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compressor selection and configuration for natural gas

compressor selection and configuration for natural gas

hi all,

i want to ask about how to choose compressor type and their configuration ( 1x100%, 2x50% or 3x30%). this is for the natural gas field development ( 96% Methane, 65 MMSCFD, specific gravity 0.57, ACFM 3025 )

at the beginning of production, in the gas processing plant, the suction pressure is around 500 psi, the lowest expected suction pressure is around 200 psi at the end of life of this plant, the discharge pressure desired is around 1200 psi. suction temperature is 100 F.

due to economies aspect, the natural gas field shall be produced for around 5 years until the pressure in the suction compressor is 200 psi.if i evaluate fig 13-3 GPSA (compressor coverage chart) there are only 2 options, either centrifugal single stage or reciprocating multistage

my questions are :

a. what should i evaluate and compare to decide to use centrifugal single stage or reciprocating multistage (single stage first and then dual stage ? ) the engine for the compressor, maintenance, cost, fuel gas consumption ? what is the most critical aspect ? because this is for natural gas processing plant which the pressure will decrease over time.

b. how about the configuration ? since the configuration will affect the horse power needed. what are the critical aspects that affect the configuration ?

i need your advice to decide the type of compressor and the configuration to compress natural gas from 200psi / 500psi to 1200 psi (7100 hp needed, mostt conservative if single stage), honestly i am not familiar with the common engine power and compressor available in the market. i need your info :)

thank you very much :)

RE: compressor selection and configuration for natural gas

You have to be very careful using GPSA figure 13-3 for anything. For some inexplicable reason, the scale is ACF not SCF. Disregarding compressibility I get 1290 ACFM on your 500 psig suction case.

Your application (assuming sea level) starts out at 2.3 compression ratios and goes to 5.7 compression ratios. 2.3 is pushing the upper limit of a single stage centrifugal, and it is pushing the bottom limit of a recip (there is no real bottom limit, but below 3.5 compression ratios the compression efficiency falls off rapidly (at 1.5 compression ratios the compression efficiency is assumed to be 50% and above 3.5 ratios it is assumed to be 92%, intermediate values can be prorated).

I like dynamic compressors on clean, dry, predictable gas like you get in a plant outlet. I've seen a lot of them in raw field gas service offshore and if there was an alternative with similar energy density they would never have been installed. Maintenance frequency and cost on those machines is horrific. They just don't deal well with changing fluids, changing conditions, changing fuel gas. But they do pack a lot of power into a very small footprint. The only reason to chose them for raw gas is if space is very limited and expensive.

Your hp at 500 psig suction is around 3600 hp. At 200 psig it goes up to around 7800 hp (both of these numbers from a compressor wheel). With 100F suction you can do the 500 psig in a single stage recip, but that will only take you down to about 275 psig before you run out of discharge temperature.

I would put a site suction controller and design my compression for 180 psig suction. That would take 8500 hp. I would probably do it with 5 CAT 3520B LE compressors driving 2 stage recips. There are much bigger machines, but all machines need maintenance, all machines break. You really do not want to shut in your whole field because a turbo ate itself.

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 and configuration for natural gas

Is the flowrate at 200psig suction press still 65mmscfd?

RE: compressor selection and configuration for natural gas

There is an important info you did not indicate;
The flow profile change with start (phase I) / end (phase II) of production.

Assuming the flow stays constant, which normally is not true with respect to behaviour of gas production well. Zdas will correct me.

Also at the end of production you need an intercooling as discharge T > 200 degC by far.
So I would consider centrifugal compressor with a two sections, i.e. LP/HP, casing type x 2 similar trains;

The flow I have in phase I is approx 2400 m3/h. In phase II it is 6250 m3/h approx. making some assumed gas composition.
Phase I : Trains operation 1x100%, the other in stand by. LP and HP sections are operated in parallel. No intercooling.
Phase II: Trains operation 2x50%. For each train, LP and HP sections are operated in serial with intercooling.

If flow profile changes with time, which I would expect, you normally have the end of production with a lower MMSCFD.
In this case you might be fine with one train subject to feasibility if all performance can fit in the envelope etc.
Most likely a variable speed application.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

RE: compressor selection and configuration for natural gas

Mother Nature gives you what it is going to give you. There is nothing constant about field production, and of course the tendency in conventional reservoirs is for flow rate to decline from day one and for unconventional reservoirs to take some amount of time (often years) for the reservoir flow paths to "stabilize" (with increasing production) before starting a period of production decline. Typically we deal with that variability offshore by taking a 200-600 psi dP across the suction controller since centrifugal compressors have a very tight suction pressure range.

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 and configuration for natural gas

dear all,

thanks for your responds :)
herewith the forecast

End of Year - Compressor Suction Pressure (psig) - Flow rate (MMSCFD)

1 - 500 - 65
2 - 450 - 63
3 - 380 - 60
4 - 300 - 58
5 - 255 - 54
6 - 200 - 50 (end of production)

* section temperature is constant 90 F, Desired Discharge pressure 1200 psi
* Onshore . 96% Methane. the gas comes from 8 wells

i am stil confused to choose between multi stage reciprocating or multistage centrifugal. but i tend to choose reciprocating because as far as i know, centrifugal compressor has a surge point or in other words it has minimum flow rate. because all wells will not on line on the same time (maybe in the beginning it is just 4 wells (30 MMSCFD), it may belows the surge point. am i right ?

how about the configuration of the compressor ? what is the criteria to decide the compressor configuration ? I think this is related with the reliability of the compressor. is it ok if i just used (2 x 100%, 1 stand by ) ? if the compressor is in Multi stage (HP & LP), do i have to provide the back up compressor for HP and LP ? or is it enough to provide 1 back up compressor ?

btw I need a software from compressor vendor and engine, i just can find the Dresser Rand software (reciprocating) for free from the internet. could you please tell me the software for centrifugal compressor available for free on the internet ? i need to know the surge point limit from stated condition. i also need the gas engine or turbine software / spreadsheet from vendor (if any) to estimate the fuel gas consumption and condition. i hope you can tell me if available :)

because to be honest i am new and not familiar with compressor and prime mover, i hope i can develop my sense of engineering

RE: compressor selection and configuration for natural gas

Okay, so we have declining production.

For wet natural gas production, recips can be problematic, since the gas is at dewpoint and it is not easy to maintain feed gas in dry conditions. Vane type demisters in upstream separators make things worse, and so do pulsation snubbers with internal low points.

RE: compressor selection and configuration for natural gas

Think you should be able to do it with variable speed centrifugal compressor.

You need two similar trains.

Each train has one casing compressor. Casing is equipped with LP and HP stages with in and out for intercooling.
Each stages can have about 4 wheels maybe spinning at 14000 rpm rated speed.

Start of production, you are low head and low flow (2400 actual m3/h roughly) so you run one train only.
It means 2 x 100%. No intercooling required. You run at low speed then like 70% or so.

End of production, you are high head and high flow. You split the flow between the two trains and you run at high / rated speed.
It means 2 x 50%. You need intercooling between LP and HP.

The feasibility needs to be checked with Vendor, in particular in terms of;
- First and foremost, the concept proposed is feasible.
- The compressor will cope with variation of production, you check all operating point vs. time are covered.
- The 4+4 wheels is challenging rotor-dynamically, needs verification. otherwise you need two casing trains.

There are other ways to do it. You could use a 3 trains configurations. In which case you run at start of production with one train (3 x 100%) with LP and HP stages configured in parallel. At end of production you run 3 trains (3 x 33%) with LP and stages configured in serial. Inconvenient of this option is:

- Use more trains. so more investment, footprint and operational costs.
- Less power efficient as you are reducing the flow capacity through each train.

Again and again, cannot afford to not check with manufacturers.

"If you want to acquire a knowledge or skill, read a book and practice the skill".

RE: compressor selection and configuration for natural gas

If you end with Multi-stage centrifugal compressor, you have to consider the following:

Anti-Surge Valve= Yes
Gear Box = Yes
Dry Gas Seal = Yes
Control System = Yes
Alarm (faults) = more
Maintenance cost= high
Nitrogen plant = Yes

Needs for Reciprocating compressor
Anti-Surge Valve =No
Gear Box =No
Dry Gas Seal =No
Control System =No
Alarm (faults) =less
Maintenance cost=Low
Nitrogen plant =No

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