4 replies to this topic

[Propacket]

Dear All,

GPSA recommends that the maximum ratio of compression permissible in one stage is usually limited by the discharge temperature. To reduce carbonization of the oil and the danger of fires, a safe operating limit may be considered to be approximately 300°F. Where no oxygen is present in the gas stream, temperatures of 350°F may be considered as the maximum. However, sometimes gas is very lean and cold such as overhead gas from De-Ethanizer. In this case, temperature of gas seldom increases to the values specified above (300°F/350°F) even if the compression ratio is very high. My question is how to limit the compression ratio per stage if temperature does not increase to 300°F/350°F while the compression ratio is very high.

Thanks

Edited by P.Engr, 30 August 2010 - 05:12 AM.

[Art Montemayor]

P. Engr:

Is this a reciprocating compressor application? I have to assume it is.

As has been discussed in our forums many times before, the ratio of compression in a reciprocating cylinder is kept at a level that ensures that the lubrication oil used in that same cylinder does not undergo decomposition or degradation. That is where the 300 and 350 ^oF maximum discharge temperatures come from. That is ONE guide to designing the operation of a reciprocating machine. There are other criteria that are also used in conjunction with fixing the number of stages of compression (and, therefore, the compression ratio of each stage) as well:

• The minimum (or optimum) amount of work done for a compression process is obtained by increasing the number of stages ((and, therefore, reducing the compression ratio of each stage);
• There are mechanical limitations on the mechanical stresses of the various components used in a reciprocating machine. These are reduced by increasing the number of stages (and, therefore, reducing the compression ratio of each stage).
• In many processes there is a process requirement to divide the total compression into stages so that other unit or process operations can take place at the intermediate stages of compression. A 2-stage ammonia mechanical refrigeration cycle with an economizer introduced at the discharge of the first stage is one example. There are many others.

I have done a lot of reciprocating compressor applications at low temperatures and have never had a dilemma with trying to “limit the compression ratio per stage”. Why are you trying to do such a thing? What is your intent or goal? What is the whole story behind this query? If you furnish this information, perhaps we can give you a more specific response.

[Propacket]

Dear Art,

Have a look on the attachment. It’s a reciprocating compressor for compression of overhead gases from de-ethanizer. As evident from the attachment, overhead gases are compressed from 240 psig to 1500 psig. The compressor power is 1613 hp and discharge temperature is 263.5 °F that is well below the limit 300°F. This implies that we can contain the compressor in one stage. But our client has suggested that it will be a 2-stage compressor due to high compression ratio. I was wondering that discharge temperature has not reached its limit. Then, why should we use 2-stage compressor?

In addition to the maximum temperature criterion, you have described three other reasons for multi-staging of compressors. Out of the three reasons you have described, the second reason about mechanical stresses might be the reason of avoiding high compression ratio as in my case. Can you explain what is the maximum compression ratio we can use in order to eliminate mechanical stresses?

Thanks

Attached Files

•  Compressor.pdf   27.01KB   135 downloads

[Art Montemayor]

P. Engr:

I respectfully request that my questions on a query be addressed or answered. It is very difficult to travel on a one-way street where there is no map to indicate where the side streets and alleys are - or if it is a dead-end.

In our endeavor to help or assist other engineers in our forums, we must take precaution that we do not inadvertently recommend or suggest something that is not only incorrect, but which can result in harm or a hazard to our members. That is why I don't like to guess or speculate when I respond to a query. We need ALL the basic data that we can get in order to supply a useful or correct recommendation or comment. And in order to accomplish that, we need ALL the story and basic data.

It is not possible for me or anyone - outside of the reciprocating compressor's manufacturer - to give a realistic and correct answer as to why the compression ratio can't be (or shouldn't be) increased. Often, as I've stated, other over-riding factors come into play -- factors that are beyond our control or knowledge. One such factor is the reality that no reciprocating compressor manufacturer goes into the business of producing these machines without an established inventory of pre-designed compressor cylinders on which his compressor frames are based and designed around. In real life, no manufacturer goes into a project thinking that he/she will design the required machine's cylinders to the user's specifications. What really happens is that the user MUST conform himself/herself to the available cylinder and frame designs on-hand, already designed for the specific application. If you were to specifiy that a manufacturer is to comply with your specification of a specific cylinder size and a specific compression ratio, I can assure you that you could not afford the resultant machine. The original, custom design and the required developmental work required would send the total manufacturing costs for the particular machine sky-high.

By increasing the number of compression stages you may be increasing the capital cost, but you will reduce the amount of work (energy) required for the compression and you will reduce the amount of stresses concentrated in less stages. The resultant machine should be a more rugged and durable model under the stated operating conditions and give more steady and consistant performance. The reciprocating forces will certainly be better contained and balanced in 2 stages than they would be in only one stage. The machines's "wear and tear" will be reduced. These proven, positive factors are worth a lot of operating cost monies and I have always had a bad time trying to convince operator/owners to invest in a rugged, dependable, and multi-stage machine rather than a cheaper, single-stage model. You apparently have a client who knows what he/she wants. I would not argue against the client's request.

[djack77494]

P,
There are no "brick walls" in engineering. As you move beyond the normal limits of what is used you tend to move into an area where economics and technical performance factors will slowly deteriorate. But, you don't "fall off a cliff". So it is with compression ratios. You may have heard of people refer to a compression ratio of 3 as being a "rule of thumb" limit. It's not, and neither is the 300F discharge temperature. They do represent regions where costs are likely to start steeply increasing as performance declines. But a compression ratio of (say) 4 might be totally feasible and even the best solution to some compression problems. So don't get hung up on the numbers.

If you want to have some fun, why don't you rerun your simulation using two stages of compression. For both cases, add in inter and aftercoolers to get the high pressure deethanizer OVHD to the desired temperature or else your not being fair and honest. Then let us know what you see. And by the way, don't underestimate the need for a much "beefier" machine to get to high compression ratios. That doesn't come cheap either.

Edited by djack77494, 31 August 2010 - 03:46 PM.