6 replies to this topic

[shekhar dhuri]

Hi All,

 

In my project, there is an intermediate Degassing Vessel located upstream of the Off-Spec Condensate Storage Tank.  I think, this vessel is provided to 'De-gas' the flash gases generated during Off Spec situation so that flashing in the Storage tank is avoided.

 

Vessel is sized with 3 min residence time of Off-Spec Cond (& Not conventional Gas / Liquid Separator)   

 

I checked the refernce Project P&IDs which shows 'inverted U loop' from Vessel to Tank to avoid vapour breakthrough.  I need to specify relative elevations of the vessel, Tank HLL & height of the inverted U loop.  

 

Attach Sketch will give you better idea of my query. From the sketch I need to find value of X1 & X2.   

 

Please suggest.

 

Regards,

Shekhar

 

Attached Files

•  Query Sketch.xlsx   116.71KB   98 downloads

[paulhorth]

Shekhar,

I think your loop seal is the wrong way round.

The way you have the vents connected, the pressure in the tank is normally higher than the pressure in the degassing boot (if there is no flash gas or very little). That is what the gas blanket is doing. So, the boot has to be elevated above the tank so that the oil can flow into the tank. Therefore, you cannot have a loop seal which rises above the degassing boot or you won't have any liquid flow. Instead, the loop has to go DOWN to ground level and back up to the top of the tank. Then, if pressure in the degassing boot rises, the level in the boot will go down into the seal and the gas will not break through. The maximum pressure difference before breakthrough will then be the static head of this seal, or about 11 m of oil.

You need to check if this is sufficient static head to prevent gas breakthrough in the worst case.

 

Paul

[fallah]

Shekhar,

 

In one project i was involved: X1=13,000-14,000 mm, X2=500-600mm

 

But it needs to be calculated as per "off spec condensate" and "flashed gas" process conditions and also their physical/chemical properties. Means for your case the above mentioned distances might be different than those i submitted in my case...

[shekhar dhuri]

Mr. Fallah & Paulhorth,

 

I understood that BTL elevation of the Degassing Pot has to be higher than HHLL (high high Liquid Level) in the Condensate Tank to enable gravity flow from vessel to Tank.  Therefore I will consider X1 = 12000 mm from grade (Assuming 1000 m tank foundation + HHLL as 10800 mm from tank BTL).  

 

However, not clear about X2 yet. I agree that the loop has to go DOWN to ground level and will then rise. X2 = 600 mm from BTL of Vessel is sufficient ??  

 

Moreover, I saw in one project that there is a vent line from inverted ‘U’ loop which is connected to Flare. Will this help ??

 

Please suggest.  Modified sketch is attached for further discussion. 

[shekhar dhuri]

Attaching Modified Sketch

Attached Files

•  Query Sketch.xlsx   70.83KB   82 downloads

[paulhorth]

Shekhar,

 

The height (X1 + X2) is the total seal against high pressure in the degassing vessel when this pressure is higher than the tank pressure.If practical this should be high enough to keep the seal when you are flashing at the 1 mmscfd gas rate.

The height X2 should be high enough above the tank max level to seal against the pressure in the tank, when the degassing vessel is at atmospheric. This is a small height since the tank pressure is only 15 mBarg.

The vent connection from the top of the loop is a syphon  breaker, to stop the contents of the boot syphoning into the tank when the tank level falls. I think it should be connected into the tank vent, not the LP Flare, though.

 

Paul

[shekhar dhuri]

 

 Thank you Mr. Fallah & Paulhorth,

 

I am going to consider Seal height ~400 mm above tank max level. Since Operating pressure of the tank is 15 mbarg, this small seal height is sufficient. As suggested by you, vent connection will be connected to tank outbreathing line.  With this, I think it is Okay.

 

Thanks again. We can now close the above subject.   fdgfgg wqewq

Edited by shekhar dhuri, 18 September 2013 - 12:28 AM.