The specific gravity of natural gas is a fundamental measurement in the energy sector, used to quantify its physical properties and commercial value. This single, dimensionless number provides a quick comparison of the gas’s density to that of a reference substance. Because natural gas is not a single compound but a variable mixture of hydrocarbon and non-hydrocarbon gases, this measurement is a summary of its overall composition.
Defining Specific Gravity for Gases
Specific gravity (SG) for any substance is defined as the ratio of its density to the density of a reference material, both measured under the same specified conditions. For liquids and solids, the reference substance is typically water, but for gases, the universal reference standard is dry air. This comparison produces a unitless value because the units of density cancel out in the division.
The calculation of gas specific gravity involves dividing the density of the natural gas by the density of dry air, with both measurements taken at a standard temperature and pressure (STP). This standard condition ensures that all specific gravity values are comparable, regardless of the gas’s actual flowing conditions in a pipeline or storage tank. Mathematically, this relationship is expressed as SG = (Density of Gas) / (Density of Air).
In practice, because the density of a gas is directly proportional to its molecular weight under standard conditions, specific gravity can also be calculated by comparing the gas’s average molecular weight to the average molecular weight of air. The average molecular weight of dry air is approximately 28.96, making it a constant reference point. Therefore, a gas with a specific gravity of 0.6 is 60% as dense as air.
Air is chosen as the reference because it is the medium into which natural gas is typically released, making the specific gravity value a direct indicator of the gas’s behavior in the atmosphere. This is particularly relevant for safety considerations and understanding how the gas will disperse in the event of a leak. Engineers use this ratio to quickly determine a gas’s physical characteristics.
Factors That Determine the Value
The specific gravity of natural gas is rarely a fixed number because the gas is a dynamic mixture of several different compounds, primarily methane. Pure methane (CH₄), which is the lightest hydrocarbon, has a specific gravity of approximately 0.55, meaning it is significantly less dense than air. However, natural gas extracted from a reservoir also contains varying amounts of heavier hydrocarbons and inert components, which all contribute to the final specific gravity value.
Heavier hydrocarbon molecules like ethane (SG ~1.05), propane (SG ~1.53), and butane (SG ~2.07) raise the overall specific gravity of the natural gas mixture. The presence of these higher-molecular-weight components causes raw, unprocessed natural gas to range from about 0.58 to as high as 0.79. Processing removes these valuable byproducts, resulting in a cleaner sales gas with a lower specific gravity, typically in the 0.6 to 0.7 range.
Non-hydrocarbon impurities also influence the final specific gravity. For example, carbon dioxide (CO₂) has a specific gravity of about 1.53, and nitrogen (N₂) has a specific gravity of about 0.97. The presence of these inert gases increases the overall density of the mixture, even though they contain no energy value. Therefore, the specific gravity acts as a fingerprint for the gas, reflecting its exact chemical makeup after it has been treated and prepared for commercial use.
Why Specific Gravity is Crucial for Industry
Specific gravity is crucial across the natural gas industry, from production to billing. One primary application is correlating with the energy content, or heating value, of the gas, measured in British Thermal Units (BTU). Gas with a higher specific gravity generally contains a greater concentration of heavier, energy-rich hydrocarbons like propane and butane, leading to a higher BTU value per volume. This direct relationship is foundational for determining the commercial worth of the gas for trading and customer billing.
In pipeline and process engineering, specific gravity is essential for flow calculations and equipment design. Engineers use this value to calculate density, determine pressure drops, and optimize the compression and flow rates required to move the gas across long distances. Specific gravity is also applied as a correction factor in gas metering, ensuring that the volume measured at operating conditions is accurately converted to a standard volume for custody transfer.
The specific gravity also has direct implications for safety protocols by indicating the gas’s buoyancy relative to air. Since most processed natural gas has a specific gravity below 1.0, largely due to the high concentration of methane (SG ~0.55), it is lighter than air and tends to rise and dissipate quickly in an open environment. This behavior is important for designing ventilation systems in processing plants and for first responders dealing with gas leaks. Conversely, liquefied petroleum gases (LPG) like propane and butane, which have specific gravities well above 1.0, are heavier than air and will sink, collecting in low-lying areas, which presents a far more serious hazard.