Anti siphon hole in dip pipe

[TOCallaghan]

Hello

We use a dip pipe in a distillate receiver vessel with an anti siphon hole. Recently we had an incident whereby vacuum was inadvertently applied to the vessel resulting in suck back of solvent to the vacuum pump. My question is should the anti siphon hole have prevented this or are they only designed to prevent gradual siphoning rather than preventing against a sudden large volume of liquid getting pulled back?

Thanks
Tracy

 

[Montemayor]

Tracy:

I have used and applied a lot of dip pipes as tank fill lines - especially on projects for companies like DuPont, ExxonMobil, Huntsman, etc., etc. The hole drilled into the dip pipe's side, directly under the tank roof is, of course, to break a potential syphoning effect. And it works! The holes I employ are 3/8" in diameter for 2" - 4" pipe - which are some of your more common sizes. If you have viscous, slurries, or fluids that tend to cover the hole, then a larger size may be called for on the larger sizes of dip pipe. But I've never had any syphoning taking place with the drilled hole in place.

Bear in mind that some "line pack" may dribble back, depending on your piping configuration. The usual configuration is to install the flanged, fill nozzle directly on the tank (or vessel) roof and insert the dip pipe inside this nozzle (with a slip-on flange welded to it at the appropriate depth) and bolt the two flanges together, fixing the dip pipe to the nozzle and to a pre-set depth. Under this configuration you will have some degree of liquid fluid remaining in the fill line all the way up to the height where the fill line makes a 180^o turn to enter the dip pipe. This could be the liquid that you identify as "syphoning back", but in reality it is draining back by gravity and not by a syphoning effect. Could this be the answer to your dilemma?

Perhaps the above doesn't address what you describe, because I've only described a generic type of dip pipe for filling purposes. What your description doesn't mention is what you are using the dip pipe for. You say you inadvertently pulled a vacuum on the vessel and had liquid drawn into the vacuum pump. If your vacuum pump is the instrument pulling the vacuum and it was being done using the dip pipe for vacuum draw, then you can pull liquid up through the dip pipe if the diameter of the vacuum breaker hole is not big enough. Your vacuum draw may be so fast that you establish sonic flow through the orifice hole and this maximizes the amount of gas that can be pulled through there - and it may not be enough. Therefore, the vacuum draw would start to "raise" the level of the liquid in the dip pipe and possibly defeat the 180^o turn at the top of the tank, and drain down the fill line by gravity. This could be another explanation to your dilemma.

 

[TOCallaghan]

The dip pipe is configured as you described above on the vessel roof. The liquid going through the pipe is butanol. It's a 2 inch pipe but unfortunately I don't know the hole diameter.
My guess was either that the siphon hole is blocked or as you described above that the strength/speed of the vacuum draw was too great to prevent the suck back.
I don't think it was due to liquid draining back as the vacuum pump saw a considerable amount of solvent.
Thanks for your help.

 

[25362]

If used to reduce electrical charge that would otherwise accumulate from solvent "free fall", dip pipes can sometimes be replaced by an attached angle iron letting the liquid flow down the angle iron, or simply by cutting and removing one metal strip off the 2-in pipe along its whole length inside the tank.
No anti-siphon holes would then be needed.

I recall this item has been covered in a previous thread on static electricity.

 

[rcooper]

When I've designed siphons, I have included a small hole in the top of the syphon, typically 2-3mm. The purpose of this hole is to allow air out of the siphon to ensure it primes correctly. Air is brought in through the hole, but as long as the velocity of the water is high enough, the air is entrained with the water and carried away. When the velocity of the water falls such that the air is no longer entrained, the siphon is broken by air entering through the hole.

Depending upon the magnitude of the vacuum applied to the vessel, the fluid flowing through your siphon may have been travelling fast enough to entrain air allowed in through the anti siphon hole, so the anti siphon hole would not have worked as expected.

Or as TOCallaghan describes, if the pressure loss through the anti siphon hole is sufficient to draw your fluid up the pipe, a siphon may form and be maintained as I have described above.

 

[FACS]

I would stick with the most obvious:
1) The siphon break is clogged.
2) The siphon break is too small for the condition you described.

Charlie

http://www.facsco.com