LPG - Propane Quality Assurance – LPG Standards & Testing
A lot of effort goes into making sure that the LPG you receive is of the highest propane quality, and it shows. Propane quality problems with LPG are very rare.
This is the result of comprehensive standards, thorough testing and strict compliance.
Why is LPG Tested?
LPG is tested to assure composition, energy content, performance, octane rating, dryness and safety.
The testing also serves to limit toxicity and emissions in use.
What LPG Properties are Quality Tested?
There are thirteen LPG properties that are tested:
1. LPG Vapour Pressure
2. Wobbe Index
3. Motor Octane Number
4. LPG Odour
5. Corrosion - Copper Strip Test
6. Dienes Limit Test
7. LPG Dryness
8. Free Water Test
9. Fluorine Test
10. Hydrogen Sulfide Test
11. Residue on Evaporation at 105°C
12. Sulphur (Sulfur) Test
13. Volatile Residue Test
Each property has specified test method and pass-fail parameters.
Each one is fully explained below.
The LPG Quality Standards - Propane Quality
Propane quality is very important for proper results. There are two sets of propane quality standards for LPG in Australia. We will just look at the propane quality specification, as butane use is extremely rare in Australia.
One set of propane quality standards is for LPG used in homes and businesses for various heating applications.
The industry Standards for this are the Gas Energy Australia LPG for Heating Use Specification 2013 and Australian Standard AS4670-2006 “Commercial propane and commercial butane for heating”.
The second propane quality Standard is the Fuel Standard (Autogas) Determination 2003.
This regulates the quality of LPG supplied in Australia for automotive use.
The two propane quality Standards are very similar but we’ll focus on the 2013 heating use specification, as it contains everything included in the autogas specification and more.
The following explains the thirteen LPG properties tested, why they are tested, test methods and pass-fail parameters…
LPG Vapour Pressure
When LPG is stored in a gas bottle, it is under pressure.
The term “pressure” refers to the average force per unit of area that the gas exerts on the inside walls of the gas bottle.
LPG vapour pressure is measured in kilopascals (kPa).
LPG pressure can vary greatly based on temperature, so a specific temperature is specified for the test.
A pressure test gauge is shown.
The Standard allows for a maximum pressure of 520 kPa at 40°C using ISO 8973 - Liquefied petroleum gases - Calculation method for density and vapour pressure.
Wobbe Index for LPG
The Wobbe Index is used to compare the energy output of fuel gases in heating applications.
It is often used as an indicator of the interchangeability of fuel gases, including LPG and natural gas.
Wobbe Index is measured in megajoules per cubic metre of gas – MJ/m3.
The standard sets both a minimum and maximum value for the test at 84.0MJ/m3 minimum and 88.0MJ/m3 maximum.
The test specified to be used for the Wobbe Index test is ISO 6976 - Natural gas - Calculation of calorific values, density, relative density and Wobbe index from composition.
LPG Motor Octane Number
High compression engines require high compressibility of the fuel without pre-ignition.
Octane ratings are indicative of this compressibility.
Motor Octane Number (MON) is one of a number of octane standards in use.
MON is a standard measure for the performance of motor fuel in resisting pre-ignition and knocking.
The composition of the LPG affects the MON results, as olefins have lower octane numbers.
So, this test is one way of regulating the composition of the LPG, by using performance criteria.
Whilst the autogas specification sets a minimum MON of 90.5, the heating specification has no minimum, as it has no relevance for heating.
When the number is required, for autogas, calculation is done by test method EN 589 Annex B - Automotive Fuels LPG Requirements and Test Methods. Method for Calculation of the Motor Octane Number (MON) from compositional analysis of LPG.
In its natural state, LPG is an odourless and colourless gas.
The distinctive smell that people associate with LPG is actually added to it as a safety measure.
Without the addition of an odourant, leaking gas could collect without being detected.
The specification is based on a person with a normal sense of smell being able to detect the distinct smell of the odourant at a specific percentage of LPG in air, simulating a leak.
The percentage is based on the lower flammable limit (LFL), which is the lowest concentration of a gas, in air, capable of producing a flash of fire in presence of an ignition source.
The LPG specification is to be able to detect the LPG vapour at 20% LEL in air, using test method EN 589 Annex A - Automotive Fuels LPG Requirements and Test Methods. Test Method for Odour of LPG.
To achieve this a suitable odourant, Ethyl Mercaptan, is added to the LPG at 25 ppm.
Maintaining a guideline value of 6 ppm, measured in vapour downstream from odourant injection, is considered to be a suitable.
LPG terminal systems often include a fail-safe mechanism that stops the LPG pump if the odourant injection malfunctions.
This level should markedly exceed the requirement to detect LPG vapour at 20% LEL in air.
Corrosion, Copper Strip Test for LPG
LPG piping and fittings are often made of copper or a copper alloy – brass.
In fact, brass (a copper-zinc alloy) is the preferred metal for LPG fittings, as it is non-sparking.
For this reason, it is important that the LPG be non-corrosive with regard to copper and brass.
The composition of the LPG affects the corrosion results and especially the presence of sulphur compounds.
So, as with MON, this test is one way of regulating the composition of the LPG.
The copper strips and colour comparison chart used in the testing are shown.
The test result must be Class 1 under test method ISO 6251 - Liquefied petroleum gases - Corrosiveness to copper - Copper strip method.
Dienes Limit Test for LPG
Dienes, including butadiene, are hydrocarbon gases containing two carbon double bonds.
Excessive diene content can increase the likelihood of residue problems in regulators and vaporisers.
Dienes can also reduce the octane rating of autogas.
The composition analysis is done using a gas chromatograph.
Dienes are limited to a maximum of 0.3 mol percent, as tested under the ISO 7941 test method - Commercial propane and butane - Analysis by gas chromatography
The dryness test is only applicable to propane and not butane.
Dryness is important in relation to freezing in pressure reducing regulators.
Propane must pass the dryness test, which is the valve freeze method applied for 60 seconds duration which is ISO 13758 - Liquefied petroleum gases- Assessment of the dryness of propane - Valve freeze method.
This is equivalent to less than 160 ppmv (parts per million of volume) of water at 0°C.
Free Water Test
The Free Water Test only applies to butane.
As with the dryness test, this relates to freezing in regulators.
Water promotes rust on internal surfaces of steel storage tanks and iron piping systems.
Rust caused by free water can also cause odourant fade and can block small openings.
In cold weather or in pressure reduction regulators, water can freeze.
This can impair the functionality or damage valves, pumps, piping, and regulators.
LPG cannot contain any free water at 0°C.
A pressure hydrometer is used to conduct the test (see image).
Nil free water is equivalent to less than 50 ppmv of water at 0°C, using the equipment described in ISO 3993 - Liquefied petroleum gas and light hydrocarbons – Determination of density or relative density. Pressure hydrometer method.
This test is only require if any component of the LPG has been sourced from a HF Alkylation Unit within a refinery.
Fluorine compounds, and particularly hydrofluoric acid (HF) and its combustion products, may be extremely destructive in any environment.
As a result, fluorine content is limited to a maximum of 10mg/kg.
No specific test method is specified
Hydrogen Sulphide Test
Hydrogen Sulphide is both toxic and corrosive.
LPG must be effectively hydrogen sulphide free.
Lead acetate paper is used to detect the hydrogen sulfide.
The test apparatus is shown in the image.
A negative finding is a level less than 2.8 ppmv (parts per million of volume), as measured by ISO 8819 - Liquefied Petroleum Gases – Detection of Hydrogen Sulphide – Lead Acetate Method
Residue on Evaporation at 105°C
Residue deposits can cause problems in regulators and vaporisers.
The residue is primarily a result of the presence of less volatile hydrocarbons.
So, once again, this test is one way of regulating the composition of the LPG.
As you would surmise from the name, this test is performed at 105°C, using test method JLPGA-S-03 - LP Gas Evaporation Residue Test Method (Mass Method).
Sulphur (Sulfur) Test
The emissions from sulphur combustion is an atmospheric pollutant, so the specification limits the amount present in the LPG.
It is measured after the odourant is added, as the odourant may contain some sulphur.
It is essential that the total sulphur content is measured in LPG as a liquid, not as a vapour.
The sulphur test apparatus is shown in the image.
The maximum amount permitted is 100mg/kg, using the ASTM D 2784 test method - Standard Test method for Sulphur in Liquefied petroleum Gases (Oxy-Hydrogen Burner or Lamp)
n.b. "Sulfur" is the usual spelling in American English.
Volatile Residue Test
If not removed, volatile residue can collect in the regulator and/or the pipework.
This accumulation can affect the performance of the appliances.
These compositional requirements limit the hydrocarbon molecules with 4 or more carbon atoms, in propane, to 4.0 mole % of butane.
The limit is 0.3 mole % for the other hydrocarbon molecules with 4 or more carbon atoms, including pentane, hexane, heptane, octane, nonane, decane, undecane & dodecane.
For butane, there is a limit of the hydrocarbon molecules with 5 or more carbon atoms to 2.0 mole %, including pentane, hexane, heptane, octane, nonane, decane, undecane & dodecane.
The composition analysis is done using a gas chromatograph (image above).
The volatile residue test used is ISO 7941 - Commercial propane and butane - Analysis by gas chromatography.
LPG Testing Frequency & Methodology
The LPG production stream is commonly sampled and tested at regular intervals and are also referred to as natural gas liquids – NGL.
Depending upon complexity, it may be multiple times in an eight hour shift or once or twice per week.
In some facilities, composition and moisture analyses may be performed on a continuous basis.
LPG in storage is typically tested again prior to delivery to pipelines or transport.
Periodic analysis for composition is typically done using gas chromatography (GC) to measure fuel composition, odorant concentration, and the presence of other chemicals.
A 95% evaporated temperature test is used to measure volatile residue.
A freeze valve test for dryness is used for LPG (propane).
Lead acetate paper is used to test for presence of hydrogen sulphide – H2S.
Pressure is measured using the Reid vapour pressure test.
Who Does the Testing?
Some facilities use their own lab, some use independent labs and some a combination of the two.
All are normally NATA accredited – National Association of Testing Authorities, Australia.
LPG Testing in Other Countries
The uses for LPG, in different countries, are virtually identical so the desired LPG quality is very similar.
Countries and regions, like the EU, frequently have their own standards organisations and standards.
The testing procedures may also vary.
However, what is tested is very similar and the pass-fail parameters are typically very close or identical.
LPG quality is closely monitored and controlled to make sure it meets quality standards.
The result is that the LPG customers receive meets all specifications and will perform as expected.
Comments, questions or feedback?
The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.