Two-Phase Separator Sizing - Hold up time criteria ....What's the concept behind it?

[Sinadi2003]

Hi all;

I am verifying the size of a two-phase separator (Gas/Liquid) vessel. There are two criteria which should be fulfilled. The terminal velocity in gas section and hold up time for liquid service.

I am struggling with the hold up time or height difference in liquid levels between each levels setting:
The criteria for liquid section; i have got here is as follow:

Description Symbol Requirement

High Level Shut Down HLSD 1-2 minutes with minimum 100 mm (from HLL to HLSD)
High Liquid Level HLL "liquid hold up time to be considered with 300 mm
min (from LLL to HLL)"
Low Liquid Level LLL 1-2 minutes with minimum 100 mm (from LLSD to LLL)
Low Level Shut Down LLSD 150 mm mini, but to becompatible with time required to close a
SDV

Now what my question is?:

Does anyone know what is the basis for 100 mm or 300 mm height differences in between the levels ? how about basis for time?

The vessel I am checking the size for, doesn't meet the above liquid criteria and I wanna to rationalize/ justify to ignoring these criteria. I mean not really ignoring, but changing a little bit in the height.

Any light in this shadow would be much appreciated.

Cheers;
Sinadi2003

 

[apetri]

the design depends from flows and properties of fluids so you may find separators of very different sizes with different operating conditions and levels,
it is difficult to answer without detailed information, however the "100 mm or 300 mm height differences" could depend from a specific process request or sensor type, for example a (magnetic) float level sensor, I have seen these spec's in petrochemical projects.

 

[25362]

Have a look at thread124-155681.

 

[Sinadi2003]

Thanks apetri and 25362 for your replies.

 

[MortenA]

I think that you focus a lot on the stated minimum distances - look more at the retention times.

Generally .- between HLSD and HL alarm you dont want that much time (hight difference) (it cost money) - but the accuracy of the instrument, waves etc should not trip your system. Also you would want some time for your operator to react.

- The volume between Nl and HL alarm(and NL and LL alarm) is where you would normally "absorb" you variations in flow. Here its important if your separator e.g. is an inlet separator where slugging from wells or pipelines may cause variation in liquid flow.

The level at NL is normally what you base your separation calculation on.

Best regards

Morten

 

[TD2K]

Those minimum guidelines are more for fabrication and control reasons.

The 150 mm minimum setting for the bottom LLSD is the distance above the lower tan line and is usually the minimum the mechanical engineers like to have nozzles and the resulting welds close to the circumferential weld for the bottom head.

100 mm minimum distance is only 4 inches. In terms of sizing your vessel and getting the total height it has a minimal impact. Depending on the instrumentation used, 4" isn't much of a margin to handle changing specific gravity and field calibration.