9 replies to this topic

[rona72]

Hi to all

I am dealing with reciprocating gas compressors design, for which i have to design piston ring. the machine is 4.75 x 7 esh. my arrangement of rings are 1 rider ring in the middle of 4 piston rings(2 rings on frame end side and 2 on outer end side. we ussually take clearance of .0625" clearance on both end of a piston.
what i wanted to know was the maximum allowed over travel for piston ring in worst case, i.e .025 clearance on frame end side.

regards

[Art Montemayor]

Rona:

I’m responding to this query because of my long and on-going romance with reciprocating compressors. I have been associated with reciprocating compressors since my first day as a young graduate, 51 years ago and have process-designed, specified, installed, operated, modified, reconstructed, and purchased many units since that time. I still like to tinker and work with reciprocating compressor applications.

I have never heard or used the term “over travel” for piston rings. Could you please explain what you mean by this term – preferably with a sketch.

I assume you mean an Ingersoll-Rand, horizontal, Model ESH, single throw,4.75” diameter cylinder x 7” stroke, non-lubricated, air compressor. Am I correct? I assume you are operating a non-lube cylinder because you have a rider band – a feature never employed in a lubricated piston. However, you may have “mini-lube” and if so, please tell us.

Are you using your machine’s original maintenance manual? If not, how are you guiding your decisions on clearance settings for the piston, rings, and cylinder? What do you mean when you state that you are “dealing with reciprocating gas compressors design”? Do you mean that you are putting up with an existing Ingersoll-Rand compressor design? Or are you saying that you are undertaking a re-design of an existing Ingersoll-Rand compressor?

Also, give us more basic data: a detailed Specification Sheet with stages, diameters, RPM, action, rated capacity, rated HP, installed HP, driver type, suction and discharge conditions on all stages, etc. etc.

[rona72]

Thanks Montemayor,

Yes you got it right, this is 4.75, x 7 ESH, horizontally mounted, Non- Lube Machine and process compressor of Ingersoll-Rand.

By term overtravel I meant the percentage of piston ring going out from line during stroke. we had a thumb rule that it can go up to 33 % of the width of piston ring (0.375" thk) outside liner.

Yes we are using original maintenance manual for clearance setting. and we usually take .0625" clearance on both side. while assembling of piston we go to 0.025" clearance at frame end side(this we call as worst case). so if I take 0.0625" at frame end and outer end the overtravel comes out to be 32% and which is acceptable as per our thumbrule. But if we go by worst method (0.025" clearance at FE), the overtravel comes out to be 42%. My question was how much overtravel is acceptable.

This is single stage machine with 655 RPM, rated capacity is 497 m3/ hr, rate power is 60.41 kw. suction pressure is 4.45 barg and discharge pressure is 16.09 barg. the gas is WET CO₂.

This much details I can furnish. hope it gives you clear indication of what I required. Do tell me if there is anything more you require; I will try to furnish those details.

Regards

[Art Montemayor]

Rona:

I am attaching a workbook with sketches that you can use to add call-outs, identifying what you mean by "overtravel" and indicate all clearances that you are referring to.

You have not told us:

• The material of the rings and rider band. This is very important information;
• The material of the piston; This also is important;
• The type of ring and rider band joint that you employ;
• Are the rings slipped onto the piston? Or are circular rings installed into a disassembled piston?
• The suction and discharge temperatures of the CO₂.

Await your response.

Attached Files

•  Reciprocating Compressor Piston Rings.xlsx   51.45KB   46 downloads

[rona72]

Thanks Montemayor

1.Carbon and carbon glass fiber filled, PTFE is the material for rider and piston ring.(DW298 Is the hoerbiger part number)
2. Piston is of cast iron
3. Angle cut, 1 piece ring
4. Yes, rings are sliped onto the piston
5. 122 F and 322 F are the suction and discharge temperature, respectively.

[size="4"][font="Times New Roman"]

Attached Files

•  untitled.bmp   151.3KB   24 downloads

[Art Montemayor]

Rona:

Thank you for the additional information. Now, I can understand what you are referring to. You made a basic important omission in failing to tell us that you are employing a cylinder sleeve (a “liner”). Not every reciprocating compressor cylinder uses a sleeve. I now understand that your leading piston ring is somehow reaching a position that puts it over the edge of the internal cylinder sleeve.

This is not a standard – nor a normal sleeve installation. There should not be so much clearance between the sleeve and the cylinder ends that it produces this effect. Did Ingersoll-Rand (or Dresser Industries) design this sleeve and piston – and the manner in which they are installed? I have a lot of issues with this mechanical design and personally would never accept it. I have never heard of / seen / or read about a piston ring (especially a Teflon one) “over-traveling the edge of a cylinder sleeve. I can see no logical or engineering reason to install a cylinder sleeve in this fashion.

I have used many sleeved cylinders in my time. I have even designed and had some fabricated and installed for me in some compressors – in so-called “developing” countries. But I would never install a sleeve that allows for overlap between its edge and a piston ring. It isn’t necessary and it is wasteful. The sleeve can be designed and fitted to avoid such a situation. Failing to minimize the clearance between the sleeve and both the crank and head ends is a major mistake because it increase the end clearance and diminishes the volumetric efficiency. The piston design is also bad. I would never allow a piston ring to be installed so close to the piston’s edge that it overlapped with the cylinder sleeve. The closest I have seen a ring installed close to the piston’s face is about ¾” – and that was a very small piston (about 2” in diameter). Your 4.75” piston diameter does not warrant, in my opinion, a piston ring installed that close to the face of the piston. I don’t know “long” your piston is, but if I-R designed it, I’ll bet that they did not allow for that to happen.

Is this the original I-R cylinder and piston design? Or has it been modified? What is the reason for not making a complete sleeve insertion into the cylinder and avoiding the end clearance that you have?

Basically, if I have understood the installation, I see no reason to accept the present situation of “overtravel”. There is no engineering reason or need for it to be present, in my opinion. There may be other factors here (I suspect there are) and perhaps those are what has caused this to occur.

I hope my comments help out.

[rona72]

Hi Montemayor,

I am afraid but still you havent got my problem. my piston ring is not at all crossing the edge of the liner. the liner has different id where piston travels, ie the id of liner is more than the id of liner where piston moves too and fro. i am attaching modified bmp, hope you will get my point after seeing that bmp file.

Regards
Rona

Attached Files

•  ARRANGEMENT.bmp   946.34KB   24 downloads

[Art Montemayor]

Rona:

I am afraid you have not read or understood my previous response:

• This is not a standard – nor a normal sleeve installation. There should not be so much clearance between the sleeve and the cylinder ends that it produces this effect.
• I can see no logical or engineering reason to install a cylinder sleeve in this fashion.
• The sleeve can be designed and fitted to avoid such a situation.
• I see no reason to accept the present situation of “overtravel”. There is no engineering reason or need for it to be present, in my opinion.

You have also not responded to my direct questions:

• Did Ingersoll-Rand (or Dresser Industries) design this sleeve and piston – and the manner in which they are installed?
• Is this the original I-R cylinder and piston design?
• Or has it been modified?
• What is the reason for not making a complete sleeve insertion into the cylinder and avoiding the end clearance that you have?

I do not see a justification for machining a “step” in the end of the cylinder sleeve. I don’t see what this accomplishes. That is why I suspect there may be other factors that are causing this effect, and we are not aware of them – just like the presence of a sleeve inside the cylinder was not mentioned in the original post.

Your sketch is fine; however, it does not explain why there has to be a step in the end of the sleeve and why the sleeve itself is not inserted such that it covers ALL of the internal cylinder surface.

[samrat]

Rona,
I have understood your question. My friend Art is trying you to understand your problem in detail, so that his answer would be more benefiting to your project.

This machine is a single cylinder single stage non-lubricated horizontal in construction.

In piston design there is a term employed as a "STACK LENGTH". In this axial length, you have got to fit the required number of piston rings and rider ring. The number of piston rings depends upon the pressure differential to which the cylinder is subjected to and the width of the rider ring is subject to the radial pressure. Generally 5psi is allowable in Non-lube construction.

In your design shown in, the piston rings are at the ends and the rider at the central portion. Since the cylinder is double acting you need to have adequate number of piston rings on FE(frame end) side as well HE(head end ) side of the compressor.

The general design guide line is that the ring projection should not overlap more than 33% of the width of the ring beyond the co-bore of the liner. What you have shown in the sketch is correct.

As far as the piston end clearances are concerned, the total clearance is divided into: 67% on HE side and 33% on FE side.

Piston design is dependent upon the construction of the cylinder(with liner or without), the head ends on both FE side and HE side. and of course the stroke of the machine.
I hope I have answered to your point.
While writing this on this blog I am equally impressed by Mr.Art's various communications on different subjects.
Mr. Art, kindly accept my salutation.
samrat

[samrat]

I do not know whether the explanations given has satisfied the presenter???
samrat