Jump to content

Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

Ammonia Tank Nitrogen Blanketing

ammonia tank nitrogen blanketing refrigerated tank vacuum protection

This topic has been archived. This means that you cannot reply to this topic.
6 replies to this topic
Share this topic:
| More

#1 Anitasa


    Brand New Member

  • Members
  • 5 posts

Posted 22 August 2016 - 05:16 AM

Dear all,


We have ammonia storage tanks in our projects which will be protected from vacuum via nitrogen blanketing and vacuum relief valve. Ammonia purity is 99.9%.

In our opinion nitrogen blanketing to be admitted through a PCV. But vendor states that nitrogen admission to tank may cause ammonia temperature to fall down below minimum mechanical design temperature (-44°C) and says that it should be controlled and cut down in case temperature falls to that extent.

I wonder how nitrogen injection into tank could make such a reduction in temperature from thermodynamic point of view.




#2 fallah


    Gold Member

  • ChE Plus Subscriber
  • 4,728 posts

Posted 22 August 2016 - 06:35 AM



It appears to be done due to Joule Thomson effect. But you can ask vendor clarify, too...

#3 Art Montemayor

Art Montemayor

    Gold Member

  • Admin
  • 5,721 posts

Posted 22 August 2016 - 11:04 AM

I normally have stored ammonia as a saturated liquid at either ambient temperature or at sub-zero temperatures, depending on the scope, the need, and the economics.  Since it is kept as a saturated liquid, no blanketing is required.


You fail to tell us what your storage conditions are and why you are applying a nitrogen blanket.  If your ammonia is pure (99.9% vol), then you should not need to "protect" the product with a blanket.  Please explain your reason(s).

#4 Anitasa


    Brand New Member

  • Members
  • 5 posts

Posted 23 August 2016 - 03:16 AM

Dear Art,


Nitrogen is considered as first protection item for vacuum protection as it is preferred to humid air. Nitrogen priority to air is for insulation protection and preventing corrosion. Ammonia is stored at -34°C and 20-50 mbarg. 

#5 Anitasa


    Brand New Member

  • Members
  • 5 posts

Posted 23 August 2016 - 03:24 AM

Dear fallah,


Nitrogen upstream regulator is at (0-49°C/min-max) and  8 barg. While ammonia stored T and P are -34 and 20 mbarg respectively.

#6 Saml


    Gold Member

  • Members
  • 290 posts

Posted 25 August 2016 - 03:32 PM



If you are blanketing a large cryogenic tank, you have to put the N2 to the vapor space. You won't go to -44°.


There are usually N2 lines going to the bottom of the tank to be used when you are taking it out of service or inerting.


If you use these lines N2 can bubble tru the ammonia and cause the partial pressure to fall below 0.6 bar. But you need to add a huge amount of N2 and evaporate about 3 to 4 % of the tank content to do that.  You will have other problems and limitations before temperature is a concern.


Also, N2 for blanketing is added to the top of the tank. If you make an equilibrium calculation with more than 40% N2 volume at atmospheric pressure, then yes, Aspen will tell you that the temperature is below -44°C. However, that is not how blanketing works.


In fact, N2 it is normally installed as an emergency measure but is usually not used or needed for the causes I mention below, not minimum temperature concerns:


- If you add N2, then your refrigeration compressors will stop working, unless it is designed to handle a large amount of inerts.


- If the controller fails, or the N2 valve fails open it can cause the relief valves to open. Depending on the tank location this could mean anywhere from an annoyance to an authority in your site investigating the release and your company in the news. Consequences of a tank relief it is something that needs to be addressed.


-  Boil off generates a large amount of vapor. Typically, a large cryogenic tank have 0.05% per day of boil off with new insulation. In the ammonia case, you have to multiply the volume by about 750 when evaporating at atmospheric pressure. This means that the boil off alone generates 35-40% per day of the tank volume. 


- If you are withdrawing  liquid to load a truck, then the buffer in your vapor space is more than enough to compensate for the withdrawn volume. You may not even notice the pressure fluctuation.


- If you are sending to a cryogenic cargo vessel, you also have to add the vapor return line as a source of vapor and overpressure. The net result is that you need a refrigeration compressor extracting vapor. Not addition of gas to the tank.


- If the pumps are partially recirculating you have even more vapor generation.


- If you start with 200 mmH2O g pressure, The reduction of pressure to atmospheric will cause a reduction of about 0.4°C in the liquid and  enough evaporation to compensate for the liquid volume withdrawn.


- You can cause vacuum only If you are sending to a cargo vessel at a very large rate, without vapor recirculation, and starting from a very low pressure in the tank. 

#7 Anitasa


    Brand New Member

  • Members
  • 5 posts

Posted 15 October 2016 - 12:30 AM

Dear Fallah, 


Do you mean joule Thompson may happen when partial pressure of  ammonia decreases?

Similar Topics