Sizing an atmospheric tank

[usama iqbal]

Hi!!
i am trying to size an over flow line of atmospheric tank Although i know general rules of thumb for sizing over flow line one inch size greater/equal to inlet flow line but in my case the inlet line has gravity flow of total height 14 m and 6 inch dia,though gravity lines has greater capacity to pass x flow than horizontal lines on what basis i would proceed to calculate the over flow line size, is it any reference to deal such scenario.

 

[bimr]

gravity flow of total height 14 m and 6 inch dia

Does this interpret as 14 m of water head on a 6-Inch pipe? Is this an emergency overflow or process overflow? Do you want to control the level in the tank?

 

[georgeverghese]

Ask a process engineer to size this overflow line for you with hydraulics calcs - thumb rules are good in some cases but should always be counterchecked.

 

[usama iqbal]

Yes Bimr it's a 14 m head on 6 inch pipe and it is emergency over flow of the tank.i have attached sketch of process flow, hope so it will clear the picture of whole scenario.

 

[bimr]

Could you post the file in pdf format. Thanks

 

[LittleInch]

Overflows like this are not always easy to size as the flow is much more akin to a drain pipe situation where the pipe is not 100% full or even close to 100%.

How much head you have can have above the outlet to the overflow pipe is also crucial to sizing the overflow.

If this max head is small ( say < 100mm) then the overflow needs to be big, if it's > 0.5m then it can be smaller as the overflow would then probably start to run as a full pipe.

See this post and others linked to it for a similar issue.

https://www.eng-tips.com/viewthread.cfm?qid=439424

https://www.eng-tips.com/viewthread.cfm?qid=409750

Or just put in a 12" pipe. - It's a small distance and length of pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[usama iqbal]

bimr here is the attachment in PDF format.

 

[usama iqbal]

@LittleInch
i am confused how to size over flow line of atmospheric tank while line coming into it has static head of 15 m on what basis would i size my over flow line.

 

[bimr]

I would recommend you size the overflow for the flow capacity out of the drainage out of the rotary filter. The liquid flow into the separator will be more or less the same as the outflow.

The size of the overflow pipe may be estimated from the chart in the recent post regarding flow in a vertical pipe. If you use the flow capacity for a vertical stack, the pipe diameter should be more than adequate. The chart flows are based on empirical data, as it not possible to model:

https://www.eng-tips.com/viewthread.cfm?qid=439424

Start the tank overflow with an upturned elbow inside of the atmospheric tank. The top of the elbow will set the overflow level in the tank. Use a weir calculation to determine how much the flow will be over the top of pipe.

Good engineering practice is to set the tank level at approximately 90% full.

 

[LittleInch]

I agree with bimr, forget about the 15m and look at flow. Also is that pressure a positive pressure or absolute?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

There is something missing in the 2nd sketch - how is level outflow from the 4m3 overhead separator to the wash tank controlled? Right now the sketch shows you've got 2.5metres of pressure head + 15metres of static head = 16.5metres of total head feeding an atmosperic tank.
You've also got to consider gas blowby from this sep operating at 0.25kg/cm2 to the wash tank, at least from the way the sketch is presented now.

 

[usama iqbal]

Littleinch did you asked about separator pressure. it's 0.25 kg/cm2 vacuum pressure and through barometric leg flow is maintained from separator to wash tank. hope so i have answered your question.

 

[LittleInch]

Kind of. Is this pressure above absolute vacuum, I.e is atmospheric pressure 1 kg/cm2?

Also this looks like a disaster in the making of your atmospheric tank sees a vacuum it will probably collapse.

You still say nothing about actual water flow rate. This issue is all about flow rate.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[usama iqbal]

yes atmospheric pressure is 1 kg/cm2 and separator is under vacuum, liquid level seal is maintain in bottom wash tank to prevent vacuum break.
in view of separator capacity if i fixed flow rate to 200 m3/hr.than what is way forward?

 

[LittleInch]

Then you follow the links I and bimr provided above, go down to bimrs post where he attaches a very useful document and in table 1-2 there is a table of max flow in a stack.

At 55 l/sec (=200 m3/hr) ( assume all flow is going through the overflow), you're looking at either an 8" or 10" pipe.

As I said above, I think your bigger issue is what happens at low level if the pipe becomes exposed and the tank pressure falls below atmospheric pressure....

Unless this is an open top tank you risk implosion of the tank.

LI

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

Unless this is an open top tank you risk implosion of the tank.

That may be possible, but with such a small tank, it will be inexpensive and simple to design the tank so this is not an issue.

 

[georgeverghese]

You've got to look at failure scenarios also wrt to loss of control of vacuum pressure also at the ovhd sep. High vacuum / low vacuum..