LPG carriers “Liquid phase Sampling systems”

Liquefied Petroleum gas (LPG) Carriers sampling system :

LPG consisting of C3/C4 and traces of C5 hydrocarbons.is used as an alternative fuel for vehicles and as a raw material for the petrochemical industry.

Designed to carry mainly butane, propane, butadiene, propylene, the LPG carriers are transporting the gas in liquid form at about -40°C and 5 barg.

The method of sampling is a quite complex operation :

  • The ISO 8943 LNG standard for vaporizing the sample does not apply.
  • The LPG heated vaporizing probes, only applicable on light (C1 /C2) LPG in refinery gas streams are not applying as well, as they will fractionate with Propane, Butene and traces of Pentane, having a higher boiling point concentration, in the LPG fraction.

The methods described here under assess the LPG different sampling practices for collecting or for online analysis.


Solution 1 : LPG cargo liquid phase manual spot sampling skid :

The Liquid phase manual Spot Sampling skid” represent a straightforward solution to collect multiple constant volume spot portable containers of 1 US gal. taken at different phases of transfer (usually 25/50/75%) of the ship’s tank level.

A cryogenic process bypass is applied, consisting of two opposite pitot tubes openings to transfer a great velocity of liquid across the “cryogenic 8200” expansible receptacle.

Further to cooling down the expansible sealed receptacle of 40 CC volume is fully compressed in a ‘locked’ state whilst the LPG liquid is being rinsed, cooled, and captured at process pressure and temperature (generally 5 barg and – 40°C) under thermal inertia below the sub-cooling state.

The sample vaporized after reheating at minimum temperature of 10°C is progressively transferred in close volume to a 1 US Gallon container previously vacuumed and charged at 2.5 barg.

This practice makes it possible to constitute a sample whose composition in close volume is not compromised by the change to gaseous state, thus generating the reference gas whose laboratory analyze will give the true and accurate molecular composition of LPG at the custody transfer with absolute tightness; zero maintenance; no seal or grease or moving piston and virtually limitless lifespan.

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Solution 2 : LPG cargo liquid phase automatic composite sampling system :

  • The ISO 3171 (ASTM D 4177) automatic composite sampling practice is applied for collecting LPG Samples with the liquid fast loop “GP 8100” sampler pump grabbing composite liquid sample to the variable volume and variable pressure “cryogenic 8200” expansible receptacle out of vapor coexistence whose sampling chamber is counterbalanced by neutral gas opposite chamber at pressure exceeding by about 4 to 5 bar the minimum the equilibrium vapor pressure of the fluid at transportation temperature in order to obtain a representative sample thermally insulated with an unprecedented reliability to the operation.
  • Then, the “cryogenic 8200” constant pressure and constant volume expansible receptacle is removed for transportation to the laboratory where the sample is maintained under balancing pressure, then mixed by external pulses, repressurized and withdrawn from the receptacle, such that the integrity of sample composition is not compromised with absolute tightness no seal or grease or moving piston to the off line chromatograph via liquid injection valve as for ASTM D2163 or other international standards.

The total sampling operation is PLC controlled to manage all necessary cycles during the course the LPG unloading from the local control station or the supervision PC in the laboratory.

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    Expansible receptacle


Solution 3 : LPG Cargo sampling for online analysis :

For online analysis, no manual operation is required but the challenge can be the ability to conduct an analysis in near real time.

As well, an installation in air-conditioned cabinet of the analyzer is required with replacement of empty carrier gas, and calibration bottles.

For this operation, the Opta-Periph “ISOPROBE 8100” True Vacuum probe first segment collects the liquefied LPG at – 40 °C generally under 5 to 6 barg to feed the “Reciprocating Fast loop Pressure cryogenic amplifier maintaining LPG above vapor pressure.

With a delta pressure of 4 barg the equilibrium vapor pressure of the fluid at transportation temperature the pressure is boosted to about 9 to 10 barg exceeding by 4 bar the equilibrium vapor pressure of the fluid maintained to below 20°C maximum temperature trough the insulated fast loop at the flow rate required by the application for sample transportation in its liquid phase.

The liquid injection valve of the online chromatography analyzer is sampled at these pressure and temperature conditions to apply the ASTM D2163 titled “Standard Test Method for Determination of Hydrocarbons in Liquefied Petroleum (LP) Gases and Propane/Propene Mixtures by Gas Chromatography” or comparable international standard to be applied.

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