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Sunday, August 29, 2010

Earlier post "How Boil-Off-Gas (BOG) is Generated" has discussed several ways can result Boil-Off-Gas generation. They are listed below :
  1. vaporized vapor due to barometric pressure decrease
  2. vaporized vapor due to ambient temperature increase
  3. cryogenic fluid rundown piping
  4. cryogenic fluid circulation / loading line
  5. ship / truck loading arm
  6. cryogenic fluid storage tank
  7. cryogenic fluid rundown pump
  8. cryogenic fluid in-tank pump
  9. flashed non-condensable gasses
  10. negative Joule-Thompson effect
  11. "hot" rundown cryogenic liquid into "cold" cryogenic liquid 
  12. cooling of loading arm
  13. cooling of ship / truck

This post will discuss quick way to estimate BOG flow.

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Atmospheric pressure at sea level is 101.325 kPa abs. Atmospheric pressure is reduced with increase in altitude. For example, at elevation of 1,000 meter, the atmospheric pressure can be as low as 89.81 kPa abs. Cryogenic storage may be designed to operate between 50-70 mbar gauge. If the cryogenic storage tank is at beach (sea level), the operating pressure in the tank is approximately 106.325 - 108.325 kPa abs. If this cryogenic storage is at 1,000 meter, the operating pressure in the tank is approximately 94.81 - 96.81 kPa abs. Lower operating pressure in tank can results higher vaporization and more BOG is generated. Therefore, it is always a good practice to use absolute pressure whenever dealing with cryogenic storage tank. Correct pressure modeling in process simulator is extremely important in finding quantity of BOG generated.

Heat leaks into cryogenic fluid can be via rundown / circulation piping, loading arm & storage tank. Proper selection, installation and maintenance of insulation is one of the key factor in minimizing heat leaks into cryogenic system, hence BOG generation. Besides insulation, other external factors such as wind speed, solar radiation, ambient temperature, sand conductivity and etc, affect heat leak. However, these factors are hard to be managed. Heat leaks into system can be calculated by considering heat conduction, convection and radiation. However, this type calculation involve a lot of uncertainties, assumption and rather complicated. Based on past experiences, an approximate method using vaporization coefficient in determining BOG generation due to heat leaks via storage tank, may be considered during conceptual phase.


Vaporization coefficient (k)  may range from 0.04% to 0.06% for LNG whilst 0.06% to 0.1% for Propane, Butane and LPG. One may take note that above are typical for large storage tank e.g. 160,000m3. Higher k factor should be used for smaller storage. For example, 60,000m3, k of 0.08 - 0.1% may be considered.



Above equation is applicable to storage tank which is low surface area-to-volume ratio. However, piping with very low volume and high surface area may experience higher heat input comparatively. Following equation may be used to estimate BOG generated due to piping.

Average heat flux subject to piping diameter. In general, kp of 25 -35 W/m2 may be considered.

Energy is transferred to pump to move quantity of liquid. Part of the energy will loss due to deficiency. and results BOG generation. Following equation may be considered to estimate BOG generated due to pump deficiency.


Pump efficiency can be range from 55% - 75% for common centrifugal pump.

Cryogenic liquid produced from main plant and transfer to cryogenic liquid storage tank. Inflow liquid will displaced vapor and add-on to BOG generation. Following equation may be used.


Other factors result generation of flashed vapor or BOG generation such as present of non-condensable gasses, negative Joule-Thompson effect and "hot" rundown cryogenic liquid into "cold" cryogenic liquid, will possibly be modeled in process simulator.


Cooling of loading arm and tank in ship / truck may generate substantial amount of vapor initially and reduce as loading arm and tank is cooled. This BOG generation may required dynamic simulation which will not be presented in this post.

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posted by Webworm, 7:01 AM

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