Good luck. Sounds like your players are invested and paying attention. Always a great sign.
I don't think any of the groups so far has had a TPK. If you're willing to go a little Meta, you might create a scale to track the heat in the boiler, and increase it by 1 for every round of time that ticks off the countdown, so the players can see how dangerous things are getting. Maybe say there's a gauge on the boiler with a reading that goes from "Safe Temperature" to "Emergency: Evacuate Immediately" to "Engine Explosion."