3 replies to this topic

[Darshan Soni]

Hi,

 

This question is just a result of my Curiosity, my be I'm totally wrong in asking this question.

 

Question: What will be the effect of increasing RPM of prime mover / Driver on the Driven machine (here specifically reciprocating compressors (CNG compressor), weather motor or engine driven) ? , If its possible to increase the RPM to enhance the performance, then which are the benifits of increasing RPM of prime mover on Compressor?

 

 

[ColinR33]

Darshan,

 

Speed control can be and is often used for pump and compressor capacity control, using either a VFD on an electric motor, governor on a steam turbine or speed control on an engine or natural gas turbine.  For centrifugal pumps and compressors there are affinity laws that help you estimate the capacity (as well as hp)  increases that will result from a speed increase.  Be aware that there are a number of limitations to the amount that you can increase speed for any piece of equipment.  A google search will provide you with many answers.

[Art Montemayor]

Darshan:

 

I presume this topic is related to another topic you raise in http://www.cheresources.com/invision/topic/20383-practical-ways-to-increase-the-reciprocating-compressor-cng-efficiency/

 

If it is, then we are discussing CNG compressors that are designed to operate intermittently, with 3 or 4 stages of compression and running at a relatively high rpm (usually directly coupled to an induction motor or a gas engine).  I do not believe it would be cost effective to contemplate converting an induction motor to a Variable Frequency Drive (VFD) on these type of small machines (approx. 150 kW).  As we say in Texas, the cost of the laundering would be more expensive than the shirt.

 

If you increase the rated speed of a reciprocating compressor, you increase the rated capacity of the machine – but you also increase:

• The demanded horsepower load for the machine;
• The interstage pressures;
• The interstage cooling requirements;
• The operating action (and wear) of all the compressor valves;
• The wear and tear of the piston rings;
• The oil lubrication for the bearings as well as the cylinders (assuming they are oil lubed);
• The piston rod loads on each of the individual stages;
• The wear and tear on all bearings, crosshead, and piston rod packings;
• The vibration and pulsation effects on the overall machine;
• The pressure drop between stages.

There may be other effects that creep in, but the above are the major ones I would be most concerned about – especially the rod loads.  I definitely would not go forth with this concept or idea of increasing your CNG capacity by simply changing to a faster (and higher Hp) driver.  In my experienced opinion your CNG compressors are not designed as heavy-duty, robust process machines.  You are limited to the nameplate capacity and operating conditions you bought them for.

[Darshan Soni]

@ Colin: Thank you so much.

 

@ Art Sir: Thank you so much.

Yes, my question was related to my previous post. anyway I got better idea from your answer.