So they were just leaving for a long flight (requiring lots of fuel), and a tank is mostly vapor at that early point in the flight?
When I was a kid, my dad used to put a coffee can, with about a 1/2 inch of gas in the bottom, under the engine oil pan of our family car on extremely cold days. He’d toss a match in the can, and the gas would burn slowly because it couldn’t get enough oxygen to burn fast, let alone explode.
He emphasized that just putting a 1/16 of an inch or less could be explosive. He said he could put 2 inches in the can and it would still burn slowly, because the fumes dominate the rest of the space in the can.
So the center tank was practically empty right after takeoff?
Yep, the center tank was mostly empty (50 gallons/300 lbs left in a 6 ft. tall tank)(was used on the incoming flight but was not refueled for the flight to Paris)...747-100 has 7 fuel tanks and airlines only load the amount of fuel needed for the flight plus an extra amount to get to the divert airport. Costs big $$ to haul around extra fuel. (TWA was in double bankruptcy at that point). 747’s have “nitrogen inerting” installed for partially filled tanks now. Another factor was A/C packs running for a prolonged time on the ground (flight delayed for over an hour)...A/C packs directly under the center tank, heating the 50 gallons of Jet-A.
https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%20120-98A.pdf