Without knowing any details my advice would be to contact Praxair and request assistance. They will request an elevation profile, your MOP, the distance you'd like to purge, and the fluid type to help you calculate a total required volume and an injection rate. A lot will depend if you are going from valve site to valve site or from trap to trap. Make sure you use more than one pig with liquid between them to minimize N2 bypass. Make sure to include pig tracking transmitters.
