Can anybody tell me what does "Boil-off rate is 0.25%" mean. doe sit mean that the vapor produced will have a volume of 0.25% of the total LNG capacity or is it the volume of the LNG that get vaporized. a quick respense is greatly appreciated

ARGH
The "Boil off rate" usually refers to the amount of liquid vaporized by heat leaking into the tank per unit time.  So in your case o.25% of the liquid will be boiled off per unit time.   Usually the time is per day, but your percentage is much lower than those shown by Air Liquide for their tanks so I think your unit of time might be hour.  
Anyway it indicates that to maintain the same pressure in the tank you need to vent at least this amount of equivalent liquid or vapor.  Less and you will build pressure, more will result in a drop in pressure in the tank, unless you have a vaporizer.

Regards
StoneCold

I agree with StoneCold. The 0.25% is a "natural evaporation rate" (NER) which is the percent of liquid boiled off per day given a full tank at atmospheric pressure and is generally based on liquid nitrogen for atmospheric tanks (ie: tanks meant to contain LN2, LO2 or LAr). I'm not sure about LNG tanks but I suspect it is meant to be 0.25% of LNG (not LN2) per day at atmospheric pressure. It does seem a bit low. Typical values are around 1% for smaller tanks (up to a few thousand gallons) and they go down to around .5% or less for very large tanks. I suspect this is a larger tank and/or has a low heat leak due to design.

Most LARGE LNG tanks vaporize at around 0.05% per day as pure methane.  That is the standard design for a LNG tank, especially full containment.  LNG ships are usually around 0.15-0.25% per day as pure methane.

The heat leak can usually be calculated by taking the % boil-off (volumetric), then assuming pure methane with a heat of vaporization around 220 BTU/lb, and whatever the density of the methane is at tank conditions. With these you can calculate the BTU/hr

  I might add it it % of the full volume of the tank not the remaining volume in the tank.

Thanks AgLNG for that information. i understand the method of calculating the heat in leak rate from the boil off rate from the heat of vaporization of pure methane. you have said that this value is around 220 Btu/lb.

will it it fair to say that the heat of vaporization for pure methane under standard conditions (i.e. 60F and 1 atm) can be used for this calculation.

Again thank you everybody for their inputs. looking forward to hearing from you guys