Station Configuration Impacts Availability, Fuel Consumption and Pipeline Capacity-Part 2 Of 2

The number of compressors installed in each compressor station of a pipeline system has a significant impact on the availability, fuel consumption and capacity of the system. Depending on the load profile of the pipeline, the answers may look different for different applications.

Among the factors to be considered is the fact that gas turbines can produce a significant amount of additional power at lower ambient temperatures. So, even for constant load of the pipeline, the relative load of the driver changes, and thus influences fuel consumption and pipeline capacity. Part 1 of this two-part article covered the more general thoughts. Part 2 discusses the application of these principles to an actual situation.

Central Asian Pipeline
For our case study, we considered a long-distance pipeline in Central Asia. The total length of the trunkline is about 7,000 km. The pipeline consists of two 42-inch parallel lines which turn into a single 48-inch line when it crosses the border. The pipeline design throughput is 30 billion m3 per year (Bcm/y) and maximum operating pressure of this pipeline is 9.8 MPa. There are 10 compressor stations planned in one area, and more than 20 stations in the receiving country. After first gas, it takes five years to build up to full capacity.

When we compare operations of the compressor station, we need to recognize two main approaches. We can assume either operation with fewer large turbocompressor units (Case A, two large units), or with a greater number of smaller turbocompressor units (Case B, four small units).

The following factors need to be considered when selection of either setup is decided. In evaluating the system reliability and maximum throughput, a failure analysis needs to be performed. If we were to consider two large 30-MW units the failure of one of them will result in 50% reduction of power available whereas if we consider four smaller 15-MW units, the failure of one of them will result in only a 25% power reduction. Figure 7 outlines the basic fact that, if the surviving units run at full load to make up as much flow as possible, the operating point for the Case B will be close to the highest efficiency island so the remaining online compressors will be working more effectively compared to Case A, when the single remaining large unit will be working in the stonewall area. It is obvious that pipeline recovery time will be shorter in case B.

Based on an analysis by Santos (2006), Case A can represent even more problems. The amount of gas that the single remaining 30-MW unit will have to process is so large that it will put this remaining unit into choke, and thus for practical purposes, out of operation. The amount of fuel that the remaining unit is going to burn will not justify that negligible increase in head that this unit will provide. So, practically, when one larger turbocompressor will be out of operation, the second will have to be shutdown and the station will be bypassed. Station configurations with the single oversize driver and either no standby or standby on each second or third station are often advocated. The arguments in favor of this method are very high pipeline availability (99.5%) and high efficiency (40-42%) of the larger 30-MW turbocompressor units. In fact, designing for a turbine oversized by 15% will lead to normal operations at part-load conditions almost all the time (99.5%) where there will be negative impact on turbine efficiency and, as a result, increased fuel consumption.