Slug Calculations and Slug catcher sizing

[MTQ80]

Hi Everyone,
I am working on Gas condensate pipelines and need support to solve one problem.
Brief description:- The 12” pipeline system has lot of NGL in the flowing gas stream, and Pipesim is predicting dropout of the liquid. The gas stream is saturated leaving the 2-phase separator at a pressure of ~ 50-70 psig, and the line goes underground and cools, causing liquid dropout. Around 300bbl liquid hold up is calculated by Pipesim.
Objective is to size the slug catcher. What pigging frequency should be considered? How fast this liquid will accumulate? What will basis of slug catcher design? How much margin shall be considered in slug volume as slug calculated by dynamic simulation would be more? Can anyone enlighten on these?
Regards,
MTQ

 

[BigInch]

you left out a critical number. You have 300 bbls of holdup, per what? 300 bbls/hour, 300 bbls/day, 300 bbls/MMSCFD?

If 300 bbls/day, then 7 days of ops would yield 2100 bbls. Still not an unreasonable sized vessel. Try 2 weeks, 4200 bbls. A month is 8400 bbls.. getting a little large. I'd use a safety factor of 2 for slug size, so 7 days of ops would need somewhere around a 5000 bbl vessel.

Always buy the biggest vessel size you can afford, or fit on the plot plan. That's a static cost, whereas operating costs go up for smaller size vessels, go on forever and will eventually surpass the cost of the vessel.

Independent events are seldomly independent.

 

[MTQ80]

Thanks.
Actually this holdup is calculated from Pipesim (SS run) and it calculates volume only. It don't tell accumulation rate and that's what I am looking for to avoid dynamic simulation at this time.
I need to know how much pigging frequency should be considered in selection of slug catcher. How fast this volume will accumulate? Is there any rule of thumb for this type of application?

Regards

 

[dcasto]

What did pipesim predict in slugging flow? It is very rare to see slug flow. The 300 bbl hold up is how many bbls of fluid there will be in the pipeline at equalibrium. Look at your oulet conditions and how many bbls/day of equalibrium flow there is and that will be your normal flow. If you pig the line, you will get all the 300 bbls of hold up in with the pig. Look a the velcity in the line and calculate the velocity in bbl/min (ft^3/min) and see how many minutes it will take to get the slug in. While the slug is coming in with the pig, you will lose gas flow for that time period.

All this gives you n idea of how fast you need to dump the fluid and some feel for the size of the vessel and dump valves.

 

[BigInch]

Slug flow is not unusual with flowlines from rich gas, or wells with associated water production. 300 bbls of liquid could be relatively light, but of course depends on the actual gas volume associated with that particular volume of liquid.

Independent events are seldomly independent.

 

[LittleInch]

Mtq 80 - You might want to avoid the use of a dynamic simulation, but this is the only real way you are going to solve your problem. The accumualtion rate could be hours or weeks - very difficult to tell without seeing the details. If the line also starts and stops then you will get a different slug flow compared to pigging the entire lot. Slug catcher sizing is a function of many things including the various operating cases and flows and crtically the liquid pump out or flow out flow rate from the slug catcher. Pigging will give you a big volume, but the usual get -out is to pig at a much lower gas flow rate when pigging and reduce pig velocity to 2-3 m/sec, hence reducing your liquid flow rate into the slug catcher. Whatever size you choose it will be wrong - either too big or too small. If you build a finger type one allow flanges and space to build another pipe or two on the end....

If you want a conservatve size for your slug catcher choose 300bbls, but in reality you could get a much smaller one once you do the design, which is a lot cheaper than a huge vessel or some type of finger slug catcher. Without a largest slug flow in either normal operation or start-up or the liquid pump out rate you can't size it properly. The other way to reduce voulme is to reduce pressure and increase gas speed to sweep more of the liquid out, but yu're starting at a fairly low pressure already so maybe that's not an option, but have you tried a different line size?. If your liquid accumulation rate is low, you could get away without a slug catcher providing you pig frequently, though this is normally not a recomended option due to the inherent hazards associated with pigging.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way