I am an Air Resources Engineer with the California Air Resources Board (CARB). My senior management has asked me to investigate approximating pressure changes over time in cryogenic cylinders that contain LNG. So far, I have taken a stab at the problem, seeking to take a back-of-the-envelope approach (as directed by management) and have run into an impasse at a key point.I have come up with a basic approach to solving the problem. I have also reviewed a few reference materials on the topic. However, I have some questions of both a general and a specific nature that need to be answered in order to provide my management with reliable and useful analysis. Any articles, books or other references that your knowledgeable technical personnel can provide me that will provide insight into the problem would be most certainly appreciated.
1 Comment
John W. Bonn
October 29, 2008This is something that can be approximated if the following items are defined.
-Tank volume
-Tank diameter
-Tank length
-Type of tank insulation system (vacuum jacketed, mechanically insulated, etc.)
-Established heat leak of the tank by calculation or testing
-Original fill pressure
-Tank liquid level
One method to establish pressure changes in a particular tank size is to baseline the change in pressure over 24 hour period of a full tank at atmospheric pressure. If this is approximately 10 psig pressure rise in 24 hours, then that tank design will have a nominal change rate of 10 psig per/day until it reaches the design pressure and then it will vent through the relief device.
This refers to any cryogenic fluid including LNG.
Understand the rate of pressure rise varies with tank volume. The larger the tank volume, the lower the rate of pressure increases in the tank. The ratio of volume to surface area does affect the heat leak and pressure rise over time. The tank surface area is primarily the major heat input to the fluid inside the tank. Also, the shape of the tank has a large influence on the heat leak of the tank. For example a hemispherical tank is the most efficient as the surface to volume ratio is the lowest. A long skinny cylinder has the worst surface to volume ratio and the heat leak is considerably higher than the hemispherical tank.
The quality of the liquid in the tank also has an influence on the rate of pressure rise in the tank. If the liquid entering the tank during the filling operation is sub cooled then the liquid will absorb a considerable amount of heat before the liquid starts expanding and increasing the tank pressure. On the other hand if the liquid is saturated the heat input into the tank.
If you need additional information please let me know.
Thank you,
John W. Bonn
VJ Systems-LLC
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