Software

The compressible module allows the motion of a pig in a gas pipeline to be simulated. Factors such as the system pressure, flow rates and pig friction are considered in the analysis. In this respect, PIGLAB Compressible can be used to simulate pigging operations such as waterfill and dewatering during pre-commissioning, or motion of pigs in gas pipelines during operations – for example, the velocity profile of an ILI inspection tool at different operating pressures.

When used in conjunction with other modules, aspects of the pig performance such as seal wear, and placement in the pipeline can be taken into account. Output includes pressure transience along the pipeline and pig velocity and position.

Lately, the tool has been used to investigate the motion of pigs in hydrogen pipelines.  Due to the low density of the product (hydrogen is the smallest molecule and subsequently has a low molecular mass), then the dampening effect of the gas is low and pigs can be expected to reach much higher velocities compared with natural gas under the same operating conditions (pressure and flowrate).

This can lead to excessive and dangerous speeds and could make it difficult to inspect a pipeline to the required accuracy.

The PIGLAB Compressible program works by dividing the pipeline up into distance increments and solving the continuity and momentum equations for the gas around the pig. The pig motion is then considered by solving the force balance across it. A moving grid is used, which is refined near the pig to allow a stable solution to be achieved.

The following sample output shows the motion of a pig in a 24-inch pipeline at 50bar with velocity excursions at various line features. The velocity signature from the pig is due to the gas flow and the effects of pig friction that introduces instability.

In single diameter lines, it is sometimes argued that it is not possible to simulate the motion of pigs using this type of technique as the frictional relationship is not always known, or the pig stops at a pipeline features such as a weld, which are not built into the program. However, this is not the case when data logger, test data and experience is available. The program can be used to determine the worst velocity conditions for a given operating scenario. It is then possible to redesign the pig or influence the operation to help avoid problem conditions.

In summary, PIGLAB Compressible is an ideal tool for investigations into the motion of pipeline pigs in gas pipelines. In conjunction with the other models, full simulation of pigging operations can be undertaken. Knowledge of pig motion can be used to: –