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Concept Question (Delta P or Differential Pressure) 2

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Poppapetro

Poppapetro

Petroleum
Sep 25, 2007
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The question I have relates to basic fundamentals of fluid dynamics. Here is my way of thinking: The driving force for fluid movement is Delta P.

If you have two tanks with an equal volume of 20 cubic meters absolutely full of water and tank 1 is at 500 kPag and tank 2 is 100 kPag. Furthermore, there is a line connecting both tanks with a valve at the direct midpoint of this pipe from T1 to T2. If I open the valve, what is the direction of flow if both tanks have NO vent lines to atmosphere. Fundamentally, one would think flow would be from high pressure to low, but what replaces the liquid. Could some explain this concept at a conceptual/fundamental level?


 
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Vzeos,
You should probably read my post more closely. His point was for the same mass flow rate a higher dP is "better" than a lower dP. He doesn't really define what he means by "better".

I don't think that anyone reading this thread would disagree that for the same pipe and fluid conditions increasing dP will increase flow rate. But we were talking about pressure drop caused by the gas doing work on the liquid in multi-phase flow (i.e., for a given flow rate, the dP is much higher than single-phase equations like the AGA equation or your simplistic equation would predict).

The equation I referenced is called many things, but originally it was the Bureau of Mines Method of calculating flow through a permeable media. Now it is mostly called the Absolute Open Flow (AOF) equation.

Your example was pretty, but pretty much not on point.

David
 
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Sailoday,

Experience. Just do a transient flow simulation using water. Pick any two completely full tanks with/at the same levels and a reasonable pipe configuration, similar to what the OP describes. Waters very small compressibility almost always guarantees that only a few oscillations will occur before things arrive at a pretty much static condition.

 
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BingInch
I find it hard to comprehend how any diameter or or pipe lengths will lead to your conclusion. Friction/no friction-geometry of source and receiving tanks, etc.
Regards



 
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Sailoday,
I think BigInch's answer helps a lot. There are times when experimentation is a far more useful approach than developing esoteric arithmetic that always runs that risk that you have assumed an important parameter out of existence.

Every engineer who has ever worked in the field has a list of seat-of-the-pants solutions to real-world issues that result in "good enough" answers (i.e., answers that are conservative enough to ensure that their application won't put anyone at risk, but no so conservative that they destroy economics for no good purpose). For example, I "know" that hydrotest water volume requirement will be about ID^2 bbls per 1000 ft (e.g., 10 inch pipe is 100 bbl/1000 ft). The arithmetic really works out that this is about 8-12% too much water, which is perfect for the real-world conditions that not every water truck will be pumped all the way dry.

I don't see any problem with an answer that an experienced engineer has observed that in a sloshing situation in liquid-full vessels that you can expect around 3 major transients, followed by some small number of minor transients (which is another way of saying "count them as they pass by").

David
 
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I don't see any problem with an answer that an experienced engineer has observed that in a sloshing situation in liquid-full vessels that you can expect around 3 major transients, followed by some small number of minor transients (which is another way of saying "count them as they pass by").

To me sloshing means that there is a space above the liquid. Is that true in the present case? If so a basis for that assumption/statement should be made--other than "experience"
Regards
 
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zdas04 (Mechanical)
My original response was
BigInch (Petroleum)Without a knowledge of lengths between the interconnecting piping to the quick opening valve nor the diameter, how can you justify the 3-4 oscillations?

You state experienced engineer has observed sloshing, etc.
How is the "quick opening" valve defined without a knowledge of interconnecting lenths or diameter.
BigInch (Petroleum) 1 Oct 07 16:59
I thought the tank was full, but that makes no difference,

Are the above "observations" seat of the pants engineering?


 
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BigInch (Petroleum) 1 Oct 07 21:03
Pause the simulator, open valve to full open positon... resume. I can't do it faster than that.

Sorry, but that is not the definition of a fast or quick opening valve. Think about the water hammer equations.
Regards
 
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