TransCanada Upgrades Pipeline Booster Station, Controls Emissions.
TransCanada PipeLines Ltd. transports the greatest volume of gas in North America. It operates 37,000 km of pipeline in Canada as well as important interests in various Canadian and American pipelines totaling another 6500 km. GE Nuovo Pignone, based in Florence, Italy, is one of the major suppliers of gas booster units to TransCanada.
TransCanada's BC System, formerly the Alberta Natural Gas (ANG) pipeline, operates a 171-km pipeline transporting gas from the Alberta-British Columbia border, through the Rocky Mountain chain in the southern part of British Columbia, up to the border of Idaho. At that point, PG&E Gas Transmission-Northwest distributes it through its own lines to the centers of consumption in California.
TransCanada's BC System has a 36-inch pipeline with three booster stations, including one located near the town of Elko, designated 2A, that has been upgraded with the installation of a Nuovo Pignone compressor driven by a PGT16 gas turbine. The power shaft is actuated by the first GE LM1600 gas generator fitted with a DLE (Dry Low Emission) combustion system capable of significantly reducing the levels of polluting elements contained in the exhaust.
This pipeline is fed with natural gas from TransCanada's Alberta System near Crowsnest, Alberta. The gas is transported to PG&E Gas Transmission-Northwest in Kingsgate, British Columbia. The line runs with the 36-inch and 42-inch pipeline of Foothills Pipe Lines (South B.C.) Ltd., also operated by TransCanada, through the Rocky Mountains to the Moyie River Valley where it is parallel to the road and the railway. At the Flathead Ridge, the pipelines reach the highest altitude, (2,100 meters) 6,900 feet, in the entire Alberta-California system.
Compression stations are located at Crowsnest (Station 1), Elko (2A) and Moyie (2B). These stations contain 11 compressors driven by gas turbines totaling 136,779 kW of power. This system is fully automated and designed to be operated unmanned from two dispatching rooms, the first at the Crowsnest station and the second at Cranbrook, British Columbia.
Since the pipeline runs through scarcely populated areas, TransCanada personnel inspect its entire length by helicopter once a month to detect possible leakage or other problems. Complete inspections are carried out each year by routine maintenance crews. All possible measures are taken to limit damage in case of accident and to reduce environmental impact.
Between 1990 and 1996 ANG increased the volume of gas transported by 62 percent, while simultaneously reducing by 17 percent (per volume unit of gas transported) [CO.sub.2] emissions. In addition, [NO.sub.x] emissions were reduced by 37 percent. This was accomplished through:
* the replacement of old compressor sets with more modern and efficient units to reduce fuel consumption and thus [CO.sub.2] emissions;
* use of dry gas seals on the new compressors and replacement of oil seals with dry gas seals on old units to reduce gas leakage;
* installation of new turbines equipped with exhaust silencers and retrofitting of silencers on the older units;
* use of water injection or DLE systems to reduce [NO.sub.x] emissions;
* equipping all new and existing units with electrohydraulic starting systems rather than gas expanders to lower the volume of gas discharged to the atmosphere; this provision is also being extended to existing units;
* installation of two new compressors fitted with magnetic bearings to increase efficiency by reducing the friction losses associated with oil-lubricated bearings;
* increasing the efficiency of control systems to minimize the release of gas to the atmosphere in case of unit shutdown and subsequent purge during the next startup;
* installation of silencers on the safety lines to reduce station noise;
* installation of interconnecting valves between main lines to increase operating flexibility and reduce gas discharged to the atmosphere during maintenance.
Within the context of these initiatives, ANG awarded Nuovo Pignone an order for the supply of the PGT16+PCL801 compression unit featuring the first GE LM1600 gas generator equipped with a DLE combustion system to control emissions without injecting water into the combustion chamber. Water injection is an effective and well-tested system, but also more complex and expensive to operate.
Prior to ANG's commitment, GE tested the DLE system on the test rig and guaranteed that required emission levels would be met. Measurements taken at the Elko station have confirmed GE's projections. The positive results obtained at Elko led TransCanada to order from Nuovo Pignone a compression unit featuring a PGT25+ gas turbine equipped with DLE system to drive a PCL802 centrifugal compressor to be installed at Compression Station #1 along the same pipeline near Crowsnest Pass in British Columbia.
TransCanada and Nuovo Pignone started doing business together in 1989, with the sale of the first four compressor units driven by 14,000 kW PGT16 gas turbines. These machines, installed in 1991, have now totaled over 40,000 operating hours each. Subsequently, another 13 units were installed in various compression stations and in 1998 TransCanada ordered a total of five compression units, four of them from Nuovo Pignone.
The compression unit installed at Elko consists of a LM1600 gas generator from GE coupled to a two-stage, heavy duty power turbine designed by Nuovo Pignone which is directly connected to the Pignone PCL801 booster compressor.
There are several factors about this installation which impose particularly severe operating conditions on this gas generator. First, the gas generator is coupled to a heavy-duty power turbine which, due to its weight, has much higher inertia than the one used for the tests. In addition, the mechanical drive of a pipeline compressor is much more demanding than a coupling to an electric generator. Furthermore, Elko is located at an altitude of 844 meters above sea level and the ambient temperature, which has a considerable influence on the performance of a gas turbine, ranges from -34 [degrees] to +35 [degrees] C. The engine was commissioned in November 1997 and seven months later required remapping of the fuel schedules when 1998 summer temperatures, exceptional for the region, caused engine flameouts.
The DLE combustion chamber, of the annular type, is built of Hastelloy and equipped with two rings of 18 burners each, which are fed with natural gas drawn from the local pipeline.
From startup to 50 percent of load only the burners on the external ring are in operation, then the inner ring burners are also ignited to reach 100 percent load. The natural gas from the pipeline is essentially methane (96 percent) with a lower heat value of 48,510 kJ/kg. This gas contains traces of [H.sub.2]S (1-4 ppm) and other contaminants. Gas heating value and specific gravity are measured by gas chromatography and transmitted to the control unit (UCP) for fine calibration of the DLE system.
To maintain [NO.sub.x], CO and HC emissions within established limits with a variable load under actual operating conditions, it is essential to measure precisely the fuel fed to the burners and the air supplied by the gas generator axial compressor. This task is accomplished by the UCP, which features a Woodward Netcon 5000 system.
To retain low [NO.sub.x] emission without increasing the levels of CO and HC, the flame temperature in the combustion chamber must be kept within a narrow "window" while the load conditions of the machine undergo change. This is done by measuring the fuel and mixing it with a certain volume of air, keeping the air/fuel ratio constant in the combustion chamber. The volume of air is controlled by adjusting the angle of the stator blading, the opening of the variable bleed valves (also provided on standard gas generators) and the opening of an additional vent valve placed on the delivery of the DLE gas generators. The fuel is measured by special metering valves which take into account several parameters, including the gas composition determined by gas chromatography.
During its test program, GE mapped the DLE parameters of emission levels for the entire power field of the gas generator. The mapping had to be done, however, for each gas generator because even a small difference between one generator and another produced on the same assembly line (a normal occurrence) can result in substantial differences in emission levels.
Accordingly, the mapping must be calibrated each time a machine is delivered to adapt the mathematical model to the machine conditions and to the installation site. Emission levels measured at Elko on the first machine featuring a LM1600 DLE gas generator have confirmed the results obtained on the test rig. The CO levels are slightly higher than those measured on the test rig, but still lower than one-third of the guaranteed levels.
The Elko turbine can be started quickly, cold, by carrying out the starting sequence. The turbine is started by a 150 kW hydraulic motor directly mounted on the auxiliary gearbox, capable of bringing the unit to a speed above that of ignition with the compressor casing under full pressure. P&GJ
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|Comment:||TransCanada Upgrades Pipeline Booster Station, Controls Emissions.|
|Publication:||Pipeline & Gas Journal|
|Article Type:||Brief Article|
|Date:||Jun 1, 2000|
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