6 replies to this topic

[Tween]

Hi,

Kindly advice which is most preferable from the following:

 

1.Stage wise recycle line for reciprocating compressors

2. Common Recycle line for two stages (Reciprocating compressor)

 

Thank you.

[Art Montemayor]

 

You fail to explain (preferably in a sketch) why you need/want to recycle gas in a reciprocating compressor.  Before accurately advising you, the following is needed:

• Where, specifically are you proposing to apply the recycle?  - from the 2^nd stage discharge to the 1^st stage suction?
• What is the temperature of the recycle gas?
• What is the purpose of the recycle?  Is it to supply capacity control on the gas compressor?  Is it to maintain a positive control on the 1^st stage suction and avoid a vacuum being pulled?

 Kindly supply a detailed sketch of what you have or propose.

[Tween]

Sir,

Sketch attached for your reference.

 

 

Thank you

Attached Files

•  Sketch.png   21.41KB   20 downloads

[ankur2061]

teesan,

 

A common recycle line from the 2nd stage discharge is OK and there is no need for stage-wise recycle line. However the recycle gas from the 2nd stage discharge (129°C in your case) needs to be cooled to the suction temperature of the 1st stage (55°C in your case).

 

Here I would like to add what Heinz P. Bloch one of the foremost rotating machinery experts of our time has to say about about capacity control using recycle or bypass for reciprocating compressors in his famous book "A Practical Guide to Compressor Technology".

 

3.1.1 Recycle or Bypass

 

One of the simplest methods of controlling capacity is to recycle, or bypass, the compressed gas back to the compressor suction. This is physically accomplished by piping from the compressor discharge line through some type of control valve and going back to the compressor suction line. To reduce the flow to process, one simply opens up the bypass line and diverts the excess flow back to the compressor suction. In addition to being simple, this system has the advantage of being infinitely controllable (within the limitation of the size of the bypass line). Many, if not most process-type compressors have some sort of recycle line so that operators can fine-tune the flow to process.

 

A recycle system does, however, have shortcomings, the greatest being its inefficiency. Consider that we are taking gas at an elevated discharge pressure after having invested considerable horsepower in compressing it to that discharge pressure. Now we are going to expand it back down to the lower suction pressure simply so that we can invest more horsepower by compressing it again. As far as the compressor is concerned, it is always running at 100% load and is consuming 100% horsepower, even though the flow actually delivered to process could be a low percentage, or even zero. Another problem is in the actual design of the bypass line. The greater the percentage of bypass, the larger the piping has to be. In addition, depending on the particular gas characteristics, it may be necessary to include a cooler in the bypass line to dissipate the heat generated in compressing and continually recycling the gas. Also, consider the case of multistage compression where multiple bypass lines may be needed. The cost of installing an extensive bypass line could become prohibitive.

 

The most practical application for the bypass line is for small degrees of fine capacity control or for limited-duration startup unloading, where a simple loop around the compressor can be opened for a short period of time to relieve the initial compression load.

 

 

Regards,

Ankur.

[Tween]

Thank you Ankur ji

[Art Montemayor]

 

With all due respect to Heinz Bloch, unless you have multiple side streams at each compressor stage, there is no need or logic for having multiple bypass lines in the case of multistage compression.  In fact, multiple bypass lines can be a process hazard – as discovered in many hazops.

 

If you have a once-in, once-out, multistage compressor, you should only apply one recycle capacity control valve – from the discharge of the last stage aftercooler directly to the 1^st stage suction drum.

 

I would not recommend a bypass capacity valve be used as a shutdown or startup instrument.  The best and most secure way of stopping and starting a positive displacement compressor is by fully unloading it in all stages.  Total venting to atmosphere or to a flare (if necessary) is the customary best way to ensure the machine is protected.

 

[Tween]

Ok.Good day