The use of compressors for the transfer of LPG and two-phase liquids in storage depots is a consolidated practice in the most experienced technologies of the gas field.
Actually compressors present a wide set of advantages which makes them preferred to pumps in the loading and unloading operations of two-phase fluids.
First of all, compressors eliminate any pump suction concerns in situations where there is low or negative NPSH (Net Positive Suction Head) availability. These situations would include typical railcars or other tanks which have no openings on the bottom of the tanks or a low NPSH. Actually, trying to pump out of a tank with only top openings will result in very poor pump suction conditions and very short vane and seal life. Moreover the pump would be very noisy and as the liquid is withdrawn form such a vessel, the pump suction conditions would continually worsen throughout the process until eventually the pump can no longer function. Considerably liquid would be left in the vessel and this might be 20 or 30 percent of its capacity.After removing all of liquid from a vessel, a compressor can then also remove vapour, while a liquid pump could never truly empty an LPG vessel. Actually even if it pumps all of the liquid out, the vessel would still be full of vapour at vapour pressure. The remaining vapour can equal about 3% of the tanks total capacity. This means that fewer loads will be required for the same overall capacity when compared with using a pump to do the transfer operation.Anytime a vessel must be opened to atmosphere, a compressor is the method of choice.
If the vessel is going to be opened for inspection or repair, then all of the liquid or vapour must be removed from it before any work can proceed. Due to environmental, safety or economic concerns, this product cannot be vented to the atmosphere nor can a pump be used to recover the product. A compressor will handle these situations readily.Some vessels have no openings at all into the liquid section of the tank. Typical home delivery tanks of under 2.000 litres (500 gallons) and typical small cylinders, such as the 5 kilograms (20 pound), only have a single opening in the top of the vessel. Also, the trucks or railcars that have been involved in accidents may be lying on their side such that none of the normally used openings in to the tank connect to the liquid section. In any of these situations, a liquid pump would be totally useless in trying to empty the vessel.Compressors, on the other hand, can easily handle all of these situations while the very best that a pump can do is to empty a vessel of liquid but still leaving it full of vapour. A compressor can always empty a tank of all the liquid and then proceed to empty the tank of virtually all the vapour.
Since pumps must be provided with properly designed suction lines, that may be impossible to do with many vessels. A compressor on the other hand is not subject to these limitations. A compressor can recover both the liquid and the vapour form virtually any vessel under virtually any situation.Finally, even though the initial cost for a compressor is of course higher than the cost for a pump, the average life of a compressor is three or four times that of a pump and its maintenance operations are carried out less frequently.With compressors the plant costs too will be reduced because the storage tanks can be installed at the ground level or just under since compressors don’t need NPSH.It results clear that compressors are much better than pumps for the unloading operations. Moreover, with an appropriate hose system, the same compressor can be used for loading and unloading operations. Otherwise two similar machines can be installed working without distinction for loading and unloading, thus giving the possibility to carry out these two operations simultaneously.