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Compressor selection - Reciprocating vs Rotary Screw

Compressor selection - Reciprocating vs Rotary Screw

(OP)
We have a booster compression package for offshore platform. The natural gas to be compressed with the process data as below:

+ Flow rate : 9 - 25 mmscfd (1065 - 1900 acfm)

+ MW: 21.8 - 22.2

+ Suction pressure: 550 kPag (min flow) - 850 kPag (max flow)

+ Discharge pressure: 1700 kPag (min flow) - 2300 kPag (max flow)

+ Compression ratio: 2.52 - 2.77

From the preliminary screening, reciprocating and rotary screw (oil free and oil flooded) compressors seem adequate for above application. IMHO,the reciprocating is not preferred due to below reasons:

+ Large space

+ High vibration problem

+ Bulky and Heavy

+ Lower availability

+ much more wearing parts

The oil free screw compressor seems to have the same problems with noise and vibration. Hence, I prefer the oil flooded screw for my project due to its higher availability. But this type still have problem with expensive lube oil, which may be saturated with natural hydrocarbon gas. However, other guys in my team said that we may have the limit manufacturer and after sale supportif we go with the screw type (the platform is located in southeast Asia).

Any advice on the compressor selection for my project is greatly appreciated.

Thanks

RE: Compressor selection - Reciprocating vs Rotary Screw

The only machine I know of that handles varying suction pressure very well is oil-flooded screws. Recips want suction pressure to be ±5% of design pressure. Dry screws can handle nearly ±8%, but a small increase in compression ratios tends to break the timing chain.

Your ratios are very very low for flooded screws in hydrocarbon service. [You mixed your units so much that I'm not sure which ones you are more comfortable in so I'll just guess.] The difference between success and failure in an oil-flooded screw is managing the condensible vapors. Both water vapor and hydrocarbon vapor. Hydrocarbon vapor is kind of a go/no-go. I haven't had much luck cooking it out of the screw oil after absorbtion, so you have to pick an oil that does not act as an NGL plant and suck the heavier hydrocarbon vapors out of the gas stream. That tends to push you towards synthetic oil. Expensive, but not as expensive as damaged oil wrecking a compressor.

The other side of this is water vapor. Screw oil is very hydrophillic. It really wants to extract the water vapor from the gas stream. If you do nothing then after a week or so you will begin losing your brown fluffy oil down the line. What you have to do is manage the temperature of the oil out of the screw. That really is the only control feature that you have to get absolutely right. The rest is pretty small beans. I want the temperature out of the screw to be 205-215°F (with synthetic you can go higher without damaging the oil, but it is going to be hard enough to get to 205 with those compression ratios). Your only tool to do that is heat of compression. With low ratios I insist on a secondary oil cooler (NOT a cooler bypass "constant temperature valve"). Build a glycol loop that goes to the compressor cooler and put a VFD on that pump. Also put an automated bypass around the oil cooler on the oil side to eliminate all cooling during startup.

If you get that right, everything else becomes simple. There are a number of excellent packagers in Asia, but getting a good packager that understands the issues and is willing to address them is the key. I really don't care which manufacturer's screw someone picks, they are all pretty darn good. I care a lot which packager they pick, most are absolutely incompetent on screw packaging for field use (the ones that say "we've packaged 500 units for plants, we KNOW what we're doing" are the worst in the world at variable conditions).

I always want an oil pump on a field machine, at your compression ratios, it is not optional.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

(OP)
Hi zdas04,

Thanks for your kind advice.

I did the process simulation for this compressor. The discharge pressure varies from 106 - 114 deg C (218 - 238 F).

We have checked with the packagers in Singapore and they tended to propose the reciprocating compressors. It might be their suppliers/manufacturers want to push that kind of machine to the offshore user, or they have limit experience in the screw paackage.

RE: Compressor selection - Reciprocating vs Rotary Screw

I see that a lot. I've had clients send me preliminary drawings from several of the big guys in Singapore and mostly they looked really lame.

I don't know what kind of temperature control you put in your simulation, but if you used a 3-way valve to selectively bypass your cooler, then your simulation is wrong. Every run I've ever seen has been incredibly optimistic about the performance of those stupid valves. Mostly people set the them at 180F (sometimes 140F), which is the right inlet oil temperature for a very limited number of possible combinations of process temperature, inlet pressure, outlet pressure, and gas flow rate. Very limited. Hysys (for example) seems to be fine with it and simulations move all the parameters in some odd orchestration. Never seen a field machine behave like the simulation predicted.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

(OP)
We are the early state of the project, hence I've just made a simple steady state HYSYS simulation. The calculated discharge temperature at this stage does not take into account the effect of the injected oil. WE need more detail information from manufacturer if we decide to go with this type of compressor.

RE: Compressor selection - Reciprocating vs Rotary Screw

You've "simulated" an oil flooded screw without the oil??????????????? And you are willing to quote numbers from it?????????? I would say that you really need help. A lot of help. So basically you used HySys to calculate the adiabatic heat of compression for your less than 3 ratios? The multi-phase actual process is a tiny bit more complex than that. I think we've discussed how complex in this forum a few times.

I posted the method I use and Art Montemayor found a small glitch and described how to fix it, I can't put my finger on that thread right now (if I ever find it I'm going to put it in my archive, I've needed it a couple of times). It might be worth your time to try to find it. There is a lengthy Q&A at thread1036-183642: Wet Gas Screw compressor that you might find helpful as well.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

(OP)
As said above, the type of the compressor is not decided yet, and we are in the very early stage of the project (still not in the FEED stage) and still need a lot of work and help to do.

RE: Compressor selection - Reciprocating vs Rotary Screw

(OP)
And thank you for helpful reference

RE: Compressor selection - Reciprocating vs Rotary Screw

(OP)
@zdas

If the discharge cooler is not required by our process (ie: actually we need hot gas of the downstream of compressor) , what is your advice on the temperature control of injected oil. Thanks

RE: Compressor selection - Reciprocating vs Rotary Screw

Discharge cooler not needed? Remember that the oil is a mostly closed loop, so as you add heat of compression it has to go somewhere.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

(OP)
I mean discharge cooler for the gas stream only. We definitely need a cooler for closed loop oil

RE: Compressor selection - Reciprocating vs Rotary Screw

We use a synthetic oil on 1500 btu casenhead gas that is a dew point at suction and 4 ratios we have no issues with the oil. The discharge temp is under 180 degrees

Screws do have issues with high frequency vibration mostly related to the gear box and some with the rotors

Recips with pockets can take mire than a plus minus 5% on suction pressure

But all of those issues, you are still are better off with a screw than the issues with recips

RE: Compressor selection - Reciprocating vs Rotary Screw

dcasto,
Let me make sure I understand. You are saying that you have raw field gas with 1500 BTU/SCF and 180F discharge and have "no issues with the oil". I must say that that is outside of my experience. I have never seen a flooded screw with the discharge pressure that low that didn't: (1) lose a lot of oil; (2) have bearing failures, and (3) controlled capacity properly. Never. Not once in hundreds of field machines. Yours must be magical. If you don't reject the water and liquid hydrocarbons that disolve in the oil, bad things always happen. At 180 F out of the screw you are not rejecting any of these bad things. Did you mean 180F into the screw? That is a pretty common setting for the bullett on the Amont, and usually results in outlet temps over 195F that will often reject light hydrocarbons (but not much water).

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

Dang, I wish this site had edit capabilities. In the last post I meant to say "discharge temperature that low", and I wanted to finish my thought.

When you say "at dew point at the suction", lets assume that suction conditions are 14.5 psia and 140°F. For methane that means you have 9400 lbm/MMSCF in the gas. Now you add 4 ratios to 58 psia and at 180°F the dew point is 7,100 lbm/MMSCF--where did the missing 275 gal/MMSCF go? That is kind of a lot of water to just vanish.

If you had allowed the process temperature to drift to 206°F then the dew point would have been 10,800 lbm/MMSCF and everything that had come in would go out (with the gas at 87% RH) and be able to drop out in mechanical separators after the cooler.

I have not had the high-frequency vibration issue, but that is likely because I won't deploy a gear box with a screw package. The one time I gave in and accepted a speed changer gear box I got the biggest maintenance headache of my career. Oil leaks. Oil foaming. Running out of oil. Broken gears and bearings. It was the only gearbox in the fleet and I was never comfortable that the operators were paying enough attention to it, but that piece of kit was a serious headache and I was really happy when I connected the well to a different gathering system and no longer needed the higher discharge pressure and could send that machine home.

As to the ±5% claim, that comes from an unpublished study done at one of the major recip manufacturers. The decision not to publish came from marketing and when I talked to the engineer that wrote it he was not happy with the decision. His study found that when suction pressure was more than 5% from design conditions you were starting to see valve stiffness concerns that caused the suction valves to open early or late. A few degrees of crank angle late can increase the discharge temp by tens of degrees. A few degrees early can run you out of hp. His statistical data showed a sharp decrease in the mean time to failure with a definite direction change after about 5%. Will a machine designed for 100 psia suction eat itself if you give it 90 psia? No. Will it break earlier than it would have if you'd kept suction pressure above 95 psia? His data said yes. There are many thousands of recips operating outside this design limitation all over the world. This engineer says that machines that should last 50,000 hours without failure are having a higher frequency of failure during their 5 year overhaul cycle. Is it one in a thousand or one in a dozen? I don't know, I didn't read the report I only got a tirade about it over too many beers. Since that conversation I've been way more careful to operate recips as close to their design conditions as I could pull off and I think (I wish I had been keeping statistics) I have had fewer failures than I had before I started paying attention to design conditions.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

The suction is 30 psig at 45 oF the discharge is 160 to 220 psig (min back pressure of 100 psig to make the a100 synthetic oil run around). Heck we have flooded our slug catchers and ingested condensate into the screws. let the sit a while and drain enough oil out to get back on then let then hang in min flow till the condensate kicks out and put the oil back. I was like you, I didn't believe it till I saw it. We do like it better as we get the ratios up and the temp gets to 200 F. Of coarse, water is not a concern with the suction conditions.

http://news.compasscompression.com/2012/11/how-to-...

RE: Compressor selection - Reciprocating vs Rotary Screw

Dcasto,
I was fine with the link until he said

Quote:

Control lube oil temperature supplied to the compressor with a 3-way temperature control valve.
then I wanted to strangle him. For raw field gas, those stinking leaking valves cause more problems than any other single thing on a flooded screw. I hate them. They give you a good result for such a limited combination of flow rate, suction/discharge pressure, and suction temp that they do more harm than good in an environment where all of those parameters change dramatically from second to second.

Looking at temperature into the screw worked OK in plants with constant suction/discharge pressure/temperature and very consistent gas make up. In blood, guts, and feathers you are looking at the wrong parameter. You need to be managing the outlet temp, not assuming an outlet temp from a managed inlet temp.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. ùGalileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

i'm wondering when the original poster is going to start talking to the pump/compressor manufacturers' rather than to an internet forum. It's too complex a problem to solve with a closed book and some other company's pipeline/oil field.

RE: Compressor selection - Reciprocating vs Rotary Screw

zdas04-
Thank you for an interesting series of posts on this topic. I get involved in screw and recip compressors now and again, and your posts included some eye openers. I'll be taking another look at some of our machines now.

RE: Compressor selection - Reciprocating vs Rotary Screw

Glad I could help. I seem to have better luck with fab shops that have never built a flooded screw package than with shops that have been building plant machines for years. The "experienced" shops always fight me on temperature control, the need for an oil pump, and the location of suction/discharge flanges (to say nothing of speed control). Shops that are new to screws only have to be un-taught the stuff in the screw manufacturers packaging guides (which universally lead to horrible skids).

David Simpson, PE
MuleShoe Engineering


In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

(OP)
I did meet some packagers from Singapore. Most of them are salesmen and have strong desired to push the recip. machines to our project. The others in my project team (with no back ground in process or mechanical, only project management in general) likely to go with the recip.

I am tired of fighting with those men and let them decide what they want.

RE: Compressor selection - Reciprocating vs Rotary Screw

Frustration with packagers is not the best criteria I've ever heard for choosing a technology.

There are people in the world that can help you with getting the package you need. When I write proper compressor specifications into the pre-FEED, it is less of a fight in the FEED and I have a much better track record of deploying appropriate technology. In my opinion you simply have too much variability in your specifications to ever be able to deploy an effective recip or centrifugal. This should be an oil-flooded screw application. Period. You need to find a consultant who believes that factoid and get a skid design that works for your specifications (if it doesn't have the ability for the PLC to change driver speed and if it doesn't have very flexible temperature control then you have the wrong consultant).

I've done this evaluation for clients all over the world and it is quite doable, but you have to start with a list of issues that you will not compromise on and then don't compromise on teem even when the fab shop salesman says "What nonsense, we do recip packages with that range all the time, you just have to manage your suction pressure" which translates into a design point at your lowest suction pressure and a BIG dP across the suction controller forever, or (even worse) the design point at your highest suction pressure and a recirc valve supplying makeup forever. I've seen both and the first one knocks 25% off the fuel-efficiency of the package and the second knocks 60% off. I see a lot of recips with a skid-edge efficiency under 30% (i.e., the work required to go from field suction pressure to field discharge pressure is less than 30% of the driver output). That is a big premium to pay for decades because someone didn't want to learn how to package a screw.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

if you want to compress natural gas, you cannot use a luboil based on conventional mineral oil because the gas will be partly absorbed in the lubricant which renders the lubricant more or less useless. what you need is a compressor specifically designed for compressing natural gas, either using a suitable synthetic lubricant that will not act as a diluent for natural gas or no lubricant at all. the lubricant free types will exhibit more wear then lubricated compressors, but might be the preferred option.

my suggestion is to talk to people that actually build natural gas compressors and have field experience with them - compressing natural gas comes with very specific problems that need a lot of attention.

RE: Compressor selection - Reciprocating vs Rotary Screw

Not quite right romke. Mineral oil has a significant affinity for butanes and heavier, but it does not tend to absorb methane and ethane. Many of the shale plays and all of the CBM plays have done well with mineral oil. Conventional and tight gas plays have had horrible results with mineral oil.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

edta,

What compression technology did your team settle on? Also, was this for an associated/flare gas booster to process or fuel gas booster for power on the platform?

RE: Compressor selection - Reciprocating vs Rotary Screw

Have yet to come across a oil flooded screw compressor in other than clean service applications. And the gas has to be completely solids free. Offshore natural gas will most likely have all sorts of corrosion inhibitors ( both in the vapor phase and in the liquid phase ) in it, so I cant begin to imagine what that does to the lube-cooling oil - the oil-water separator will not work with emulsion stabilising chemicals in it.

RE: Compressor selection - Reciprocating vs Rotary Screw

This problem with poor oil-water separation will persist even with a synthetic oil is used to avoid co absorption of heavy hydrocarbons from the gas stream. These emulsions are chemically stabilised and will resist resolution with enhanced liquid liquid separation devices such as coalescers which may be built into the oil-water separator.

RE: Compressor selection - Reciprocating vs Rotary Screw

georgeverghese,
Come visit the San Juan Basin of Northern New Mexico/Southern Colorado. Before I retired in 2003 I operated (quite successfully by the way) 50,000 hp of oil-flooded screws in a CBM field that produced several tons of coal (through the compressor) into the gas gathering system every week. We pigged out nearly as much coal as water, and we got a lot of water. Our operating costs (including maintenance, oil replacement, and fuel) was just under $0.025/MSCF and by 2003 (year 6 of the project to install screws) we were below one failure/10,000 operating hours. Today this basin has something like 200,000 hp of oil flooded screws running in raw gas service.

My consultancy is global, and I see new screws being installed in raw-gas service all over the world.

I don't even know where to start with your emulsion comment. Mostly we don't see them often and when we do they seem to be less of a problem than condensible vapors.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Compressor selection - Reciprocating vs Rotary Screw

Guess a lot rides on the well casing and tubing corrosion management strategies in the application - it may be that there is little or no corrosion inhibitor / production chemical injection in these onshore CBM fields you've worked in?

For offshore oil and gas fields, it is very likely that CI injection plays a big part in reducing the CAPEX for the gas and oil wells with carbon steel outer casings, especially ones with high CO2 and/ or H2S. From what I've seen so far, gas produced from CBM fields is lean and is low on CO2.

RE: Compressor selection - Reciprocating vs Rotary Screw

Saturated with water (around 10,000 lbm/MMSCF) and 28% CO2. Never seen CO2 corrosion in CBM (every time someone claims that it is there it turns out to be MIC). Injecting corrosion inhibitor into gas lines is a recipe for spending lots of money with zero return (the gas cannot transport the chemical to where it is needed so it accumulates in early low spots). Lots and lots of solids. Screws are the perfect choice.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

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