After dry docking or cargo change the vessels will have to be gassed-up by new product either on top of compatible product or after purging the vessel with Nitrogen/ Inert gas.

Depending on the method of gassing-up the vessel will loose a certain amount product to the flare or to the atmosphere.

The minimum gassing-up parcel to be ordered is thus to be the gas vapour weight of the product in the cargo tanks upon completion of gassing up + the loss during the gassing-up procedure.

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We cannot gas-up unless we can release the Nitrogen/ gas mixture (explosive) to "Flare, "Shore vapour return" or "atmosphere.

To limit the product (gas) loss when gassing-up Nitrogen filled cargo tanks, we do not introduce product into tanks until last minute when the "Flare", "Shore vopur return" or "atmosphere" release is fitted.

1. A release possibility as mentioned above is necessary to achieve a stratification layer inside the tank between the product and the Nitrogen and thus release as little gas as possible in the process. (The product (gas) expands when introduced into the tanks and is a lot heavier than Nitrogen why the product will slowly push the Nitrogen out of the cargo tanks from bottom to top)

2. If we introduce the product without having the "Flare", "Shore vopur return" or "atmosphere" release ready, we will achieve a total dilution of gas/Nitrogen inside the cargo tanks and we will thus flare-off much more product and use more time to complete the gas-up operation.

 

Depending on time available, the type of vessel, gassing-up method, shape and size of cargo tanks to be gassed up, Eitzen Gas’s vessels will have a cargo loss in the range 2 – 20 mt.

Cylindrical cargo tanks are easier to gas-up than Bi-lope tanks, and semi-ref vessels with cooling plant capacity can minimize cargo loss better than fully-press vessels without cooling capacity.

 

The vessels in the small segment (<3000 m3) will ventilate 2 to 5 mt during the gassing-up process, while the medium segment (3-6000m3) will ventilate 4-10 mt, and the bigger segment (>6000 m3) will ventilate 8-20 mt

 

 

The Vapour weight of the product in the cargo tanks upon completion of gassing-up can be calculated as per below (Cargo Vapour Calculation)

NOTE:

In the  gas formula below - the P(abs) figure most likely to be used after dry docking is 1.1 Bar absolute (slight over pressure)

 

 

 

To be able to calculate the vapour weight of the various gasses, we need to know the molar volume from Avogadro’s Law, the product molecule weight, the product temperature, the cargo tanks vapour pressure absolute and the gas volume of the cargo tanks.

 

 

Avogadro's law states that: `Equal volumes of different gases contain the same number of molecules at the same pressure and temperature'.

The volume of 1 mole of gas (6,02·10²³ gas molecules) is called the molar volume of the gas. According to Avogadro's law the molar volume is equal for all gases, and this has been shown to be correct.

At 0deg Celsius and at atmospheric pressure the molar volume is 22,414 litre/mol.

 

 

Once you know the chemical formula of the product to be calculated, you can find the Atom weights in the scheme ‘’The Elements’’ in section ‘’Intro to gas trade’’ – and from the formula you will be able to calculate the molecule weight.

 

Example:

Atom weights: Hydrogen – 1.008 g/mol, Carbon – 12.01 g/mol, Chloride – 35.45 g/mol

 

Propane – C3H8

3 x Atom weight of Carbon + 8 x Atom weight of Hydrogen

Molecule weight = 3 x 12.01 + 8 x 1.008 = 44.094 g/mol

 

Molecule weights of various products

 

 

All temperatures used in final formula for calculation to be measured in degrees Kelvin.

Kelvin is a temperature scale measure related to absolute zero (- 273 deg Celsius)

Degrees Kelvin = (273 + deg Celsius)

 

All pressures used in final formula for calculation to be measured in Bar absolute

Bar absolute = Pressure gauge + 1 Bar

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To calculate gas vapour weight in Kilos please use the below formula as follows:

 

                        Molecule weight x 273 x P(abs) x Volume

                                    22.414 x (273 + Temperature)

 

Molecule weight = as calculated above

P(abs) = Tank pressure + 1 bar

Volume = Gas volume in cargo tanks (m3)

22.414 = molar volume as found above

(273 + Temperature) = 273 + Cargo tank temperature in deg Celsius

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C-vessels:

Calculate the gas vapour weight of 1970 m3 Propane vapour at temperature (+10 Celsius) and pressure 0.1 bar gauge. (This is estimated condition after gassing-up and before loading)

 

                        44,094 x 273 x 1,1 x 1970   which equals 4,112 Kilos

                                    22,414 x 283

 

From experience we know that this vessel will ventilate up to 2,500 Kilos when gassing up.

 

Always liaise with the vessel’s Master - especially if in doubt of ventilation loss experience factor.