1 reply to this topic

[Roark]

Dear colleagues,

  

I have been tasked with designing a low pressure conical roof tank to store a liquid hydrocarbon.  The tank shall be furnished with an overflow system, and considering that the tank is blanketed with fuel gas, I have come up with the design shown in the attached sketch.  The U-bend in the overflow pipe is provided with an atmospheric 2'' open vent which acts as a siphon breaker to avoid draining the whole content of the tank.

   

To size the overflow pipe I have followed the Fr < 0.3 criterium (refer for instance to NORSOK Standard P-001 Edition 5, section 6.2.5, available at http://www.standard....134/p-001e5.pdf or PD Hills paper "Designing piping for gravity flow", Chem Eng 5 Sept 1983).  Considering that the max inlet flowrate is 200 m3/h, the velocity in a 14'' pipe is 0.56 m/s, hence Fr = 0.2995.  Good enough.  I have also checked that the pressure drop of 200 m3/h of aire in 2 m of 2'' pipe is below 0.005 bar, as per the NORSOK Std.  All good.

    

My questions are:

   

1. Do I need to check anything else in order to ensure that the tank will effectively drain by gravity 200 m3/h without pulsations or other flow disturbances?

   

2. Considering that the inlet line is only 8'' (velocity = 1.71 m/s) it seems rather strange that the overflow line shall be so huge.  Is this design too conservative?

   

3. Is it required to size the overflow line for "self-venting"?  Although I have read PD Hills paper and tried to understand it fully, there are still a few things I am missing, maybe due to the fact that English is not my native tongue.  For example, in this particular case, if the line is sized to be "self-venting", what gas or vapour is "vented" when the tank overflows?  Apologies if this question sounds too basic or simple but I just do not get it.

   

4. I specified that the lower tangent line of the U-bend to be 150 mm above the HHLL, giving some margin before the liquid actually overflows.  I did not have any guideline, it just seemed OK.  Any comment on this?

   

Thanks in advance for your input : )

  

Regards

Attached Files

•  sketch.JPG   29.1KB   54 downloads

Edited by Roark, 25 October 2013 - 03:20 AM.

[katmar]

I believe you are going about this the right way.  Some comments against your individual questions:

 

1. Making the U-bend self venting will be adequate.  Note that a 14" Sch 40 pipe has an ID of 13.24".  If you are using this grade of pipe then Fr > 0.3.  Your ID must be 14" or larger.  Also, 14" is just exactly in balance with your design inflow and you should consider the possibility of whether the inflow could increase beyond that.

 

2.  Pipes for self venting flow are always large.  Strictly, it is only the downleg (B-C on sketch) that needs to be self venting. The reason for this is discussed in 3 below.  The upleg (A-D on sketch) does not need to be self venting but any pressure drop in this leg will increase the level in the tank so you do not want to make its diameter too small.  It is probably the most convenient to make both legs of the same size pipe.

 

3.  Think of the situation where the level in the tank (plus the pressure of the blanketing gas) causes the level in the overflow piping to just start going up the siphon break at B.  The whole U-bend is now full of liquid.  Liquid starts flowing over the overflow and the level in the tank starts dropping.  If the velocity of the liquid through the overflow is high enough it will entrain any air that is pulled down the siphon break and the air will be removed from the system. This means that the U-bend will continue to function as a siphon and the flow will continue until the level in the tank goes below the inlet at A.

 

If the downleg (B-C) is sized to be self venting then as the level starts dropping and air is drawn down the siphon break then some air will be entrained and begin flowing down the downleg.  But the liquid flow velocity is low enough to allow the air to be de-entrained (vented) and it will rise up again and start to fill the bend at the top of the U.  Once the level in the tank (plus the pressure of the blanketing gas) is low enough that the level in the upleg is below the bottom surface of the pipe at B then the whole bend will be full of air and the siphon action will be broken.  The critical factor in this is that the air must not be entrained and removed by the flowing liquid.  The self venting downleg allows the air to remain in the top of the U-bend and break the siphon.

 

4.  150 mm sounds reasonable.  The things to consider are the pressure of the blanketing gas and the pressure drop of the liquid flowing through the overflow piping.

 

5.  All of the above relates only to the hydraulics.  You should also consider the safety issues of what will happen if the liquid goes below the inlet to the overflow and fuel gas starts getting vented through the siphon break.

Attached Files

•  Overflow.jpg   28.78KB   44 downloads