TL;DR
Flame thrower turrets should use fuel constanly .What ?
Flame thrower turrets should constanly use fuel. They barely have any disadvantages, you only need to keep the fuel supply up.You can place as many as you want with no downside, only the ones that are attacked actually use fuel.
I had the idea for this suggestion, when I saw the pilot light which even burns when there is no fuel.
Something like 1 unit per minute could be a good start to discuss an appropriate amount of drain.
Why ?
Flame thrower turrets became the most important defense entity. Laser and gun turrets were degraded to support defense.They make area damage and especially in endgame it is hard to make a working defense without fire. (0.17.70)
A higher fuel usage encourages the player to setup an automated supply. Defense should have a price.
Failing to bring fuel in time will be punished by the biters quickly.