In air or in Vacuum Apart from the units, weight is measured in air or in Vacuum. Even though on ships it is more common to measure the cargo weights in the air, sometimes you may find that the charterers would give the requirements for measuring weight in Vacuum. Remember, for stability and draft calculation we still would need to use the weight in air. Coming back to the topic, can you guess for the same amount of cargo which weight would be more? Weight in air or weight in Vacuum? Well, the weight is Vacuum is always more than the weight in Air.
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In air or in Vacuum Apart from the units, weight is measured in air or in Vacuum. Even though on ships it is more common to measure the cargo weights in the air, sometimes you may find that the charterers would give the requirements for measuring weight in Vacuum.
Remember, for stability and draft calculation we still would need to use the weight in air. Coming back to the topic, can you guess for the same amount of cargo which weight would be more?
Weight in air or weight in Vacuum? Well, the weight is Vacuum is always more than the weight in Air. This is because, like with water, air and any other medium in which the weight is present would offer some kind of buoyancy which reduces the weight.
In the vacuum, there is no buoyancy and hence the weight is more than the same weight when measured in air. Converting weight in Vacuum to Weight in Air and vice-versa Ok, so now here is the first thing that we can learn.
How to convert weight in Vacuum to weight in Air? The first page of the ASTM table 56 provides the factor for converting weight in vacuum to weight in air and vice versa.
Basics of cargo calculations Ok, now let us get back to basics of cargo calculations on tankers. And it is not that complicated. We also measure the temperature of the cargo preferably at three levels and take the mean of these three temperatures to get the temperature of the cargo. So here is what we have. Now we get the volumes for each of these tanks for the corrected ullage that we have got.
This will be the volume at the observed temperature. Remember volume changes with temperature. Let us say we got the volumes from the ullage tables and the volumes for each tank are as per below. As the volume changes with the temperature, this cannot be the measure of how much cargo we have loaded or discharged. We need to convert the volumes to the weight of the cargo in each tank.
We need the density of the cargo to convert the volume of cargo to the weight. And as the density also changes with the temperature, we would need the density of the cargo at the cargo temperature to convert the observed volume to weight. If that was not enough, humans on this planet earth have managed to confuse it further. Volumes are measured in cubic meters at some places and barrels like in the US in other Weight is measured in Metric tons at some places and in long tons at other places and barrels at 60 deg F at other places.
Take a deep breath and read on. First, check what cargo surveyor has provided you. Density at a particular temperature and correction factor So let us say that cargo surveyor has provided us with the density at a particular temperature and correction factor. This means that Density at 31 Deg C would be: 0. Here is how the ullage report will look like.
A Table of densities at different temperatures The cargo surveyor may provide a table of densities at different temperatures.
This is even easier than the previous section that we discussed. The density table could look something like this. The cargo calculations, in this case, are also easy. We just take the density of the cargo to the corresponding cargo temperature that we measured.
Rest of the calculations is the same as what we discussed in the previous section. If the cargo temperature is between two values in the density table, we just interpolate to the get the density at the desired temperature.
Density at 15 Deg C and ASTM table to use The previous two methods are useful and applicable for cargoes the density for which changes proportionally with temperature. These methods are mostly used for calculation of chemical cargoes. But for petroleum products and crude oils, ASTM tables are used for calculating cargo weights.
Let us say cargo surveyor provided the density at 15 deg C as 0. Let us use the same volumes and temperatures that we have used in our initial example. So as we can see for temperature 34 Deg C, the volume correction factor is 0. Similarly, we need to find the VCF for cargo temperatures of other tanks.
Here is how the ullage report would look like so far. Now at many places may be using the standard volume instead of weight. The standard volume of the cargo would also remain same as this is the volume at the fixed temperature 15 Deg C. But in any case, we still need the weight of the cargo as the stability calculations need the weight of the cargo in each tank and not the standard volume.
Getting the weight from standard volume is simple. We have the volume at 15 Deg C and we have the density at 15 Deg C. If we multiply these two, we get the weight by a simple formula. But wait. The density at 15 Deg C is always the density in Vacuum. So if simply multiply this density with standard volume, we get the weight in Vacuum. So we then need to either convert the weight in vacuum to weight in the air as we discussed earlier or we can simply convert the density in Vacuum to density in Air.
There is a simple co-relation between density in vacuum and density in the air. And we call this as weight correction factor WCF. So in our case, the WCF would be: 0. When we apply this WCF to the standard volume, we get the weight of cargo in Air. In above ullage report, I have applied the WCF to the Gross standard volume but we can easily make one additional column and apply the WCF to the standard volume of each tank to get the weight in the air for each tank.
And as you might have guessed correctly, these ports also do not measure the temperature in Deg C but in Deg F. Also, the volume is measured in Barrels and not in cubic meters.
So when in these ports, we need to have the volumes in Barrels and temperature in Deg F. This is not so difficult a task. There is a simple formula to convert these. So for these ports here is what the volumes and temperatures in ullage report would look like.
Following the same principle as earlier, we need to bring this volume to volume at 60 Deg F. And to do that we need to apply the volume correction factor.
Let us say the cargo surveyor has provided the API gravity at 60 F to be So, in the same manner, we get the VCF Volume correction factor for other required temperatures that we have measured in each tank. And when we multiply the volume at observed temperature with VCF, we get the standard volume, this time the volume at 60 Deg F.
We need to apply Weight correction factor WCF to the standard volume to get the weight of the cargo. Let us say we are interested in calculating the weight in Metric Tons in the air. So as we found out the weight conversion factor for API 66 is 0. We can apply this WCF to the standard volume to get the weight of cargo in the air. Now the final ullage report will look like this. But there are other ASTM tables that supplement these tables that we discussed so far.
But what if we are provided with API gravity at some other temperature, say at 80 deg F? While the ASTM tables that we discussed in previous sections are the one that is used mostly, there are other ASTM tables that supplement these main tables.
And even for the main ASTM tables, the information about which table need to be used for cargo calculation is provided by cargo surveyor. We need to follow the information provided by the cargo surveyor because that would be the table that is used for shore calculations and we need to use the same to avoid ship shore quantity difference. Conclusion Cargo calculations are sometimes tricky.
Not because these are difficult but because there are so many variations to it. But we need to understand that at the very basic level, we calculate the volume from ullage tables and we need to be provided with density at the same temperature as the cargo. We multiply both and we get the weight of the cargo. In this case, we need to get the volume correction factor VCF to convert the volume at the observed temperature to the standard volume which is volume at 15 Deg C or Volume at 60 F respectively.
We then need to apply the weight correction factor WCF to convert the standard volume to weight. There are different ASTM tables for crude oil and for product oils. The one with letter A is for crude oils and the one with letter B is for product oil. ASTM tables without any letter are common for both crude oils and product oils. Get your hands on ASTM tables and you will find that cargo calculations are not as difficult as it seems.
Tabel ASTM 53 .pdf
ASTM Table 53 / 54
ASTM Tables Usage & Procedure of Calculations V. 1.0, 1.1, SuperASTM 4.0