Pipeline Compression: Multistage vs. Single Stage

[megonzalez]

Hi everybody!
I would like to know if any of you have worked with Pipeline Centrifugal Compression.
A couple of P/L Engs. have agreed on Single Stage Compressor stations are more efficient than Multistage when you are evaluating the P/L system. They have also agreed on 1.4-1.5 Compression Ratio per station (single stage i.e. no need of aftercooling).
I assume the aftercooling makes the whole system inefficient due to the energy required to cool the gas down.
Do you have any experience/ personal opinion on P/L Compression Stations? Low CR and multiple compressor stations along the P/L, or Higher CR with aftercooling and fewer stations?

 

[pmover]

yes and many!

Most PL compression units are single stage units (low-head, high flow units) at the compression ratios you stated. Aftercooling requirements depend upon pipe coating, cathodic protection requirements, and it is more efficient to cool the gas (takes less HP to compress the gas).

Another factor to consider is whether or not the compression unit needs a surge control system. There are compression stations with centrifugal compressors with and without surge control systems. In the event of a low-flow scenario or operation, some of the gas may be recycled, thus raising the inlet temp to the compressor. If the station or unit is to be tested (emission testing, unit performance/compliance test, or otherwise) and the capacity is not available at the time of test, then gas cooling with recycle may be needed.

hope this helps! good luck!
-pmover

 

[dcasto]

Ratios of less than 1.7 typically have better efficiencies. So thats a correct value.

There have been lots of studies on the ratio versus stations, versus pipe cost, versus operating costs. These all come out with the same 1.7 ratio of less ratio on each station as the optimum. Station distances optimizations come out at around 100 miles. You can do your own economic analysis with these few starting points.

You should cool the gas as much as you can before going back into the pipeline to minimize pressure drop. A hotter gas will have a lower density and a higher velocity and more pressure drop, which costs you compression energy.

 

[ad90]

Most Pipeline Booster Stations in the UK are single stage machines, I can't think of any that have after-coolers.

The most efficient machines are the axial inlet type; Dresser-Rand Pdi and Rolls Royce RFA.

I would suggest that every cetrifugal compressor requires an Anti-Surge system. If the station/machines are designed correctly i.e. they are not ruinning in partial recycle and the loop is long enough to provide the heat sink for start-up, there should be no issue with requiring to cool the re-cycled gas.

The restrictionn in going for less/mile compressors and higher pressures are the design codes for the country you are in e.g. IGE TD1 in the UK. Offshore/sub-sea pipelines operate at considerably higher pressures, the one I am working on right now is rated at ~190barg, as opposed to the maximum onshore Uk pipeline being rated at ~85barg.

Regards

Andrew