A natural gas compressor is a machine that acts to reduce the volume of natural gas by increasing the pressure of the gas. This is generally done for one of two reasons.
- To reduce natural gas volume and store larger quantities in tanks or bottles for storage or transport.
- To use the pressurization and attendant volume reduction to move natural gas through pipelines for distribution over larger areas, such as pipeline transportation or distribution to customers.
Due to possible impurities in the natural gas, such as moisture and particulates, it is necessary to filter and process the natural gas prior to it entering the compressor.
The gas may be subjected to heating and then cooling in machinery. This causes any water and hydrocarbon liquids to condense out of the natural gas, which dries it out.
The resulting liquids will then be collected and disposed of or used for other purposes.
Strainers and filters are also used prior to the natural gas being compressed. This will remove dirt and byproduct solids from the gas. This reduces potential wear and pitting on the inner surfaces of the natural gas compressor and downstream piping systems.
Natural Gas Compressor Designs
Depending on the specific application, the physical design and mechanical means of gas compressors will vary.
One of the most common designs is that of a positive displacement, reciprocating compressor design.
This is a series of pistons, somewhat like cylinders in a car engine. Here, the natural gas is drawn into an enclosed cylinder space and compressed by the mechanical action of a piston.
It is then discharged from the cylinder into a storage container or into a pipe for transportation.
Another common design is one that uses rotating vanes instead of reciprocating pistons to raise gas pressure and reduce volume.
This design is referred to as a dynamic natural gas compressor.
Gas Compression Engines
Besides the classification of the type of compressing cycle, a natural gas compressor can have a variety of sources for the driving of the mechanical process.
This is the “prime mover” or engine of the compressor.
There are three common engines used to drive the compressing portion of the natural gas compressor:
- Gas-fired turbine
- Electric motor
- Gas-powered reciprocating engine
Both turbine and the electric motor are used to centrifugal compressors. These fit into the dynamic compressor category described above.
The gas-powered reciprocating engine is driven by natural gas, often from the pipeline itself and compresses other gas as described above for the reciprocating compressor.
Reciprocating compressors are often used when incremental adjustments are necessary to the amount of gas being compressed and sent along a main pipeline or distribution network.
This allows for supply adjustment in accordance with customer needs.
Gas Pipelines
As previously mentioned, natural gas compressors are often used to compress gas in order to move it along supply pipelines.
Friction inside the pipes and distance will cause the gas to lose pressure and flow to decrease. Therefore, many gas pipelines have multiple compressor stations along the pipeline to maintain flow.
These stations can be isolated and not frequently visited by personnel.
Natural gas compressors require means of both local and remote monitoring to ensure they are functioning safely and can be secured both manually and automatically in case of emergency.
Gas Measurements
A natural gas compressor has multiple electronic and mechanical instruments that measure moisture content of the gas, cylinder temperature, lubricating oil temperatures to the bearings and moving mechanical parts as well as alarm and control functions.
The instrumentation will first sound local and remote alarms at a monitoring station. This will occur if any measurement moves above or below it normal range.
This allows operators to take corrective actions first without having to shut the compressor down.
In the event of a reading moving beyond alarm set points or a sudden catastrophic failure of a component or system, the control devices will secure the compressor while notifying the remote monitoring station as to what instrument reading caused the shutdown.
This protects the equipment from further damage.
VOC Emissions
An important consideration of natural gas compressors is maintenance of the seals between the compression area, whether rotary or reciprocating, and the outside atmosphere.
This leakage is referred to by the Environmental Protection Agency as Volatile Organic Compound (VOC) emission. Federal regulations mandate the monitoring of natural gas and methane emissions from a natural gas compressor in the pipeline compressor stations. They also madate those in operation at natural gas wellheads, storage facilities and transfer stations.
This requires the operators to have gas monitoring systems in addition to the monitoring systems previously discussed.
Seals in rotating machinery degrade over time and with continuous use, require strict monitoring and maintenance programs. This is to prevent the release of hydrocarbons into the atmosphere from compressor leakage.
In addition to monitoring and reporting requirements to the EPA and various federal and state utility groups, federal rule initiatives from 2012 and 2016 gave recommendations for reducing emissions from natural gas compressors through combinations of improved seal installation, cover design and venting systems to reduce compressor leakage emission by 95 percent.
As a world leader in natural gas production, as well as using natural gas as a substantial source of domestic electric production, the United States relies on high capacity compressed natural gas storage and delivery systems.
A natural gas compressor is an integral component of these systems.
The compressors provide mechanical volume reduction and pressure increase of the natural gas in hundreds of thousands of pipelines and distribution systems.
They are monitored for wear, component failure and damage. This minimizes downtime and ensure the delivery system remains in operation for heating and electrical needs.
In addition, as a potential source of methane and gas leakage, they are constantly monitored for emission data. That data is provided to state and federal regulators. This ensures minimal environmental impact from compressor operation.
Natural gas compressors also require a trained and skilled labor and maintenance force to ensure timely repairs and proper operation.
Any rotating machinery has operational hazards and natural gas compressors are compressing potentially flammable gas as well.
Natural gas compressors are a vital piece of industrial machinery and a key piece of the natural gas transportation and distribution network.