Dip Pipe: Anti sypon hole distance from vessel Shell ? Min dist ?

[jamesbanda]

Dear all,

sounds an odd question..

what is the guidance for min distance to avoid erroision from an anti suphon hole to the vessel wall? i've heard of an incident in the 90's at my site that at vessel failed after 20 years service from this but unfortunatly we have lost the incident and recommendations..? so i dont know if it was 1 inch or 30 inches away ! (how annoying..)..

i've a tank which is 600mm away from the vessel wall to the anti syphon hole which seems very reasonable to me..but i'd like to have a reference/ advice etc..

 

[Latexman]

This "pee distance" is calculable. In terms of difficulty, it's about a 1 martini problem. Here's a reference that is similar, but not exactly like your application:

NONCOMPRESSIBLE FLOW THROUGH A HOLE IN SHELL

However, it can be modified to apply, replace P1 with dP across the hole.

In most situations when backpressure at the hole is low, there is no need to even calculate it, IMO. If there is concern though, design the dip tube to always point the hole to the tank's center and opposite wall. In most situations, that will be good enough. If backpressure at the hole is high (lots of flow restiction downstream of the hole), just design a way to divert the stream down (provide a larger tank nozzle and weld an elbow pointed down on the hole).

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[jamesbanda]

Thanks,

I'm embarrised to say that rather than one anti syphon hole we have two, it was drilled straight through.. I dont think its an issue becuase i think the flow is govered as below.

Point 1: Bottom of tank / dip pipe
Point 2 : Dip pipe hole above liquid surface
h1= height from liquid surface to bottom of dip pipe
h2= height from base of dip pipe to anit sypon hole

Pressure at Point 1 Considering liquid surface = P1=rh1g+Ps = (from liquid static head basis)
Conisdering the flow equation. P1= P2+rgh2 -DPf (from in flow basis)

Equate P1=P1 both ways
P1=rh1g+Ps = P2+rgh2 -DPf (from in flow basis)
Therefore P2 = (rh1g - rgh2 )+DPf +PS
DP through orifice DP (e.g out of the dip pipe hole) = (rh1g - rgh2 )+DPf
Or = +DPf -rDh12g
Worst case is when the liquid is at the top tan line.

so in most cases we dont get flow from the anti syphon hole.. but this does not match my experience on atm tanks.. when i look i see a dribble out of the anti syphon hole.. why ?

 

[gr2vessels]

James, for some reason it keeps ringing in my ear the '1 martini' thingo, refer above. The anti-siphon hole is meant to absorb air/gas from the vapor space to break the siphon flow on the suction line. You might experience a bit of reversing flow from the returning flow, but I can't even give you a logical explanation for that fenomena. I doubt it that will cause any erosion on a block of butter. Perhaps a foot valve with a strong spring might be the cause of a bit of return flow through the hole, but not more than a soft dribble. Erosion? I'm going to get a martini..I'll buy you one. Speaking of, the embarrassed is spelled this way.
Cheers, haha
gr2vessels