Pipeline emptying + liquid flashing at subatmospheric pressure

[Peter C]

It seems easy, yet it’s quite complicated.

Considering a full pipeline (e.g., water) close on both ends and without any venting valve. A drain is opened at a location of the pipe allowing fluid to flow out just by gravity. It is clear that all product in the vicinity and in a higher level than the hole will drain out, but what about the rest? Is there a simple way to evaluate it as well as the final liquid configuration in the pipe?
Furthermore, I’ve thinking about two other points so far: siphon effect and, most importantly, at what point the moved column will generate enough sub-atmospheric pressure to vaporize some of the fluid, like in the example of figure 1 (

https://files.engineering.com/getfi...ae70-9b8d4f95eddc&file=Flashing_induced.png).

Regards

 

[1503-44]

Bernoulli customarily evaluates total energy between two points at any one given time, As there is no change in time, dynamic changes are meaningless, however the equation can be extended to include time by reverting to the general energy equation form. Both potential energy and kinetic energy are part of Bernoilli.

https://engineeringlibrary.org/reference/bernoullis-equation-fluid-flow-doe-handbook

You simply have to balance them over a time interval. The individual terms describing the changes in pressures and velocity at each point will also balance. ΔP = M*(V1-V0)/Δt/A
. That equation can be written for point 1 and for point 2. Unbalanced forces, or pressures, F/A, are then accounted for and you will see the effects of accelerations as soon as time starts counting.


Yes, there is no way that water x=0-1 can physically drain to anywhere. Not sure if I implied it would, but if I did, that would have been in error.

I'm not understanding why you are putting 14.7 air at the high point. The problem doesn't have to start with that condition and it adds some unnecessary complexity, as then you have to keep track of air volume and pressure in the pipe as it expands. It's pressure will be reduced immediately as the drain is opened. As for how it affects the rest of the system, it really just adds 14.7 psi initial pressure to every liquid particle in the system. Before we added it at the end of the "simulation" just to get the last of the water out. It does however add air pressure to force water out from the beginning time =0 and a better possibility of developing a siphon, at least temporarily. It is dependent on how much air you add. A small amount will lose pressure very fast as it expands and not push a lot of water out before its pressure is negligible. A large amount could blow all the water out, or the initial air volume might even fill the pipe almost completely leaving little to drain at all. How much air and at what pressure do you want to add to x=1?

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

I think I agree with everyone.

The other issue about the lack of vent is that even if a little bit dribbled out the section form 6 to 7 is flat and therefore any air entering would have no driving force to send it up to 10.

The LHS in any case would be draining until you reached equilibrium as mr 44 says about x = 2.3

RHS 6-10 won't drain
0-1 won't drain and even if you stuck a vent a 0 wouldn't drain as the air would just bypass the liquid column.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Great.

Yeah. You'd have to but an air bomb at 0 and hope it blows over x=1.

Did I ever tell you that Site 7 stopped pumping with a diesel and gasoline interface on the high high point plain, which dropped pressure overnight and they mixed about 10,000 bbls inside the line when the gasoline vaporised? They were wondering what the hell happened when the big interface mix got to the DPS/RS at Medinah. I went back and did a study on that, which they found hard to believe, so we proved it with some gas and diesel in a clear hose in the sun.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

That doesn't surprise me. Stopping with the I/f anywhere is never great, but top of the hill!

In Algeria on those surface laid lines in the fields they were complaining about erratic flow between about 2 am and 5am.

We had a look and they were well in the hydrate zone so basically making slushy ice in the line.... Sun came up and it went away.

One of my very first line clearance they were very worried about one particular valley where there was barely enough Nitrogen pressure when the pig was at the bottom of the valley to climb the next hill. Middle of the night I was the only engineer there and the operators decided to "open her up a bit" to make sure that there was minimal back pressure on the high point.

I was busy calculating back pressure and flows every 10 mins and thought I was doing it wring because my back pressure hydraulic line was quite a bit lower than the hill. As the pig then made its way up the hill everything started moving faster and then all of a sudden the receipt pressures went hair wire as the vacuum they had pulled collapsed. We barely managed to not trip on high pressure d/s the control valve.

From readings I later proved we had drawn something like 2km of vacuum which when it collapsed gave us a real shock.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Runaway trains and pigs have a lot in common.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Snickster]

I'm not understanding why you are putting 14.7 air at the high point. The problem doesn't have to start with that condition and it adds some unnecessary complexity, as then you have to keep track of air volume and pressure in the pipe as it expands. It's pressure will be reduced immediately as the drain is opened. As for how it affects the rest of the system, it really just adds 14.7 psi initial pressure to every liquid particle in the system. Before we added it at the end of the "simulation" just to get the last of the water out. It does however add air pressure to force water out from the beginning time =0 and a better possibility of developing a siphon, at least temporarily. It is dependent on how much air you add. A small amount will lose pressure very fast as it expands and not push a lot of water out before its pressure is negligible. A large amount could blow all the water out, or the initial air volume might even fill the pipe almost completely leaving little to drain at all. How much air and at what pressure do you want to add to x=1?

My assumption is that if the pipeline is full and closed then the only way to get it full is to fill it from say point 0 while the line is vented to the atmosphere at point 1. You have to be venting the pipe while filling it to get it full. So when it is completely full and the only pressure in the pipe is due to static pressure of liquid, the vents are closed to atmosphere with no air at all in the system but still have atmospheric pressure initially at highest point - but no air. It seems like that is how the OP originally meant the system to be at time = 0?

 

[1503-44]

OK, so basically no air to speak of and Liquid pressure just begins at 14.7 psia. That's not as complicated without air. Then near immediate depressure of the liquid and a flash at the high point shortly after the drain opens. Yeah. Why not. You quickly get to the same place.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Snickster]

Exactly.