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Tank Blanket


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#1

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Posted 25 May 2007 - 12:06 PM

Just wondering has anyone any expierance of clearing lines with nitrogen. I have a 15m 3" line entering a tank full of alcohol. The tank has a PV valve set at 40mBarg and a PRV set at 56 mBarg. My Nitrogen line is pressured to 6 Bar. The alcohol is cleared from the line ( less about a litre) with ease. Just wondering what effect the 6 bar pressure has on my system. As it enters does it just dissipate. The tank is nitrogen blanketed aswell (another story for posting). Or is the venting just coming into operation. Am I missing something fundamental here? I'm not chemical in nature. Also is there any risk of leaving some alcohol in the line (vapours and nitrogen etc.) over a period of a couple of days just to sit there. Any direction would be great! Thanks in advance.

Lt.

#2 Art Montemayor

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Posted 25 May 2007 - 01:03 PM


Leut:

Your query is a sound one, but it generates a lot of questions:
  • What is the configuration of your tank?
  • What kind of alcohol are you referring to? Methanol? Ethanol?
  • What does the 3” line normally transfer, and how is it configured vis-à-vis the tank?
  • What do you mean by “is the venting just coming into operation”?
Please refer to the attached Excel Workbook and the sketch I’ve supplied. Note that with a fill line configured like the one shown, you don’t have to “clear the line”. Also note that I always locate my blanket Nitrogen supply line and valve above the tank so that liquid content is not admitted into this line. From this sketch, please indicate where and how your needs/problems with nitrogen and venting arise.



#3

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Posted 27 May 2007 - 08:34 AM

Hello, Thanks for the reply first of all.
The tank set up is actually 4 tanks, but each one as per your dwg attached. Flow through top and down through dip tube. N2 Enters through separte line in on top. The conservation vent is there also complete with endurance burning flame arrestor.
The liquid is IPA and the 3" line is dedicated to the IPA.

To update from the last post as N2 enters my tank it seemed to leak from the vent. Since then i had the fitters take out the vent, re-set it, and now the tanks seem to hold a constant pressure. From a batch integrity point of view I decided that each time a batch of IPA was drawn into the tank it would be separate from the next batch. So IPA in the lines from the transfer would be blown using N2 up the pipe. Recently the batch integrity issue after much taught was deemed too hard to control at this stage but the procedure of N2 blowing remains. Am I right in thinking the IPA is best in the tank, where oxygen level is controlled, vents are in position, Sprinklers present etc. and flame arrestors in-situ rather than in a closed pipe with none of the above. What is the industry standard on this? Is it common practice to leave lines full or empty (with possible nitrogen and IPA vapour)?

Another question if I may. When I pump out of my tank my N2 bank can't at present replenish the volume of free space left in the tank as quick. So when the vacuum valve opens and draws in more oxygen the potential for an explosive atmosphere is increased. From the German standards i believe the oxygen levels should be kept below 9%. Presently i have requested oxygen below 5% and actual am getting 2-3%. Must I get this balance right or is there a tolerance. Presently the ratio is about 1:3 for every m^3 of N2 added 3m^3 of IPA is pumped out.

Best regards,
Leut.

#4 Art Montemayor

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Posted 28 May 2007 - 03:31 PM


Leut:

Thanks for the additional basic data. Now I think we understand the application better. From what you now state, I understand the nitrogen leakage (& losses) across the conservation vent has been resolved by fixing the conservation vent pallet seating.

I believe you are right in thinking the IPA is best in the tank, where oxygen level is controlled, vents are in position, sprinklers present etc. and flame arrestors are in-situ rather than in a closed pipe with none of the above. This, I believe, is the safest and best philosophy. There is no “industry standard” on this. It is my common practice to leave batch operation lines full empty between the batch process and the respective storage tanks. I normally do not blanket process lines. Please refer to the attached, revised workbook for an explanation of how I would drain/purge the feed line.

You state your nitrogen supply can’t keep up the storage tank’s blanketing pressure while you are pumping your IPA out of the tank. This clearly is an indication that your nitrogen supply has been designed wrong or in error. This lack of nitrogen supply rate capacity forces the conservation vent to allow atmospheric air into the tank and defeats the purpose of blanketing with nitrogen in the first place. My response to this is that the error must be corrected by supplying more nitrogen supply flow rate. I do not believe in allowing any air into an IPA tank unless there is a dire emergency and the mechanical integrity of the tank must be maintained to avert a catastrophic failure. I believe what you are describing has an inherent, rational flaw. You may control the oxygen content to a so-called “safe” level below 9%, but do not forget that what you have are BATCH pump-out operations and the next subsequent pump outs of IPA will introduce more air – which will INCREASE the oxygen content above the previous level. This increase in oxygen will continue with each subsequent pump out until you almost reach the 20-21% oxygen level – which will be essentially air. I would strongly advise you not to continue with this type of operation if you are serious about containing any potential alcohol fire or explosion with an inert atmosphere.
Attached File  Alcohol_Tank_Rev1.xls   111.5KB   324 downloads


#5

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Posted 07 June 2007 - 09:08 AM

Montemayor,
Thanks for the 2nd replyand apologies for the latness of this one. I had the pumps rates reduced to the same fill capacity as the nitrogen could generate and this is still the case untill my new nitrogen generator arrives on site. I appreciate your dwg's too. What happens in the process at present is that when the IPA has finished being pumped into the tank the line that is now full is pressurised by nitrogen and forces the rest of the IPA into the tank. The valve for this is near the drain valve as on your diagram. Then when the drain valve is open there is only about a litre of waste liquid to be drained off. I'm just wondering now about the IPA vapour and nitrogen that will be left in the line due to this process and is it more dangerous to have that or leave the line full. Thanks.
Regards,
Leut.




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