12 replies to this topic

[go-fish]

I have to design an overflow line for a demin water storage tank. The tank has the nitrogen blanketing. It has been suggested to add an overflow (attached sketch) with a siphon break.

 

I am trying to understand how this arrangement will work. Please can somebody explain.

Attached Files

•  Overflow line.pdf   13.15KB   286 downloads

[thorium90]

erm, why do you need N2 blanket for a demin water tank, its not flammable.

 

Where does your overflow go to? Back to some utility water tank? To a waste water pit?

 

I think the size of that overflow pipes are important too. Dont want it to create a vacuum when the water overflows out otherwise you will have a weird looking tank...

Edited by thorium90, 29 January 2013 - 11:55 PM.

[fallah]

thorium90,

 

The N2 blanketing is installed to prevent air ingress into DM storage tank by maintaining a little bit positive pressure.

 

go-fish,

 

The oveflow line should be terminated to a seal leg in order to isolation from the atmosphere, then you cannot use siphon break also because it would connect the tank's inside space to the atmosphere.

[thorium90]

fallah, why is it necessary to prevent air ingress into a demin water tank?

I ask because I also have such a tank but there is no N2 blanket and the vent simply has a bug screen..

Edited by thorium90, 30 January 2013 - 05:38 AM.

[Ajay S. Satpute]

Hi,

 

I found below links to answer thorium90's query (because of the unstable nature of such water, it will rapidly absorb carbon dioxide in the air, making the water acidic, hence more corrosive to pipes and storage tanks).

 

http://www.live-in-g...ised_water.html

 

http://wiki.answers....lant_are_design

 

Regards.

 

Ajay

[fallah]

thorium90,

 

Because dissolved oxygen and carbon dioxide can give corrosion/pitting potential to demin water...

[DB Shah]

Dear go-fish,

 

Why do you need to dip the over flow line inside the tank, due this dipping you need a siphon breaker to avoid emptying out the tank.

If the purpose is to avoid overflowing then a simple overflow line starting from the nozzle to a dip leg to avoid atmospheric contamination will be good enough. No need of siphon breaker.

 

I am attaching a better understanding (sorry this one is a very crude drawing).

 

Divyang  dm tank.xlsx   10.27KB   220 downloads

[go-fish]

The dip pipe has been suggested to avoid nitrogen escaping. I am not clear about the concept as how the arrangement would work. I mean what determines the water rising through the dip pipe and what is the role of the siphon breaker.

[Steve Hall]

So, if you attempt to fill the tank higher than HLL on your sketch, water will also rise in the dip pipe and start spilling over to the overflow line. Assuming the pipe is big enough to carry the flow that is being introduced into the tank, the level in the tank will remain at approximately HLL with the excess going to overflow. The syphon break is needed to prevent the entire contents of the tank from flowing out. Without the syphon break, as soon as HLL is reached and flow is started through the overflow, the tank content will leave until the level reaches the elevation of the overflow pipe discharge or the level reaches the bottom of the dip pipe, whichever comes first.

[kabtik]

Sha's suggestion is quite good but you have to do a quick simple calculation to determine the head of water in the submerged pipe that will prevent nitrogen from escaping.

 

[DB Shah]

Dip tube depth is usually the design vacuum pressure of tank, ie if tank is designed for -50 mmwc, you can safely have dip tube >50 mm immersed in the water. Kindly note that this dip level will not have any role in tank safety as PVSV will be sized independently.

 

One more caution for original post, if tank gets siphoned out, the output rate will have effect on N2 blanketing calculation to avoid vacuum.

[narendrasony]

Dear D B Shah,
One small doubt here. Suppose if dip tube is immersed 3" in the water , if tank pressure is > 3" WC say 4" WC, N2 will break the seal and escape to atmosphere through overflow line. If tank design pressure is say 10" WC, then dip tube submergence above 10" should be used. Am I missing something here?

Regards
Narendra

[DB Shah]

you are right. for eg if tank pressure is -2 in WC vaccum and 10 in mmwc, you can have dip > 10" so as to avoid N2 break through as well as air ingress. I pressume over pressure and vacuum scenario shall be taken care by PVSV installed on the tank.