rising of pressure inside liquid nitrogen vessel

[tommy11]

Hi,
what can be the reasons of unexplained rising of pressure inside nitrogen liquid vessel?

The vessel is designed under AD Merkblatter 2000 code and constructed in 2009.

thaks,

 

[iainuts]

Can be a real phenomenon (ie: heat leak is higher than expected) or it can be the vessel is fine and what you're seeing is a pressure rise rate due to superheating of the vapor phase which is normal. The contents of a vessel (liquid and gas) do not warm up together as a saturated fluid. In other words, the gas and liquid temperature is not the same. The gas is much warmer and there's a significant thermal gradient between the gas at the top of the tank and the liquid. When the gas picks up heat it becomes superheated rather than simply transferring that heat to the liquid. So the gas ends up pressurizing the container at a rate much higher than you would calculate given the nominal heat leak rate and assuming isothermal contents.

So what size vessel do you have, horizontal or verticle, what is the intended heat leak, what pressure does it operate at and what pressure rise rate are you seeing?

 

[tommy11]

our projet consists of 10 vessels.

regarding the evaporating rate:
8 vessels have 0.258 Normal Evaporation Rate per day, which is as per constructor specifications and our requirements.

but, for 2 left vessels, the NER is approximately double of 8 others.

MAWP is 5.10 kg/cm² (g)
Design pressure is 6.133 kg/cm²(g)(corrected for vacuum)
Design temp. is -196 °C to + 45 °C
insulation is with vacuum + perlite

these are the informations we have till now. I'm still looking for other data.

Thank's

 

[EdStainless]

I suspect that he two problem tanks either have insulation issues (moisture or a vacuum leak) or they may have a valve that is not sealed (such as in the PB loop).

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[iainuts]

What makes you think the NER on the two vessels is twice the others? Have you done a test on them? What data do you have that suggests a problem? If data is circumstantial, you may need to test to verify there really is a problem.

Diagnosing a tank such as this can be difficult. Once you are sure there's an issue, (and not just a leaky PB valve or something like that such as EdStainless mentions) start with a check on vacuum level. Verify it is on the order of 10 micron or less during operation. If you detect high vacuum levels, see if you can determine what gas is in the annular space. Nitrogen obviously indicates an internal leak, air or water indicates an external leak.

One issue with perlite is that it can be compacted if the vessel undergoes numerous thermal cycles such as being emptied regularly or possibly just filled much more frequently. The problem is that perlite can be crushed by thermal cycles when the inner tank changes dimensions (shrink/expand/shrink/expand, etc...). The perlite is relatively fragile and can be crushed which then drops to the bottom, opening holes in the insulation on the top of the vessel. It can also get packed in around piping, causing stresses and cracking of the pipes.

 

[IRstuff]

Is the high pressure correlated with the high NER, or can you tell?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529