Filling a 36' tank efficiently

[Focal]

Hey Guys,

I haven't done fluid dynamics for a few years and I am really rusty with fundamentals. What I want to know is if I am filling a tank and I have two options of top fill and bottom fill, how does flow work? To clarify. Let's say the tank is 20% full and thus the height of liquid is like 8 ft. Where as the top fill has a 2" pipe running up 32'. The vertical head from the top fill would be higher than the that of the bottom fill right? But is that the determining factor in my flow rate, or is the fact that the tank has a greater volume of liquid sitting over the bottom fill that it will resist fill more? So should fill the tank via the top fill, bottom fill, or both? BTW the tank has been vented to atmospheric pressure. If someone could help me grasp what factors are affecting my flow and which is the best option, that would be great.

Thanks in advance guys. Image attached

 

[LittleInch]

Flow works by overcoming static head and friction losses to make the liquid move.

Taking your questions in turn and thanks for including a sketch - I wish more people would....

The head from the top fill would be more than the bottom fill, but would also be constant on the basis that if just tipped out of the pipe as you show in your sketch and is not submerged in the liquid. If it was submerged then it would act partly as a syphon and could end up drawing a vacuum at the top if the fill rate was quite low / ceased due to the high static head (depends what the fluid is - water / diesel would be ok, petrol / naptha wouldn't as it has a high vapour pressure)

The bottom fill pipe would see only the static head which would gradually increase as the tank fills up. This can cause excess flow at the start and normally the inlet pipe is covered to a minimum depth before you go for full flow to prevent turbulence or excess vapour.

Normally tanks are bottom fed, mainly to avoid excess spray / vapourisation of the liquid which for hydrocarboncs can generate explosive vapour and potential for static discharge = big bang, not to mention loss of product out of the open vent. Also you don't have to hang the pipe off the tank shell.

The only advantage of a top fill as you have shown it is that there is no method for back flow from the tank as ther eis an air gp between the end of the fill pipe and the liquid level. This then doesn't rely on non return valves or other devices and is sometimes required by municipal water companies to avoid accidental contanination of the water system if the feed water pressure looses pressure or breaks.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Focal]

Thanks LittleInch, there is a detail that I missed which changes things significantly. It's a Cryo tank holding N2. So the pressure building that occurs changes things. Sorry

 

[LittleInch]

Not really as it affects both options the same as pressure increases. The top fill will be a constant head plus tank pressure whereas bottom fill will be a variable head plus tank pressure. Do you mean that this becomes a pressurised tank or that it is controlled to a low presusre regardeless of liquid level?

All the comments about vapour and spray remain the same. I can't see why you would want to top fill.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Focal]

So I've learned that the technique is to bottom fill like you've said but in order to keep pressures low it is required to use top fill. The Cryo fluid cools tanks vapors thus lowering pressure whereas bottom fill increases it. A balancing act is done between both fills with bottom being the primary focus. Thank you so much for your help.

 

[zdas04]

One additional reason for top fill is to minimize disturbing the volume at rest. Oil field tanks on wellsites are nearly always top fill to reduce the amount of flash gas from a heated tank when the separator dumps.

Since you have pressure on the tank from the boiling nitrogen, the height of the column of fluid is not really a factor in pump performance. Without that, if I was using a dynamic pump I'd want to come in the top (constant discharge head); with a PD pump it doesn't much matter.

With top entry you are blowing your fill volume through the nitrogen equivalent of saturated steam. I wonder if that heat transfer component (condensing the gaseous nitrogen) would drive a decision towards bottom fill?

David Simpson, PE
MuleShoe Engineering

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