Gas gathering is collecting gas from the wellhead and moving it to a processing plant or to a major transmission line. From the well, the natural gas goes into gathering pipelines. The gathering lines become larger in diameter closer to the central collecting point. This application uses either small, individual wellhead compressors or large central units handling several wells or an entire field. The small compressors can be as small as five horsepower. The large central units can be several thousand horsepower. As the field pressure drops due to depletion and the suction pressure at the compressor drops, the ratio of compression increases. The discharge pressure can also vary greatly depending on the flowing pressure of transmission pipelines in the area. Typical suction pressure ranges for reciprocating compressors in gas-gathering services are from about 10 to 150 psig. Typical discharge pressures for reciprocating compressor in gas-gathering services are from 150 to 1200 psig. Machines are expected to handle a wide range of these conditions and keep the driver reasonably well loaded.
Gas-gathering applications probably account for the majority of installed reciprocating compressor horsepower in the oil and gas industry. Compressor units are generally sold to the user. The rental of compressor units is growing rapidly and is itself becoming a viable service industry.
The nearer the compressor is to the wellhead, the more likely the gas will be saturated. Some tolerance of this liquid is desirable in the design of a field unit. Reciprocating compressors are generally utilized in gas-gathering applications. Some rotary positive units are used in low-pressure applications, and both can be used as wellhead gas compressors.
Some natural gas gathering systems include a processing facility. The processing facility removes impurities like water, carbon dioxide or sulfur which could corrode a pipeline. They also remove inert gases which could reduce the energy value of the gas. Processing plants can also remove small quantities of propane and butane.
From the gathering system, the natural gas moves into the transmission system. This system is composed of about 272,000 miles of high-strength steel pipe ranging from 20” to 42” in diameter.
These large transmission lines for natural gas can be compared to the nation's interstate highway system for cars. They move large amounts of natural gas thousands of miles from the producing regions to local distribution companies (LDCs). The pressure of gas in each section of line typically ranges from 200 pounds to 1,500 pounds per square inch. These ranges depend on the type of area in which the pipeline is operating.
Compressor stations are located approximately every 50 to 60 miles along each pipeline. These stations boost the pressure lost through the friction of the natural gas moving through the steel pipe. Many compressor stations are completely automated, so the equipment can be started or stopped from a pipeline's central control room. The control center also can remotely operate shut-off valves along the transmission system. The operators of the system keep detailed operating data on each compressor station. They also continuously adjust the mix of engines running to maximize efficiency and safety.