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Optimization of Reciprocating Compressors to Maximize Throughput

Reciprocating compressors are well-suited for delivering high pressure compressed gas in single or multi-stage configurations. These pieces of equipment are expensive and energy intensive and as a result, it is important to operate them at their “optimal point” regardless of natural gas price.  The interpretation of “optimal point” depends on the application and priorities of the organization and could refer to maximizing throughput, improving fuel efficiency, minimizing emissions or maintaining reliable operating conditions. Optimization studies like these can be found in literature (see references) and are often based on studying the operating curves of the compressors.

This article shows an example of how to use Symmetry to simulate the reciprocating compressors and using these models to maximize throughput. The optimization strategy consists of finding the optimal point where the compressor works at its maximum capacity and the well production is stimulated while maintaining a feasible operation. Generally speaking, the production of a natural gas well can improve with lowering the suction pressure while an increase in the suction pressure increases the compressor throughput. Therefore, it is imperative to find an optimal point where both of the well production and the compressor throughput can benefit from it. 

Methodology

To perform the optimization study, the reciprocating compressor performance curve should be generated. The curve could be divided into three main segments; the cylinder capacity section, the knee and the horsepower section as shown in Figure 1. In the cylinder capacity section, cylinder capacity utilization is maximum which means no clearance is added to the first stage cylinder(s) and the driver runs at maximum speed. The horsepower section of the curve represents a horsepower utilization of 100%. In this section, clearance devices are used to unload the driver. The knee point is the only point where the cylinder capacity utilization and the horsepower used are 100% and is the optimal operating point of the compressor.

Compressors_1.png

Figure 1. Operating Curve of Reciprocating Compressors

Case Study

A compression station consisting of multiple compressors in a natural gas plant is evaluated. Figure 2 shows a schematic of the reciprocating compressor configuration. This 3-stage compressor package has a capacity of 6.66 mmscfd, a suction pressure of 80 psig, a discharge pressure of 1275 psig, and operates at a speed of 900 rpm with a horsepower consumption of 800.

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Figure 2. Reciprocating Compressor Unit Schematic

The optimization study consists of building the performance curves of the equipment and comparing against the current operation point of the equipment. These curves can be built in Symmetry by performing parametric studies in the case study tool by following the methodology described in the previous section. The curves for this study are shown in Figure 1.

The current operation point for this example is below the capacity curve and can be moved closer by increasing driver speed and/or reducing suction pressure as shown in Table 1. Three different configurations are evaluated here. In the first scenario, capacity is increased by running the driver to its maximum speed and keeping the suction pressure unchanged. This condition does not have any effect on the well production. In the second scenario, the suction pressure is decreased and the compressor capacity is not changed but the well production is stimulated due to the decrease in suction pressure. In the third scenario, the driver works at its maximum speed and the suction pressure reduces to 71.5 psig. In this condition, the compressor capacity increases by 13% while the suction pressure decreases. The optimal operating point depends on the specific response from the well but is likely somewhere close to the third scenario. The increase in capacity can now be combined with an economic analysis to quantify the potential benefits of changes in operation. 

Compressors_3.pngTable 1. Reciprocating Compressor Optimization Results

Conclusion

Depending on the production well response and using the compressor performance curves, the reciprocating compressor unit can be optimized and evaluated for opportunities to increase the driver speed, reduce added clearance, and optimize suction pressure. The outcome will be an increase in throughput.

Symmetry can be used to generate reciprocating compressor performance curves which are used to evaluate and optimize the operation.

Please contact your local VMG office for more information.

Arash Behrang, Ph.D., VMG Calgary

Raul Cota, Ph.D., P.Eng., VMG Calgary

References

Mino, J. (2013). Natural Gas Reciprocating Compressor Optimization. Society of Petroleum Engineers. 

Bourn, G. and Sutton, M. (2016). Anadarko’s Midstream Compression Optimization Program. GPA Annual Convention.

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