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Reciprocating Compressor: Manufacturer's Compression Ratio


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#1 sheiko

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Posted 05 December 2016 - 06:28 AM

Hello gentle cheresources people (I'm back)

I would like to know if it is possible to get the actual/real compression ratio of a reciprocating compressor (one stage, one double-acting cylinder, size 8.5", rated capacity= 4775 Nm3/h, rated RPM = 370, Dresser-Rand), before starting up the machine?
Process people have specified compression ratio of 1.86 but I assume the real compression ratio is a function of the actual mechanical design from the manufacturer...right?

In addition I wanted to confirm that this compression ratio is independent of capacity. I mean is the compression ratio valid at 50/75 or 100% of rated capacity?

Thanks and regards

Edited by sheiko, 05 December 2016 - 06:39 AM.


#2 Bobby Strain

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Posted 05 December 2016 - 12:04 PM

And what is your real concern? Because you can get the information you ask by reviewing the compressor datasheet provided by the manufacturer.

 

Bobby



#3 sheiko

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Posted 05 December 2016 - 05:58 PM

Sorry Bobby my query is unclear
I would like to know:
1/ If compression ratio on the mechanical data sheet is the real mechanical compression ratio
2/ most of all, If compression ratio depends on flowrate (positive displacement pumps for example: discharge pressure is back pressure so depends on flow rate)

Edited by sheiko, 05 December 2016 - 07:48 PM.


#4 Bobby Strain

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Posted 05 December 2016 - 07:41 PM

1. Reasonably so. There are losses entering the cylinder and leaving the cylinder. The manufacturer provides appropriate power and capacity allowances for these losses.

 

2. The capacity is a function of compression ratio and inlet pressure. The pressure difference is important for rod loading.

 

So, what is your concern?

 

Bobby



#5 sheiko

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Posted 05 December 2016 - 07:55 PM

Thanks Bobby

Well my concern is that we have revamped an existing compressor with the following process data (I just changed job two weeks ago, now in operations after 5 years doing process economics, so I was not involved in the study). Design basis is as follow.
- suction operating pressure : 14.8 barg
- discharge operating pressure : 28.5 barg (compression ratio = 1.86)
- discharge PSHH = 32 barg
- PSV set pressure = 33.2 barg (while design pressure is 37.7 barg...and by the way designed with relieving flow rate twice bigger than 4775 Nm3/h because at the beginning of the project there were 2 cylinders...no comment)
- No discharge pressure control

Problem is that during startup the compressor suction pressure will be 20.8 barg (then decreased to 14.8). As a result if compression ratio remains at 1.86, then discharge pressure will be higher than PSHH and compressor will trip (if PSHH not desactivated). Process people say if we start at 50% capacity, the compressor will not trip as compression ratio should be lower than 1.86...I do not disagree since I believe compressors deliver flow and not pressure (so compression ratio will vary with flow), but I have read some of my old university courses and found that compressors deliver constant compression ratio...That is my concern.


Edited by sheiko, 06 December 2016 - 10:31 AM.


#6 Bobby Strain

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Posted 05 December 2016 - 11:48 PM

I hope you have operators who are experienced with reciprocating compressors. And, surely, you have a startup procedure. As well as an operating procedure. The compressed gas must be going somewhere and the destination pressure is somehow controlled. You should search for a good explanation of how compressors work. You and your process engineer colleagues are missing some fundamental knowledge in your analysis. You can find good information at Ariel website. When in doubt, enlist the services of the revamp company for startup assistance.

 

Bobby



#7 sheiko

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Posted 06 December 2016 - 07:18 AM

Bobby

 

Thanks for the advice. Yes there have been design errors. Of course we have all procedures and assistance.

 

My query was specifically about compression ratio and its evolution at different % of rated capacity. I was just confused by a statement (found in a high school course about reciprocating compressors) that compression ratio was constant whatever flowrate...but anyway...


Edited by sheiko, 06 December 2016 - 07:26 AM.


#8 Bobby Strain

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Posted 06 December 2016 - 10:52 AM

You will find good answers to your query here. See the discussion following EQ. 7.

 

https://pgjonline.co...ng-compressors/

 

Bobby



#9 sheiko

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Posted 07 December 2016 - 06:37 AM

Thanks Bobby
That article has cleared my doubts as "Reciprocating compressors will automatically comply with the system pressure ratio demands, as long as no mechanical limits (rod load, power) are exceeded." and that's make sense.
I will make some hydraulic calculations to better evaluate the situation

Edited by sheiko, 07 December 2016 - 06:55 AM.


#10 vigneshdk

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Posted 12 March 2017 - 11:12 AM

Hello

 

I have newly joined the group. just to add to above discussion I believe the compressor would be capable of handling the increased suction pressure however an increase in flow would be seen at the discharge end hence work done would be increase hence power... There could be a possibility to control the flow using clearance pockets if they are part of scope of supply... Team please give your thoughts on this.

Regards
Vignesh



#11 sgkim

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Posted 21 March 2017 - 11:22 PM

Hi Vignesh, 

 

Reciprocating compressors can be operable also at an elevated suction pressure, the increase in flow due to increased suction pressure shall be controlled by the control system provided such as suction unloading, discharge bypassing, motor speed control, etc.   Selection of these flow or pressure control system will be done by the manufacturer.   By these control devices motor overload and discharge over pressure can be avoided.     

 

Compression ratio could be determined by the optimization to minimize the operation (compression power and inter-cooling) cost.  But there are also constraints in discharge temperature due to the maximum service temperature of construction materials.  

 

~ Stefano     



#12 rychurek

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Posted 01 April 2017 - 03:46 AM

Compressor ratio is a design thing ( discharge temperature limitation, mechanical stress protection and so on). Discharge pressure has effect of performance but it's not capacity control parameter, lower performance in higher discharge pressure ( volumetric efficiency effect ) Discharge pressure is a function of system counterpressure, if you start from 14 barg suction pressure and no restriction on discharge you will have 14 barg on Discharge as well and capacity according to volumetric performance. Now you can/ gas consumer can throttle/ increase downstream pressure and your discharge pressure increase accordingly so volumetric performance drop a bit, compressor discharge temperature increase.




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