Pipeline Engineering

Natural Gas Line Purging - AGA Procedure

"Purging Principles And Practice" published by AGA has developed a gas purge procedure (Section 5.3(f)) for pipe greater than 4" to displace air with gas in the line prior to putting it back to service. The procedure requires to maintain the minimum gas inlet purge pressure during the estimated purge time. AGA has calculated minimum gas inlet purge pressures based on various pipe and blowdown sizes using a hydraulic model. In the model, the main pipeline segment is considered as an isothermal pipe with friction, while the inlet and blow-off piping are modeled as adiabatic pipes with friction. We have developed this tool based on mimimum inlet pressures calculated by AGA for various pipe/blowdown sizes and the recommended 2 minutes per mile of pipeline purge velocity. This tool is intended to provide an easy way to support your gas purge planning. Please refer to Figure 5-3 of the "Purging Principles And Practice" for additional modeling basis used by AGA.

Typical purge velocity is assumed to be 2 mins/mile (equivalent to 44 ft/s or 13.4 m/s) per AGA.
Typically range from 50% to 200% of the initial purge duration per AGA.
Per AGA Table 5-1. Add 5psig (or 34.5kPa) to the inlet pressure if purging is done through a crossover arragement and the pressure is measured at the crossover valve.