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Gas Properties and Operating Conditions
Contribution to the System Curve

By Joe Honeywell



System curves are often prepared for new facilities utilizing estimated gas properties and operating conditions. The gas characteristics and expected operating limits are frequently selected for a narrow range of conditions. It simplifies the facility design and equipment selection for the design engineers. However, once the facility is operational and the actual gas properties are known, the system curve is forgotten. All too often, however, the properties and operating conditions change over time and so does the resulting system curve. Plant engineers are left to deal with the many issues that occur now that the facility is operational even though they deviate from the original design criteria.

The actual gas characteristics can have a far reaching effect on a facility design and the resulting system curve. Many processes are very sensitive to fluid properties by limiting the operating conditions and gas throughput. They can also affect the performance of some equipment and its operating limits. Plant engineers sometimes fail to consider even the importance of gas properties and their influence while troubleshooting equipment and operating problems. Changes from the original design and actual fluid characteristics should be one of the first considerations when debottlenecking plants or evaluating equipment operating conditions.

Changes in certain operating parameters are well understood by engineers. Pressure is one of these parameters. It plays a major role in gas throughput and power requirements. However, changes in other operating parameters such as temperature and gas composition also play a role in a facility and are less likely to be understood. As is often the case, it is the combination of many changing parameters, not just the select few, like pressure, that contributes to changes in the facility operating conditions. Therefore, the contribution of each parameter to a system curve should be carefully considered in the design and operation of a facility.

This paper will examine the affects that various parameters have on a pipeline system curve and how changes to each parameter will affect the power and gas throughput. Representative curves are prepared for selected gas physical properties and operating parameters that contribute to line pipe friction. Their effects are also evaluated on the power requirements. The various system curves and power levels are calculated and graphically illustrated. The results are analyzed and recommendations are presented.

While this paper was specifically prepared for pipeline systems, the principles could be applied to process facilities, equipment sizing and troubleshooting of operating problems. The basic methodology for determining the system curves for pipelines or process facilities are the same. Hopefully, the message of system changes resulting from variations in design parameters will be better understood for all types of installations.


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