4 replies to this topic

[Stefano]

Hi Everybody, I need to design an overflow device for an atmospheric tank containing water and blanketed with fuel gas. The tank is existing and the dimension are 18.3m (60ft) diameter and 12.2m (40ft) height.
The operating temperature is 48.9°C (120°F) and the maximum expected flowrate is 400 m3/h.
I have read the valuable article “Design piping for gravity flow” P.D. Hills and all your previous posts about the argument, but some doubts still remain.

There are two main problems:
1) avoid the fuel gas exiting from the tank during normal operation.
2) the evaporation pond is quite far from the plant: 1400m away from the tank, 20m below the base tank level

For the item number 1) a good solution could be install an internal line with a siphon break to the atmosphere but there are two problems: the blanketing gas is fuel gas and should be diverted to safe area or cold vent, and, as the tank is existing, complex internal works should be avoided. An external lateral overflow is a more practical solution, but it’s necessary a sort of hydraulic seal.

For the item number 2) an approach could be calculate the vertical part of the pipe for self venting service and, after a certain number of diameters (to be calculated), reduce the pipe diameter to a smaller size and consider flooded flow.

I can’t find a really convincing solution and any help is appreciated.
Tanks

[katmar]

It seems that you have done a good job of the background reading so I guess you have read Justin Spencer's query at http://www.cheresour...rflow-line/
If not, read through it as well

On your point 1. - The outlet and siphon break do not have to be internal to the vessel. You can have an outlet near the bottom and then put a vertical leg up to the siphon break, all outside the tank. The siphon break will not vent fuel gas under normal operation, but it could happen if the tank level went below the side outlet. You have to comply with your safety regulations and do it properly. Don't forget that under really bad circumstances the fuel gas could come out at the evaporation pond. It wouldn't be the first time an evaporation pond has caught fire.

On your point 2. - Yes, I agree with that approach. The downleg from the siphon break needs to be self venting for about 2 metres below the top, and then below that you could go to a smaller pipe. Check that the line to the evaporation pond never goes higher than the bottom of the tank, or you will get water backing up the downleg. Check also for local high points along the way to the pond. The recommendation for ensuring that any air in local high spots is flushed out is that the flow rate should be at a Froude number of at least 0.55. This looks like it would require a 14" line, but from the head you have available you could probably go smaller than that - 12" or even 10". So there shouldn't be problems there.

[Stefano]

Katmar, thanks for your analysis and your suggestions.
So the idea could be to put the outlet of the overflow pipe under the low low level switch, then install a rising pipe till the overflow level, connected with a siphon break discharging in the blowdown manifold. The size could be 14" for the vertical section (upward and downward), then provide a 14"x12" reduction before the horizontal section. The only remaining concern is about the size: from the reading of the article and the various posts, I understood that the requirement for a self-venting pipe is Fr < 0.3, but now you write Fr < 0.55. Maybe 0.3 is too conservative number leading in an oversizing? Is it right?
Thanks again for your consulting

[katmar]

If you have a low level switch then putting the side outlet below that would be ideal.

Fr < 0.3 is the correct criterion for self venting flow, which means you need a 20" or 22" downleg from the siphon. In this section of the pipe you want air to NOT be entrained in the water. The upleg (from the tank outlet up to the siphon break) could be a bit smaller, depending on how close you want the level in the tank to match the level of the siphon break. It is a simple Darcy-Weisbach calculation to determine the pressure drop in the upleg, and this will be the difference in head between the level in the tank and the siphon break. It would probably be simpler just to keep it all 20" or 22".

The pipe from the tank to the evaporation pond must not be self venting. You want that section of pipe to have a high enough velocity to ensure that any air in the line (from start up or whetever) is flushed out of the line. The criterion for this is Fr > 0.55

The downleg and the horozontal line to the pond have different requirements. In the downleg air must not be entrained and must be allowed to escape from the water. In the horizontal section air must be entrained by the water to remove it from the pipe.

The horizontal pipe is a far more difficult design to get right than the vertical section down from the siphon break. For the vertical section you simply make the pipe big enough to be self venting for the maximum flow rate, and you know that for any flow rate less than that it will still be self venting. In the horizontal section if you make the line size correct to flush the air out at the maximum flow, then maybe the air will not be flushed out if the flow drops to half of maximum. It all depends on whether there are any high points along this line where air could get trapped. If air gets trapped you can get slack flow, and then the water will back up in the vertical section until it has sufficient head to flush the air out. This may or may not be a problem. A good resource to learn about slack flow is www.aplv.org/files/docs/AirInPipesManual.pdf

If the line has a constant slope from the tank to the pond then it is not a problem and under low flow conditions the pipe will just not run full.

[Stefano]

Ok, now your explanation is very clear to me. I have to check the terrain slope. I'll also read the document you attached with lot of interest.
Thank you very much, if any other issue I'll let you know
Stefano