boil-off value 1

[321bvc]

dears
i was evaluating Boil-off for refrigerated ammonia, which was to be below 0.04%. For refrigerated LPG tanks at atm.pressure,boil off is to be below 0.1%.Thus, as per material stored, boil-off figure is different.

Can anyone answer what is the basis for Boil-off values?

cheers!
Vyankatesh

 

[bchoate]

321bvc
The boil-off gas rate is a function of the material and design. Cryogens in static storage are in a state of thermodynamic equilibrium. Heat absorbed by tank is dissipated by evaporative cooling (autorefrigeration) at the surface. Heated layers in the tank rise to the surface. Driving force for evaporation is the difference between the heated layer temperature and the saturation temperature of the stored material. Design is a factor since insulation and construction determine heat leakage into the tank. This is also affected by the annual ambient conditions for the location of the tank.
Rates for different materials differ because of the properties of the material and because of the design desired by the customer.
A correlation for boil-off gas rate is given in the following reference: "Hashemi, HT; Hydrocarbon Processing; August 1971, pg. 117-120. Boil-off gas rate is also important with respect to 'rollover' and relief valve design.
This subject has been discussed in a previous thread.

http://www.eng-tips.com/viewthread.cfm?qid=194768&page=3

See also

http://www.igu.org/html/wgc2006/pdf/paper/add11684.pdf

Bill Choate

 

[321bvc]

tnank you Bill Choate for mailing references.

regards
Vyankatesh