3 replies to this topic

[jprocess]

Dear All:
As you know for sizing of 2 phase separators the designer first needs to determine terminal velocity which is a function of vapor and liquid density.
There are special cases that we have only one phase that entering the separator and the liquid discharge line is normaly non flow.Such cases may meet at compressor suction drums and suction drums that exist in boil off gas recovery sytems of lpg storage tanks.
So in these cases we have not any liquid phase,consequently we have not any liquid density value and so we can not calculate terminal velocity!
Any hint?
Thanks in advance.

[pleckner]

The reason you have a separator drum upstream of a compressor is to remove any chance of liquid droplets from getting into the compressor and destroying it. If you were 100%+ sure you don't have liquid there would be no reason to have this drum. Since you are never sure there won't be liquid, you need the drum. So, look at the process and determine what liquid you could have if something in the process just wasn't working correctly; carryover, something condensing out in the line if conditions were just right, etc.

[djack77494]

Phil's comment is right on the money. Consider off design conditions, start-up and shutdown, etc. As an example of what could happen, consider a compressor with an anti-surge recycle line. That line originates at the (or one of the) discharge lines of the compressor. There the gas is hot and high pressure. The recycle line is stagnant and may lose heat, resulting in some condensing. By having a suction drum, that condensate would be harmlessly removed, rather than feeding into the compressor suction line.
Doug

[Art Montemayor]

Jprocess:

You are getting your terms mixed up and creating a logic problem in the process.

There is no "Terminal velocity" involved in the Brown-Souders equation you refer to and the same one employed in the calculations for a 2-phase vapor/liquid separator. You cite the existance of this ficticious velocity as if we all knew about it. I don't know where you got your version of the Brown-Souders equation, but it is interpreted (or corrupted) wrongly.

The velocity involved in the Brown-Souders equation is the Maximum Allowable Superficial Velocity. All the adjectives related to the subject are important to note:

Maximum Allowable = do not design for a higher value;
Superficial Velocity = the calculated velocity of the separated vapor/gas as it travels vertically through the vertical separator, assuming that there are no internal obstructions within its internal diameter. In most cases, this is a ficticious velocity but it is the basis for the design.

I have never come across any so-called "terminal velocity"; I presume this is someone's invented terminology and subject to correction.

Phil and Doug have already expounded on the other wrong concepts involved in this query. Allow me to cite still another one (& one which I have been recently involved in resolving with respect to 4 reciprocating natural gas compressors that ruptured their suction valves due to liquid ingestion into the compressor cylinders):

"boil off gas recovery sytems of lpg storage tanks" immediately indicates that this gas stream is what is designated as a "wet gas" stream - a term popularly employed in the hydrocarbon industry to indicate a gas stream eminating from a saturated liquid and, as such, a saturated gas that is prone to partially (or wholly) condensing further downstream should the pressure and temperature conditions allow it to do so. I can tell you from personal experience that a reciprocating compressor used to compress "boil-off" gas in Middle East or African climatic conditions will be prone to have liquid condensate formed within the suction lines as soon as the sun goes down - especially at night when the temperatures can be lower than 50 ^oF.

And this is one of the effects that both Phil and Doug are alluding to. You must design for this effect if the climatic conditions warrant it. I have seen a lot of compressors ruined by not taking this condition into consideration. Therefore, if you apply common sense, the density of the liquid is indeed available - it is the density of the saturated liquid condensed from the saturated vapor.

I hope this experience helps you out.