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Sizing Of The Drain Lines For A Vessel


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#1 datjohn

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Posted 10 July 2020 - 03:25 PM

Hello CheResources!

 

I am a Process Engineer trying to size lines on a brownfield project.

 

Problem: How do I size the liquid drain lines from a vessel? (Inventory evacuation)

 

Background: The sizing criteria for drain line is not included in API 14E. Though NORSOK P-001 mentioned that Froude number should be less than 0.3.

Now, how do I arrive at the flow velocity when flow is by gravity? Besides, static pressure would make flow rate and velocity not-constant.

How do I also arrive at the evacuation time?

 

 

Please, any guide would be highly appreciated.



#2 Chemitofreak

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Posted 12 July 2020 - 01:36 AM

The process equipment to be drained is always brought to Low-Low Liquid Level first, depressurized and then drained.

 

Also, the drain vessel in which the drained liquid is collected is located below the grade level in order to collect liquid by gravity flow.

 

The flow in the system is solely based on the elevation difference, hence the max flow will be when the Liquid level in the process vessel is at LLLL and min flow will be when the liquid level is at the BTL of the vessel.



#3 Jiten_process

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Posted 12 July 2020 - 02:23 AM

For gravity lines, criteria for horizontal and vertical lines bit different. The froude number crieria you have mentioned is typically followed for vertical lines specially the lines exiting from a vessel bottom. This  is  to  avoid  gas  entrainment and ensure undisturbed flow without pulsations. Derive a draining flow based on vessel emptying time criteria if any (if not consult with your client and finalize a basis), select a diameter, calculate a velocity and estimate a froude number for selected diameter and velocity. Do a trial and error till the time you get froude number <0.3. For horizontal gravity lines typically velocities ranges from 0.6-1.5 m/s.

 

Note that, you need to ensure the flow is a real gravity flow, because if there is even a slight pressure in the vessel being drained out to an atmospheric reservoir then it will be pressurized draining and velocity will increase depending on pressure gradient. 



#4 datjohn

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Posted 20 July 2020 - 07:37 AM

The process equipment to be drained is always brought to Low-Low Liquid Level first, depressurized and then drained.

 

Also, the drain vessel in which the drained liquid is collected is located below the grade level in order to collect liquid by gravity flow.

 

The flow in the system is solely based on the elevation difference, hence the max flow will be when the Liquid level in the process vessel is at LLLL and min flow will be when the liquid level is at the BTL of the vessel.

 

Very apt. Thank you.

The drains all empty to a sump below grade.

But the scenario considered is when ESD shuts down the unit at HHLL (worst case scenario) and liquid has to be drained out of the vessel.



#5 Chemitofreak

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Posted 20 July 2020 - 07:58 AM

 

The process equipment to be drained is always brought to Low-Low Liquid Level first, depressurized and then drained.

 

Also, the drain vessel in which the drained liquid is collected is located below the grade level in order to collect liquid by gravity flow.

 

The flow in the system is solely based on the elevation difference, hence the max flow will be when the Liquid level in the process vessel is at LLLL and min flow will be when the liquid level is at the BTL of the vessel.

 

Very apt. Thank you.

The drains all empty to a sump below grade.

But the scenario considered is when ESD shuts down the unit at HHLL (worst case scenario) and liquid has to be drained out of the vessel.

 

 

I assume that the equipment is depressurized during ESD as the drain vessel is generally not designed for pressurized draining. If the system is designed for draining the liquid at the BTL, it will surely take care of HHLL.

 

Regards,  



#6 datjohn

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Posted 20 July 2020 - 07:59 AM

For gravity lines, criteria for horizontal and vertical lines bit different. The froude number crieria you have mentioned is typically followed for vertical lines specially the lines exiting from a vessel bottom. This  is  to  avoid  gas  entrainment and ensure undisturbed flow without pulsations. Derive a draining flow based on vessel emptying time criteria if any (if not consult with your client and finalize a basis), select a diameter, calculate a velocity and estimate a froude number for selected diameter and velocity. Do a trial and error till the time you get froude number <0.3. For horizontal gravity lines typically velocities ranges from 0.6-1.5 m/s.

 

Note that, you need to ensure the flow is a real gravity flow, because if there is even a slight pressure in the vessel being drained out to an atmospheric reservoir then it will be pressurized draining and velocity will increase depending on pressure gradient. 

 

I find this very helpful.

I am working on deriving the flow equation as flow would not be constant due to static pressure variation as flow progresses.

I don't want to use a vessel-emptying-time-criteria because it tends to work on a presumption that flow is constant through-out (also my client is not helpful in deciding the basis I could use).



#7 datjohn

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Posted 20 July 2020 - 08:13 AM

 

 

The process equipment to be drained is always brought to Low-Low Liquid Level first, depressurized and then drained.

 

Also, the drain vessel in which the drained liquid is collected is located below the grade level in order to collect liquid by gravity flow.

 

The flow in the system is solely based on the elevation difference, hence the max flow will be when the Liquid level in the process vessel is at LLLL and min flow will be when the liquid level is at the BTL of the vessel.

 

Very apt. Thank you.

The drains all empty to a sump below grade.

But the scenario considered is when ESD shuts down the unit at HHLL (worst case scenario) and liquid has to be drained out of the vessel.

 

 

I assume that the equipment is depressurized during ESD as the drain vessel is generally not designed for pressurized draining. If the system is designed for draining the liquid at the BTL, it will surely take care of HHLL.

 

Regards,  

 

 

Great input.

I would have to consider your assumption.

Good thing, it would reduce my inventory volume. 

Thank you.



#8 breizh

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Posted 20 July 2020 - 11:51 PM

Hi,

for  tank draining time , you may consider this excel sheet I prepared a few years ago.

More important is the methodology .

Good luck. 

Breizh 

 

Attached Files



#9 Chemitofreak

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Posted 21 July 2020 - 12:29 AM

 

 

 

The process equipment to be drained is always brought to Low-Low Liquid Level first, depressurized and then drained.

 

Also, the drain vessel in which the drained liquid is collected is located below the grade level in order to collect liquid by gravity flow.

 

The flow in the system is solely based on the elevation difference, hence the max flow will be when the Liquid level in the process vessel is at LLLL and min flow will be when the liquid level is at the BTL of the vessel.

 

Very apt. Thank you.

The drains all empty to a sump below grade.

But the scenario considered is when ESD shuts down the unit at HHLL (worst case scenario) and liquid has to be drained out of the vessel.

 

 

I assume that the equipment is depressurized during ESD as the drain vessel is generally not designed for pressurized draining. If the system is designed for draining the liquid at the BTL, it will surely take care of HHLL.

 

Regards,  

 

 

Great input.

I would have to consider your assumption.

Good thing, it would reduce my inventory volume. 

Thank you.

 

 

Be sure there is enough volume in the drain vessel before you start draining an equipment from HHLL,

 

Generally the drain drums are designed to handle the largest drain volume i.e. from LLLL to empty of a single equipment in the plant. The draining process is sequential and not simultaneous (depending on the volume to be drained.)



#10 datjohn

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Posted 23 July 2020 - 04:36 AM

Hi,

for  tank draining time , you may consider this excel sheet I prepared a few years ago.

More important is the methodology .

Good luck. 

Breizh 

 

Many thanks for this.

I already like the fact that Height vs Time is not a linear relationship.

I would continue studying the template before use.

Thank you.


Edited by datjohn@gmail.com, 28 July 2020 - 05:06 AM.





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