Low pressure gas transfer 1

[Rauf30]

Hi all,
I'll really appreciate some help regarding equipment selection.
We are about to design an Associated Petroleum Gas Treatment plant. The plant will be located approximately 1.5 km from Gathering Station, and it is planned to lay two pipelines (high pressure 7 barg and low pressure) from GS to APGT.
Gas pressure at tie-in point to (inside of GS) low pressure pipeline is almost atmospheric (200 - 600 Pa), with max flow 4000 sm3/h. We are now considering the following options to transfer AP gas to the APGT plant.
1. Liquid ring compressor (preferably installed at APGT)
2. Blower or fan installed at GS and screw compressor or sliding vane at APGT
3. Ejector at GS (probably multiple ejectors) and centrifugal at APGT
4. Your option
Any experience sharing in transferring low pressure gas is highly welcomed.

 

[LittleInch]

Rauf,

You need to understand what it is exactly you're transporting.

Is this simply water saturated gas or does it have some free liquid ( water or H/C) in it? Makes a big difference to decide how you're going to transport it.

There may be 250kg/hr coming back but if this is all condensate stripped out of the gas it's a lot different to free water in the pipeline.

If it's a mist type liquid then you might need a coalescer or cyclone or maybe just a good de-mister pad in a separator or Knock out drum upstream your compressor.

If you want to look at types of compressors look at air compressors. They start at atmospheric conditions and compress gas to 7-8 bar very commonly. You can't just use an air compressor to compress hydrocarbons, but the types of compressor are similar. Just be wary of the temperature you will have d/s any compressor.

At 1500m of pipe you're not really looking at a big difference in cost and if there is no space or power at the GS, then you could just lower the pressure at the APGT below atmospheric pressure by using a blower, axial fan or other compressor. A lot depends on what happens to the gas when it gets to your APGT as doing it that way you have no capacity to run the gas through any filters, coalescers etc so you will end up with some liquid in your pipeline.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

It seems that some low pressure separator will be needed. Maybe something like a very large "drip pipe" separator that can be suctioned out every day or two.

 

[Rauf30]

LittleInch,

Feed gas is water saturated at operating P and T of 60C at tie-in point.

GS do not want to update their design docs, so it preferred to locate any required equipment at APGT.

[highlight #FCE94F]lot depends on what happens to the gas when it gets to your APGT as doing it that way you have no capacity to run the gas through any filters, coalescers etc so you will end up with some liquid in your pipeline.[/highlight]

Due to low pressure, I have no capacity to run the gas through any equipment even at GS site. That is why I am looking for proper compressor to increase the pressure.

 

[1503-44]

A drip pipe, or other type of simple separator won't need much pressure, so you can at least get the liquids and sand out before going into the compressor. At low pressure the bigger the separator the better. It needs low velocities to work properly.

 

[1503-44]

The above flow and pressures were for compressor at the GS.

If you place the compressor at the GT, the 4000m3/h through the 250mm pipeline has a high velocity of 47m/s or, going through the 500mm the velocity is 10m/s
Using a compressor discharge pressure of 2 barg at the GT requires around 200kW.

 

[Rauf30]

1503-44,

Thanks you for your calculations. Diameter of pipeline will be calculated on later stage, after decision of compressor type and location.

There is about 10 meter elevation difference sloop to APGT, so liquids in the pipeline theoretically can be collected at APGT area.

 

[georgeverghese]

Oil free rotary screw compressors may work okay at GS only if you are gathering and boosting clean LP casing head gas into HP line, and not production gas. If these are oil flooded screws, you can expect all sorts of mechanical problems with lube oil contamination with heavy hydrocarbons in the gas. Oil free screws have limited life span even when they work well, so you'll need sufficient inline and offline spare machines.

 

[1503-44]

You have to know pipeline diameter or you will not know your compressors suction pressure and/or discharge pressure. The items cannot be designed separately. They work together.

 

[Sesquipedalianist]

We had a similar problem which was easily solved by compressing the LP gas to HP value at the GS.

 

[Rauf30]

Hi Sesquipedalianist,

Could you please share a little bit more information? Like suction pressure, compression method and etc.

 

[horacio Torres]

From the process facilities and economics, install a centrifugal compressor at gas station with discharge pressure close to the amine gas plant treatment.

Be aware of the liquid condensation, Is water o c5+ ?????

Horacio