What device must be used instead of gas itself to clear a line?

The use of a pig is a standard practice in the maintenance and clearing of gas lines. A pig is essentially a device inserted into a pipeline that moves through the line to remove debris and obstructions. The term "pigging" refers to the process of using these devices, which can be solid or functional with scrapers and brushes, to clean and inspect the internal conditions of the pipe without needing to use gas itself.

This method is crucial for ensuring the integrity and safety of gas lines, as it allows for the detection and removal of blockages and buildup that could pose a hazard if left unaddressed. Other devices like pumps, blowers, or compressors may serve different functions but are not specifically designed for the task of clearing lines in the same effective manner as pigs are. These alternatives do not focus on the internal cleaning and inspection process that is vital for maintaining safe gas transmission. Thus, the pig stands out as the appropriate tool for this specific task.