18 replies to this topic

[Gennarocariulo]

Hi, i have this exercise to solve 

A vessel is fed from the bottom with a fixed stream Q. The exit is generated by an overflow in the vessel itself as shown in figure. i need to design the exit tube diameter in order to avoid accumulation in the tank (The liquid level must be fixed). I tried various ways but couldn't find an answer, can someone help me?

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[breizh]

Hi,

same size than inlet pipe !

my 2 cents

Breizh 

[Gennarocariulo]

Hi,

same size than inlet pipe !

my 2 cents

Breizh 

 

Tell me more about this!

[breizh]

Hi,

Flow in = flow out  right?

Breizh 

[Gennarocariulo]

Hi,

Flow in = flow out  right?

Breizh 

 

Sure, but it looks too much simple for me 

Isn't there any calculation that can be done? 

[Pilesar]

Fluid flow occurs with a pressure drop. Calculate the pressure drop available between the vapor portion of your tank and the outlet discharge to an outside drain. For a tank at atmospheric pressure, the pressure drop between the two points is zero! Larger outlet piping reduces the friction component of the pressure drop. If the outlet piping is small, the friction component of pressure drop will be higher and the liquid level will rise in the tank above the outlet so that the pressure drop between the inside of the tank and the outside is still zero. The level inside the tank may need to be higher than the tank itself, and then the designer is embarrassed! Ideally, you don't want the overflow line to fill with liquid so that you do not have a liquid level above the outlet location. Practically, the outlet diameter is often selected to be 'one pipe size larger' than the liquid inlet as a short cut to the calcs. But you can perform the calcs to determine how large the line needs to be for total outlet pressure drop of zero.

 

[I withdraw my 'one pipe size larger' comment and replace it with 'two pipe sizes larger'. See my post a couple of hours after this one for explanation.]

Edited by Pilesar, 25 May 2021 - 09:20 AM.

[breizh]

Hi,

Pilesar is correct , I should have written > or equal to the inlet pipe . 

 

https://www.irbnet.d...nda/CIB4613.pdf

 

Good luck

Breizh 

Edited by breizh, 25 May 2021 - 07:32 AM.

[Gennarocariulo]

Hi,

Pilesar is correct , I should have written > or equal to the inlet pipe . 

 

https://www.irbnet.d...nda/CIB4613.pdf

 

Good luck

Breizh 

 

Could you help me figure out what h is in the formulas presented in this article?

Fluid flow occurs with a pressure drop. Calculate the pressure drop available between the vapor portion of your tank and the outlet discharge to an outside drain. For a tank at atmospheric pressure, the pressure drop between the two points is zero! Larger outlet piping reduces the friction component of the pressure drop. If the outlet piping is small, the friction component of pressure drop will be higher and the liquid level will rise in the tank above the outlet so that the pressure drop between the inside of the tank and the outside is still zero. The level inside the tank may need to be higher than the tank itself, and then the designer is embarrassed! Ideally, you don't want the overflow line to fill with liquid so that you do not have a liquid level above the outlet location. Practically, the outlet diameter is often selected to be 'one pipe size larger' than the liquid inlet as a short cut to the calcs. But you can perform the calcs to determine how large the line needs to be for total outlet pressure drop of zero.

Very nice explanation, i will try to do some calculations!

Second, for one size larger you mean like from IN20 to IN25?

Edited by Gennarocariulo, 25 May 2021 - 07:38 AM.

[breizh]

Hi,

Fig 4 of the document. 

Breizh 

Edited by breizh, 25 May 2021 - 08:10 AM.

[Gennarocariulo]

Hi,

Fig 4 of the document. 

Breizh 

 

So it is a degree of fredoom i can chose.

So to do the calculation, i use the formula (3) which should be referred to my design, put h=10mm for example, Q=38L/min (my stream) and i calculate d, then verify if h<d is correct, is that right?

[Pilesar]

That looks like a good article Breizh supplied. I noticed the statement in it 'it is general to assume the size of the overflow pipe to be two size-ups of the size of water supply pipe.' So I withdraw my 'one pipe size larger' comment and replace it with 'two pipe sizes larger'. 

 

Because piping is available in standard sizes, engineers calculate the minimum diameter they need and 'round up' to the next standard pipe size to be cost effective. Rounding up provides a bit of safety factor to cover the uncertainties of fluid flow calculations. Usually, a larger pipe size does not hurt the process performance but it can be tricky with two-phase flow where pipe diameter affects the flow regime of the fluid.

[breizh]

hi,

Don't forget to add a vent on top of your tank , goose neck should be adequate.

Breizh

[Gennarocariulo]

 

Hi,

Fig 4 of the document. 

Breizh 

 

So it is a degree of fredoom i can chose.

So to do the calculation, i use the formula (3) which should be referred to my design, put h=10mm for example, Q=38L/min (my stream) and i calculate d, then verify if h<d is correct, is that right?

 

Thanks everyone, can someone tell me if this is right please?

[breizh]

Hi,

Yes ,sounds correct . 

As an engineer you need to apply a safety factor , refer to the discussion above.

Good luck.

Breizh 

[Gennarocariulo]

I have another question.

We want to run the vessel in such way because we want the level in the tank to be fixed.(it's a reactor, so we want a fixed volume and fixed residence time). Would it be good, in this case, to install the right tube inclined? Like, also 45°? Will it help to empty the exceeding part of the liquid?

[breizh]

Hi,

I and probably others don't understand what you are asking ! Please wok like an engineer with a clear scope .

Start with the big picture before asking for minor things.

Good luck

Breizh 

[Pilesar]

The initial question was about 'a tank' with a side overflow 'as shown' with one liquid inlet and a overflow liquid discharging with an air gap into an open-top receiver. The answers you received apply to your original description, but may not apply to your equipment or process if the description was inadequate. I suggest you begin a new question about level in a reactor and describe the process including details like reactants, products, catalyst, reaction type, pressure, temperature, flowrates, vessel size. Is there other information we would need to know? Such as 'the reactor is swept with nitrogen' or 'the fluid is flammable'.  Your reactor question is very different from the original question and deserves its own thread. There does not seem to be any open issues with your original question.

Thanks for asking your original question and bringing a new question. Without questions, this forum would not exist! You are helping the chemical engineering community of the world in a small way. Beginning a new thread with your new question will increase the value of your contribution to the profession.

[latexman]

Similar to breizh, please work like an Engineer.  Draw a dimensional sketch with the new idea, 45 degree pipe.  Analyse the two cases.  What are pros and cons of both cases?  Make a decision, then do detailed design and implement.

 

Don't just ask SGOTI.

[katmar]

If you want to keep the liquid level as constant as possible, the usual method is to install a segmental weir that feeds into a trough (or pan) that in turn feeds your overflow pipe.  The longer you can make your weir, the less height variation you get with changes in flow rate.  This is the same concept that is used to control the discharge from a distillation tray into the downcomer in such a way that the liquid level on the tray is held "constant"..