Reciprocating vs. Centrifugal Compressors 1

[BusyCEO]

Can anyone please tell me the relative benefits & drawbacks of reciprocating vs. centrifugal compression. Also, which ones are more common for offshore/onshore applications?

thanks,
Mack

 

[CRG]

What is the application, fluid, pressures, etc?

 

[zdas04]

Reciprocating compressors are very effecient, very rugged, and very well understood by field operating people. Single stage recips are the baseline for adiabatic effeciency. For design purposes I use 4.5 compression ratios per stage as about the max although some machines are designed for somewhat more or less. Two stage machines require about 6% more Hp per cubic foot compressed. Three stage machines need about 14% more. A recip will develop very high temperatures on the cylinder outlets that must be accounted for in the skid design. The weakest component in a recip is the valves - compressor manufacturers like for operating conditions to be within +/- 5% of the design conditions which is doable in plants and pretty tough in field, non-air service (in most compressed-air applications the suction pressure never varies much at all). What I see as the other main drawback is balancing stages. If you get a machine set up (i.e. the right valves, clearances correct, etc) for one set of conditions and then suction or discharge changes you often end up doing all the work in one stage and really reduce the capacity of the machine while increasing the risk of mechanical failure. I like to use 40 psig as the lowest suction pressure where a recip is my first choice.

The two most common rotary compressors are centrifugul and screw. Centrifigul compressors are very common in service where the discharge must be oil-free (recip's inject oil into the process stream to lubricate the cylinders). It is an easy thing (during design) to stack stages to get any desired ratio of skid suction to skid discharge pressure. Temperatures must be carefully managed across the stages. A centrifigul is about as effecient as a 3-stage recip. The are often used at the inlet to a process where oil contamination would be a problem.

Screws come in dry and oil-flooded. Dry screws are slightly less effecient than a centrifugul and are typically used in small applications where a centrifugul would be too large. Dry screws connect the male rotor to a driver and use a timing gear to drive the female rotor. They have all the temperature problems of a centrifugul, but the high temp is limited to the single discharge stream. A dry screw can do up to about 6 compression ratios. They can more easily change operating conditions than a centrifugul since the screw is positive displacement and a centrifugul is not.

Flooded screws have been around for almost 30 years and are my first choice for suction pressures below 40 psig. They have an oil flood that seals around the rotors, lubricates surfaces, acts as a cushion that allows the mail rotor to drive the female without a timing gear, and carries heat away from the screw. Ten compression ratios will increase oil-temperature about 35F in a flooded screw while the temperature increase in a dry machine for 10 ratios is closer to 380F. I use flooded screws anywhere the suction pressure needs to be between a moderate vacuum (say 12 inHg) and 40 psig and suction conditions could reasonably be expected to fluxuate. They can usually handle 10-15 compression ratios without much problem. Flooded screws are just slightly less effecient than single stage recips (the oil seal and the lower temperatures account for the dramatic improvement in effeciency). The biggest drawback is is the oil. It is expensive, you have to carefully manage the temperature to keep from collecting water in the oil, separation of oil and gas is harder than most skid packagers understand, it takes a non-trivial amount of hp to move it, and it is really a pain to deal with if it gets off the skid.

Hope this helps.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[olmedo]

Reciprocating compressores are limited to smaller capacities while for larger capacities use centrifugal. Also reciprocating compressors produced more vibrations and noisier.

 

[ccfowler]

Reciprocating compressors can adapt most readily and economically to applications where the flow requirement varies widely. Some provide unloading cycles where the power requirement is very small. Others can operate with incrementally adjustable flow capacities with very good efficiency.

In an application where there is a substantial "base load" flow requirement but significant fluctuations in the flow, a combination of a centrifugal base load compressor with reciprocating compressor(s) to cover the varying portion of the load is likely to provide the optimum solution for both initial cost and operating costs.

Within reason, relatively large slow speed reciprocating compressors are likely to be the most efficient alternative for a given duty, and both efficiency and durability are improved by having multiple stages with suitable interstage cooling. Weight, size, and initial cost are usually the main negative issues for them.

Bear in mind that all of the advice and comments here are generalizations, and every type of compressor (and combination of compressors) should be given due consideration for any specific application. The only certainty is that there is no single "best" type of compressor.

 

[zdas04]

ccfowler,
I don't think I understand how "reciprocating compressors can adapet most readily and economically to applications where the flow requirement varies widely". I've heard this before, but the detailed explaination never supported the basic assertion.

For example, take a well making 1 MMCF/d with a design suction of 20 psig and a design discharge of 200 MCF/d. My compressor wheel says this is a 130 Hp, 2-stage recip doing almost 6.5 ratios. Let's say we configure a recip to go from 20 to 70 in the first stage (2.5 ratios and 61 Hp) and 65 to 200 in the second (69 Hp and 2.7 ratios). Now a downhole event happens that causes the liquid level in the wellbore to drop 80 ft and the compressor now sees 1.4 MMCF/d and 50 psig suction (some of the gain in hydrostatic backpressure is eaten up in increased friction). This is now a single stage application (needing 106 Hp and 3.4 ratios). What happens in real life is the first stage takes us from 50 to 146 (still about 2.5 ratios) and the second stage goes from 141 to 200 (1.3 ratios and the Hp is off the wheel low) which will put the first stage in rod load and the second almost in blow-by. These numbers are in the realm of what happens 20 times a day in a typical wellsite. I've run manufacturer diagnostics on hundreds of 2 and 3 stage recips on wellsite applications and they ALWAYS look something like this example.

Same scenario with a flooded screw operating with a turn valve (or slide valve it doesn't matter) controlled by the PLC and at design conditions it is setting at 85% - when the downhole event happens, the turn valve goes to 70% and everything is just fine. No rod load, no non-reversal event, and no valve problems.

Call me strange, but the screw example seems more adaptable.

One interesting aside, I was looking at the MMBTU/hp vs. engine load curve on a "large low speed integral" recip last week and was suprised to note that at 60% load, the fuel requirement is more than twice the fuel requirement at 100%. These machines have a strong reputation in the Oil & Gas industry for operating at a wide range of conditions, but is it justified?

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[ccfowler]

zdas04,

Reading your comments reminded me of a very important omission in my comments. The required operating conditions! My comments were intended to apply where the suction and discharge conditions remain reasonably stable. Substantial variations in either of these can have dramatic implications as you very rightly pointed out.

Suitably applied, those big, slow reciprocating compressors can have seemingly infinite service lives with very good efficiency and relatively modest maintenance requirements. The important concept here is "suitably applied!"

Unfortunately, the well-deserved reputation that recips have for durability and ruggedness can be unwisely translated into a presumption that they can be casually applied with little risk of failure. That can be a very costly and needless mistake.

A common problem for many situations now seems to be an excessively simplistic emphasis on either efficiency or initial cost (or sometimes both) without corresponding concern for true suitability, durability, the implications of transient or off-design conditions, etc.

 

[zdas04]

CCFowler,
I can think of a lot of cases where a recip is the best (often only) compressor for a particular job. My problem keeps being that folks want a rubber-recip that will seamlessly handle a wide variation of suction pressures and flows. There is a company (I can't remember their name) that has recently started selling a "screw-cip" which has a flooded screw and a single-stage recip driven off the same driver. I haven't seen one of these in real life, but the concept is the closest thing I've ever heard of to a rubber compressor.

Technology is not good or evil (in a moral sense), it just fits a given application or it doesn't. I've had a lot of folks look askance at me for deploying almost 40,000 hp of flooded screws in 1997, but today that field has lower reservoir pressure, higher rates, and better run times than any of the folks that tried to do the same task with two-stage recips. Flooded screws really are ready for prime time, and I have an awfully hard time convincing folks that screws might be a better answer for a particular problem than a big, slow, reliable, integral recip. I'll keep banging this drum, but followers are slow to get out of their chairs.


David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

 

[BusyCEO]

David/CCFowler,

Thanks for all the info - sounds like you guys have quite a bit of experience with compression. Im wondering, for gas gathering systems, what percent of installations would you say are recips versus centrifugals? Are centrifugals more commonly used in offshore applications or downstream of a pressure regulator or separator (ie. no pressure fluctuations)?

thanks,
Mack

 

[zdas04]

Centrifugals are used offshore a lot. This is generally because a centrifugal with a gas-turbine drive has much lower moments and couples that have to be braced than you would see with a recip and/or a piston engine. You could probably do the same thing with a flooded screw with better skid effeciency, but tradition is a tough nut to crack. Centrifugals also very common for plant-inlet compression (for very good reasons). Other than that, if I see a centrifugal onshore I'm pretty confident that the guy who spec'ed learned everything he knows about compression offshore.

For gas gathering systems onshore, recips make up something like 90+% of the installed horsepower. Many times this is simply sound engineering that picked exactly the right machine for the service, but sometimes it is purely cultural and someone crammed a 3 stage recip in flucuating-suction service when the "right" answer would have been a flooded screw in front of a single-stage recip.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com