report points to the failure of the oil pump being the cause of the failure, which results in the seizure,
A subtle point I agree, but to me a seizure is a result not a cause.

We are in danger of getting bogged down in semantics, which is prob' missing the main point. But what you are saying is in effect...
"It seized when it seized!"
I'd rather you said
"It seized when the oil pump failed causing the bearings to run dry, overheat and seize.
Do you see what I mean?"

Having read it further.

Regarding the charge of being 'superficial':-

Maybe you should look further into the cause of the oil pump failure, after all it should be swimming in oil. Why did the bushes seize/fail/run dry or whatever happened?
Apply the same rigour that you applied to the whole compressor to the specifics of the oil pump until you can say an oil way was blocked, or the design doesn't feed enough oil to the bushes, or the design clearences are excessive, or the pump had run in excess of it's design life etc.
Del

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#14
In reply to #13

Re: Piston Compressor

02/10/2013 7:23 AM

The oil pump is not the problem here, but it also got damaged because there was no oil in the compressor sump, meaning it ran dry. Like I said, the oil might have washed away and the compressor pressure switch not working, because as the oil pressure drops the safety switch is supposed to cut the compressor off, which in this case it did not happen. secondly, when we got to site there was no oil in the compressor sump.

It seized when the oil pump failed causing the bearings to run dry, overheat and seize.
Do you see what I mean?" you got a point there.........

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#15
In reply to #14

Re: Piston Compressor

02/10/2013 7:38 AM

The oil pump is not the problem here, but it also got damaged because there was no oil in the compressor sump, meaning it ran dry.Ah! Why didn't I know this?
Ok I only skim read it, but a report should state the conclusion simply and concisely at the beginging and at the end.
Where did the oil go, why was there no oil in the sump... just keep asking why until you get an answer or you reah a point where it can't be answered.

So can we assume the sequence of events was.
Oil ran out (for reasons unknown) Oil pump siezed... bearings seized?
Del

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#16
In reply to #15

Re: Piston Compressor

02/10/2013 3:19 PM

I get your point, thanks man, I really appreciate your input and will take all the advice and use it. Thanks again..

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