CHALLENGES OF PURGING AIR WITH NATURAL GAS AND HYDROGEN BLENDS IN PIPE SEGMENTS

The aspiration for blending hydrogen (H-2) into natural gas (NG) in gas transmission systems is high and is happening globally. However, several design and operational aspects need to be developed to ensure the safe and reliable delivery of these blends to the end users. One of these aspects is the development of an effective purging procedure of air with NG+H-2 blends during the commissioning of a pipe section. The present research examines various aspects of the purging procedures and attempts to address salient parameters and conditions when the NG+H-2 blend is the purging gas. Three principal aspects were investigated, namely: 1) The stratification velocities of the air and gas wave fronts for different NG+H-2 blend ratios, 2) The interfacial turbulent diffusivity coefficients between air and NG+H-2 blends that determine the extent of the mixing spread as purging progresses, and 3) The applicability of the minimum stratification velocity to smaller pipe sizes <= NPS 4. The present investigation found that the stratification velocity of the air wave is higher than that of the NG+H-2 wave, and both deviate from the classical value corresponding to Fr # = 0.707 assumed in AGA Purging Principles and Practices. Initial results of the diffusivity coefficients show an upward deviation from that currently in use in the gas transmission industry.