How to measure LPG density?

Have you ever thought that if you do not have enough gas, you cannot finish cooking your meal? Or that it will stay cold in your house or the water in the shower will be freezing due to insufficient gas in your gas cylinder? LPG (liquefied petroleum gas) is part of our everyday life. How can LPG producers make sure that they fill the right amount of LPG into their tankers, cylinders, and containers?

The answer is: by measuring its density. Read more about how easily this can be done.

Why measure LPG density?

The density of LPG is an important feature for quality control in processing and transporting these substances. LPG density is used in custody transfer quantity calculations or to satisfy transportation, storage, and regulatory requirements. The LPG density is required to allow conversion from mass flow to volume flow measurements.

Different methods for measuring  the density of LPG:

There are three different ways to determine the density of LPG. All three methods have advantages and disadvantages. Here you can find a summary of the three different LPG analysis methods together with pros and cons:

The pressure hydrometer is a hydrometer or thermohydrometer in a transparent plastic pressure cylinder which withstands pressures of up to maximum 28 bar and is surrounded by a steel mesh. This cylinder is equipped with three valves, a pressure gauge, and a thermometer reporting the temperature to the nearest 0.2 °C. Before the measurement starts, the pressure hydrometer has to be verified with reference liquids: propane and butane according to ASTM D1657. The measurement starts with purging the cylinder with a portion of sample. Afterwards it is filled with the sample, until the hydrometer floats freely. The whole setup is placed in a constant temperature bath. After reaching the equilibrium temperature, the density value and the temperature are read off the hydrometer. If needed, the readings are converted to the density at the reporting temperature using the appropriate Adjunct to D1250 Guide for Petroleum Measurement Tables (API MPMS Chapter 11.1). It is important to empty the cylinder after every measurement.

Pros

  • Price
  • ASTM standard : ASTM D1657

Cons

  • Accuracy of hydrometer measurement: between 1 kg/m3 and 0.2 kg/m3. For this application the accuracy required is better than 0.1 kg/m3.
  • Limitation to vapor pressure of 14 bar (200 psi)
  • Calibration of hydrometer needed
  • Verification of pressure hydrometer with reference liquids needed
  • Results are temperature-dependent
  • Safety problem for user – in some countries the cylinder is still made of glass
  • Time-consuming measurement due to thermostatting

The LPG sample is analyzed by gas chromatography via either liquid or gas sampling valves. The resulted chromatogram of the sample is interpreted by comparing peak retention times and areas with those obtained for the reference standard mixture or pure hydrocarbons under identical operating conditions.

The component distribution data of liquefied petroleum gases (LPGs) can be used to calculate physical properties such as relative density by summing up the mass of each component.

Pros

  • Measurement is independent of the user
  • ASTM standard: ASTM D2598, D2163

Cons

  • Expensive instrument
  • Expensive reference standards for calibration
  • Time-consuming preparation of the column and time-consuming measurement
  • Inaccurate determination of density
  • Results depend on the data interpretation of the peak areas

The LPG sample is filled with the help of an adapter, which can be directly coupled to the LPG pressure vessel into the oscillating U-tube density meter. The density meter is adjusted with propane and butane as reference liquids and is set to the required measuring temperature, for example 15 °C. The filled LPG is kept in the measuring cell by closing the outlet valve. After a few minutes the result will be displayed. No further manual conversion or calculation has to be done.

Pros

  • Accurate results: 0.1 kg/m3
  • No pressure limitation to 14 bar
  • No human influence of results
  • Fast measurement: 10–12 minutes duration
  • Measurement and result is independent from user
  • Easy and cheap cleaning
  • Good thermostatting and accurate temperature measurement (0.03 °C)

Cons

  • No ASTM standard available

Now you know the most important things about the three different methods of determining the LPG density. The oscillating U-tube method was shown to be the fastest and most accurate one and the measurement results will help you save time and money. Are you interested in more details? Have a look at the free application report.

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