Understanding Natural Gas Distribution – A number of stages make up the system that moves gas from underground wells to stove tops, hot water heaters, furnaces, and other consumption points. While each stage relies on the same principle, gas flowing from higher to lower pressure, the purpose and structure of the design change along the way.
From the well into gathering
Like inverted branches on a tree, thin gathering lines work deep within the well to pull gas upward through gradually widening pipes. Most of this movement is done using the pressure of the gas alone, but some gathering systems use natural gas-powered field compressors to increase pressure in the system.
Once to the surface, the gas is directed to a processing facility to remove impurities such as water, CO2, and sulfur, and to separate out inert gases like helium. Small amounts of butane and propane can be produced directly at the processing plant.
Hundreds of thousands of miles of wide-diameter high-strength steel pipe move large quantities of gas from gas-producing regions to local distribution companies across the nation. Given that the pressure within transmission system pipes ranges from 200 to 1,500 PSI, piping is engineered to handle double the PSI needed for transmission to maintain safety.
The transmission network functions in the same way as the highway system, with extra capacity available through a looped design that runs parallel pipes along high-demand corridors. Natural gas can move through the system at speeds up to 30 mph.
Compressor stations boost pressure along the way
Every 50 to 60 miles along the transmission system, gas enters a compressor station. The purpose of compressor stations is to add pressure to the line that is lost through the friction that slows gas as it flows through the steel pipes.
Most compressor statins are automated and controlled remotely from a central control room. Operators in the control room monitor equipment data to make adjustments to compressor station engines to maximize efficiency. If needed, valves throughout the transmission system can be closed remotely for safety.
Using pressure to modify the line pack
The gas stored in any section of line between two compressor stations is known as the line pack. Depending on the size of the pipe and the pressure, hundreds of millions of cubic feet of gas can be contained in just one 50-mile stretch of the transmission line.
By increasing or lowering the pressure within a pipeline segment, the size of the line pack will get larger or smaller. In this way, the companies that manage pipeline distribution can store more or less gas in the line to adjust for fluctuations in demand using sophisticated software and modeling. Natural gas demand shifts are accounted for down to the hour, to deliver gas when and where it is needed without over-pressurizing the system.
Entering local gas utility systems
Transferring natural gas from transmission pipelines to local gas utility lines requires a significant reduction in pressure. This pressure reduction is accomplished at gate stations. Most utilities have multiple gate stations, receiving gas from different pipelines.
Gate stations serve three major functions:
- Reduce line pressure to distribution levels (0.25 to 200 PSI)
- Add odorant to gas for consumer safety
- Measure flow rate to track how much natural gas has entered the utility’s system
The utility’s distribution system
Once through the gate station, distribution lines ranging from 2 to 24 inches in diameter begin to carry the natural gas closer to the customers. As lines move nearer to the gas’s final destination, the size of the distribution lines, also called mains, is reduced.
Similar to how the line pack is monitored and adjusted, gas utility companies track flow rates and pressures at various points in the system to make necessary adjustments for optimal delivery capacity. Distribution systems are designed to handle pressure 5x the maximum allowed pressure for added safety. As gas flows through the system, regulators open and close to maintain the desired pressure, and relief valves are installed to vent gas to the atmosphere where necessary.
Natural gas mains are laid out in a grid pattern to allow for the redirecting of gas via many different routes. This allows utilities to use detours to avoid customer disruption and keep the gas flowing during maintenance outages and emergencies.
Ready for consumption
Service lines run from the main into the gas meter outside of businesses and residences. Here, at the meter, the responsibility of system equipment and maintenance passes from the utility company to the owner of the building.
At the meter, a pressure regulator reduces the natural gas for final consumption. 0.25 PSI is just enough higher than ambient air pressure to force gas to flow out of the burner when a valve is opened, add a spark for a clean blue flame.
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