Liquefied Petroleum Gas, commonly referred to as LPG, is a versatile hydrocarbon fuel that plays a significant role in today’s energy landscape. Derived either as a by-product during crude oil refining or sourced directly from natural gas fields, LPG primarily comprises a mixture of propane and butane. Due to its high calorific value and reduced emission profile, LPG stands as a compelling alternative to traditional fossil fuels.
LPG consists of a mixture of hydrocarbons, predominantly C3 (Propane) and C4 (Butane), sometimes with trace amounts of C2 (Ethane) and C5 (Pentane).
Liquefied Petroleum Gas (LP Gas or LPG), the term applies to a mixture of certain light hydrocarbons derived from petroleum which are gaseous at normal ambient temperature and atmospheric pressure but may be condensed to the liquid state at normal ambient temperature by the application of moderate pressure.
LP gases mainly consist of one or more of the following hydrocarbons:
a) Propane (C3H8)
b) Propylene (C3H6)
c) n-butane (C4H10)
d) Isobutane (C4H10)
e) Butylene (C4H8)
Small quantities of one or more of the following hydrocarbons may also be present:
a) Ethane (C2H6)
b) Ethylene (C2H4)
c) Pentane (C5H12)
d) Pentene (C5H10)
IS 4576:2021 standard specifies three types of LP gases, based on their principal constituent(s):
a) Commercial Butane: A hydrocarbon product composed predominantly of butanes, butylene or their mixtures.
b) Commercial Butane – Propane Mixture: A hydrocarbon product composed predominantly of a mixture of butanes and/or butylene with propane and/or propylene.
c) Commercial Propane: A hydrocarbon product composed predominantly of propane, propylene or their mixtures.
LPG shall contain a minimum quantity of odorant, generally a mercaptan, to detect a leak.
Subject to agreement between the purchaser and the supplier, the material shall also pass the agreed limits of residue when tested according to IS 1448 (Part 70).
Let’s go through the properties of each component of LPG which is mentioned in IS 4576:2021
| Gas | Chemical Formula | Molar Mass (g/mol) | GCV at 60 °F (kCal/kg) | Boiling Point (°C) |
| Propane | C3H8 | 44.0972 | 12034 | -42.11 |
| Propylene | C3H6 | 42.0814 | 11695 | -47.72 |
| n-Butane | C4H10 | 58.1230 | 11807 | -0.50 |
| Isobutane | C4H10 | 58.1230 | 11807 | -11.72 |
| Butylene | C4H8 | 56.1072 | 11584 | -6.28 |
| Pentane | C5H12 | 70.1340 | 11712 | 36.06 |
| Pentene | C5H10 | 72.1498 | 11512 | 21.17 |
| Ethane | C2H6 | 30.0694 | 12396 | -88.89 |
| Ethylene | C2H4 | 28.0536 | 12029 | -103.72 |
Requirement for Liquefied Petroleum Gases as per IS 4576:2021
| Sl No. | Characteristics | Butane | Butane-Propane mixture | Propane |
| 1 | Density at 15°C, kg/m3 | Report | Report | Report |
| 2 | Vapour pressure at 40°C, kPa, gauge, Max (Note: 100 kPa = 1 Bar) | 520 | 1050 | 1550 |
| 3 | Composition, liquid volume percentage | |||
| a) C2 hydrocarbons | – | Report | Report | |
| b) C3 hydrocarbons | Report | Report | 95.0 Min | |
| c) C4 hydrocarbons | Report | Report | 4.0 Max | |
| d) C5 hydrocarbons and heavier, Max | 2.5 | 2.5 | 0.2 | |
| OR Volatility: | ||||
| Evaporation temperature in °C for 95 percent by volume at 760 mm Hg Pressure, Max | 2.2 | 2.2 | -38.0 | |
| 4 | Total volatile Sulphur, mg/kg (ppmw), Max, | 140 | 140 | 150 |
| 5 | Hydrogen sulphide, Max | 5 ppm | 5 ppm | 5 ppm |
| 6 | Moisture content (Dryness Test) only applies to propane | – | – | Pass |
| 7 | Free water content only applies to butane/butane-propane mixture | None | None | – |
| 8 | Caustic test | Pass | Pass | Pass |
1. Density/relative density may also be determined by analyzing the gas by gas chromatograph and using composition and density factor data as per ISO 8973/IP 432.
2. Vapour pressure may be determined at any other temperature and converted to 40°C/65°C by means of suitable vapour pressure-temperature graph. The same can also be determined by analyzing the gas by gas chromatograph and using composition and vapour pressure data as per ISO 8973/IP 432.
3. Each consignment of commercial butane-propane mixture shall be designated by its maximum vapour pressure in kPa at 40°C and shall not exceed 16.87 kg/sq. cm at 65°C for transportation through road tank trucks.
Further, if purchaser and the supplier agreed, the minimum vapour pressure of that mixture shall not be lower than 200 kPa gauge compared to the designated maximum vapour pressures and in any case the minimum for the mixture shall be not lower than 520 kPa at 40°C (Note: 100 kPa = 1 Bar).
4. Total volatile sulphur limit include sulphur compounds added intentionally as odorant/for odouring purposes.
5. Free water test only applies to butane/butane-propane mixture. Water promotes rust on internal surface of steel storage tanks and iron piping system systems. Water can also cause odorant fade and can block small openings. LPG can not contain any free water at 0°C and shall be determined by visual inspection of sample using equipment on which density is determined. Subject to agreement between purchaser and supplier, limit, temperature and method for water content determination may be changed for specific applications like shipment of refrigerated LPG etc.
6. Free water content can also be determined with the following procedure:
Take 100 ml of liquid in dry graduated tube and allow to vaporize the LPG. When about 2-3 ml residue is left, shake and pour the residue on wattman filter paper. Allow the residue on filter paper to evaporate; if wet spot not seen, free water is absent.