Compressor selection - Reciprocating vs Rotary Screw 3

[edta]

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

 

[zdas04]

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

 

[romke]

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.

 

[zdas04]

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

 

[Holocron007]

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?

 

[georgeverghese]

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.

 

[georgeverghese]

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.

 

[zdas04]

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

 

[georgeverghese]

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.

 

[zdas04]

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