Two phase flow

[GeorgeGrande]

All,

I thoroughly red thread469-22342 and it almost gave me all the answers i needed.
Yet, I am designing a 700m horizontal 20% quality, 10-16 bara, two phase line (steam and water).
I am trying to optimize pressure drop, velocity and flow regime. There is a disagreement between the designers regarding the philosiphy of this optimization.
The simulation programm i am using give DISP flow at 45 ft/s yet veteran eng claim that in this kind of quality the vapour velocity must be greater than 20 m/s (~65 ft/s) to avoid slugs and keep the line concrete safe from hammering.
I used different kinds of correlations while studying this issue, Some gives different solutions.
Can some one give me a minded answer that i (or he) can back up?
Can some one share his experience?

 

[hanon]

To predict the flow regime you can use the Baker correlation / chart that are available in many fluid books (for example in Practical Process Enginerring by Sandler & Luckiewicz ), and to estimate the pressure drop use the Lockart-Martinelli charts. Slug flow, as far as I know, is undesiderable and may be avoided by increasing velocity (this is reducing diameter) ...but taking care of the pressure drop.

To avoid hammering try to design good isometric for the pipe to avoid liquid seal in the downsides, and use steam traps and ...although I am young ...obey to veterans eng

 

[zdas04]

The flow-regime map is a useful tool as long as you keep it firmly in mind that there is no such thing as steady-state multiphase flow. Each point on the map happens at a discrete total energy. If you trade pressure for velocity (for example), or lose heat to the surroundings then you will shift to a different place on the map. As the system moves from mist flow through the steps to slug flow, things change dramatically.

When I was working on my Masters I read a lot of current research (as of 1993) in multiphase flow and I was quite distressed by the disclaimers in fine print at the end of many dissertations that "the equations presented should be accurate to +/- 70%" -- you can do better by flipping a coin. I've tried to stay kind of current in the field and the bottom line is that I've never seen multiphase flow arithmetic (including CFD and other FEA techniques) that I would risk my life on (to say nothing of my reputation, such as it is). At the ASME World Congress last fall, a bunch of grad students presented their state-of-the-art CFD work and by and large I found it to be pointless with no applicability to a pipe design.

The best I've ever done in condensing flow is to put a lot of places to accumulate and remove liquids. Then the single-phase arithmetic does well enough.

David

 

[GeorgeGrande]

Thanks for your kind reply.

Hanon - this is one of the methods I used before I applied the forum.

David - Thanks for your comment. I will flip the coin....

 

[KernOily]

Hi George. I have spent boatloads of time sizing two-phase oil patch steam lines and I'll tell you what we do because it works.

I use the Pipephase software with the BBM correlation. This gives me pressure drop predictions of ±15-20% match against actual field data. That is plenty good enough.

We design for mean velocity of 65 ft/sec and annular flow regimes for lines carrying 50% to 70% quality. This velocity and flow regime gives reasonable pipe sizes at reasonable pressure drops across the lengths in question, which is usually several thousand feet. Why annular? Because annular provides the best phase-split quality at junctions, e.g. a tee, more or less independent of flow rate out the two legs of the tee. This is for a regular tee only, and does not apply to a modified tee e.g. a tee with inserts or some other type of phase-split mitigation technology. Those have different requirements.

If you are not worried about good phase splitting, then you might do better with a different flow regime. But I have to worry about getting 65% out both legs of a tee, independent of the rate out each tee (more or less).

Stay away from Dukler, L-M, and all the other 'steam' dP correlations you see out there in the literature. They do not work well, or at all, in this application.

Been doing it this way for nigh on 20 years now and it works. There, I just gave away the ranch. Hope this helps! Pete

 

[GeorgeGrande]

Pete,

Thanks for sharing this experience. I will try to implement it in my work. Yet I think annular flow in 20% quality is a little ambitious. Do you think 65 ft/sec should be the preferred mean velocity at any quality?
The two phase line is leading into a separator so i dont mind the phase split in the pipe yet i must avoid slugs or wavy flow to keep the line and keep off "Batch" separation.

Thanks for your kind reply anyway.