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Evaluation, Design, and Optimization of Centrifugal Compressor Performance Maps
Federal regulation of interstate gas transmission in the United States is regulated by the Federal Energy Regulatory Commission (FERC). FERC’s main concern is assuring the maximum seasonal flow capacity of an interstate gas transmission system. Therefore, compressor stations must have sufficient available driver horsepower to meet compression horsepower requirements for each compressor installation regardless of whether full driver horsepower is needed or used. This criterion must be met for FERC to certify the interstate transmission system’s maximum seasonal capacities.
FERC does not directly address oper-ational flexibility and efficient operation to certify compliance and therefore pipe-line compressors rarely operate near their design point. Operational flexibility and efficient operations are left to individual interstate transmission companies since system throughput varies throughout the year due to daily and seasonal fluctua-tions of supply and demand.
This article exposes shortcomings to traditional manufacturer centrifugal compressor performance maps. Yet it is also apparent that these performance maps are not utilized effectively for their gas turbine drivers. This analysis indicates that maximum gas turbine horsepower is not sufficient to utilize the complete compressor performance map because compressor manufacturers generally choose to provide maximum flow design conditions at operations around 80% turbine speed. Therefore, larger horsepower turbine drives are being specified and purchased to satisfy FERC certification. The maximum horsepower available of a large turbine drive may never be utilized since the compressor manufacturer intentionally chose the FERC certified capacity to occur at a reduced speed (80%).
A gas turbine’s available horsepower depends on atmospheric pressure and temperature. The required compression horsepower depends on pipeline flow conditions (i.e. gas composition, pressure, temperature, etc.).
Centrifugal compressor performance maps provided by vendors provide plenty of operational information (i.e. speed, head, flow, and efficiency); however, this is not necessarily sufficient to determine if the operation of the gas-turbine/compressor unit can satisfy the operational requirements dictated by ambient and pipeline conditions.
The traditional Head versus Flow performance map identifies the performance envelope to determine compression horsepower. These maps may not provide a clear picture of the possible operation available to the user in terms that are commonly used throughout the industry. For the past seventy years, there has been little improvement in user communication via the compressor map format.
Therefore, new methods for evaluating, designing, and optimizing centrifugal compressor maps used in natural gas transmission systems have been developed. Based on these methods, the most commonly used driver that powers many pipeline compressors lacks the flexibility to effectively and efficiently utilize the complete speed and efficiency ranges provided by the centrifugal compressor manufacturer.
Compressor manufacturers develop performance maps after receiving information based on specified design conditions from pipeline design engineers. This includes the maximum capacity at inlet and outlet design conditions needed to meet FERC certification.
Compressor manufacturers commonly locate the design point (Figure 1) using the design conditions along the highest efficiency line (82.5%) and at or above 80% speed. Yet pipeline compressors rarely operate near their design point resulting in operations at reduced compressor and driver efficiencies, and gas recycling.
The forty-two intersection points within the boundaries of surge and stone-wall can be considered valid operating points from the compressor vendor’s standpoint. But they are not necessarily valid operating points unless the driver can provide the required compression horsepower.
A centrifugal compressor map provides the pipeline design engineer with information to determine where possible compression operations exist given head and flow coordinates. However, the map does not necessarily indicate if operation is possible since ambient conditions dictate if the gas turbine can provide the compression horsepower needed. It would be more useful to have a snapshot of all theoretical and physically possible operating points based on pipeline supply/demand conditions, daily or seasonal variations in pipeline operations, and changes in ambient conditions. These conditions could be substantially different from design condition. A clearer picture of what the compressor/driver can do under off-design conditions would be helpful.
Using the information in Figure 1, Figure 2 provides physically possible operations at the appropriate intersecting speed and efficiency lines. Fortunately, the operation at design conditions is possible at 80% speed as predicted by the compressor manufacturer.
Years of gas industry operating experience with gas turbine-driven centrifugal compressors has shown that operations should not be considered below the 80% speed due to increased maintenance and operational costs. As general rule-of-thumb, a 20% reduction in driver speed translates to a 50% horsepower reduction and an increasing gas turbine fuel rate.
Inspection of Figure 2 shows only the operations with sufficient driver horsepower while Figure 1 shows possible compressor operations without the driver information. Figure 2 is more useful as it identifies those operations that are physically possible due to sufficient available driver horsepower.
Pipeline design engineers use terms like head, ACFM, efficiency, and speed when dealing with compressor performance maps. These terms do not clearly communicate the operation of a centrifugal compressor installation to other functional company groups. Figure 2 lacks the terminology commonly used between functional groups such as volumetric flow at standard conditions, Qstd (MMscfd), suction pressure, Ps (Psia), and discharge pressure, Pd (Psia). More informative compressor performance maps are presented in Figures 3 & 4 with terms commonly used throughout the natural gas industry.
Changes in ambient temperature can readily be presented in this format. Decreasing the ambient temperature from 120⁰F to 32⁰F provides additional operation at 90% speed and three additional operating points at 100% speed (Figures 3 and 4). These charts provide operating points on the compressor performance map where there is sufficient driver horsepower from 50% to 100% speed but without a horsepower amount.
Figure 5 provides additional information concerning available (BHPavail) and utilized (BHPutil) horsepower versus volumetric flow rate (Qstd). Many operating points utilize less than half of the available driver horsepower which limits efficient driver operations. There is a substantial difference in available and utilized driver horsepower at lower speeds. At 50% RPM, utilized horsepower is 17% of available driver horsepower; 99% of available horsepower is utilized at 90% speed.
In the 32℉ ambient temperature case (Figure 6), the utilized horsepower can be as low as 18% and as high as 97% of available horsepower at 50% and 100% speed, respectively. Figure 6 depicts an increase in speed range from 90% to 100% due to the lower ambient temperature with three out of six points on the 100% speed line having sufficient driver horsepower.
A season needs to be selected for optimizing the manufacture’s compressor map. To comply with FERC certification, the winter season (32℉) was selected. The summer season (120℉) was used to bracket optimization.
Maps were developed to show operation before and after optimizing the manufacturer’s compressor map. Three values were generated: 1) BHPreqd: The required compressor horsepower, 2) BHPutil: The utilized driver horsepower needed to physically satisfy compression needs, 3) BHPavail: Maximum available driver horsepower available at ambient conditions. These three values are plotted against Qmax (MMscfd) (Figures 7 and 8) for a 120⁰F ambient temperature.
The following conditions need to be satisfied at all 42 points for complete map operation to be physically possible: BHPreqd =BHPutil ≤ BHPavail. Figure 7 shows substantial levels of BHPreqd above 90% speed. Therefore, operations above 90% speed are not possible. Figure 8 shows after optimization of the initial manufacturer’s compression map additional operations are available at 90% speed.
Prior to optimization, results show an operational improvement at 32℉ (Figure 9). After optimization, all of the required horsepower, BHPreqd, is equal to the utilized horsepower, BHPutil.
As Figure 10 (32℉) indicates, the entire optimized compressor map can be utilized. Figure 10 supports the fact that sufficient driver horsepower is available to allow operations within the optimized compressor operating envelop. The final optimized compressor map with all possible operating points for Winter (32℉) is shown in Figure 11.
A sensitivity analysis was also performed by varying the design inlet pressure +/-50 Psia (Figures 12 to 15). The results indicated no problems in achieving the design flow rate at the design discharge pressure (Pd) with the available horsepower (BHPavial) at 32℉.
As stated earlier, operations below 80% speed should be limited due to increased maintenance and operational driver costs. Even with compressor map optimization, there remains the problem that most possible physical operations exist below 80% gas turbine driver speed.
An alternate compressor driver capable of providing complete and efficient compressor map coverage would be needed to obtain operations on the complete, optimized compressor map. But years of operating experience with gas turbine-driven centrifugal compressors has shown that operations should not be considered below the 80% speed. Thus,
1. Optimization of existing and new compressor maps provides insight into limitations in operational flexibility of large ISO horsepower gas turbines (7,150-31,050hp) given driver constraints.
2. The replacement of a gas turbine driver with a driver that can perform efficiently and effectively at 50% to 100% compressor speed levels is needed to utilize the entire centrifugal compressor map based on the results provided. The complete compressor performance map will be made available at lower speeds in the event that such a driver becomes available.
3. Analysis indicates larger capital-intensive gas turbines are specified because compressor manufacturers chose to provide maximum flow design conditions at inefficient operations around 80% turbine speed. Therefore, users purchase larger horsepower turbine drives and never utilize their horsepower capabilities.
Reconfiguration of interstate natural gas compressor stations from gas-driven compression only to electrical power generation for gas compression and electrical power sales can benefit optimum full-load operation of the existing gas turbines. The excess electrical power produced could supplement renewable energy derived from wind, solar, geothermal or nuclear sources supplying electrical power in the absence of a conventional power station during events such as the 2021 Texas Electrical Reliability Council of Texas (ERCOT) rolling-blackout since the gas supply is available directly at the compressor station(s).
Additionally, if the variable speed electric motor drive becomes an option, replacement of the currently installed compressor(s) with new hermitically sealed electric motor driven compressors should be considered along with adding new electrical generator(s) coupled to the existing gas turbine(s). The existing gas turbine(s) could now operate at 100% load powering the electrical generator(s) to supply power for either gas compression or to the electrical grid. There would be times during seasonal demand for gas compression (Winter residential heating) or power generation (Summer – electrical power for cooling). And if a viable variable speed electric motor drive option becomes available in the 50% to 100% speed range then the electric motor driver’s power characteristics should be used to develop an optimum compressor/driver map. There will be no need to simulate seasonal horsepower changes due to ambient conditions since seasonal changes do not affect the horsepower rating of an electric drive.
Roberto A. Hein, P.E., retired engineer.
Phone: (915) 276-4346