January 2019, Vol. 246, No. 1
Features
Compressor Stations Doing the Difficult Work
By Nicholas Newman, Contributing Editor
Compressor stations compress and push gas from the field where it is extracted or from an LNG re-gasification import port, along transmission pipelines, to end customers, such as refineries, power stations or LNG export plants.
Compressors, housed in stations located at 40- to 100-mile intervals along a pipeline, run continuously and allow gas to be rerouted into gas storage areas during periods of low demand. An average station can ship 700 Bcf/d of natural gas, while the largest are reported to be able to send 4 Tcf/d of gas.
Compressor stations also house pig launchers and receivers, which are maintenance and inspection robots, needed to maintain the efficiency, integrity, and safety of the pipeline and station.
ABB, Siemens and Voith, the top three suppliers of compressors, operate in the oil and gas compressor market valued at about $24.18 billion in 2015, which is projected to reach $34.11 billion by 2021, by researchers at Markets and Markets.
How Many?
The United States has an estimated 1,200 operational natural gas compressor stations sited along its more than 3-million-mile pipeline network, delivering 25 Tcf of gas to 74 million customers, according to a 2018 EIA report.
In contrast, the smaller Russian gas pipeline network of 106,000 miles, operated by Gazprom, has 254 compressor stations for gas transmission.
At the compressor station, gas is passed through scrubbers and filters to extract any remaining liquid and solids from the gas stream. To handle the scheduled system flow requirements the gas may be passed through a series of compressors.
Most compressor units run in parallel, with each unit adding pressure before redirecting the gas back into the pipeline at full pressure. Sometimes, several compressor units are operated serially in order to achieve high pipeline pressure rates before release to the next section of pipeline.
Typically, compression of gas generates heat. As a rule of thumb, for every hundred increase in psi, the temperature of the gas stream increases by an added 7 to 8 degrees. In fact, most compressor stations are equipped with an aerial cooler system, like a truck’s radiator, to dissipate any excess heat.
Types of Compressors
Compressor stations can be divided into gathering stations, which collect gas from several wells in a field, such as the Bakken or offshore in the Gulf of Mexico, and export gas to the long-distance grid and those that support pressure along a long mains transmission network.
Examples of these can be found in Europe on the Nord stream sub-sea pipeline between Russia and Germany or the soon-to-be-completed Trans-Adriatic Pipeline linking Turkey with Italy.
In addition, there are four main types of compressor: centrifugal, axial, screw and reciprocating (Table 1).
Table 1: Comparison of different compressor types
|
Compressor Type |
Advantages |
Limitations |
|
Centrifugal |
High-efficiency |
High initial cost |
|
Axial |
High peak efficiency |
Difficult to manufacture |
|
Screw |
Simple design, few moving parts |
High rotational speed |
|
Reciprocating |
Simple design, easy to install |
Higher maintenance costs |
Source: IPIECA
Compressor Stations
These are designed to cope with small diameter pipelines of up to 20 inches, which commonly move gas from the wellhead to a natural gas processing facility or to an interconnection with the larger mainline pipeline. In the U.S., individual state regulations apply whereas in Europe, national regulations operated by national regulators apply.
Compressors need to cope with gas from different wellheads and a variety of incoming pressures. As Rita-Wei Fu, of Swiss Swedish engineering giant ABB, pointed out, “Compressors supplied by ABB have to be modularized and scalable to meet market needs.”
One purpose of the compression station is to regulate and standardize gas pressures, (generally between 800 and 1,200 psi) before onward transmission. Therefore, when a gas field is expanded, the compression station also must be scaled-up. Typical gas compression stations occupy sites varying between five and 15 acres.
In the U.S., transmission pipelines between 20 and 48 inches in diameter and pressurized from 800 psi to 1,200 psi, carry natural gas from producing areas, such as Bakken and Marcellus shale fields, to major markets like New York or Boston.
To ensure a continuous flow gas must be periodically compressed and pushed through the pipeline. Friction and elevation differences slow the gas and reduce the pressure, so compressor stations are placed 40 to 70 miles apart along the pipeline to boost the pressure.
Because they are only providing a boost in pressure, interstate transmission system compressors are generally smaller, with just two compressor units (one operational, the other a back-up) within a single building.
Innovations
“There have been incremental improvements to the operation, design and monitoring of compressors that has made them more efficient and have cut their carbon emissions,” said Rita-Wei Fu, of technology company ABB. “In terms of innovation, the arrival of new affordable technologies mean that you can now base microchips everywhere to deliver information.”
Operations have benefited from the application of digital tools like edge computing and internet-of-things-equipped sensors, which make it easier to monitor and diagnose issues, whilst new materials and designs, are making it easier to relocate, refurbish and even decommission compression stations at the end of their life.
However, the latest significant innovation has been to use electricity rather than gas, to power both on-and-offshore compressors, which has reduced methane emissions resulting from leaks in the gas supply line to the engine, incomplete combustion or a “system upset.”
A good example of a gas gathering electrically powered compressor station can be found in Norwegian gas pipeline operator Gassco’s Gas gathering and processing plant near Bergen. It has a daily processing capacity of up to 143 standard cubic meters and 69,000 barrels of NGL, piped in from the Troll, Kvitebjørn and Visund fields.
Moreover, an increasing number of offshore compressors are now being powered either by sub-sea cable to the mainland, or electric-powered compressors on offshore rigs lower carbon emissions from industrial processes and have a smaller footprint.
However, Fu cautions, “careful planning must take place to make sure that there is enough flexible grid capacity to meet compressors operational needs.” In the future, offshore compressors could perhaps be powered by floating wind turbines, as envisaged by Norwegian energy company, Equinor.
Grid-Connection Benefits
According to EPA PRO fact sheet No. 103, methane emissions reductions of 32,800 Mcf per year have been achieved in a demonstration pilot of grid connected compressors, running five c
compressors: two of 2,650 hp, two of 4,684 hp and one of 893 hp reciprocating grid-connected compressor.
Operators have reported that electric motors reduced gas losses and saved gas.
In summary, it must be noted that most of the escaped gas during a “system upset” comes from compressor blowdown emissions and is the same for both gas engine and electric motor-driven compressors.
Nevertheless, given the importance of keeping the world’s temperature within check and the contribution of methane emissions to rising temperatures, the displacement of gas by electrically powered compressors is a useful and welcome innovation.
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