2 replies to this topic

[dminkel]

I need to be able to calculate the weight of an 18-wheeler before he is loaded with LPG.

 

Company X comes into our facility to be loaded with LPG. He provides his Tare weight (Empty Weight) as he has just been scaled at the CAT scales somewhere close by.

 

Of course we also have the capacity of the trailer in gallons (generally 10,500 - 11,500)

 

On an Excel spreadsheet I have:

 

calculations for measuring/calculating the vapor pressure based upon the analysis/composition of the product

 

I have the calculations for determining the Gravity of the product based upon the following gravities (assuming they are correct:

 

Methane               0.03

Ethane                  0.356

Propane                0.5075

Propylene             0.527

Isobutane             0.563

Normal Butane.    0.584

Iso-Butylene.        0.6001

Trans-but              0.601

Cis2-Butylene       0.6271

Iso Pentane          0.624

Normal Pentane   0.631

Hexane                 0.664

 

Truck Drivers can only load a maximum of 80,000 pounds (total weight of everything - tractor,trailer,bottle)

 

I'm doing something wrong in my calculations. My job is to get those bottles filled so that they weigh as close to 80,000 pounds as possible. Usually, with what we're loading right now, that comes out to around 70%-73%.

 

I'm calculating the weight per gallon as per the components of the anaylsis. Right now that weight is right at about 4.77 pounds/gallon

 

I'm taking 80,000 pounds and subtracting the tare weight (so we have an inbound weight and when he leaves to go scale right down the road he gets the outbound weight)

 

An example would be

 

Water capacity gallons - 11,500

Percent Loaded -             64%

Calculated Gravity  -        0.594

Vapor Pressure -             30.9

Temperature                    95

Temperature Factor        0.966

 

One more thing.... something that seems to be much more complicated....  I need to be able to calculate how much actual product is in the bottle when it is 'empty' but has 50# or 75# of pressure. I assume the variables above would be enough to come up with that number.   So how many pounds OR how many gallons does that vapor pressure equal at 75# pressure on an 11,500 trailer?

 

 

[Profe]

Hi Dminkel

 

For your calculation, you need the composition of LPG by Gas Chromatography

 

for the calculation follow the procedure attached

 

I think that will be useful.

 

Fausto

Attached Files

•  SG LPG.pdf   19.42KB   21 downloads

[Art Montemayor]

Dminkel:

 

Your query seems to be written badly or just confusing.

 

You state: " I need to be able to calculate the weight of an 18-wheeler before he is loaded with LPG."  Then you proceed to tell us the tractor+rig is weighed prior to being filled with LPG and the tare weight is known.  To me, that tare weight should answer your question about the weight of an 18-wheeler before it is loaded with LPG!  What is the problem?  Or is your opening statement written wrong?  Or is our understanding of the word "tare" wrong?

 

You state you have calculations over the vapor pressures and the "gravities" and that you are doing something wrong in your calculations.  If you have any calculations, why don't you simply submit them on this thread?  That would be so much simpler in trying to understand what you are doing or trying to do.  What do you call "gravities" - Specific Gravity?  Or densities?  If Specific Gravity, then what is the temperature and pressure of the referenced substance?

 

You state: "Truck Drivers can only load a maximum of 80,000 pounds (total weight of everything - tractor, trailer, bottle)".  That, to me, means that your local road limits are a total of 80,000 pounds on the pavement for the total tractor+rig (presumably in Texas).

But then, you follow with: " My job is to get those bottles filled so that they weigh as close to 80,000 pounds as possible.  Usually, with what we're loading right now, that comes out to around 70%-73%."  If what you mean by "bottles" is the actual LPG portable pressure vessel or tank, then this statement is false according to your first statement!  You have stated that you have a maximum road load on the total tractor+rig of 80,000 pounds, so how can you justify filling the trailer with 80,000 pounds?  What do you mean by "70%-73%".  Citing percentage figures means nothing if you don't identify what they refer to.  I've loaded a lot of liquefied gas - including a lot of LPG - in my career and all that ever mattered in doing so were 2 major items:

1. The maximum legally locally permitted fill volume of the subject LPG vessel;
2. The maximum permitted NET LPG weight loaded in that vessel.

I have to presume that you are trying to determine the second of the above two items.  But you have to take into consideration that you have to leave a specified ULLAGE in the subject pressure vessel in order to allow for liquid expansion and saturated vapor compression within the vessel as the ambient temperature may vary AND for a potential hazardous overheating situation outside the vessel.  Normal ullage values for transport vessels that I have worked with are between 15-20%.  Rarely have I worked with as little as 10%.  So what are your local ullage limits?

 

How are you measuring the amount of LPG loaded in the trailer?  With a weigh scale (which is the standard and most accurate means) or with a flow meter?

 

You also ask: " I need to be able to calculate how much actual product is in the bottle when it is 'empty' but has 50# or 75# of pressure.  I assume the variables above would be enough to come up with that number.  So how many pounds OR how many gallons does that vapor pressure equal at 75# pressure on an 11,500 trailer?"

 

I think you want to know the residual LPG saturated vapor left in the trailer after it is off loaded OR the amount of LPG saturated vapor existing in the trailer prior to it being loaded with liquid product.  If that is the question, then calculate the density of the saturated LPG vapor at the existing conditions and multiply it by the total volume in the vessel - assuming that ALL of the liquid LPG has been pumped out (which can't be really true, unless you run your off loading pumps dry).