Natural gas is a primary energy source used globally for heating, electricity generation, and industrial processes. Understanding its physical properties is fundamental for every stage of its journey. Specific gravity (SG) is a measure that provides insight into the gas’s density, composition, and behavior. This dimensionless number influences how the gas is processed, transported, and safely utilized.
Defining Specific Gravity for Gases
Specific gravity (SG) is a relative density measurement, representing a direct comparison between the density of a substance and the density of a reference substance. For gases, SG is defined as the ratio of the density of the gas sample to the density of dry air. Both densities must be measured at the same standard conditions of temperature and pressure, typically 60°F and 14.73 pounds per square inch absolute (psia).
SG is a unitless number that indicates how much lighter or heavier a gas is compared to air (SG = 1.0). If a gas has an SG less than 1.0, it is lighter than air and will rise and disperse when released. Conversely, an SG greater than 1.0 means the gas is heavier than air and will sink and pool in low-lying areas.
SG can also be calculated as the ratio of the gas’s molecular weight to the average molecular weight of air (approximately 28.96 pounds per pound-mole). This simplified calculation is often used for ideal gas approximations and quick estimations.
Typical Specific Gravity Values for Natural Gas
Processed pipeline natural gas typically has a specific gravity between 0.55 and 0.80. Methane (\(\text{CH}_4\)), the primary component, is very light and has an SG of approximately 0.554, serving as the lower boundary for commercial gas.
The SG varies significantly because natural gas is a mixture of various hydrocarbons and non-combustible gases. A higher specific gravity indicates a richer gas containing a greater proportion of heavier hydrocarbon molecules. Heavier components increase the overall average molecular weight, driving the SG closer to 1.0 or higher.
- Ethane (\(\text{C}_2\text{H}_6\)): 1.05
- Propane (\(\text{C}_3\text{H}_8\)): 1.56
- Butane (\(\text{C}_4\text{H}_{10}\)): 2.067
Non-combustible impurities also influence the SG; for example, carbon dioxide (\(\text{CO}_2\)) is about 1.53, while nitrogen (\(\text{N}_2\)) is close to air at 0.97. The final measured SG is a weighted average reflecting the gas stream’s chemical composition.
Why Specific Gravity is Crucial for Gas Handling
Specific gravity is a fundamental input for accurately calculating the quantity of gas moved and its potential energy content.
Custody Transfer and Energy Content
In custody transfer, where gas is bought and sold, SG is used alongside temperature and pressure measurements to determine the density of the gas. This density is then used in flow equations, such as those governing orifice plate meters, to precisely calculate volumetric and mass flow rates.
The SG is also directly linked to the gas’s energy content, often measured in British Thermal Units (BTU). Since heavier hydrocarbons have higher BTU values than methane, a higher specific gravity generally correlates with a higher energy value per unit volume. Accurate SG measurement is necessary for billing purposes to ensure consumers pay for the exact amount of energy received.
System Design and Safety
Specific gravity data is used to size various pieces of equipment, including compressors, fans, and burners. Compressors require precise SG data to calculate the power needed for compression and ensure optimal efficiency and safety during operation. Burner nozzles and air mixers are tuned based on the SG to ensure the correct air-to-fuel ratio for complete combustion.
SG has direct implications for safety protocols, particularly concerning leak detection and ventilation. Since natural gas typically has an SG below 1.0, it is lighter than air and tends to rise and disperse. This behavior favors placing exhaust vents near the ceiling, contrasting with heavier gases like propane (SG > 1.0), which require floor-level sensors and ventilation.
How Specific Gravity is Measured
The specific gravity of natural gas can be determined using several specialized techniques, depending on the required accuracy and whether the measurement is taken in the field or a laboratory.
Continuous Gas Gravitometer
One common method involves the use of a continuous gas gravitometer. These instruments work by mechanically comparing the torque produced by a fan spinning the gas sample against the torque produced by a fan spinning a reference sample of air. The ratio of these two forces provides an instantaneous reading of the specific gravity.
Effusion Method
Another technique is the effusion method, which relies on the principle that the rate at which a gas escapes through a small opening is related to the square root of its density. This method uses a device called a gravity balance to compare the time required for a fixed volume of the gas sample to escape against the time taken for the same volume of air to escape under identical conditions.
Gas Chromatograph (GC) Calculation
For the most precise determination, the specific gravity can be calculated mathematically after obtaining the full molecular composition of the gas using a gas chromatograph (GC). A GC separates the gas mixture into its individual components, measuring the percentage of each. By knowing the molecular weight of each component, a weighted average molecular weight for the mixture is calculated, which is then divided by the molecular weight of air to yield a highly accurate specific gravity value.