9 replies to this topic

[Balaram]

Hi,

Is there any thumb rule available for sizing liquid overflow lines in storage tanks?

In my case the max. incoming liquid line size is 4".

[vinod]

Bala

Thumb rule is that u have to have a overflow nozzle size one diametr more than the largest inlet/outlet flow connection.

Regards

Vinod

[ankur2061]

Bala

Thumb rule is that u have to have a overflow nozzle size one diametr more than the largest inlet/outlet flow connection.

Regards

Vinod

Vinod,

A slight correction to your thumb rule:

Over flow line size equal to or greater than the largest inflow connection to the tank.

Regards,
Ankur.

[proinwv]

No. Rules of Thumb are usually not engineering or scientifically sound. They may work for a very specific situation, but not all.

What you need to do is determine the following:

1. Maximum inflow rate to the tank.
2. Fluid physical properties
3. Tank MAWP
4. Tank vent settings

Then design a piping overflow that will not create enough backpressure that would overpressurize the tank.

[Art Montemayor]

To all reading this thread:

Paul is 100% correct and his advice should be heeded. The issue of gravity flow (such as an overflow from a storage tank) is not as simple a drain problem as one would imagine. It is full of potential problems and miscalculations that can cause a severe situation – especially with respect to atmospheric storage tanks. The configuration of the piping, the size, the layout, the correct venting, and the location can all be critical factors. Please refer to P.D. Hills’ famous article on this subject. This issue has been a major point of discussion in some of the world’s biggest engineering houses. My personal advice on the subject is: do not underestimate the complexity of the problem.

As Hill points out, Gas entrained in liquid flowing by means of gravity from a vessel can reduce the outlet pipe's capacity and cause flow to surge cyclically. These problems can be avoided by carefully designing for either full-liquid or two-phase flow.

The most conservative design is to ensure self-venting of the drain line. This means that the correct size has to be calculated by other than “rules-of-thumb”.

I recall discussing this issue a couple of years ago in the Eng-Tips Forum with respect to dropping a liquid down a pipe into a mine. The correct answer was arrived at as explained above. Again, P.D. Hills explains everything in his article.
 Art__s_Fluid_Flow.xls   942KB   2398 downloads

[Balaram]

Paul and Montmayor,

Thanks for your valuable information.

Now I clearly understood the principle behind the sizing of liquid overflow line.

Regards,
Bala

[Hhussain]

But i have question, Hazen and Williams equation 3.9, Gravity flow for fully flooded lines, can we relate this equation with P.D.Hills paper figure 3 to calculate flowrate Q?

Regards

Hi,

Where can i get hold of a copy of the above mentioned “Designing Piping For Gravity Flow” by PD Hills.

Or would anyone be able to send me a copy please on PDF it would be most helpful.


Thanks

Jon

[Art Montemayor]

Jon:

If you are really interested in this subject, download the Excel Workbook I attached to this thread and read it. The P.D. Hill Article is in the workbook, in easy to read script and illustrations as done by me. I made the effort just to help guys like you. But you have to read through the threads and the attachments to get the information.

[wisepeppy]

Art,

I downloaded your workbook, and reviewed the gravity flow section by PD Hills. It is indeed very straight forward, and I'm able to use it quite readily to size a suitable self-venting overflow for a tank, but I'm a bit confused by the lack of any fluid properties within the analysis. Certainly a fluid with higher viscosity will require a larger overflow pipe than a low viscosity fluid, no? The JL factor seems to be determined solely on volumetric flow rate and pipe diameter - why are fluid properties not a factor? e.g. a fluid flow of 150 gpm (34 m3/h) would require a ~7" overflow pipe (175 mm), based on "Curve 1" for entrance conditions. Am I to believe this is as true for molasses as it is for alchohol?