Independent cargo surveyors are often employed by cargo buyers or sellers and the survey companies provide personnel to check cargo operations and cargo quantities both on board ship and within terminals. Such activities can include cargo measuring and cargo sampling.
During cargo operations, it is important to maintain a log of events. Ship’s officers and shore personnel should ensure that their time records are in agreement and that, when independent cargo surveyors are used, similar times are recorded in the Surveyor’s Time Sheet.
The following is a widely practised procedure using Sl units when considering a standard temperature of 15°C.
(i) Find the average liquid and average vapour space temperatures (°C) and the vapour space pressure (barg or mbarg).
(ii)Read the liquid level and calculate the liquid volume (Vt) at tank conditions (t°C) using the ship’s calibration tables and making corrections for temperature and the ship’s list and trim (Vt in m3).
(iii) Obtain the liquid density noting the temperature at which it is determined and, by ASTM-IP Table 53, convert this to liquid density at 15°C (D15 in kg/m3). Note that in Table 53 density is given in kg/litre. For all practical purposes the values are equal to tonne/m3 and should be multiplied by 1,000 to obtain kg/m3.
(iv) Using the liquid density at 15°C (expressed in kg/litre) and the average liquid temperature, enter ASTM-IP Table 54 to derive the appropriate volume cor-rection factor to convert Vt to the volume at 15°C (V15 in m3).
(v) Calculate the liquid mass as Mliq (kg) = V15 x D15
(vi) Calculate the vapour volume at tank conditions (Vt) in m3 by subtracting the apparent liquid volume at calibration temperature from the tank total volume at calibration temperature and applying to the difference the necessary vapour space contraction factor.
(vii) Determine the vapour density at vapour space conditions (DVT) by the following calculation based upon the ideal gas laws.
(viii) Calculate the vapour mass (m) as the product of vapour volume and vapour
density:m= Vt x Dvt (kg).
(ix) Add the liquid mass, Mliq, and the vapour mass, m, to give the total mass, MT;
MT = Mliq + m (kg).
(x) Convert the total mass to weight-in-air by means of the appropriate conversion factor found by entering the left hand side of the short table in the introduction of ASTM-IP Table 56 with the liquid density at 15°C.
Sources : Liquefied Gas Handling Principles On Ships and in Terminals – McGuire and White