# Boil-Off Gas (Bog) Calculations For Cryogenic Liquefied Natural Gas Tanks

Boil-off Gas has been discussed on "Cheresources" in several posts in the past.

Recently, I came across an article on Boil-off Gas from LNG tanks which I found to be quite interesting. Today's blog entry is related to boil-off gas calculations based on this article for cryogenic LNG storage tanks of some standard capacities such as 140,000, 160,000, 180,000 and 200,000 m3 (mentioned as kL or kilo-liters in the article).

The article provides heat leakage (in other words heat ingress) values for the above mentioned four tank capacities for cryogenic LNG storage tanks and the BOG is calculated based on a simple heat balance from the heat leakage to the tanks. The heat leakage values are provided in Table 1 of the article and the link for the article is:

http://www.enggjourn...10-02-04-30.pdf

Please note that the blog entry does not provide calculations or basis of the heat leakage values as provided in the article and the values for heat leakage have been considered on an "as-is-where-is" basis provided in the article.

Let us go to the calculations for the various tank capacities:

140,000 m3

The heat leakage value (Q) as provided in the article for this capacity is 169,919 W.

1 W = 0.001 kJ / s

or

169,919 W = 169.919 kJ / s

The latent heat of vaporization (λ) of commercial LNG is assumed to be 512 kJ / kg

A phase change heat-balance can be represented by:

Q = m*λ

or

m = Q / λ

where:

m = Boil-off Gas rate, kg/s

Q = Heat Leakage, kJ / s

λ = Latent Heat of vaporization of LNG, kJ / kg

Calculations

m = 169.919 / 512 = 0.3319 kg / s

or m = 0.3319*3600*24 = 28,676 kg / day

160,000 m3

The heat leakage value (Q) as provided in the article for this capacity is 168,243 W.

1 W = 0.001 kJ / s

or

168,243 W = 168.243 kJ / s

The latent heat of vaporization (λ) of commercial LNG is assumed to be 512 kJ / kg

A phase change heat-balance can be represented by:

Q = m*λ

or

m = Q / λ

where:

m = Boil-off Gas rate, kg/s

Q = Heat Leakage, kJ / s

λ = Latent Heat of vaporization of LNG, kJ / kg

Calculations

m = 168.243 / 512 = 0.3286 kg / s

or m = 0.3286*3600*24 = 28,391 kg / day

180,000 m3

The heat leakage value (Q) as provided in the article for this capacity is 166,552 W.

1 W = 0.001 kJ / s

or

166,552 W = 166.552 kJ / s

The latent heat of vaporization (λ) of commercial LNG is assumed to be 512 kJ / kg

A phase change heat-balance can be represented by:

Q = m*λ

or

m = Q / λ

where:

m = Boil-off Gas rate, kg/s

Q = Heat Leakage, kJ / s

λ = Latent Heat of vaporization of LNG, kJ / kg

Calculations

m = 166.552 / 512 = 0.3253 kg / s

or m = 0.3253*3600*24 = 28,105 kg / day

200,000 m3

The heat leakage value (Q) as provided in the article for this capacity is 163,253 W.

1 W = 0.001 kJ / s

or

163,253 W = 163.253 kJ / s

The latent heat of vaporization (λ) of commercial LNG is assumed to be 512 kJ / kg

A phase change heat-balance can be represented by:

Q = m*λ

or

m = Q / λ

where:

m = Boil-off Gas rate, kg/s

Q = Heat Leakage, kJ / s

λ = Latent Heat of vaporization of LNG, kJ / kg

Calculations

m = 163.253 / 512 = 0.3188 kg / s

or m = 0.3188*3600*24 = 27,544 kg / day

It is interesting to note that as the capacity of the LNG tank increases the BOG rate decreases.

Well this is what I have today for the readers of my blog and I hope to get some comments on this blog entry by process engineers who are actively involved in LNG storage and terminal design.

Regards,
Ankur.

Zauberberg
Interesting article, thanks for sharing.

I have seen different rules of thumb (or calculation methods) in LNG plants where I used to work, and most often the designers used a fixed figure of 0.05 wt% of maximum tank volume as the BOG generation rate. It makes sense to fix the evaporation percentage irrespectively of actual LNG volume inside the tank since heat gain is essentially always the same, making the equivalent mass of evaporated LNG to balance the heat gain also the same.
ashish_usct
hi ankur

while working on DFR for one of RLNG terminal i found that min. send out of terminal can be optimized with BOG handling system, and reconderser operating pressure. Though i didn't go into detail of trade off between these. request you to throw some light on this and what could be mathematical approach for this?
ankur2061

hi ankur

while working on DFR for one of RLNG terminal i found that min. send out of terminal can be optimized with BOG handling system, and reconderser operating pressure. Though i didn't go into detail of trade off between these. request you to throw some light on this and what could be mathematical approach for this?

ashish,

Other than this blog entry on BOG calculations which is a theoretical calculation I have no operating or design experience related LNG or LNG re-gasification terminals. Sorry to disappoint you.

Regards,
Ankur.
ravinder.chemical
Hi All,

Currently I am currently working on a project for Flare Radiation & Dispersion Study.

Could anyone help me out and let me know, how can I calculate the thermal Radiation from DNV PHAST Software, when both HP and LP flare are flaring together.???

Regards,
Ravinder
jrtailor09
Dear Ravinder,

This is not right place to post your question.

Are ( LP flare & HP flare) at same location or different ?

You have to calculate radiation by considering co -ordimnates x=0 & y=0 for both the flare according specified co-ordinate for the other structure.

We have to consider the one flare on at a governing condition to calculate radiation effect. we did calculation in same way & also done by one of the reputed engineering consulting company.

Regards,

Jatin Tailor
shzx603
Hi,everybody!

I have the actual experience to work with LNG tank designer. I am process designer and am lucky to communicate with the famous LNG tank contractors in the word, such as KOGAS from KOREA, TEG from GERMAN, CBI from AMERICA.
The basic method to calculation the BOG is very stable and the contractors almost have the same philosophy. The normal precedure is as follows:
• The prevail BOG rate in the word is 0.05%(wt) based on the gross volume of LNG inner tank and pure methane. The smaller the BOG rate, the smaller the equipments(BOG compressor and the BOG pipe in the LNG terminal) used to cope with the BOG. Maybe somebody think the BOG rate should be reduced to the least to reduce the BOG. But in fact, the LNG designer usually set the BOG rate according to the running project experience, such as 0.05%. If the BOG rate is set to 0.03%, lots of insulation material such as perlite is required to installed, especially in the tank deck, tank inner wall, which will increase the cost of LNG tank. So it is not a good thing to reduce the cost by reducing the BOG rate.
• The LNG designer simulates the required the structure and insulation material to guarantee the set BOG rate. During the process, the designer will simulate the heat ingress from the atomsphere to LNG tank every 24 hours. The tank usually is divided 3 main parts to simulate the heat ingress: the tank dome, the tank wall and tank bottom. The method and the mathematic are easy and simple by the heat conduction, convection and radiantion according the most highest temperature due to the sun radiation in the day and the most highest temperature in the night.
• At this time, you can simulate easily the BOG rate according the above total heat ingress. We must make clear that the BOG rate is simulated based on pure methane and the gross inner tank volume. IF the BOG rate is bigger than the set point, then the designer will make some changes in the structure and simulate again until the perfect result.
• So, this is the basic and normal procedure to design BOG rate in the LNG tank. I just make some comments about this topic. I am intrested in LNG tank.
Regards,

Gary!
Wael B.

Hi,everybody!

I have the actual experience to work with LNG tank designer. I am process designer and am lucky to communicate with the famous LNG tank contractors in the word, such as KOGAS from KOREA, TEG from GERMAN, CBI from AMERICA.
The basic method to calculation the BOG is very stable and the contractors almost have the same philosophy. The normal precedure is as follows:

• The prevail BOG rate in the word is 0.05%(wt) based on the gross volume of LNG inner tank and pure methane. The smaller the BOG rate, the smaller the equipments(BOG compressor and the BOG pipe in the LNG terminal) used to cope with the BOG. Maybe somebody think the BOG rate should be reduced to the least to reduce the BOG. But in fact, the LNG designer usually set the BOG rate according to the running project experience, such as 0.05%. If the BOG rate is set to 0.03%, lots of insulation material such as perlite is required to installed, especially in the tank deck, tank inner wall, which will increase the cost of LNG tank. So it is not a good thing to reduce the cost by reducing the BOG rate.
• The LNG designer simulates the required the structure and insulation material to guarantee the set BOG rate. During the process, the designer will simulate the heat ingress from the atomsphere to LNG tank every 24 hours. The tank usually is divided 3 main parts to simulate the heat ingress: the tank dome, the tank wall and tank bottom. The method and the mathematic are easy and simple by the heat conduction, convection and radiantion according the most highest temperature due to the sun radiation in the day and the most highest temperature in the night.
• At this time, you can simulate easily the BOG rate according the above total heat ingress. We must make clear that the BOG rate is simulated based on pure methane and the gross inner tank volume. IF the BOG rate is bigger than the set point, then the designer will make some changes in the structure and simulate again until the perfect result.
• So, this is the basic and normal procedure to design BOG rate in the LNG tank. I just make some comments about this topic. I am intrested in LNG tank.
Regards,

Gary!

Dear All

as i am now in a project facing long term shutdown due to gas supply and we do have a problem that loss of LNG in our LNG Storage tanks rate increased to reach about 0.6% wt of total cargo inside the tank hence the capacity of the tank is 160,000 M3 of course we need to do long circulation to preserve the temp. profile of the rundown line to loading header as a normal standard in this case.
- we use IHI BOG compressor loader capacity running in this case about 25%
- we are doing circulation every 4 Hrs with flow volume about 70 m3 /hrs .
- we are using BOG to run two Hitachi Gas turbine with fuel consumption 6 nm3 /hr .

My question is : is there any possibility to reduce LNG losses by decreasing a number of circulation as increasing the number of circulation increasing the rate of evaporation as well .

RGDS

Wael
Anjaney Shukla
Ankur, good literature. Do u have idea about design guidelines and procedure for cryogenic tanks
Erwin APRIANDI
Hi,

I'm a bit confused here, in the paper it is said that the bigger the tank the lower the BOG?
In common sense the bigger the tank means bigger area to transfer between environmental to the fluid inside, means higher heat can be transfered.

And for the rules of thumb which most vendor use, is in 0.05% kg/kg per day (wt), or based on volume? since it is said "based on the gross volume of LNG inner tank and pure methane"

I'm currently not in the LNG business but it is always good to know new things.

And an addition, I have read some article that said According to International Maritime Organization (IMO) requirement, BOR (Boil-Off Rate) for 125000 m3 membrane LNG carrier is 0.12% kg/kg per day. With typical value of 0.1% to 0.15% kg/kg per day depending on the insulation properties and sea conditions.

Hi

With reference to the interesting findings above. Is there any knowlegde how the BOG formation is related to operational pressure in a pressurized storage tank (e..g. max pressure 8 bar,g) and also the influence of any inerts (esp. Nitrogen content). If just using heat input to a pressurized storage tank (say abt 500m3 volume) , heat input from pump energy during filling, heat losses through insulation (normally vac. insulated tanks), and cold down operation of tank filling lines. Logistiscs have of course an impact (filling of vessel frequency as well as supply to LNG trucks (which reduce the pressure).

It seems that high pressure would reduce the BOG formation considerably since normal experience seems to be that it takes quite some time before "venting" is neccessary compared to using heat input to the storage tank and corresponding heat of evaporation. Seems to be a dynamic simulation - does anyone has some experince hereto.

BR

LO

potatoteddy

Ankur, good literature. Do u have idea about design guidelines and procedure for cryogenic tanks

Try EN 14620, API 620, NFPA 59A, NFPA 15

potatoteddy

Hi,

I'm a bit confused here, in the paper it is said that the bigger the tank the lower the BOG?
In common sense the bigger the tank means bigger area to transfer between environmental to the fluid inside, means higher heat can be transfered.

And for the rules of thumb which most vendor use, is in 0.05% kg/kg per day (wt), or based on volume? since it is said "based on the gross volume of LNG inner tank and pure methane"

I'm currently not in the LNG business but it is always good to know new things.

And an addition, I have read some article that said According to International Maritime Organization (IMO) requirement, BOR (Boil-Off Rate) for 125000 m3 membrane LNG carrier is 0.12% kg/kg per day. With typical value of 0.1% to 0.15% kg/kg per day depending on the insulation properties and sea conditions.

The paper is probably trying to say the bigger the tank, the lower the vol% (or wt%). Try to think of the "surface area/volume" concept and maybe it will make more sense.

As a side note, I know of LNG tank designers/contractors claiming to have maximum heat leak of less than 0.05vol%/day for tanks in the range of 200,000m3 and above.

LNG carriers BOG rate is much higher due to the different tank design and insulation properties as compared to onshore LNG tanks. 0.05vol% is normally used for onshore tanks.

PSDC_YU

Hi everyone

Thank you the meaningful discussion.

I have been working lately on the unloading and loading scenario of LNG tank using Aspen Hysys. BOG calculation is involved in the task. BOG generation occur due to several factors though i am considering transfer files, tank walls heat gain.

During unloading scenario a constant flow of LNG from the tank is made, some part of the flow is recycled back to the tank after cooling transfer lines (which acquires lot of heat).  In my calculations the tank take 14 days to fully unload.

My problem with the simulation is that i assume different heat transfer coefficient for wet and dry part of the tank for heat leak calculations. However hysys tank model does not give the liberty of using different overall U (pic attached ). Thus to sort this problem i tried two tank model (one acting as liquid tank and other as vapor tank) now in two tank system the height lose by liquid tank must be equal to the height gain of vapor tank. This in turn changes the heat transfer ares and help in capturing the real dynamics. Does it make sense to use two tank model system and is there any easy way of doing the same in hysys.

I have to calculate the followings.

1. Rate of liquid height change with time

2. Rate composition change with time

3. Gas temp change over time

4. Liq phase composition change with time

5. BOG rate over time

A piece of knowledge i learned is that the "aspect ratio (height-to-radius) of a tank for minimum rate of heat gains should be approximately unity".

Thanks in anticipations.

astro

To add to this worthwhile contribution, I found this offering that outlines some potentially useful engineering criteria for some of the other contributions to BOG:

https://amarineblog....ng-of-bog-rate/

I can't vouch for the validity of some of the data but the principles appear sound to me at first glance.

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