From the following document - see Fig 1 on page 10

https://www.linde-gas.com/en/images/13152_MT_CE_In...

I would suggest pressuring the header to 0.5barg with near pure N2, then blowdown, then repeat another cycle of pressuring to 0.5barg and blowdown again. That will prevent the subsequent methane fill of the header from combusting. The second cycle is to account for non ideal mixing of the header contents prior to blowdown.

Start with CEN/TR 15281, AGA's Purging Practice and Principles, NFPA 56, NFPA 69, IGE/SR/22.
For large pipe inert gas slug method would be more economical - see link
https://files.engineering.com/getfile.aspx?folder=...

Fig 1 on page 10 of that doc tells me methane at any concentration is not flammable in a N2 / O2 mix where O2 concentration is less than 13% v/v.

This document indicates a lower O2 concentration of 10-11% v/v for flammability of methane, close to the previous value cited. See page narrative and Fig 3 on page 4 of 7

https://www.cdc.gov/niosh/mining/userfiles/works/p...

The 2 cycles of pressurisation should get you down to < 10 % v/v O2.

AS flare knock out drum (KOD) is designed for low pressure so following procedure may be adopted for purging the flare system before taking it in line:
1. Close flare KOD outlet valve.
2. Open N2 from all sweep points in flare headers. (can remove RO for in N2 sweep points for purging purging purpose)
3. Take 0.5-0.7 bar pressure upto flare KOD
4. Open flare KOD outlet valve and depressurize upto 0.1-0.2 bar pressure.
5. Repeat this 4-5 times.
6. Take gas sample from flare KOD in bladder and analyse for oxygen percentage.
7. If oxygen in less than 1 %, can take flare in service.
8. install RO in N2 sweep points.