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velocity of fluid in piping system
3

velocity of fluid in piping system

velocity of fluid in piping system

(OP)
Hi
I red somewhere that there is a minimum velocity for twophase flow in pipe, in function of density.

Would someone give me the correlation?

I need the maximum velocity correlation too.
Tx

RE: velocity of fluid in piping system

2
There isn't a hard and fast answer to your question.  There was some work done in Norway a couple of years ago that suggests that below a superficial gas velocity of about 3 m/s, gas will lose its ability to carry liquid.  The problem is that this minimum number seems to be more pressure dependent than the authors of that paper would suggest.  

Above about 1000 kPa, the 3 m/s seems to work really well regardless of how much water you have.  

Above 500 kPa it seems more dependent on GWR and if you get very much liquid then 3 m/s isn't fast enough.  

Below 500 kPa, 3 m/s won't shift any liquid.  You need to be over 150 m/s to reliably shift liquid and that is a lot faster than most companies will allow for the maximum velocity.

For a design point in low pressures I don't specify a minimum, I specify pigging facilities.  Above 1 MPa I specify a minimum velocity of 3 m/s.

At the other end it is simply a trade off between pipe size and compressor power requirements--the faster the fluids go, the more pressure drop due to friction that you have to make up in compression.

Many companies have internal standards that list a max design velocity.  They all list some kind of a justification for the max (usually it is to minimize erosion of the passivisation layer), but most of them are just words that don't have much basis in reality.  I see 30 m/s pretty often, but I really think that they just took the 100 ft/s from the old standard and slavishly converted and rounded off.  No one knew the basis for the 100 ft/s, and they don't know the basis for the 30 m/s.

My answer is "how much compression hp do you want to waste?", set the max velocity to match that value.

David

RE: velocity of fluid in piping system

David,

I'll humbly admit my ignorance and ask you, on what basis the limit of the 100 fps limit was set...because I've wondered that myself and haven't gotten a good answer.  I'm guessing that is has some reference in limiting reaction forces at turns and bends.
 

RE: velocity of fluid in piping system

Could be any combination of two things, pressure drop and wear.

rmw

RE: velocity of fluid in piping system

I think you are too kind rmw.  I believe it came solidly from the anal extract table.  Someone pulled a number out of "the air" and it has stuck.  I've been looking at this stuff really hard for several decades and I've never found an explanation of that number that held water.

What I look at is how much hp I can afford to throw away on friction.  Every millibar of dP in the pipe must be replaced by a power-consuming compressor on the other end.  For low pressure lines (i.e., under 50 psig on either end) I live with 15 psi/mile.  For lines above 150 psig I like to use 8 psi/mile.  For lines above 500 psig I use 4 psig/mile.  For the LP lines this is usually much higher than 100 ft/sec, for the MP and HP lines it is well under.  

These numbers are not set in concrete, for example if someone wants to increase the flow in a big, HP line by 25% and it results in the dP/mile being 8 psi/mile I'll probably sign off on it making sure that someone understands that their power consumption at the compressor station just went up 55% (25% because of the extra volume and 30% because of the lower pressure at the inlet).

David

RE: velocity of fluid in piping system

(OP)
here the equations

max velocity of gas:
v [m/s] = 120 / (density [kg/mc])^0,5

min velocity of mixed flow liq/gas:
v [m/s] = cost / (density [kg/mc])^0,5
where cost = 75 (i dont remember)

can you confirm ?  

RE: velocity of fluid in piping system

(OP)
my problem is that if i use this equations in vacuum line, where density go down 0,3, i must set a very speed flow with relatively small pipe size...

v max 219 m/s (very high)
and
v min 137 m/s (very high)

any tip?

RE: velocity of fluid in piping system

Those equations are worse than worthless for vacuum flow.  The underlying assumptions all require incompressible flow (i.e., less than 10% pressure drop from start to finish of the line, if your pressure at the head is 100 kPa, then the equation is invalid at 90 kPa).  If you calculate the Reynolds Number for the flow you'll probably find that it puts you in a region that the equations don't support as well.

Your 219 m/s number is nearly 0.7 Mach in natural gas at 15C--that would be compressible flow at any upstream pressure.  Like I said above, none of your equations are going to work below about 500 kPa(g) and I'll add that they won't work with velocity greater than about 0.5 M.

At your pressures, you simply cannot rely on gas flow to reliably shift liquids.  It will occasionally launch a slug, but that really is the worst of all worlds.

David

RE: velocity of fluid in piping system

(OP)
thanks for your replies very helpful

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