Fluid dynamic problem
Fluid dynamic problem
(OP)
Gentlmen,
I have a problem with a pump circuit.
A centrifugal pump has sends a neutralization effluent with density 960 kg/m3 from ground to the top of an atmospheric drum (Bottom T.L. = 6 m).
Before the drum there is a filter with pressure drop of 4 bar that is in elevation, at 21 m.
The drum is exactly below the filter. The filter is complitely full of liquid.
I don't understand what kind of flow is in the pipe going from the filter to the drum.
If the pipe if full of liquid, the liquid flows for pressure difference, but in this case the filter should operate under vacuum. The pump should be calculated considering the atmospheric drum as arriving point.
In this case I will calculate a low static head, because the drum upper tangent line is at 10 m.
If the pipe is not full of liquid, I must consider the filter atmospheric. In this case I will obtain an higher static head for the pump (since the arriving point is the filter at 21 m).
At the following link you'll find a sketch of the circuit:
http://img822.imageshack.us/img822/4707/eqo.pdf
Thank's a lot for your replies.
Sergio
I have a problem with a pump circuit.
A centrifugal pump has sends a neutralization effluent with density 960 kg/m3 from ground to the top of an atmospheric drum (Bottom T.L. = 6 m).
Before the drum there is a filter with pressure drop of 4 bar that is in elevation, at 21 m.
The drum is exactly below the filter. The filter is complitely full of liquid.
I don't understand what kind of flow is in the pipe going from the filter to the drum.
If the pipe if full of liquid, the liquid flows for pressure difference, but in this case the filter should operate under vacuum. The pump should be calculated considering the atmospheric drum as arriving point.
In this case I will calculate a low static head, because the drum upper tangent line is at 10 m.
If the pipe is not full of liquid, I must consider the filter atmospheric. In this case I will obtain an higher static head for the pump (since the arriving point is the filter at 21 m).
At the following link you'll find a sketch of the circuit:
http://img822.imageshack.us/img822/4707/eqo.pdf
Thank's a lot for your replies.
Sergio





RE: Fluid dynamic problem
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RE: Fluid dynamic problem
RE: Fluid dynamic problem
Independent events are seldomly independent.
RE: Fluid dynamic problem
I think that the cascade flow is more probable solution.
However, I've calculated Froude number for a vertical downflow (fluid velocity is 1 m/s and line size is 1.5 in with 80S schedule).
The result is Fr = 2.7 .
I know that for Fr > 2 (if liquid doesn't seal the bottom of the pipe), the total cross section of the pipe should be water filled. So, we don't have a cascade flow but
a liquid moving for pressure difference. As result, the filter will be under vacuum.
What is my mistake?
RE: Fluid dynamic problem
BI has it exactly right in his description of events. The only thing you can really do is create more frictional losses in the section from the filter to the tanks such that the pressure required at the top of the pipe is greater than 0 barg. for a flowing system this could be either a smaller pipe than you currently have, an orifice plate at the bottom or a control valve maintaining a set back pressure in the incoming pipe of around 1 barg. The disadvantage of the first two is that they will drain down when flow stops.
My motto: Learn something new every day
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RE: Fluid dynamic problem
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RE: Fluid dynamic problem
Your F number calc isn't correct, because I believe that you've assumed a steady state flow of 1 meter/sec throughout the system, however gravity will accelerate that flowrate in the downcomer, hence at a faster velocity there, that pipe will not flow full, it will cascade down increasing velocity at 9.81 m/sec2 x time it takes to get to the bottom, while it continuously reduces its cross-sectional area of flow. As the fluid flows downward, the pipe will flow less and less full to make up for the steady mass flowrate, but at faster and faster velocities.
Independent events are seldomly independent.
RE: Fluid dynamic problem
Luckily our predecessors solved this problem for us long ago. At the high point after the filter you must install a vent to ensure that the pressure there is always atmospheric. This removes the problem of the pressure at that point varying with flow rate. The vertical downflow section from the vented point down to the tank should be designed for self venting flow. This is achieved by keeping the Froude Number below 0.3. By doing this you ensure what BigInch has called "Cascade Flow" occurs all the way down. It also make the sizing of the pump a whole lot easier because the outlet pressure of the filter is fixed at atmospheric.
It is unusual for me to disagree with either of the Inch Brothers, but in this case I have to query LittleInch's assertion that the velocity will increase in the downflow section. If the pipe is full, by continuity the velocity has to be equal all the way down. If the pipe is not full the friction losses are less than the change in static head and become irrelevant.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Fluid dynamic problem
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RE: Fluid dynamic problem
Does the continuity equation really apply here? If I dump a bucket of water from 20 m, the velocity of that lump of water will increase continuously until it hits the pavement. I think of cascade flow the same way--each lump ("control volume" if you want to be less colloquial) of water that overtops the hill in a line that is not full will fall under the influence of gravity disconnected from the last lump or the next lump. "Under the influence of gravity" means that it is accelerating at 9.81 m/s2. It really can't accelerate downwards at the same time it is "maintaining a constant velocity". I know that the continuity equation does not apply to channel flow where fluid can "stack up or starve" the flow. Any time you have a partially full line you have that situation.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"
RE: Fluid dynamic problem
And that's exactly why I'm correct; continuity DOES apply here (nobody's adding mass flow anywhere) and, just as it does to all steady state mass flow, including steady open channel flow, to maintain the same mass flow under accelerating velocities in a VSLC (very slightly compressible liquid) the cross-sectional area of flow must reduce to compensate for the increased velocity. Otherwise A x V x DENS = mass flow would not be constant, and nobody was adding any mass flow anywhere in that system.
In open channel flow, steady state flow at constant depth occurs where the down-slope of the channel imparts just enough energy to the flow to counter the tendency for the fluid to accelerate as it decreases its elevation. A shallower slope will cause the velocity to slow down, due to lack of sufficient gravitational energy to keep up the speed, but if steady mass flow is maintained, the depth must then increase. Likewise, a steeper slope will add more gravitational energy to the fluid than what was previously being expended, and again, if no mass is added to the stream (it's not raining right), the constant mass flow will be maintained during the increase in velocity by a reduction in the depth of flow.
Independent events are seldomly independent.
RE: Fluid dynamic problem
It's really splitting hairs though. In the calculation of the Froude Number we are not interested in whether the pipe is full or not. The velocity in the determination of the Froude Number is the superficial velocity, which assumes that the pipe is full of the phase we are considering (i.e. liquid in this case). We happily use the Fr < 0.3 criterion for self venting flow - where the pipe is most definitely not full of liquid because the vapor is flowing countercurrent up the pipe.
Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Fluid dynamic problem
Independent events are seldomly independent.
RE: Fluid dynamic problem
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"
RE: Fluid dynamic problem
Independent events are seldomly independent.
RE: Fluid dynamic problem
Since we assume that the pipe is not full. The fluid cross section decrease when we approach near to the tank, so fluid velocity has to increase.
I've considered 1 m/s to calculate Froude (this is the result that we obtain for pipe full of liquid).
For this reason calculated Froude number is the lowest that we can obtain, and it is yet higher than 0.31 (or 1... I've not understood if there is a difference in Froude limit if the liquid seals the bottom of the pipe). It means liquid full pipe... How validate the cascade flow?
RE: Fluid dynamic problem
As I understand it, Froude number will simply tell you whether the flow is self-venting. However, for the purposes of calculating head, the Froude number at the value you are calculating is irrelevant, since all the Froude number is telling you is that the opening of the downpipe is fully covered by water, and does not tell you whether the pipe itself is filled.
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RE: Fluid dynamic problem
second, if you are going to calculate froude, you need to know what the normal depth is. you can't just assume a velocity based on full pipe flow if the pipe is not actually flowing full - note that the pipe will probably never flow full without venting, the vacuum will prevent it from happening. Ever heard your bathroom sink drain make funny burping noises? that's because the vent is not working properly creating a vacuum... third, a vent or combination air/vac valve is very commonly used in force mains where pumping over a hill and would be recommended for this application in order to reduce the pumping pressure and prevent vacuum.