2 replies to this topic

[MFAHOS]

Gents

I'm about sizing compressor for unloading liquid LPG from Truck to Spheres.

I have seen a compressor manufactrer tables in which, one can assume that the actual volume flow at the compressor discharge is the same as the liquid unloading volume flow rate, and the compressor differential pressure is the same as the pressure drop in liquid system (from tank to sphere)

- can this approach be followed to size a compressor for Lpg unloading ?


In fact the compressor will be used for unloading propane and butane separately, can this be achieved without problem in operation? knowing that the operating pressures are differents (suction pressure in case of propane 13barg, 4barg for butane)

Thanks

[Art Montemayor]

MFAHOS:

You state that you are sizing a compressor for unloading LPG. You also have been looking at specific manufacturer’s capacity tables. One thing that I continue to be quite confused about is the fact that many engineers who post questions on our forums consistently fail to reveal ALL of their basic data – as if the answer to the query were to be more “expensive” with more data revealed. Doesn’t everyone understand that the Forum issues FREE advice and that the more detailed and concise basic data that we have, the clearer and the more accurate will be the answer? For example, the identity or location of your compressor manufacturer would be a key source of basic data in this case – but you haven’t told us any more information. The answers to your two questions are:

• As the manufacturer of the compressors you are evaluating states, you can indeed transfer liquefied hydrocarbons from one vessel to another. You do this by simply displacing the vapor from one tank to another. The liquid product will flow because of the displacement. Therefore, the answer to question #1 is: yes, you can certainly transfer LPG with an unloading compressor system. (Note that I said “system” – not just “compressor”.)
• If you buy the correct type of quality unloading compressor system you should be able to transfer both Propane and Butane – at different times and under different conditions. But why did you even mention “LPG” in the first place? LPG is a totally different product and you have now confused us about what it is that you are really planning to unload. The correct answer as to whether you can indeed use the same compressor system to unload both Propane and Butane lies in the compressor manufacturer’s line of compressors and their mechanical design. We cannot answer that because you haven’t told us the name of the manufacturer.

Be mindful of the fact that the unloading compressor system involves vapor-liquid separators and instrumentation for controls and operation. Before you size the compressor you should design the total unloading system. If you use a reputable compressor manufacturer, they may help and assist you in the system design.
I hope this experience helps you out.

[MFAHOS]

Art Montemayor,


Thanks for your reply

I am sorry for not being clear in my questions

I should have formulated the question as follows:

IN TRUCK UNLOADING WITH COMPRESSOR(refer to drawing) IS THE ACTUAL VAPOR VOLUMETRIC FLOW RATE ENTERING THE TRUCK EQUAL TO THE ACTUAL LIQUID VOLUMETRIC FLOW RATE LEAVING IT ?

I need to know if this is true to perform hydraulic calculation and size the compressor.


Trying to find a response to this question, I looked in the compressor manufacturer document I mentiond previously, I found that the Actual vapor flow rate in the compressor discharge is equal to liquid unloading flow rate, but I wanted to discuss and confirm this with other people to be sure.


Regards

Attached Files

•  TRUCK UNLOADING.JPG   20.56KB   121 downloads